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Electrokinetic properties of silica, alumina, and montmorillonite

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Title:
Electrokinetic properties of silica, alumina, and montmorillonite
Creator:
Horn, John Milton, 1951-
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Language:
English
Physical Description:
xiv, 153 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Adsorption ( jstor )
Aluminum ( jstor )
Charge density ( jstor )
Hydrogen ( jstor )
Ions ( jstor )
Montmorillonite ( jstor )
pH ( jstor )
Sodium ( jstor )
Streaming ( jstor )
Surface areas ( jstor )
Aluminum oxide ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF
Materials Science and Engineering thesis Ph. D
Montmorillonite ( lcsh )
Silica ( lcsh )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis--University of Florida.
Bibliography:
Bibliography: leaves 145-152.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by John Milton Horn, Jr.

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University of Florida
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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE










By

JOHN MILTON HORN, JR.


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY










UNIVERSITY OF FLORIDA

1978


1























































To My Lovely Wife, Barbara


1
















ACKNOWLEDGEMENTS


Deep appreciation and many thanks are extended to the

author's graduate supervisory committee which included

Dr. G. Y. Onoda, Jr., chairman; Dr. L. L. Hench;

Dr. R. W. Gould; and, Dr. D. O. Shah. Special thanks go to

his advisor, Dr. G. Y. Onoda, Jr., without whose many

helpful and lengthy discussions, this work would not have

been possible.

The author wishes to thank Mr. Fumio Ouchi for the

Auger data presented in Chapter 7. Also, thanks go to

Mr. Peter Curreri and Mr. Jim Adair for helpful discus-

sions, and to Mr. Nick Gallantino for technical

assistance in the lab.

Finally, the author wishes to acknowledge the

National Institute of General Medical Sciences grant

#GM21056-02 and the National Science Foundation

grant #AER76-24676 for partial financial support for this

work.


iii

















TABLE OF CONTENTS


Page

ACKNOWLEDGEMENTS....................................... iii

LIST OF TABLES ........................................ vi

LIST OF FIGURES........................................ vii

ABSTRA CT .............................................. xi

CHAPTER
1 INTRODUCTION .............. .................. 1

2 STREAMING POTENTIAL AND NONCREEPING
FLOW IN POROUS BEDS. ......................... 8
Introduction.............................. 8
Materials and Methods..................... 11
Results and Discussion.................... 16
Conclusions.............................. 24

3 CALCULATION OF SURFACE CHARGE DENSITY ERROR
DUE TO COMPLEX SPECIES FORMATION IN
SOLUTION .................................... 25
Introduction ............................. 25
Procedure................................ 28
Results ................................. 35
Discussion ............................... 41
Conclusions.............................. 47

4 DETERMINATION OF ADSORPTION CHARACTERISTICS
FOR FINE PARTICLE SYSTEMS FROM ELECTRO-
PHORESIS MEASUREMENTS ........................ 49
Introduction. ........................... 49
Materials and Methods..................... 50
Results and Discussion.................... 51
Conclusions.............................. 64













TABLE OF CONTENTS continued.



Page

CHAPTER
5 REVERSIBILITY OF ALUMINUM ION ADSORPTION
ON FUSED SILICA ............................. 66
Introduction ............................. 66
Materials................................ 70
M ethods .................................. 71
Results and Discussion................... 76
Conclusions................................ 84

6 THE EFFECTS OF AGING ON THE ZERO POINT OF
CHARGE OF ALUMINA........................... 85
Introduction............................. 85
Materials and Methods...................... 87
Resu lts .................................. 90
Discussion................................ 97
Conclusions.............................. 101

7 ELECTROKINETIC PROPERTIES OF
MONTMORILLONITE.............................. 103
Introduction ............................. 103
Materials................................ 107
Methods.................................. 109
Results and Discussion................... 111
Conclusions ................................ 136

APPENDIX A ............................................ 140

APPENDIX B ............................................ 142

BIBLIOGRAPHY........................................... 145

BIOGRAPHICAL SKETCH................................... 153
















LIST OF TABLES


Table Page
I Standard Free Energy of Formation Values
at 2980K (Kcal/mole)............................ 29

II Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2SiO3, and Their Equilibrium
Constants (K) ................................. 31

III Activity Coefficients for Ionic Species
at Various Solution Concentrations............ 32

IV Calculated Values for Disturbed Layer
Thickness (t) and Soluble Si02 at
Equilibrium Using Data from van Lier
et al. (ref 22) .............................. 46

V Values of Co for Various Zeta Potentials
and Weight Percent Solids ..................... 55

VI Values of C, F and log (C/F) for Various
Zeta Potential Values.......................... 59

VII Surface Areas of A-16, A-17, T-61, C-30 DB
and Gamma Alumina Powders...................... 91

VIII Compositions of Montmorillonite Clays 1608
and 1613 in Weight Percent .................... 108

IX Chemical Conditions for Zero Zeta
Potential (ZZP) for 1608 and 1613
Montmorillonites.............................. 125
















LIST OF FIGURES

Figure Page

1 Streaming potential cell; a) PMMA tube;
b) threaded PMMA plugs; c) platinum leads
to electrodes consisting of perforated
disks with attached platinum mesh;
d) platinum electrode; e) solution flow
entrance; f) solution flow exit;
g) porous bed; h) O-ring ..................... 13

2 Streaming potential solution flow system..... 15

3 R-C circuitry used in streaming potential
experiments.................................. 17

4 Streaming potential vs. pressure using
the R-C measuring circuit.................... 19

5 Streaming potential vs. pressure without
the R-C measuring circuit.................... 21

6 Flow rate vs. pressure for fused silica...... 22

7 Surface charge density error (Ao) vs. pH in
10-1, 10-2, 10-3 M/L NaCI solution for
vitreous silica using Bolt's experimental
conditions.................................... 36

8 Surface charge density error (Ao) vs.
total surface area for amorphous silica
at pH = 7, 8, 9 and 10....................... 37

9 Surface charge density error (Ao) vs. pH
in 10-1, 10-, 10-3 M/L NaC1 solutions
for hydrated silica using Tadros's and
Lyklema's experimental conditions............. 39

10 Surface charge density error (Ao) vs.
total surface area for precipitated
silica at pH = 7, 8, 9 and 10................ 40


vii













LIST OF FIGURES continued.


Figure Page

11 Zeta potential vs. concentration of
A1C13-6H20 for 0.00125, 0.0025, 0.025,
0.2 and 1.0% weight percent montmoril-
lonite........................................ 52

12 Apparent aluminum ion concentration Co
vs. weight percent montmorillonite for
zeta potential = 0 at pH = 4.0............... 56

13 Concentration of aluminum Co minus the
equilibrium concentration (C) of aluminum
in solution vs. weight percent montmoril-
lonite for zeta potential = +15, +10, +5,
0, -5, -10 mV at pH = 4.0.................... 57

14 Zeta potential and log adsorption density
(F) vs. equilibrium concentration (C) of
aluminum in solution for montmorillonite
at pH = 4 .0 .................................. 60

15 Log of the ratio of C and F vs. zeta
potential for montmorillonite for the
experimental data............................ 63

16 Streaming potential apparatus modified
to maintain constant flow pressure;
a) secondary reservoir; b) primary
reservoir; c) pump; d) solution flow
valve; e) cell; f) electrometer;
g) recorder; h) solution head height......... 73

17 Zeta potential vs. concentration of
sodium citrate and aluminum chloride......... 74

18 Streaming potential-pressure ratio vs.
stream time for untreated, heat treated,
base treated heat treated and base
treated and non-aluminated fused silica...... 77


viii













LIST OF FIGURES continued.


Figure Page

19 Zeta potential vs. concentration of
sodium citrate for fused silica in
supporting electrolytes of 10-3 M/L
NaCl and 10-3 M/L NaC1, 10-4 M/L
A1C13-6H20.................................... 81

20 Streaming potential-pressure ratio vs.
stream time for aluminated fused silica
for untreated, heat treated, base
treated and heat treated and base
treated surfaces............................. 83

21 Zeta potential vs. pH for y-alumina
after aging for 0, 1, 8, 16, 33 and
97 days in water ............................. 93

22 pH of the zero point of charge (pHZPC)
vs. aging time in water for T-61, A-17,
C-30 DB, y, and A-16 aluminas................. 94

23 Solution pH vs. weight of alumina added
for A-16 alumina.............................. 96

24 Zeta potential vs. pH for unwashed T-61
alumina (coarse particles) aged in water
for 1, 2 and 3 days.......................... 100

25 Zeta potential vs. pH for 1608 and 1613
montmorillonite............................... 112

26 Zeta potential vs. pH for 1613Na and
1613Ca montmorillonite....................... 113

27 Zeta potential vs. log concentration of
NaC1, CaC12-2H20 and A1C13-6H20 solutions
for 1608 montmorillonite..................... 115

28 Zeta potential vs. log concentration of
NaCI, CaC12-2H20 and AlC13-6H20 solutions
for 1613 montmorillonite..................... 116













LIST OF FIGURES continued.


Figure Page

29 Zeta potential vs. log concentration
of NaC1, CaC12-2H20, and A1C13-6H20
for 1613Na montmorillonite.................... 117

30 Zeta potential vs. log concentration
of NaC1, CaC12'2H20, and A1C13-6H20
for 1613Ca montmorillonite................... 118

31 Zeta potential vs. pH for 1608 montmoril-
lonite in 0, 10-5 and 10-4 M/L A1C13-6H20
solutions .................................... 122

32 Zeta potential vs. pH for 1613 montmoril-
lonite in 0, 10-5, 10-4 M/L A1C13-6H20
solutions .................................... 123

33 Auger peak height ratio vs. pH for
vitreous silica after one hour exposure
to 10-4 M/L A1C13-6H20 solution.............. 128

34 Zeta potential vs. pH for 1613Mat in
10-4 M/L A1C13-6H20 solution. SSH-O
means single solution method, clay
plus Al solution aging time = 0 hours.

35 Zeta potential vs. pH for 1613Mat in
10-4 M/L Al(N03)3-9H20 solution.............. 130

36 Zeta potential of 1613 montmorillonite
in 10-4 M/L A1C13-6H20 and water at pH =
6 vs. clay equilibrium time at pH = 4,
6 an d 8 ...................................... 132

37 Solution pH vs. total weight of montmoril-
lonite added to 25 ml of water............... 135
















Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy



ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE

By

John Milton Horn, Jr.

March 1978

Chairman: George Y. Onoda, Jr.
Major Department: Materials Science and Engineering

The surface chemistry of montmorillonite, vitreous

silica and alumina is investigated by electrokinetic

methods of streaming potential and microelectrophoresis.

Also, improved analytical techniques are developed to

obtain these electrokinetic data and adsorption

information for these oxide materials.

Many investigators find that linear streaming

potential-flow pressure relationships do not pass through

the zero potential-zero flow pressure origin. One source

of this error, electrode polarization, is eliminated

by introducing an R.C. circuit in the system. Simulta-

neous study of flow pressure versus flow rate reveals a

nonlinear curve suggesting the existence of noncreeping













flow in the pressure range studied. However, streaming

potential-flow pressure relationships remain linear and

pass through the origin when electrode polarization

effects are eliminated. Therefore, zeta potential

values calculated from the Smoluchowski equation under

noncreeping flow conditions are as valid as if creeping

flow conditions existed.

Using mass balance concepts, surface charge

density error incurred by assuming that all hydrogen

or hydroxide ions which cannot be accounted for as free

ions in solution adsorb to the surface rather than form

part of complex ions in solution is calculated for five

silica phases in three electrolyte solution concentra-

tions. The error increases as thermodynamic stability

of the silica decreases, as pH increases and as the total

surface area present in the experimental system decreases.

A new method is developed for determining adsorption

isotherms directly from electrophoretic measurements.

The unique feature is gathering zeta potential data as a

function of ion concentration for various solids

concentrations. The adsorption of aluminum ions onto

montmorillonite clay is presented as an experimental

example. The values of adsorption densities and equili-

brium concentrations of aluminum ions in solution after


xii













adsorption determined by this method fit an analytical

form of the Stern equation.

The desorption of aluminum ions from thermally

and/or chemically treated vitreous silica surfaces is

investigated by observing the changes of the streaming

potential-flow pressure ratio as a function of streaming

time. Aluminum ions desorb from surfaces whose treatment

has created adjacent silanol groups whereas they remain

on surfaces which contain isolated silanol groups.

Analogous to interpretations of adsorption-desorption

phenomena for fine particle systems, the mechanism of

adsorption onto coarse particle surfaces is hydrogen

bonding of the aluminum ion to isolated hydroxyl groups

on the silica surface.

The aging of alumina powder surfaces is studied by

observing changes in the zero point of charge (ZPC) with

aging time in water using electrophoresis measurements.

Alumina powders age due to the changing hydration state

of the surface. Grinding of T-61 powder from relatively

coarse to fine powder causes the ZPC to shift from

pH = 7.0 to pH = 9.5. Nonalpha phase aluminas age much

more slowly but to a greater extent than alpha phase

powders.

The surface chemistry properties of montmorillonite,

an aluminosilicate clay mineral, are investigated by


xiii













electrophoresis. Ion exchange of sodium, calcium and

hydrogen ions in solution for similar cations in the

clay controls the electrokinetic behavior of the

montmorillonite. Aluminum ions are found to specifically

adsorb from solution onto the surface. However, the

degree of aluminum ion adsorption as shown by electro-

phoresis measurements depends on the equilibration time

and pH of the clay in water whereas the electrokinetic

behavior of the clay in the absence of aluminum ions in

solution is independent of equilibration time and pH

in water.


xiv















CHAPTER 1
INTRODUCTION


The surface chemistry of montmorillonite, an

aluminosilicate clay mineral, and its two major con-

stituents, silica and alumina, is investigated by

electrokinetic methods of streaming potential and micro-

electrophoresis. Using these techniques, many authors

have studied the changing characteristics of the elec-

trical double layer surrounding the particles or colloids

as a function of adsorption of ions from solution, ion

exchange, and/or aging of hydrated surfaces. However,

proper use of these two techniques can also yield

important information about the mechanisms of these

processes. Opportunities are provided for improving

these techniques which can then be used to develop new

methods for determining adsorption properties of oxide

surfaces.

The electrokinetic parameter used to describe the

nature of the electrical double layer is the zeta

potential. Essentially, the zeta potential is the

electrical potential at the Stern plane in solution. The

Stern plane separates the diffuse (Gouy-Chapman) layer of











counter ion charge (the ions in solution which electrical-

ly balance the charge on the solid surface) from the

layer of adsorbed ions (Stern layer). The magnitude of

the zeta potential is determined by the concentration of

ions in the Stern layer and/or the concentration of

ions in the diffuse layer. Zeta potentials can be

determined from streaming potentials using coarse

particles or from electrophoretic mobilities of colloids.

In the past, two practical problems of noncreeping

flow and electrode polarization have limited accurate

measurements of streaming potentials. In a streaming

potential experiment, solution is forced to flow through

a porous bed of coarse particles. A linear relationship

between the streaming potential, E, and solution flow

pressure, P, is predicted by the Smoluchowski equation

which is used to calculate zeta potentials as described

in Chapter 2. In practice a linear relationship is

usually observed. However, Chapter 2 also shows that

a nonlinear relationship exists between volumetric flow

rate and flow pressure. This suggests that noncreeping

flow exists in the pressure range used in many typical

streaming potential measurements. Since the Smoluchowski

equation assumes creeping flow, zeta potentials calculated

from streaming potential values obtained under noncreeping












flow conditions may be invalid. Chapter 2 shows why

streaming potential values obtained under commonly

encountered noncreeping solution flow conditions can

still be used to calculate zeta potential values using

the Smoluchowski equation.

The second problem commonly experienced in streaming

potential measurements is the nonzero intercept of the

streaming potential-solution flow pressure relationship.

Theoretically, when the flow pressure is zero, the

streaming potential must be zero. However, a finite

rest potential is commonly observed at zero pressure.

This problem may be due to electrode polarization.

Therefore, Chapter 2 presents a method for measuring

streaming potentials without the undesirable effects of

electrode polarization.

Calculation of surface charge densities (a) of the

solid oxide surface from potentiometric titrations using

potential determining ions is used by many investigators

to determine the variations of a with pH. In a

potentiometric titration, the concentration of hydrogen

ions before and after addition of a known amount of

titrant is recorded. The quantity of hydrogen or

hydroxide ions which cannot be accounted for in solution

is assumed by most investigators to be adsorbed onto the












solid surface. However, if dissolution of the solid

occurs, some of the ions may not adsorb to the surface,

but may be part of complex ions in solution. However,

they are not free hydrogen or hydroxide ions. Their

absence cannot be detected by pH measurement. Therefore,

absolute values of surface charge densities calculated

by most investigators are too large. For silica, surface

charge densities are negative values since the surface

is negatively charged in water above pH = 3.0. Therefore,

Chapter 3 describes the method used for aqueous silica

systems to calculate the surface charge density error due

to use of mass balance expressions which are incomplete

when hydrogen or hydroxide ions which are part of complex

ions in solution due to dissolution of the solid are

neglected.

Accurate adsorption density values which are used

to calculate surface charge densities are easily

determined for oxide systems since hydrogen and hydroxide

ions are potential determining ions. However, more time

consuming and sometimes less accurate methods must be

used to determine adsorption densities of other ions on

the oxide surface. Chapter 4 presents a new method for

determining this information from electrophoresis data.

The unique aspect of this method is the use of various













solids concentrations in gathering data of zeta potential

versus apparent ion concentration in solution. From

these data, adsorption densities and equilibrium

concentrations of ions in solution after adsorption can

be determined. The Stern equation is then used to

determine if these data fit the expected form for common

adsorption isotherms.

The techniques discussed in Chapters 2 and 4 are

used to study the electrokinetic properties of an

aluminosilicate system. The system chosen was montmoril-

lonite clay, which is a major component of phosphate

slimes. The slow settling of phosphate slimes due to

fine particle size clays such as montmorillonite is a

major problem in the phosphate mining industry. Dense

coagulation of clay particles would result in faster

slime settling rates and higher final sediment densities.

The degree of coagulation of the particles is partially

controlled by their electrical double layer properties.

According to DLVO theory (1), coagulation will result

when the electrical repulsive forces between particles

are small enough to allow van der Waal's forces of

attraction to cause particle coalescence. Since zeta

potentials are a measure of the degree of these electrical

forces, Chapter 7 is devoted to finding chemical













conditions of zero zeta potential (ZZP) of the particle

surfaces.

To fully understand the montmorillonite system, its

two major components, silica and alumina, were studied

independently. Chapter 7 shows that montmorillonite has

two important properties. First it specifically adsorbs

aluminum ions under certain conditions. Since silica also

has this property, a detailed study of the desorption of

this ion from silica surfaces treated with chemical and

thermal agents known to alter hydration (2) is

presented in Chapter 5. This is accomplished by using

a slightly modified streaming potential technique from

that described in Chapter 2. Information obtained from

these desorption studies gives insight to the adsorption

mechanism of aluminum ions onto silica surfaces.

The second major property of montmorillonite

described in Chapter 7 is that equilibration time and pH

in water affects its capacity to specifically adsorb

aluminum ions. This appears to be a phenomenon caused by

varying degrees of hydration of the clay particles.

Another "aging effect" of this type is described in

Chapter 6 for aluminum oxide surfaces. It is possible

that hydration of clay particles is controlled in some

manner by the hydration of alumina in the clay. Therefore






7





Chapter 6 presents evidence of alumina surface hydration

by studying the change of the zero point of charge

with aging time in water.















CHAPTER 2
STREAMING POTENTIAL AND NONCREEPING FLOW IN POROUS BEDS


Introduction

The measurement of streaming potential on porous beds

is an important method for determining zeta potential (3).

The method is convenient because many materials cannot

readily be shaped as capillary tubes. A common practice

for porous beds is to use the Smoluchowski equation (4)

4T XE []
EP

to calculate the zeta potential () from the measured

streaming potential (E), the viscosity (n), the specific

conductivity (X), the dielectric constant (E), and the

driving pressure (P).

Equation 1 was originally derived for simple

capillaries assuming laminar flow conditions. Boumans (5)

has shown that the E/P ratio in simple capillaries is

smaller under turbulent flow than under laminar flow.

A change in flow behavior with increasing pressure is

also known to occur in porous beds. The relationship

between flow rate and pressure changes from linear to

nonlinear at a certain Reynolds number. Many streaming

potential measurements on porous beds, in the past, have













been made under conditions near the linear to nonlinear

transition zone. This raises questions about the

validity of using the E/P ratio to calculate the zeta

potential when the flow condition is nonlinear. The

purpose of this investigation was to determine whether

the E/P ratio changes with increasing P, as flow changes

from linear to nonlinear. If no changes occur,

Equation 1 is valid in the nonlinear region as well as

the linear region. Only a rather narrow pressure range

was investigated since this was the range of interest in

typical streaming potential measurements.

In porous beds, a Reynolds number, Re, has been

defined (6) as

D vp 1
Re = -- (-) [2]

where D is the mean particle diameter which is equal to

400 microns for the particles used, v is the superficial

velocity found by taking the ratio of volumetric flow

rate and cross sectional area of the bed, p is the

solution density, i is the solution viscosity and E is the

bed porosity.

At Re values below around 10, flow rate and pressure

are linear (7). This is the region of creeping or Darcy

flow. At high Reynolds number, a nonlinear relationship












develops (the region of noncreeping or nonDarcy flow).

This change is a result of the formation of standing

eddies behind the particles. The Ergun equation (6)

describes the flow behavior over the range of interest:

D 2
(P P) (=) ( ) = 150 D (1-) [3]
2-7 L D (vp/) [J
vp p

where AP is the pressure drop across the bed, L is the

bed length and all other variables as defined in Equa-

tion 2.

Accurate E versus P measurements cannot be obtained

without recognizing and dealing with the experimental

problems of electrode polarization. Ball and

Fuerstenau (3) cite electrode polarization as the

probable cause for E versus P curves not passing through

the origin. Somasundaran and Agar (8) proposed that the

instantaneous change in the voltage when solution flow

is initiated is the true streaming potential. Korpi and

deBruyn (9) incorporated a recorder into their system to

aid in the measurement of the instantaneous voltage

changes during flow initiation and termination.

In the present investigation, a R-C circuit is

introduced which directly nulls the background potentials

(rest potential and electrode polarization potentials).

This greatly facilitates the measurement of the true













streaming potential, especially under conditions where

the background potential is large and varies with time.

A modified streaming potential apparatus is described

that is suitable for low pressure studies and for use on

materials that are considerably reactive. A new cell

design is introduced that is easier to pack, easier to

clean, and is less fragile than previous designs.



Materials and Methods

1. Materials

The bed materials used for this study were fused

silica* and an invert silicate glass (denoted as

"bioglass") whose preparation is noted elsewhere (10) and

whose composition is as follows: 45 wt.% Si02,

24.5 wt.% CaO, 24.5 wt.% Na20, and 6.0 wt.% P205. This

glass has certain reactive properties which makes it an

interesting material for biological implant studies (10).

The fused silica and bioglass were ground and sieved, and

the -20+45 fraction (0.0833-0.0354 cm aperture) was used

in the experiments. The fused silica was acid washed by

conventional methods used by other investigators (11).

Due to the reactivity of bioglass, no acid washing was



*Vitreosil,Thermal American Fused Quartz Co., Montville,
New Jersey.













performed. Instead, bioglass was only washed with con-

ductivity water.

The bed lengths were 4.1 cm and the cross sectional
2
area of the beds was 2.38 cm The bed porosity was

calculated to be 36%.

Water used to rinse the bed material and to prepare

solutions was obtained from a water deionization system*

with the following specifications: a resistivity of

1.87 x 10 ohm-cm, dissolved solids at the parts per

billion level, dissolved gases removed, and organic

removed. Since dissolved gases were removed, equilibra-

tion with air was required which caused the pH to drop to

5.5-6.0 due to absorption of CO2. This is the pH range

at which the streaming potential experiments were

performed. Sodium chloride** solutions of various

concentrations were prepared with this water and used in

the experiments.


2. Apparatus

The cell for the streaming potential system is shown

schematically in Figure 1. It consists of a thick walled



*Continental Water Conditioning Co., Inc., Gainesville,
Florida.
**ACS reagent grade, Scientific Products, Ocala, Florida.




































Figure 1. Streaming potential cell; a) PIMA tube;
b) threaded PMMA plugs; c) platinum leads to
electrodes consisting of perforated disks with
attached platinum mesh; d) platinum electrode;
e) solution flow entrance; f) solution flow
exit; g) porous bed; h) 0-ring.













polymethylmethacrylate (PMMA) tube with ends tapped to

receive two threaded PMMA plugs. This design allows

easier cleaning between experiments and easier packing of

the porous bed material. The platinum leads to the

electrodes* protrude completely through the PMMA plugs

and are sealed by a press-fit using a Teflon spacer. The

platinum electrode consists of a perforated disk for

easy solution flow and an attached platinum mesh for

increased electrode surface area. Because the electrodes

are able to slide through the PMMA plug, adjustments can

be made to obtain a tight packing of the bed.

The solution flow system is shown schematically in

Figure 2. The solution reservoir is a 25 liter poly-

ethylene container with a mechanical on-off flow valve.

The height of the outlet tube is varied to produce dif-

ferent hydrostatic pressures across the cell. This

design allows for E versus P measurements at pressures as

low as 1.0 cm Hg. Also, flow rates can be measured at

the outlet tube exit.

The potentials from the electrodes were measured with

an electrometer.** The output from the electrometer was



*Englehard Industries, Carteret, New Jersey.
**Keithley, Model 602.






















Solution
reservoir


I
head


on-off
flow valve
{


-- adjustable
outlet
tube


cell


Figure 2. Streaming potential solution flow system.












passed through an R-C circuit (details of which are dis-

cussed in the Results and Discussion section) into a strip

chart recorder.* E versus P curves were generated by

plotting the peak height value on the recorder cor-

responding to the applied pressure.



Results and Discussion

1. Circuitry for Measuring Streaming Potential

A R-C circuit shown in Figure 3 was introduced into

the system for the purpose of directly measuring only the

true streaming potential on the recorder within a 99% ac-

curacy by nulling out all background potentials. Two

different circuits can be employed by changing the switch.

When the liquid is not streaming, the switch is in

position 1. The rest potential is rapidly stored in the

capacitor, nulling the signal to the recorder. The time

constant of this circuit is 2.2 seconds and so more than

99% of the nulling occurs in 10 seconds.

After charging the capacitor with the rest potential,

the switch is turned to position 2 and flow through the

cell is initiated within two seconds. The time constant

for the new circuit is 220 seconds. Therefore, the rest


*Hewlett Packard Model 680.

















105a
10 Li


670 n Recorder
with 2 x 10I
resistance


330a


._ output

Electrometer
input :
from
cell


Figure 3. R-C circuitry used in streaming potential
experiments.












potential stored in the capacitor does not decay more than

1% in two seconds. The potential received by the electro-

meter when flow is initiated is the sum of the rest

potential and the true streaming potential. However,

since the rest potential has already been stored in the

capacitor, the rest potential contribution of the input

voltage is nulled out and only the streaming potential is

recorded. Because of the arrangement of the resistors,

the input voltage to the recorder is 1/100 of the output

from the electrometer (which is one volt full scale).

Therefore, the recorder is kept on a 10 mV full scale

range.

Streaming can be terminated after around five seconds

of flow. Then the switch is placed in position 1. The

capacitor again charges rapidly to the rest potential and

a new measurement can be initiated.

Using the R-C circuit, streaming potential measure-

ments were carried out on fused silica using water and

NaCl solutions of 10-5, 10 and 10- mol/L as streaming

solutions and on bioglass using water as the streaming

solution. The E versus P curves obtained are shown in

Figure 4. Within experimental error, all curves are

observed to be linear and pass through the origin.

Similar findings have been observed for a variety of














-6.0- --z
HO

-5
> -5.- 10 M NaCI --
44
o 10 M NaCI .

x -4-0- 10 M NaCI 'b

*^ Bioglass
0 -3.0- H 0
2.0 3.0 4.0


E -2.0 -


0
-1.0 -= \0
0O


0 1.0 2.0 3.0 4.0

Pressure, (cm.Hg)

Figure 4. Streaming potential vs. pressure using the R-C
measuring circuit.













materials and solutions without deviations in linearity

or intersection with the origin.

Without the R-C circuit, under otherwise identical

conditions, straight lines passing through the origin

are not obtained in the E versus P curves as shown in

Figure 5. This can be attributed to the fact that rest

potential contributions were not taken into account.


2. Creeping Versus Noncreeping Flow Conditions

During the E-P measurement described above, solution

flow rates were also determined. Volumetric flow rates,

Q, versus pressure are given in Figure 6. A nonlinear

relationship between Q and P was observed.

In Equation 3, v can be converted to Q, since
2
Q = vfr2, where r is the radius of the bed. The Ergun

function becomes

2 1.75 2
Ap k Q + Q [4]
kP k1
1 1

where

k3 D 2 4
k1 P

and

2
k2 150 ( )(Tr p)
2 DP
3
Since = 0.36, D= 0.04 cm, L = 4.1 cm, p = 1 g/cm,
p












































Figure 5.


Pressure (cm. Hg)

Streaming potential vs. pressure without the R-C
measuring circuit.














A IU M NOCI ..I .. ,,
-4
10 M NaCI
8.0- -3
- 0o 10 M NaCI

-* o
6.0-



o 4.0



2.0- _
A


0 A
0 1.0 2.0 3.0 4.0


Pressure, (cm. Hg)

Figure 6. Flow rate vs. pressure for fused silica.













p = 0.009 poise, and r = 0.87 cm, the values of kI and k2

are calculated to be 0.0040 and 51, respectively.

The data of Figure 6 fit an equation of the form

given by Equation 4 if k1 and k2 are 0.00375 and 48, re-

spectively. This is in close agreement with the re-

spective calculated values. Thus, the nonlinear behavior

follows what is expected from the earlier flow studies.

From Figures 4 and 6, it can be seen that the E/P

ratio remains unchanged even though solution flow

becomes nonlinear. The streaming potential is

proportional to the streaming current. The streaming

current is given by the integral over space of the product

of the charge density and velocity (projected in the net

flow direction) at every point (12), and so this integral

must be proportional to pressure. However, the charge

density distribution does not change with pressure. This

strongly suggests that the velocity of the liquid at every

point near the surface increases in direct proportion

with the pressure. This conclusion would be in agreement

with the current views that streamline flow near the

particle surfaces remains even after standing eddies

develop.














Conclusions

In the porous beds that were studied, noncreeping

flow did not appreciably affect streaming potential-flow

pressure relationships at the pressures studied. This

suggests that streaming potential experiments in the

past which have been performed under similar noncreeping

flow conditions would have had the same E/P ratios as

would have been measured under creeping flow conditions.

For those cases, the calculated zeta potentials would have

been as valid under noncreeping flow as under creeping

flow conditions.

Electrode polarization effects were nulled out by

using a R-C circuit which allowed only the true streaming

potential to be recorded. This method greatly facili-

tated the measurement of streaming potentials of reactive

materials such as bioglass.

Combined results of the electrode polarization and

noncreeping flow studies showed that electrode polari-

zation and not noncreeping flow was the reason for E

versus P curves not passing through the origin as found

by previous investigators. This was demonstrated by the

fact that E versus P curves were linear and passed through

the origin under conditions of noncreeping flow as long

as electrode polarization effects were accounted for,

vis., by using the described R-C circuitry.
















CHAPTER 3
CALCULATION OF SURFACE CHARGE DENSITY ERROR DUE TO
COMPLEX SPECIES FORMATION IN SOLUTION


Introduction

Calculations of surface charge densities are made

from adsorption density values of potential determining

ions on oxide surfaces (13-16). Potentiometric titration

of the aqueous-oxide system with potential determining

ions are used to determine adsorption density values as

a function of pH. For aqueous-silica systems, different

negative values of surface charge density at any given

pH are found depending on the phase of silica used in the

titration. Using precipitated silica, Tadros and

Lyklema (17) found that absolute values of surface charge

densities were an order of magnitude higher than those of

Bolt (14), who studied amorphous silica. Tadros and

Lyklema suggest that a gel structure exists on the

precipitated silica surface and that extension of

surface and counter ion charge inside the pores of this

gel structure causes higher charge densities. However,

Yates and Healy (18) suggest that precipitated silica does

not have a gel structure in the surface but that it

contains an incompletely condensed layer of polysilicic













acid. They also suggest that surface roughness con-

tributes to high inner layer capacities creating high

surface charges due to slight interpenetration of

potential determining and adsorbed counter ions.

An alternative hypothesis for anomalously high

surface charge densities can be investigated by examining

the mass balance equation which describes an aqueous-

oxide system in a titration experiment. The real

adsorption density (Freal) is defined as the excess moles

per unit of surface area of hydrogen over hydroxide ions

on the solid surface. The correct mass balance equation

for this value is

[(H-OH)initial+AH-(H-OH) solution-C]V
real A [5]

where (H-OH)initial is the total excess moles of hydrogen

over hydroxide species per unit volume of solution in the

system before titrant addition, AH is the moles of titrant

per unit volume of solution added to the initial system,

(H-OH)solution is the excess moles of free hydrogen over

hydroxide ions per unit volume in solution, V is the

solution volume, A is the total surface area of oxide

powder used in the titration and C is the excess moles

per unit volume of hydrogen over hydroxide ions which are













part of complex species in solution due to dissolution

of the solid. Essentially Equation 5 says that those

hydrogen or hydroxide ions after titrant addition which

cannot be accounted for as free ions in solution or in

solution as part of complex species must be adsorbed to

the solid surface. Only relative values of Freal can

be determined if titrations are performed in one elec-

trolyte concentration since the quantity (H-OH) initial

is unknown. However, by performing titrations at various

electrolyte concentrations, several adsorption density

versus pH curves can be obtained which intersect at one

point. In the absence of specific adsorption of counter

ions to the oxide surface, rreal is zero at this point.

Therefore, from Equation 5, (H-OH)initial can be

determined and absolute values of Freal can be calculated.

Many workers choose to neglect the C term in Equa-

tion 5 since they assume that the concentration of complex

species which form in solution is negligible compared to

the concentration of free hydrogen or hydroxide ions in

solution. However, this may not always be true especially

under conditions of a thermodynamically unstable solid

phase in solutions of high pH. If "C" is neglected, then

[(H-OH) initia+AH-(H-OH) olution]V
apparent A













where apparent is the adsorption density calculated from

potentiometric titration data by most investigators. If

Equation 6 is subtracted from Equation 5, then

VC
Ar = r r = [7]
real apparent A [7]

C will be the amount of error incurred in the adsorption

density value when these hydrogen or hydroxide ions

assumed to be free in solution actually compose part of

complex solution species. Therefore, this chapter serves

to determine C in terms of experimental parameters so

that real apparent and, therefore, surface charge

density error real Capparent, can be calculated for the

silica water system.



Procedure

1. Formation of Complex Species

It is possible to calculate the equilibrium solu-

bility of silica from thermodynamic information. All

that is required is knowledge of the free energy of

formation of the solid and soluble species at the

appropriate temperature. Table I shows the standard free

energy of formation for five solid silica phases, a sodium

silicate, a neutral aqueous silica species and two aqueous

complex ionic species. Also shown are the values for












Table I. Standard Free Energy of Formation
Values at 2980K (Kcal/mole)


N Species AG (Kcal/mole)
1 Si02(quartz) -192.4

2 Si02(cristobalite) -192.1

3 Si02(trydymite) -191.9

4 Si02(vitreous) -190.9

5 Si02(hydrated) -187.8

H2Si03(aq) -242.0

HSiO^ ) -228.36
3(aq)

SiO(aq) -212.0

Na2SiO3(c) -341.0

Na(aq) -62.6

H20(aq) -56.69

H 0
(aq)

OHaq) -37.6
(aq)

Sources: Pourbaix, ref. 14 and
Dickerson, Gray and Haight,
ref. 20.













H20 Na H and OH (19,20) Equa-
2(aq)' a(aq)' (aq)' (aq) 20) Equa-
tions are written describing the formation of the neutral

soluble silica species (H2Si03). Since five different

silica phases are considered, five equations yielding

five equilibrium constants (K[N] where N = 1-5) are

obtained. The reactions and values for their equilibrium

constants are shown in Table II.

The reactions for the formation of the various ions

from H2SiO3 species are shown in Equations 8-10. Their

equilibrium constants are calculated from data in Table I.

H2SiO3 = HSiO3 + H KA = 1.01 x 10-10 [8]


H2SiO3 = SiO + 2H KB = 9.92 x 1023 [9]


2Na+ + H SiO3 t Na2SiO3 + 2H+, KC = 2.81 x 10-27 [10]

Activity coefficients for the various ions were

incorporated into the final calculations of surface

charge density error. The values for these coefficients

were obtained from calculations by Klotz (21) using the

Debye-Hueckel theory for strong electrolytes. Table III

shows the values for ions used in the calculations at

several ionic strengths. The values for HSiO3 and Si03

were not directly available from Klotz (21), but were

chosen for ions of similar size as HSiO and SiO3.
33













Table II. Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2Si03, and Their Equilibrium
Constants (K)

Reaction K[N (where N=1-5 from Table I)
Si2(quartz)+H2H2SiO3 K 1 = [H2Si03] = 6.31 x 10-6

Si02(crist.)+H20=H2Si03 K 2 = [H2Si03] = 1.05 x 10-5

Si02(trid.) +H20=H2SiO3 K 3 = [H2SiO3] = 1.47 x 10-5

Si02(vit.) +H20=H2SiO3 K 4 = [H2Si03] = 7.49 x 10-5

Si2(hyd.) +H20=H2SiO3 K 5 = [H2Si03] = 1.47 x 10-2












Table III.


Activity Coefficients for Ionic Species
at Various Solution Concentrations


Solution Concentration (moles/liter)
Species 0.1 0.01 0.001
HSiO3 0.750 0.898 0.964

SiO 0.360 0.660 0.867

H+ 0.830 0.914 0.967

Na+ 0.770 0.901 0.964

Source: Klotz, ref. 21













The excess of hydrogen of hydroxide species which

is part complex species is directly related to the

concentration of each complex species in solution. From

Equations 8-10 it can be seen that for every HSiO3 ion

formed, the excess of hydrogen over hydroxide is -1.

Accordingly, for Si3 the excess is -2 and for Na2SiO3

it is -2. Therefore,

C = -([HSiO3] + 2[Si03] + 2[Na2Si03]) [11]


2. Development of the Working Equation for Ao

By writing the equilibrium constant expressions for

Equations 8-10 and substituting K[N] from Table II for

[H2Si03], the equilibrium concentration of each complex

species can be written in terms of experimental

parameters. That is,

[HSiO3] + K[N] [12]



S KB K[N] [13]
[SiO2 [13]
2 +2


KC K[N] YNa+ [Na ]
[Na2Si3] = 2 2 [14]
YH+ [H ]

Now, C in Equation 7 can be expressed in terms of experi-

mental parameters:













V KA K[N] + 2KB K[N]

S HSiO YH+ [H+] Si0 YH+ [H]
3 3

+2 2
2KC K[N] [Na +]2 +
+ 2 2 [15]
YH+ [H ]

Adsorption densities can be converted to surface charge

densities by the relation

a = 106FT [16]

where F is Faraday's constant (9.65 x 104 coul/mole) and

106 is the number of microcoulombs per coulomb. There-

fore,

6 V
Ao = 10 F A(C) [17]

where C is the term in parentheses in Equation

Calculations of Ao were made for 10-3, 102 and 10-

M/L NaC1 solutions for the five different silica species

for a pH range of 7-10 in increments of 0.1 pH units.

Since the calculations are numerous, a computer program

was written and is appended. The program is written

specifically for the silica system. However, as the

appendix shows, the program can be generalized for any

material as long as the free energy of formation data for

the various reacting species are known.













Results

Figure 7 shows the calculated values for Aa as a
03 -2 -1
function of pH for 10-3, 102, 10 M/L NaC1 solutions

for vitreous silica using Bolt's (14) experimental

conditions of surface area and solution volume. It can

be seen that as the ionic strength of the electrolyte

solution increases, the absolute value of AC increases

for any given pH value. This trend is consistent with

the experimental surface charge densities measured by

Bolt.

Comparison of the calculated values with Bolt's

absolute values in Table I of his paper (14) for amorphous

silica shows that little error is incurred in neglecting

complex species formation in solution. This is not an

unexpected result since Bolt used a high surface area

to volume ratio in his experiments. Therefore, Bolt's

data represents real as well as Oapparent since Ao is

negligible for the pH range studied.

Using Bolt's data it is possible to calculate at

what value of total surface area significant error would

have occurred for any given pH value. Figure 8 shows

Aa as a function of total surface area for pH = 7, 8, 9,
5 2
10. For pH 9 and total surface area of 10 cm 10% error
7 2
would have occurred. Bolt used 5.4 x 10 cm total






















(J
O
x





U
E







b
O


E



















Figure 7.


Surface charge density error (Ao) vs. pH in
10-2, 10-3 M/L NaCI solution for vitreous
silica using Bolt's experimental conditions.






























E
o -2
_- 10 -
10
--





0 10
.2


b -3


pH
*7
4o8
10 09





-5

105 106 107 10


Total surface area (cm2)



Figure 8. Surface charge density error (AF) vs. total
surface area for amorphous silica at pH = 7,
8, 9 and 10.












surface area in his studies. Therefore, even at pH = 10,

less than 1% error should be expected based on his

experimental values of surface charge density. This

calculation quantitatively demonstrates the importance of

the experimental condition of surface area in a

titration experiment if the formation of complex

species is to be neglected. To achieve greatest accuracy

a high surface area to volume ratio must be used.

Tadros and Lyklema (17) studied the surface charge

density as a function of pH for precipitated silica.

The absolute values range from 15 to 200 micro coul/cm2

in the pH range 7-10. Figure 9 shows that values of

Aa calculated in the present work reach 30 micro coul/cm2

which suggests that at pH = 10, 10-15% error is incurred

by Tadros and Lyklema by neglecting the complex species
6 2
formation. Tadros and Lyklema used 8 x 10 cm surface

area in their experiments (20g of 40 m2/g per 100 cc).

Figure 10 shows a plot of Aa versus total surface area.

At a surface area of 8 x 106 for pH = 10, 10% error should

be expected based on their value of a. Tadros and
8 2
Lyklema therefore should have used 10 cm of total sur-

face area to achieve 1% error or less at pH = 10 if they

wanted to neglect the complex species formation.

From data such as presented in Figures 8 and 10,

the amount of error in surface charge density incurred as






















(J

E





b
F <















Figure 9.


Surface charge density error (Aa) vs. pH in 10
10- 10-3 M7L NaCI solutions for hydrated
silica using Tadros's and Lyklema's experi-
mental conditions.

























































6 7
10 10


Total surface area, (cm2)


Figure 10.


Surface charge density error (Ao) vs. total
surface area for precipitated silica at
pH = 7, 8, 9 and 10.


CoJ
E
O
_-
0
0


E

b
<













a function of pH for any given surface area used can be

determined. The error becomes more significant as the

pH increases and surface area decreases. This should be

expected since soluble complex species form in greater

quantities at higher pH's due to increased ionization of

the neutral soluble silica species.

According to the calculations, based on the data

of Tadros and Lyklema, significant error should be

expected by using 2g of 40 m2 /g material in 100 cc of

solution. However, these authors claim no significant

difference in the amount of OH/g SiO2 adsorbed when 2, 10

or 20 grams of solid were used in the same volume of

solution. To observe this, they must have used much

less volume than 100 cc, which was the solution

volume V used in the calculation in the present work.

However, no indication of solids concentration was given.



Discussion

These calculations demonstrate the importance of

knowing the amount of each phase present in an aqueous

silica system. In an experiment using the same total

surface area for all five phases less error in surface

charge density will occur for quartz than any other solid

silica phase. The order of least to most significant













error is the same as the most to least thermodynamically

stable. The calculations show that in a mixed phase

solid of 99.99% quartz and 0.01% hydrated silica, the

amount of soluble complex species formed from the hydrated

phase will be the same as the amount formed from the

quartz.

One occurrence of a mixed phase solid has been

addressed by van Lier et al. (22). In their quartz

solubility studies, they confirmed the existence of a

disturbed layer on ground quartz particles by dissolution
0
studies at high pH values. The thickness of 300 A which

they calculated from their results agreed well with

Gibb et al. (23). Van Lier et al. found that abnormally

high solubilities in water and high pH solutions of

quartz are obtained if the disturbed layer is not re-

moved. However, removal of this layer yielded normal

solubility data. This suggests that the layer is not

quartz but probably a more thermodynamically unstable

phase.

Van Lier et al. were not able to identify the

disturbed layer phase. Using their experimental condi-

tions and the free energy of formation values in Table I

of the present work, it is possible to calculate the

theoretical thickness of the layer assuming the silica

phase is each of the five solid phases considered.












To make the thickness calculations, dissolution

reactions must be written. These are

Si02 + OH = HSi03 [18]

SiO2 + 20H = SiO0 + H20 [19]

Using quartz as an example, the standard free energy of

reaction for Equation 18 is 1640 cal/mole and for Equa-

tion 19 it is -1100 cal/mole assuming the data in

Table I are correct for quartz phase. Neglecting activity

coefficient, the equilibrium constant expression for each

equation is

-AGo [HSiO3]
K18 = exp( RT = Sio2 OH- [20]


-AGo [H20][Si0=]
19 = exp(fT-) = 2o]' [21]
[SiO2] [OH]

K18 is calculated to be 0.061 and K19 is 6.53. Since K18

and Kl9 are known, the concentrations of HSiO3 and Si0O

can be calculated using pH = 12.30 from experimental

conditions of van Lier et al. These values are [HSiO3] =

1.2 x 10-3 M/L and [Si0O] = 2.6 x 10-3 M/L. The equi-
-4
valent concentrations of Si02 are 9.4 x 10 M/L and

2.05 x 10-3 M/L. Therefore the total concentration of

quartz (Si02) present in solution after dissolution is

2.99 x 10-3 M/L or 0.18 g/L. Since van Lier et al. used













0.030 liters, the final weight of powder present in the

system after dissolution was 0.3854g-0.0054g = 0.3800

grams, where 0.3854 is the initial weight of powder used

by van Lier et al. before dissolution.

To calculate the thickness of the disturbed layer,

two assumptions are needed. First, the particles are

assumed to be spheres. Secondly, the number of particles

in the system remains constant. The total initial volume

Vi of particles in the system is related to the initial

particle radius ri by

3
V. = Ni 4/3rr3 [22]
1 1 i

where Ni is the initial number of particles. The same

equation can be written relating the final total volume

Vf and final particle radius rf after dissolution. That

is,

Vf = Nf 4/3rr3 [23]

where N is the final number of particles after dis-

solution. Since

W
P = V [24]

then

W. = N.p 4/3Br3 [25]
1 1 i


and













W = Nf p4/3irr3 [26]

where p is the density of the solid phase, Wi and Wf are

the initial and final total weight of powder respectively.

Since N. = Nf, then



1 f
W. Wf
-3 3 [27]
p4/3Trr~ p4/3irf3
-4
A value for r. = 1.5 x 10 cm is chosen since it is mid-

range of the particle size range used by van Lier et al.

Therefore,

Wf 1/3
r = ( ) ri [28]
-4
For quartz the value of rf is 1.493 x 10 The

thickness t of the disturbed layer would be t = ri rf =
-7 0
7 x 10 cm = 70 A.

Similar calculations for the four other silica

species were made. Table IV shows values of t and the

total concentration of soluble SiO2 at equilibrium for each

species. The values of t in Table IV represent the amount

of the particle which could be dissolved away if the

layer was each of the phases studied. They also represent

the thickness which the disturbed layer must have for the

solution to become saturated with respect to each phase

considered. On this basis, the disturbed layer cannot












Table IV. Calculated Values for Disturbed Layer
Thickness (t) and Soluble Si02 at
Equilibrium Using Data from van Lier
et al. (ref. 22)

2 o
Silica phase SiO2 x 10 (moles/liter) t (A)

Quartz 0.30 70

Cristobalite 0.50 119

Tridymite 0.70 167

Vitreous 3.88 968

Hydrated

*Calculations show total dissolution













have a quartz, cristobalite or tridymite structure since

the values of the thickness calculated are lower than
0
300 A. That is, the solution becomes saturated with

respect to quartz, cristobalite or tridymite when 70, 119
O
or 169 A of thickness are dissolved away. However, the

disturbed layer could have either an amorphous or hydrated

structure since saturation cannot occur in a system

containing silica particles with a disturbed layer thick-
O
ness of 300 A.



Conclusions

It has been shown that care must be taken in

adsorption density measurements using potentiometric

titrations to use high surface area powder to solution

volume ratio systems if one is to neglect complex species

formation. Guidelines for such experimental conditions

have been presented by using experimental data presently

available in the literature for the basis of the calcula-

tions. It is important to note that more experimental

error will be incurred by not considering complex species

formation for less stable silica phases especially at

higher pH values where the ionization of the neutral

soluble silica species occurs in greater quantity.

These calculations have also shown that as little

as 0.01% hydrated phase present in quartz forms equivalent













concentration of complex species as all of the quartz

itself. Hence the importance of careful consideration

of the solid phase under study cannot be over emphasized.

The thickness of the disturbed layer on quartz has

been calculated using data on the solubility of quartz

and thermodynamic data for solid silica species. The

calculations show that the layer cannot be quartz,

cristobalite or tridymite.
















CHAPTER 4
DETERMINATION OF ADSORPTION CHARACTERISTICS FOR FINE
PARTICLE SYSTEMS FROM ELECTROPHORESIS MEASUREMENTS


Introduction

Different types of adsorption information can be

obtained for fine particle-aqueous systems depending on

the type of experimental method used. The method com-

monly used to calculate adsorption densities of oxide

systems from potentiometric titration using potential

determining ions to determine the zero point of charge of

the surface was described in Chapter 3. To determine

adsorption densities accurately for ions other than

potential determining ions, other techniques such as

solution depletion (as measured by atomic adsorption or

emission) must be used. These measurements require

synthesis of calibration curves which essentially doubles

the amount of work involved in determining an entire

adsorption isotherm.

The unique feature in the method presented in this

chapter for determining adsorption isotherms from electro-

phoresis measurements is the use of various solids

contents. By obtaining zeta potential behavior as a

function of apparent ion concentrations in solutions for












different solids concentrations, adsorption densities

and equilibrium solution concentration of the ions can be

calculated. The montmorillonite clay-aluminum ion solu-

tion system is used to obtain these experimental data.

These data are then plotted on the basis of their

relationship in the Stern equation (24) to determine if

they fit an analytical form of this equation.



Materials and Methods

Montmorillonite 1613 was used as received from the

supplier.* A stock suspension of 2% solids was prepared

by ultrasonic dispersion of the clay in water and was

equilibrated for one week at ambient temperature. After

equilibration, 0.5, 0.05, 0.005 and 0.0025% suspensions

were prepared by diluting the stock suspensions with

water described in Chapter 2.
-1 -2
Aluminum chloride solutions of 2 x 10 2 x 10

2 x 103, 2 x 10, 2 x 105 and 2 x 106 M/L were

prepared from one molar stock solution. Fifty ml of each

solution was adjusted to pH = 4.0. This was mixed with

50 ml of clay suspension previously adjusted to pH = 4.0.

This mixture constituted the electrophoresis solution.


*Georgia Kaolin, Inc., Elizabeth, N.J.













This procedure was carried out for each of the solids

suspensions listed above. Therefore, electrokinetic data

as a function of aluminum ion concentration for various

solids content could be obtained.

Electrokinetic data were generated using microelectro-

phoresis using the Riddick cell. The methods of

determining electrophoretic mobilities and calculating

zeta potentials are discussed elsewhere (25).

All chemicals used in this study were certified

reagent grade materials.



Results and Discussion

1. Calculation of C and F

Figure 11 shows the results of zeta potential as a

function of aluminum ion concentration C for various

solids contents. This concentration is the number of

moles of aluminum ions added to the system divided by

the solution volume. It is not the actual concentration

of aluminum ions in solution since adsorption occurs onto

the montmorillonite particle surfaces. Therefore, it is

the apparent concentration if no adsorption takes place.

It can be seen that zeta potential-concentration curves

for various solids contents differ except for very dilute

suspensions. This indicates that, at higher solids















Weight percent solids


* 1.0
o 0.25
o 0.025
S0.0025
S0.00125


K~


pH =4.0


[AICI36H20] moles/liter
20 /liter


Figure 11.


Zeta potential vs. concentration of AlC13*6H20
for 0.00125, 0.0025, 0.025, 0.25 and 1.0%
weight percent montmorillonite.


V


oI 0-


20h


- -


------------------~t
















contents, the equilibrium concentration, C, of aluminum

ions in solution is significantly lower due to adsorption

onto the clay particle surfaces.

A mass balance equation can be written which de-

scribes the system at equilibrium as follows:

C = C + A(W) [29]
L

where Co is the apparent concentration of aluminum ions in

solution if no adsorption occurs, C is the concentration

of aluminum ions in solution after adsorption to the

particle surface, F is the adsorption density of aluminum

ions on the clay surface, VL is the solution volume, Ws

is the weight of solids in the system, and A is the

specific surface area of the clay. Ws/VL can be replaced

by the weight percent by the following argument: weight

W
percent (W/o) of solid is defined as s x 100 where W =
W +W L
s L

weight liquid and W = weight of solid. If WL > Ws then


W7/ = -x 100. Since WL = PL VL where pL = density of
WL


liquid, when W1% s x 100. Since p = 1.00/cc, the


right side of Equation 29 must be multiplied by a factor












1000 cc/liter. Therefore

C = C + 10PL AF(W%) [30]

and the units of each term in Equation 30 are moles/liter.

If enough solid is present in the system, most of the

ions in solution will adsorb to the surface, and

C << 10PLArFW; therefore Equation 30 becomes

Co = 10pLAF(W/o) [31]

Taking the log of both sides of Equation 31 yields

log CO = log(lOpAr) + log(W%) [32]

Table V lists the values of C0 for zeta potentials =

15, 10, 5, 0, -5, -10 mV.

A plot of concentration C as a function of weight

percent using data at zeta potential equal to zero mV

from Figure 11 is shown in Figure 12. For high solid

contents the experimental points approach a straight line

whose slope is 1.0. At low solids contents, the points

approach a line of zero slope. The C value corresponding

to this line is C. Rearrangement of Equation 30 and

taking logarithm yields

log(C -C) = log(lOpLAr) + log(W7o) [33]

Therefore if Equation 32 correctly describes the system

under concentration,then plots of C -C versus W% should

yield straight lines for each zeta potential. As

Figure 13 shows, straight lines are obtained.













Table V. Values of Co for Various Zeta Potentials
and Weight Percent Solids


wt.%
C(mV) 0.00125 0.0025 0.025 0.25 1.0
-10 3.5x10-5 4.5x10-5 1.1x10-4 7 x10-4 2.9x10-3

-5 8.5x10-5 9.3x10-5 2.1xl04 1.3x10-3 5.4xl0-3

0 2.2x10 2.3x104 4.5x104 2.3x10-3 10-2

+5 5 xl0-4 5.1x104 8.8x104 5.4xl0-3 1.8x10-2

+10 1.2xl0-3 1.2x103 1.8x103 8.5x103 3 x102

+15 2.7x10-3 2.8x10-3 3.6xl03 1.3x102 5 xl0-2























E pH=4.0



E -3
10
o









-4

0.001 0.01 0.1 10



Weight percent solids


Figure 12. Apparent aluminum ion concentration Co vs.
weight percent montmorillonite for zeta
potential = 0 at pH = 4.0.













-2
101TT
o0 I I 1 IIII I I I I l ll T111

-Zeta potential, (mV)
-10
-5
00
A 5

-3 a 10
10 -
o 15








-4
10
-4



pH 4.0





-5
10
0.002 0.01 0.1 1.0


Weight percent solids


Figure 13. Concentration of aluminum Co minus the
equilibrium concentration (C) of aluminum in
solution vs. weight percent montmorillonite
for zeta potential = +15, +10, +5, 0, -5,
-10 mV at pH = 4.0.












From Equation 33, at 1% solids, C-C = 1pLAF.

Therefore, the adsorption density, r, can be determined if

the specific surface area of the solid is known. For

montmorillonite, a generally accepted theoretical specific

surface area of 800 m2/g was used in the calculations.

Table VI lists the values of C, F and log(T) for each

zeta potential considered. A plot of c and F vs. C is

shown in Figure 14.


2. Application of Experimental Data to the Stern
Equation

Stern (26) has derived an adsorption isotherm

relating the surface charge/cm2 (o) to the Stern potential

(4s). The Stern equation is
NN ve

1+Fr exp( kT-)

where 01 is the surface charge/cm2 associated with the

adsorbed ionic layer, N1 is the number of adsorption sites
2
per cm at surface, v is the zalence of the ion adsorbed,

e is the charge of the electron, N is Avogadro's number,

M is the molecular weight of the solvent, n is the number
3
of ions per cm far from the surface, s is the potential

at the Stern plane, C is the specific adsorption potential

of the adsorbed ion, k is Boltzmann's constant and T is

al MC
temperature. Since r and n = then
ve 100'












Table VI. Values of C, r and log (C/F) for Various
Zeta Potential Values


(mV) -10 -5 0 +5 +10 +15

C 3.5x 8x 2x 5x 1.2x 2.7x
(moles/1) 10-5 10-5 10-4 10-4 10-3 10-3

F 3.63x 6.75x 1.25x 2.25x 3.75x 6.25x
(moles/cm) 10-11 10-11 10-10 10-10 10-10 10-10

log 2.92 3.07 3.20 3.35 3.51 3.64
(C/F)







60












-9.00 E
-10 _
0

E -9.40
-

-9- --9.80
0 C
"S 10- ""\


S- -10.20 o
N
20C

Zeta potential vs c o
-10.60
a Log adsorption density vs c
30
I5 I04 2i3 i2
10 10 10 10



Concentration c, (m/,)



Figure 14. Zeta potential and log adsorption density (F)
vs. equilibrium concentration (C) of aluminum
in solution for montmorillonite at pH = 4.0.












N
r = [35]
1000 ve q) s
+MC exp( T )

Multiplying the numerator and denominator of the right

MC veis +
side of Equation 35 by TT exp-( kT ) yields

MC veqs +
nMj N1 exp-( kT- )
S MC s[36]
y-- exp-( kT )+i
es M -
If y = kT K = 100 exp(kT), and ys = C, then

N1 C K exp-(vy)
r = [37]
C K exp-(vy)+l [37]

Taking the inverse of both sides and rearranging Equa-

tion 37 yields

N -r
C( -) exp yv [38]

Since the total number of adsorption sites (NL) is much

greater than occupied adsorption sites, N1 >> F. There-

fore,

C exp(vy) [39]
F NIK

Taking the natural log of both sides and converting to

common log,

log(C 1 v
log(-) = log N1K + 2 [40]
F N K' 2_.y3[40]













The experimental data (C and F) fit the Stern equa-

tion, since Figure 15 shows that a plot of log(y) vs. y

yields a straight line. According to Equation 40 the

slope should be 1.3 if v = +3. However, the straight line

obtained has a slope of 0.8.

A plausible explanation for the discrepancy between

the theoretically determined and experimentally obtained

slope must come from examining the factors involved in

v, the valence of the adsorbed ion. Errors in any other

term lead only to shifts in the position of the line but

no change in slope. In calculating the theoretical

slope, v was assumed to be +3 for the aluminic ion.

However, for the slope to decrease to the experimentally

determined value, v must be lower than three. That is,

the effective valence of the adsorbed ion must be smaller

than 3. This is possible only if aluminum ions are in-

volved in ion exchange for sodium ions (+1 valence, making

the effective vAl, +2) or calcium ions (+2 valence, making

the effective vAl, +1). Therefore the vAl would have a

value between 1 and 2. The effective valence calculated
0.8
from Figure 15 is 7-(3) = 1.8.

Energetically, the work required in bringing an

aluminum ion from solution to the surface is partially

supplied by the "negative" work involved in releasing a


























Olk 3.2
0
-J
3.0


2.8


2.6


2.4 1 1 1J11
-0.6 -0.4 -0.2 0 0.2 0.4 0.6


y



Figure 15. Log of the ratio of C and F vs. zeta potential
for montmorillonite for the experimental data.













sodium or calcium ion from the surface to the solution.

Therefore, v in the Stern equation is related not just

to the valence of the adsorbed ion, but also to the net

work involved in bringing the ion to the surface during

ion exchange reactions.

The intercept of the straight line with the y-axis

in Figure 15 yields the specific adsorption potential if

N1 is known. However, since the slope can vary due to

ion exchange processes, the specific adsorption potential

will also vary with the type and degree of ion exchange

reaction.

The data treated in this chapter cover only one

pH value. Future work involving the entire pH range

should give more insight into ion exchange reactions of

aluminum for other ions in the clay. Also, the influence

on the Stern slope by aluminum ion hydrolysis as the pH

increases can be investigated. However, this type of

study would have to be performed using a material other

than montmorillonite so that the contribution by ion

exchange to Stern slope characteristics is minimal.



Conclusions

A new method for determining adsorption isotherms

from electrokinetic data of zeta potential vs. ion






65





concentration for various solids content has been

described and tested experimentally. It was found that

the experimental data fit an analytical form of the Stern

equation. However, the experimentally determined slope

was lower than the theoretical Stern slope. This was due

to the lowered effective valence of the aluminum ions

involved in ion exchange reactions.
















CHAPTER 5
REVERSIBILITY OF ALUMINUM ION ADSORPTION ON FUSED SILICA


Introduction

Considerable interest has been generated in the past

ten years in the coagulation and flocculation behavior of

silica sols. It has been shown that of utmost importance

in this behavior is the hydration state of the surface.

It is now generally accepted that the surfaces of silica

sols are covered by silanol groups with a density of 5 OH/
O
100 A (27), whereas a quartz surface is relatively inert,

being covered by siloxane groups (28,29). However, some

silanol groups have been shown to exist on quartz,

especially on ground or milled material (22,30).

Lange (31) found that the amount of water hydrogen bonded

to precipitated silicas was about 38% of the total pos-

sible adsorption capacity. This meant that 62% of the

silanol groups were hydrogen bonded to each other. Using

an explanation involving ion exchange of hydrogen ions

for metallic ions, Allen and Matijevic (32) found that

the hydrophilic nature of silica decreased in the presence

of simple electrolytes.

Recently, Tschapek and Sanchez (33) studied the

amount of NaC1 required to coagulate different silica













sols as a function of suspension pH. It is clear from
their results that the absence of isolated silanol groups

lowered the amount of NaC1 required for coagulation at low

pH's. Also, the absence of hydrogen bonded silanol groups

did not lower the amount of NaC1 required for coagulation.

They concluded that isolated silanol groups were primarily

responsible for the stability of silica suspensions

calcined between temperatures 110C and 8000C. Con-

sequently, hydrogen bonding between silica particles

could not have been the mechanism of their coagulation.

Removal of silanol groups from the surface by heat

treatment also removes the mechanism of surface charge

development in solution. Their removal is highly

irreversible as shown by Rubio and Kitchener (34).

Removal of the charge development mechanism consequently

decreases the repulsive forces between particles to a

point which may allow close interaction of the primary

particles and ultimately their coagulation. This

coagulation would be facilitated by hydrophobic bonding.

If this is true then the results of Tschapek and

Sanchez (33) for untreated silica sol and silica sol

calcined at 800C suggest that only those silanol groups

which are not hydrogen bonded to each other can contribute

to the formation of adsorption sites for ionic species in












solution. Adsorption site formation occurs by ionization

of the silanol group. This could be the reason why

isolated silanol groups stabilized their suspensions,

i.e., the ionization of the isolated silanol group

created repulsion between the particles. Only in this

way could these two sols have identical coagulation

characteristics.

The previously mentioned mechanism for sol stability

demonstrated how thermal treatment rendering the silica

surface hydrophobic created the possibility of hydro-

phobic bonding between silica particles. Rubio and

Goldfarb (35) showed that chemical treatment can also

create hydrophobic surfaces. Their results suggest that

if a number of quaternary ammonium cations attach to the

surface by means of a nitrogen atom the surface will

become hydrophobic. The greater the surface coverage

the more hydrophobic the surface becomes. They state that

in this way, aggregation of the particles may be effected

by hydrophobic bonding. This type of mechanism is sup-

ported by the fact that quaternary ammonium salts have

been used as flotation collectors for quartz

particles (36).

Adsorption characteristics of species from solution

will be affected by surface hydration states of silica













since surface hydroxyls act as primary adsorption sites

for polar molecules (37). Conventional methods used to

study these phenomena require the use of finely divided

materials. In this chapter, a technique is introduced

which allows investigations of silica material which has

a size range of 300-800 microns. The results obtained

using this technique are interpreted in the same manner

as those obtained when fine material is studied.

Adsorption of aluminum ions from solutions by silica

surfaces is of particular interest to glass corrosion

studies. It has been shown that aluminum ions decrease

corrosion of silica when they are present either in the

glass itself (38) or in the corrosion medium in contact

with the glass (39). Weyl (40) proposed that a require-

ment for corrosion inhibition for the latter case is that

aluminum ions adsorb to the surface and not form a more

soluble compound than the glass itself. Lyon (41) found

that rinsing of a container glass with aluminum ion

solutions and subsequently rinsing with water did not

inhibit alkali extraction from the glass which suggests

that aluminum ion adsorption may not always be permanent.

Aluminum ion adsorption to a glass surface may

theoretically affect either ion exchange or network

breakdown. In his glass solubility work, Iler













investigated the effects of adsorbed aluminum ions on the

dissolution of fused silica. Using fused silica

eliminated the possibility of the ion exchange mechanism

since no alkali were present in the glass. Therefore only

network breakdown was involved. Iler's results showed

that as the amount of aluminum ions in solution increased,

the dissolution of silica decreased for a given period of

time. However, Iler exposed his samples to solutions

containing a 1:1 molar ratio of aluminum to citrate. He

presumed that negatively charged aluminosilicate sites

were created. The inhibition mechanism proposed was

repel of hydroxide ions from the negatively charged sites.

The present work will show that this presumption is

incorrect. Also since Lyon's work on container glass

suggests that aluminum ion adsorption is not permanent,

this chapter considers certain conditions under which

aluminum ion adsorption may be reversible or irreversible.

The conditions considered are those which are known to

alter the hydration state of the silica surface (42).



Materials

Silica used for this investigation was commercially

obtained.* Just before use in the experiments, 10 grams



*Vitreosil, Thermal American Fused Quartz Co., Montville,
New Jersey.













of a -20+45 mesh fraction (833-350 micron) was boiled in

100 ml concentrated hydrochloric acid until no discolora-

tion of the acid was observed. The sample was then rinsed

with conductivity water until all chloride ions were

removed.

All solutions used in these investigations were

prepared from Certified ACS reagent grade chemicals.

Water used to prepare the solutions was obtained from a

deionization system previously described in Chapter 2.



Methods

Electrokinetic theory can be applied to study

adsorption of electrolytes near the solid surface (12).

Theory predicts the existence of an electrical double

layer near the surface consisting of ions present in the

solution. The double layer contains an immobile (Stern

Layer) and mobile portion (Guoy-Chapman diffuse layer).

The electrical potential at the plane separating these

two layers is the zeta potential.

Usually zeta potential values are calculated from

the Smoluchowski equation (Equation 1, Chapter 2). As

shown in Chapter 2, to obtain zeta potential information

streaming potential experiments are performed. However,

modified streaming potential techniques can be used to













study not only adsorption but also desorption of aluminum

ions from the solid surface. Studying desorption

characteristics yields information about the reversibility

of adsorption. This is accomplished by using an apparatus

in Figure 16. Desorption studies were accomplished by

first exposing the particles in the cell (e) to a 10-3 M
-4
NaC1 solution containing 10 M aluminum ions. Figure 17

shows that a positive zeta potential value resulted at

this aluminum ion concentration indicating specific

adsorption of aluminum ions to the surface. The solution

in reservoirs a and b was changed to a 103 M NaCI

solution containing no aluminum ions. This solution was

streamed through the cell at constant pressure [made

possible by maintaining a constant hydrostatic head in

(b)] for an extended period of time. Solution flowed

continually during the desorption experiment except when

data were taken. To obtain these data, the solution flow

valve (d) was turned off and on causing a deflection in

the electrometer (f) indicating the streaming potential

at that particular time. This operation required only a

few seconds resulting in only momentary interruption of

the desorption phenomenon. The signal from the electro-

meter was recorded on a strip chart recorder (g) and the

time was noted. A special circuitry discussed in







































Figure 16. Streaming potential apparatus modified to
maintain constant flow pressure; a) secondary
reservoir; b) primary reservoir; c) pump;
d) solution flow valve; e) cell; f) electro-
meter; g) recorder; h) solution head height.





















-40-


40


S40-


80- ACIC'6H O


120-



-6 -5 -4 -3
10 10 10 10

Concentration, (moles/tr

Figure 17. Zeta potential vs. concentration of sodium
citrate and aluminum chloride.













Chapter 2 was used to eliminate undesirable effects of

electrode polarization.
-3
By using 10-3 M NaCI as a supporting electrolyte for

adsorption of aluminum ions, no significant change in

E/P due to varying solution conductivity as predicted by

Equation 1 should occur during the desorption experiment.

Also since desorption studies were performed under

conditions of constant pressure, no change in the

streaming potential due to varying pressures as predicted

by Equation 1 should be expected. Therefore, during the

desorption studies, the change in E/P should only be due

to change in the zeta potential (C) due to removal of

aluminum ions from the silica surface.

Desorption characteristics of several types of

silica surfaces were studied by this technique. One set

of samples remained untreated. A second set of samples

was heat treated at 8000C for 8 hrs. in vacuum.* A third

set was treated with 1 M NaOH solution for 24 hrs. at

220C. Finally, a combined thermal and chemical treatment

of the glass particles was performed using the agents

described above.


*Centorr hot press vacuum chamber, 10-5 Torr.













Results and Discussion

Figure 18 shows that major differences exist in the

desorption behavior of aluminum ions from silica after

various surface treatments. It can be seen that most of

the aluminum ions desorb from the untreated silica.

However, some aluminum ions remain on the surface even

after 60 minutes of streaming as shown by the fact that

the E/P value does not reach the E/P value for non-
-3
aluminated silica streamed only with 10 M NaC1 solution.

Chemical treatment of the silica particles in one molar

NaOH solution caused most of the aluminum ions to desorb

after approximately 80 minutes of streaming.

For heat treated silica desorption of aluminum ions

was much less than for untreated or chemically treated

silica. Only a small amount of aluminum ions appeared to

reversibly absorb to the surface.

Chemical treatment of the heat treated samples with

one molar NaOH solution reestablished the reversible

adsorption capacity of the silica. It can be seen that

almost complete desorption of aluminum ions occurred since

the E/P value approaches the value for nonaluminated
_Q
silica streamed in 10-3 M NaCl solutions.

It should be noted that data for nonaluminated
_3
silica samples streamed in 10-3 M NaC1 solution were in-

dependent of the various surface treatments.
















cn 3'"\ Heat treated and
I base treated
E
E Non-aluminated SiO2
E I -in 13 M NaCI
- 0
wa- -



-5-


-5-

0 10 30 50 70 90

Stream time, (min.)
Figure 18. Streaming potential-pressure ratio vs. stream
time for untreated, heat treated, base treated,
heat treated and base treated and non-aluminated
fused silica.













The major difference between untreated and heat

treated silica samples is that the former is dominated

by adjacent silanol groups while the latter contains

isolated silanol groups (2). The data show that aluminum

ions reversibly adsorb to a silica surface containing

adjacent silanol groups and irreversibly adsorb to a

surface containing isolated silanol groups. However,

since initial E/P values for the two samples were the

same as seen in Figure 18 at stream time equal to zero,

there appears to be little difference in the amount of

aluminum initially adsorbed. If only those silanol

groups which are not hydrogen bonded to each other can

become adsorption sites, as suggested earlier, then only

those which are not hydrogen bonded to each other can

be true specific adsorption sites for aluminum ions. When

aluminum ions adsorb, the double layer characteristics

change as indicated by changes in zeta potential. If

zeta potential characteristics change in the same way (as

indicated at stream time = 0 for untreated and heat

treated silica) for these two surfaces having different

hydration states, then the aluminum ion adsorption site

must be present in the same amount on both surfaces. This

trend is supported by the results of Tschapek and

Sanchez (33) which showed identical coagulation












characteristics for untreated silica sols and silica sols

calcined at 8000C.

An atomistic view of the adsorption-desorption

process may occur as follows. The pH of the solution

containing 10- M aluminum ions was 4.2-4.5. In this

range the aluminum ions are mostly in the aluminic

(Al ) form with some probability that some Al8(OH)20

species exist. This complex species has been postulated

and shown to exist by several workers (43,44). When

desorption begins using a 10- M NaC1 solution whose

pH = 5.5-6.0, more formation of the complex aluminum

ion species near the surface is favored. The contribu-

tion of the double layer characteristics of this species

will be greater than Al since they are quatrivalently

charged. Hence, their contribution to the double layer

characteristics will be the same for both types of

surfaces at time = 0. However, if these ions are to

remain on the surface during the desorption experiment.

they must hydrogen bond to the surface. They can hydrogen

bond only to the surface containing isolated silanol

groups since the surface containing adjacent silanol

groups cannot hydrogen bond with solution species. There-

fore, during the desorption experiment those aluminum

species which cannot hydrogen bond are streamed away and

the E/P value with time decreases significantly.













Similar interpretations of adsorption from solution

of poly(ethylene oxide) (PEO), a neutral species, onto

silica has been reported by Rubio and Kitchener (34).

Their results clearly show that isolated silanol groups

provide the best adsorption sites for PEO. Adsorption

occurs by hydrogen bonding of the ether oxygen with the

hydrogen of the silanol group. They found that complete

dehydroxylation by heat treatment rendered the surface

incapable of adsorbing PEO. Also, the hydrated surface

could not adsorb PEO since its hydrogen bonding capacity

was exhausted in its effort to hydrogen bond with another

silanol group on the surface.

Iler (39) was not able to show whether or not

citrate ions subsequently adsorbed to negatively charged

aluminosilicate sites. In fact, he assumed they probably

did not since adsorption between two negatively charged

species should not be expected. However, since

adsorption of aluminum ions onto silica creates positively

charged sites as shown by the results in Figure 16,

citrate ion adsorption may be possible.

Figure 19 shows that the zeta potential is reversed

back to a negative value when aluminated silica is

exposed to increased concentrations of citrate ion

solutions. Also shown is the curve for citrate on














-120-


-80-


-40-

E

S40

- ^Supporting electrolyte
a -3
80 @10 M NaCI
N 80-
10 M NaCI, 10 M AICI 6H20

120


160-

-6 -5 -4 -3
10 10 10 10

Concentration No citrate 2H20, ( mole/liter)

Figure 19. Zeta potential vs. concentration of sodium
citrate for fused silica in supporting electro-
lytes of 10-3 M/L NaC1 and 10-3 M/L NaC1,
10-4 M/L A1C13-6H20.












nonaluminated silica. The negative zeta potential values
-5
for concentrations greater than 3 x 10 M citrate for the

aluminated silica curve could be due to one of two pos-

sible effects. Either aluminum ions were removed from

the silica surface to form aluminum citrate complexes in

solution or citrate ions adsorbed to positively charged

aluminosilicate sites. Study of Figure 19 alone does not

help to eliminate either of the possible mechanisms.

However, if the desorption characteristics of the

system are studied, one mechanism can be eliminated.

Therefore, untreated silica samples were exposed to a

10- M NaC1 solution containing 10 M citrate ions and

10- M aluminum ions. Then a 10- M NaC1 solution void

of citrate and aluminum ions was streamed through the

cell. Figure 20 shows the results obtained. If aluminum

ions were removed from the surface to complex with citrate

ions in solution, then the E/P value should decrease to
_3
E/P value for nonaluminated sample streamed with 10 M

NaC1 solution. However, the E/P increased at early

times indicating the removal of a negatively charged

specie from the surface. The only negatively charged

specie present in the system capable of specific

adsorption was citrate ions. These results indicate that

citrate ions first adsorbed to aluminosilicate sites and






















oase TreaTea
I
E -

o
E 0





-3-

-3
SiO ,10 M NaCI

-5-

0 10 30 50 70 90

Stream time, (min.)

Figure 20. Streaming potential-pressure ratio vs. stream
time for aluminated fused silica for untreated,
heat treated, base treated and heat treated and
base treated surfaces.













then desorbed upon streaming the particles with 10-3 M

NaCI solution. At longer streaming times a slight

decrease in E/P occurred indicating removal of aluminum

ions from the silica surface.



Conclusions

A method for studying the desorption behavior of

large particles has been presented. This method yielded

results which can be interpreted in terms of the hydration

state of the surface of the silica particles.

Aluminum ions reversibly adsorbed to surfaces con-

taining adjacent silanol groups and they irreversibly

adsorbed to surfaces containing isolated silanol groups.

The mechanism of irreversible adsorption proposed for

aluminum ions was hydrogen bonding of the complex aluminum

species to the isolated silanol surface groups.

Contrary to Iler's results (39) aluminum ions adsorb

to fused silica forming positively charged rather than

negatively charged sites. Also, aluminum ion adsorption

was shown to activate the silica surface for specific

adsorption of citrate ions.
















CHAPTER 6
THE EFFECTS OF AGING ON THE ZERO POINT OF CHARGE OF ALUMINA


Introduction

Parks (45) has compiled, a considerable amount of

information from many different authors concerning the ZPC

of several oxides and hydroxides including those of

aluminum. Knowledge of the ZPC allows prediction of the

sign of the surface charge at any pH of a solution in

contact with the oxide. Also, knowledge of the ZPC is

important for understanding coagulation processes since

minimum surface charge (minimum interparticle repulsive

forces) is necessary for maximum coagulation as predicted

by DLVO theory.

The ZPC for aluminum oxides ranges from pH = 6.0 to

9.5. The scatter in the ZPC is indicative of varying

experimental conditions and solid phase used. Excluding

adsorption of impurities, Parks (45) and others (46) have

attributed most of the scatter to varying degrees of

surface hydration. Since the ZPC of the oxide depends

on the degree of surface hydration, aging phenomena of

alumina can be studied by observing changes in the ZPC

with aging time, and, in this way, surface hydration can













be studied. Robinson et al. (47), O'Connor et al. (46),

Johansen etal. (48,49), and Schuylenborgh et al.(50-53)

all agree that treatment which leads to bulk or surface

dehydration results in a more acid ZPC than for oxides

which are hydrated. Those treatments which dehydrate the

surface (e.g. heat) lower the ZPC whereas treatments

which increase hydration of the surface (e.g. grinding)

increase the ZPC.

Most of the ZPC information on aluminum oxide com-

piled by Parks was obtained on either naturally occurring

minerals or on synthetic materials prepared in the

author's laboratories. However, little ZPC information

can be found for commercially prepared aluminas.

Information of this nature could be beneficial to both

suppliers and users of commercial aluminas particularly

if the powders are subjected to aqueous environments

during processing. Parks (45) has shown that very small

levels of adsorbed impurities such as phosphate and

sulfate as well as certain organic greatly affect the

ZPC of the oxide. If the impurities were undesirable,

they could easily be detected by measuring the ZPC of

the oxide. Appropriate steps could then be taken to

eliminate the impurities during processing.




Full Text
Zeta potentiaI, (mV)
Ionite.
115


80
60
40
20
0
20
40
:0
1613 Mat
I0"4 AI(N03)3 -9H20 (m/,)
SSM -0
o DSM -0
SSM 20
a DSM 20
_i I l 1 I I I I
3.0 40 5.0 60 70 8.0 9.0 10.0 11.0
pH
Figure 35. Zeta potential vs. pH for 1613Mat in 10"^ M/L
Al(NO3)39H20 solution.
130


57
-2
Weight percent solids
Figure 13. Concentration of aluminum C0 minus the
equilibrium concentration (C) of aluminum in
solution vs. weight percent montmorilIonite
for zeta potential = +15, +10, +5, 0, -5,
-10 mV at pH = 4.0.


31
Table II. Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2S03, and Their Equilibrium
Constants (K)
Reaction
K[NJ (where N=l-5 from Table I)
Sl02(quartz)+H20=H2Si03
K 1 = [H2Si03] = 6.31 x 10"6
Sl02(crist.)+H20=H2Si03
K 2 = [H2Si03] = 1.05 x 10"5
Sl02(trid.) +H20=H2Si03
K 3 = [H2Si03] = 1.47 x 10-5
S102(vit.) +H20=H2Si03
K 4 = [H2Si03] = 7.49 x 10"5
S102(hyd.) +H2O-H2Si03
K 5 = [H2Si03] = 1.47 x 10-2


98
damaged surfaces undergo a more extensive hydration of the
type A^O^ + 3H2O = 2Al(0H)g, yielding a surface layer of
gibbsite. For less severely damaged surfaces, a less
complete reaction probably occurs, yielding a surface
corresponding to A10-0H. Polarization of the hydroxyl
groups by the aluminum cation determines the degree of
their dissociation in water. The hydroxyl group in
A10-0H should be more highly polarized since the 0:A1 =
2:1. In A10-0H, therefore, the OH group is slightly
acidic relative to the OH group in AlCOH)^-
O'Connor et al. (46) found that freshly crushed
powder always had a positive zeta potential in water.
This suggests that the ZPC for alumina is greater than
the pH of the water. However, heating the alumina to high
temperatures eventually reversed the sign of the zeta
potential to a negative value. They attributed this
change to dehydration resulting in a surface approximately
A10-0H or y-alumina. If the alumina was placed in water
for a certain period of time, the zeta potential again
became positive indicating rehydration. These phenomena
were observed for samples heated to temperatures <1000C.
For powders heated >1000C, rehydration is very slow as
shown by Robinson etal. (47). O'Connor et al. (46)
suggest that heating to temperatures >1000C creates


9
been made under conditions near the linear to nonlinear
transition zone. This raises questions about the
validity of using the E/P ratio to calculate the zeta
potential when the flow condition is nonlinear. The
purpose of this investigation was to determine whether
the E/P ratio changes with increasing P, as flow changes
from linear to nonlinear. If no changes occur,
Equation 1 is valid in the nonlinear region as well as
the linear region. Only a rather narrow pressure range
was investigated since this was the range of interest in
typical streaming potential measurements.
In porous beds, a Reynolds number, Re, has been
defined (6) as
Re
yp
y
[2]
where is the mean particle diameter which is equal to
400 microns for the particles used, v is the superficial
velocity found by taking the ratio of volumetric flow
rate and cross sectional area of the bed, p is the
solution density, y is the solution viscosity and e is the
bed porosity.
At Re values below around 10, flow rate and pressure
are linear (7). This is the region of creeping or Darcy
flow. At high Reynolds number, a nonlinear relationship


7
Chapter 6 presents evidence of alumina surface hydration
by studying the change of the zero point of charge
with aging time in water.


LIST OF FIGURES continued.
Figure Page
29 Zeta potential vs. log concentration
of NaCl, CaCl2*2H20, and AlCl3*6H20
for 1613Na montmorillonite 117
30 Zeta potential vs. log concentration
of NaCl, CaCl2-2H20, and AlCl3*6H20
for 1613Ca montmorillonite 118
31 Zeta potential vs. pH for 1608 montmoril
lonite in 0, 10~5 and 10-4 M/L AlCl3*6H20
solutions 122
32 Zeta potential vs. pH for 1613 montmoril
lonite in 0, 10~5, 10~4 M/L AlCl3*6H20
solutions 123
33 Auger peak height ratio vs. pH for
vitreous silica after one hour exposure
to 10-4 M/L AlCl3*6H20 solution 128
34 Zeta potential vs. pH for 1613Mat in
10"4 M/L A1C136H2O solution. SSM-0
means single solution method, clay
plus A1 solution aging time = 0 hours.
35 Zeta potential vs. pH for 1613Mat in
10~4 M/L Al (NO3) 3 9H20 solution 130
36 Zeta potential of 1613 montmorillonite
in 10-4 M/L AlCl3*6H20 and water at pH =
6 vs. clay equilibrium time at pH = 4,
6 and 8 132
37 Solution pH vs. total weight of montmoril
lonite added to 25 ml of water 135
x


88
Surface areas of each powder were determined using
a surface area-pore volume analyzer.* The results of
these measurements are compared to those stated by
Flock (54). This author classifies surface area as
"useful" (when surface area is produced only by physical
adsorption of ^ molecule due to van der Waal's forces)
and "nonuseful" (when the surface area is produced by the
reactive chemical surface of nonalpha agglomerates). A
third type of surface area is produced by reactive
alumina agglomerates. This is attributed to weak elec
trostatic charges due to incomplete phase conversion,
i.e. lattice mismatch and/or localized cation defect
structures. Since they are alpha phase, the higher
surface areas are "useful" and correlate with increased
thermal reactivity, i.e. lower firing temperatures.
Flock has used the petrographic microscope to identify
the existence of these phases in alpha aluminas.
In this chapter further evidence of the existence of
nonalpha phases is given by studying the change in ZPC
with aging time. The advantage of using a chemical method
along with an optical method is that not only crystallo
graphic differences but also chemical purity of the
powders can be studied.
*Quantachrome Corp., Greenvale, N.Y.


28
where r is the adsorption density calculated from
clU p cl L 011 L
potentiometric titration data by most investigators. If
Equation 6 is subtracted from Equation 5, then
at = r i r
real apparent
VC
' A
[7]
C will be the amount of error incurred in the adsorption
density value when these hydrogen or hydroxide ions
assumed to be free in solution actually compose part of
complex solution species. Therefore, this chapter serves
to determine C in terms of experimental parameters so
that r i r and, therefore, surface charge
density error o a can be calculated for the
V631 cippcilT0n L
silica water system.
Procedure
1. Formation of Complex Species
It is possible to calculate the equilibrium solu
bility of silica from thermodynamic information. All
that is required is knowledge of the free energy of
formation of the solid and soluble species at the
appropriate temperature. Table I shows the standard free
energy of formation for five solid silica phases, a sodium
silicate, a neutral aqueous silica species and two aqueous
complex ionic species. Also shown are the values for


APPENDIX B
LIST OF SYMBOLS USED IN THE COMPUTER PROGRAM
IN APPENDIX A


10
develops (the region of noncreeping or nonDarcy flow).
This change is a result of the formation of standing
eddies behind the particles. The Ergun equation (6)
describes the flow behavior over the range of interest:
(
AP P) (#) () = 150 y
L 1-e Dp(vp/y)
2T
v p
[3]
where AP is the pressure drop across the bed, L is the
bed length and all other variables as defined in Equa
tion 2.
Accurate E versus P measurements cannot be obtained
without recognizing and dealing with the experimental
problems of electrode polarization. Ball and
Fuerstenau (3) cite electrode polarization as the
probable cause for E versus P curves not passing through
the origin. Somasundaran and Agar (8) proposed that the
instantaneous change in the voltage when solution flow
is initiated is the true streaming potential. Korpi and
deBruyn (9) incorporated a recorder into their system to
aid in the measurement of the instantaneous voltage
changes during flow initiation and termination.
In the present investigation, a R-C circuit is
introduced which directly nulls the background potentials
(rest potential and electrode polarization potentials).
This greatly facilitates the measurement of the true


121
-4 ++
solution of 10 M/L Ca which had contained no clay.
_| |_
This evidence confirms that ion exchange of Ca in
solution for H+ or Na~*~ in the clay controls the electro-
kinetic characteristics of this clay.
Of the three salts studied, aluminum ions decreased
the zeta potential (i.e. repulsive forces between
particles) to zero at the lowest concentration as pre
dicted by the Schulze-Hardy rule. However, as shown in
Figures 27-30 the pH range for the A1 studies was
restricted to 3-4. Therefore, zeta potential character
istics of 1608 and 1613 clays from pH = 3-10 in constant
aluminum ion concentrations (10 10 ^ M/L) were studied.
The samples for electrophoresis were prepared by the
double solution method (DSM).
Figures 31 and 32 show the results for 1608 and 1613
respectively. For 1608 10 M/L AlCl^^O does not
reverse the sign of the zeta potential for any pH in the
range studied. However, 10 ^ M/L A1C1^6H20 reversed the
sign from negative to positive at pH = 4.1 and back to
negative at pH = 8.5. For 1613, 10 M/L AlCl26H20
reversed the sign from negative to positive at pH = 5.6
and back to negative at pH = 7.1. In 10 M/L AlCl2-6H20
for 1613, the sign reversed from negative to positive at
pH = 4.1 and back to negative at 8.2-8.3. Also shown in


97
The discrepancy in the equilibrium pH value and
pHzpc fr C-30 DB indicates a positive ion impurity
whereas for A-16 it indicates a negative ion impurity.
Positive ion impurities create a more basic ZPC and
negative ion impurities create a more acidic ZPC.
Sources of positive ion impurities are most likely iron
for C-30 DB whereas sources of negative ion impurities
are probably phosphate or sulphate for A-16.
Discussion
O'Connor et al. (46) give a detailed account of the
sequence of events during hydration and dehydration for
alumina. From an atomistic viewpoint, the alumina lattice
consists of oxygen ions arranged in a hexagonal close
packing with aluminum ions occupying two-thirds of the
available octahedral holes. Each aluminum ion is sur
rounded by six oxygens and each oxygen by four aluminum
ions. Coordination at a freshly fractured surface will
be incomplete but must eventually be satisfied. Aluminum
ions adsorb hydroxyl ions from solution while dangling
oxygen ions bonded to surface aluminum ions adsorb
hydrogen ions. Subsequently, surface hydroxyl groups
ionize to produce the surface charge. O'Connor et al.
(46) also state that it is likely that more severely


40
Total
surface area,
(cm2)
Surface charge density error (Act) vs total
surface area for precipitated silica at
pH = 7, 8, 9 and 10.
Figure 10.


r^>
Pressure (cm. Hg)
Figure 5. Streaming potential vs. pressure without the R-C
measuring circuit.


80
Similar interpretations of adsorption from solution
of poly(ethylene oxide) (PEO), a neutral species, onto
silica has been reported by Rubio and Kitchener (34).
Their results clearly show that isolated silanol groups
provide the best adsorption sites for PEO. Adsorption
occurs by hydrogen bonding of the ether oxygen with the
hydrogen of the silanol group. They found that complete
dehydroxylation by heat treatment rendered the surface
incapable of adsorbing PEO. Also, the hydrated surface
could not adsorb PEO since its hydrogen bonding capacity
was exhausted in its effort to hydrogen bond with another
silanol group on the surface.
Her (39) was not able to show whether or not
citrate ions subsequently adsorbed to negatively charged
aluminosilicate sites. In fact, he assumed they probably
did not since adsorption between two negatively charged
species should not be expected. However, since
adsorption of aluminum ions onto silica creates positively
charged sites as shown by the results in Figure 16,
citrate ion adsorption may be possible.
Figure 19 shows that the zeta potential is reversed
back to a negative value when aluminated silica is
exposed to increased concentrations of citrate ion
solutions. Also shown is the curve for citrate on


pH
Figure 34. Zeta potential vs. pH for 1613Mat in 10-1^ M/L
AlCl3*6H20 solution. SSM-0 means single solu
tion method, clay plus A1 solution aging time =
0 hours.
129


64
sodium or calcium ion from the surface to the solution.
Therefore, v in the Stern equation is related not just
to the valence of the adsorbed ion, but also to the net
work involved in bringing the ion to the surface during
ion exchange reactions.
The intercept of the straight line with the y-axis
in Figure 15 yields the specific adsorption potential if
N-^ is known. However, since the slope can vary due to
ion exchange processes, the specific adsorption potential
will also vary with the type and degree of ion exchange
reaction.
The data treated in this chapter cover only one
pH value. Future work involving the entire pH range
should give more insight into ion exchange reactions of
aluminum for other ions in the clay. Also, the influence
on the Stern slope by aluminum ion hydrolysis as the pH
increases can be investigated. However, this type of
study would have to be performed using a material other
than montmorillonite so that the contribution by ion
exchange to Stern slope characteristics is minimal.
Conclusions
A new method for determining adsorption isotherms
from electrokinetic data of zeta potential vs. ion


11
streaming potential, especially under conditions where
the background potential is large and varies with time.
A modified streaming potential apparatus is described
that is suitable for low pressure studies and for use on
materials that are considerably reactive. A new cell
design is introduced that is easier to pack, easier to
clean, and is less fragile than previous designs.
Materials and Methods
1. Materials
The bed materials used for this study were fused
silica* and an invert silicate glass (denoted as
"bioglass") whose preparation is noted elsewhere (10) and
whose composition is as follows: 45 wt. % SO2 ,
24.5 wt.7o CaO, 24.5 wt.% ^2*3, and 6.0 wt.% P2O5 This
glass has certain reactive properties which makes it an
interesting material for biological implant studies (10).
The fused silica and bioglass were ground and sieved, and
the -20+45 fraction (0.0833-0.0354 cm aperture) was used
in the experiments. The fused silica was acid washed by
conventional methods used by other investigators (11).
Due to the reactivity of bioglass, no acid washing was
*Vitreosil,Thermal American Fused Quartz Co., Montville,
New Jersey.


133
10 ^ AlCl^'l^O was added at pH = 6, whereas little change
in the zeta potential is observed after aluminum ions
are added at clay equilibration time = 0 hours. Equili
bration at various pH values also affected the results of
adding aluminum ions to the solution. However, zeta
potential measured at pH = 6 is independent of equili
brium pH of the clay in water when no aluminum ions are
added. Therefore, not only equilibration time but also
equilibration pH affected the adsorption capacity of the
clay for aluminum ions. These results suggest that
variations in the hydration of the clay affect the
adsorption capacity for aluminum ions.
Mathers et al. (84) have studied the effect of acid
and heat treatment of clays on structure and cation
exchange properties of montmorillonites. They found
that hydrogen montmorillonite converted to an aluminum
montmorillonite when stored moist at 30C. That is,
aluminum ions from lattice position became exchangeable
cations with time. The clays used by these authors were
purposely converted to hydrogen clays by acid treatment.
However, it is known that both sodium and calcium clays
when suspended in water increase the pH indicating ion
exchange of these cations for hydrogen ions in solution.
In this way, the sodium and calcium clays used in the


151
70. C. Ho and R. L. Handy, "Electrokinetic Properties
of Lime Treated Bentonites," Clays and Clay Minerals
12, 267 (1963).
71. A. M. Gaudin and D. W. Fuerstenau, "Quartz
Flotation with Anionic Collectors," Trans. AIME
202, 68 (1955).
72. R. F. Packham, "Some Studies of the Coagulation of
Dispersed Clays with Hydrolyzing Salts," J. Colloid
Interface Sci. 20, 81 (1965).
73. S. L. Swartzen-Allen and E. Matijevic, "Colloid and
Surface Properties of Clay Suspensions: II. Elec
trophoresis and Cation Adsorption of Montmoril-
lonite," J. Colloid Interface Sci. 59, 143 (1975).
74. J. Kamil and I. Shainberg, "Hydrolysis of Sodium
Montmorillonites in Sodium Chloride Solutions,"
Soil Sci, 106, 193 (1968).
75. D. C. Nearpass, "Exchange Adsorption of 3-Amino--l,
2, 4 Triazole by Montmorillonite," Soil Sci. 109,
77 (1970).
76. A. W. White, "Water Sorption Properties of Homoionic
Montmorillonite," Clays and Clay Minerals 3, 186
(1954).
77. F. Bernstein, "Distribution of Water and Electrolyte
Between Homoionic Clays and Saturated NaCl Solution,"
Clays and Clay Minerals 8, 122 (1959).
78. C. R. O'Melia and W. Stumm, "Aggregation of Silica
Dispersions by Iron (III)," J. Colloid Interface
Sci. 23, 137 (1967).
79. P. Bar-On, I. Shainberg, and I. Michaeli, "Electro
phoretic Mobility of Montmorillonite Particles
Saturated with Na/Ca Ions," J. Colloid Interface
Sci. 33, 471 (1970).
80. E. Matijevic and L. J. Stryker, "Coagulation and
Reversal of Charge of Lyophobic Colloids by
Hydrolyzed Metal Ions: III. Aluminum Sulfate,"
J. Colloid Interface Sci. 22, 68 (1966).


38
surface area in his studies. Therefore, even at pH = 10,
less than 1% error should be expected based on his
experimental values of surface charge density. This
calculation quantitatively demonstrates the importance of
the experimental condition of surface area in a
titration experiment if the formation of complex
species is to be neglected. To achieve greatest accuracy
a high surface area to volume ratio must be used.
Tadros and Lyklema (17) studied the surface charge
density as a function of pH for precipitated silica.
2
The absolute values range from 15 to 200 micro coul/cm
in the pH range 7-10. Figure 9 shows that values of
2
Aa calculated in the present work reach 30 micro coul/cm
which suggests that at pH = 10, 10-15% error is incurred
by Tadros and Lyklema by neglecting the complex species
6 2
formation. Tadros and Lyklema used 8 x 10 cm surface
2
area in their experiments (20g of 40 m /g per 100 cc).
Figure 10 shows a plot of Ao versus total surface area.
At a surface area of 8 x 10 for pH = 10, 10% error should
be expected based on their value of a. Tadros and
8 2
Lyklema therefore should have used 10 cm of total sur
face area to achieve YL error or less at pH = 10 if they
wanted to neglect the complex species formation.
From data such as presented in Figures 8 and 10,
the amount of error in surface charge density incurred as


104
exposed to aqueous environments. The exchange phenomenon
gives rise to the cation exchange capacity (CEC) of the
clay. Clays such as montmorillonite with a high degree
of isomorphous substitution have high CEC's.
The neutralizing counter ions in a clay-liquid
system form an electrical double layer on the particle
faces. Sodium-saturated montmorillonites have a more
negative zeta potential than calcium montmorillonites
since calcium ions penetrate the Stern layer to a higher
degree than sodium ions.
Early work by Freundlich et al. (68) showed sign
reversal of the zeta potential for montmorillonite when
contaminated with small amounts of thorium and aluminum
chlorides. Additions of small amounts of potassium and
sodium hydroxide resulted in more negative zeta
potentials. Oakes and Burcik (69) were unsure whether
these effects were due entirely to either adsorption in
the Stern layer or ion exchange processes. These authors
also studied electrokinetic characteristics of a Wyoming
bentonite which had equilibrated in water for several
months. They found that calcium ions produced a less
negative zeta potential rapidly but at a decreasing rate
as more salt was added. They compared their results to
those of Freundlich et al. (68) and found good qualitative


Zeta potential,(mV)
Log concentration,
(moles
/
)
liter
Figure 28. Zeta potential vs. log concentration of NaCl,
CaCl2'2H20 and AlCl3*6H20 solutions for 1613 montmoril-
lonite.
116


V PENS
[1] KA+-1.0177 10
[ 2 ] KE-*-% 9 2/. 2 3
[3] KC<-2 81 3/7 2 7
[4] /O 6.31 /7~ 6 1 0 577" 3 1.47/7 5 7.9 4/7 S 0.0147
[ 5 ] P<7+-9 65 0 0 00 000 0
[ 6 1 17/1 '
[ 7 ] 77,1 <-
rol (7,7'
r o i <7/mi
[ i o ;i an'
cm <7/mi
r 1 2 ] (7/7/1'
[13] <7/7/Ml
[14] (](?+-0
[15] /C/M/i/ t( (7//X (7/1 )
[1G] A'M/f/? (<7/? x (<7//* 2 ) )
[17] K Z<-(KC*( Gi'A *?.)):( GU 2 )
[18] ;/<-o
[19] 1/'
[20] i/^n
[21] SA'
[22] /7/M1
[23] /'
[24] /mi
[25] 77/ 7 <77/ x 1/
[26] / <77/ ?
[27] '7'
[28] y<-n
[29] PI!'
[30] P/M]
[31] X1
[32] Ml
[33] ITA<-YpX
[34] RETURN: N<-H +1
[35] //<-l 0* PI!
[36] Q <- ( V / ) x ( ( 2 x KY x K [ N ] ( // 2 ) ) + ( XX x K [ // ]://) + ( 2 x X % x 77 [ // ] x ( /// 2 ;
[37] QQ+-QQ ,i>
[38] -*-(//< 5) /RETURN
[39] ??l<-?r?[l + i7]
[40] (Of? 2<- [41] 00 3 --(7(7 [ ( ( 2x7 ) + l ) + i 7]
[42] [43] (>0 5 <-(?(,? [ ( ( 4 x 7 ) + 1 ) + i 7 ]
[44] <7i?f?l^f7(71 x/T
[45] <7(7 (7 2 -*-(7f? 2 xP(7
[46] CQQ3+QQ3*FC
[47] [48] <7 [49] 'DORE'
7
:(//* 2 )) )
140


107
Materials
Montmorillonite clays of various compositions were
obtained from a commercial supplier.* Table VIII lists
the composition of two as-received montmorillonites used
in the present work. It can be seen that, relative to
each other, 1608 is more nearly a sodium clay and 1613
is more nearly a calcium clay. Also, 1613 has much less
iron than 1608. Because of this, 1613 was chosen for
sodium or calcium saturation. These saturated clays
were designated 1613Na and 1613Ca. Their preparation
consisted of conditioning as-received 1613 clay in 1 M
NaCl or CaC^^F^O for one week with a clay-solution
ratio of 20 ml/gram clay. The clays were rinsed in
water to remove excess salt. For 1613Na only two rinses
could be achieved since the clay remained suspended even
after centrifugation** at 1750 rpm for 20 minutes after
the second rinse. For 16130a multiple rinses (at least
10) were possible. After rinsing the clays were dried in
an oven at 90C. After drying, both clays were ground and
used in electrokinetic experiments. A second sodium clay
was prepared by the method described by Swartzen-Allen and
Matijevic (73) and was designated as 1613Mat.
-'Georgia Kaolin, Inc., Elizabeth, N.J.
**Damon/IEC HN S-II centrifuge.


92
nonalpha phase material may have caused the slightly
higher surface area determined in the present work. The
T-61 used in the present investigation probably does not
contain nonalpha phase material. Instead, the difference
may be due to the degree of grinding. This might be
expected since the sample used in the present work came
from a batch whose agglomerate size was designated as
-325 mesh. The data described by Flock probably came
from powder which had better particle size characteriza
tion and consequently from narrower particle size ranges.
Table VII also shows that the surface area of C-30 DB
increased 1000 fold when the powder was heated to 500C
for 24 hours. The high surface area of the heated powder
is strong evidence for the conversion to a transition
phase. X-ray diffraction data of the powder before and
after heat treatment showed that the heat treated powder
was gamma alumina.
2. ZPC Determinations
Figure 21 is an example of the type of data obtained
when zeta potential as a function of aging time in water
is studied. Figure 22 shows the results of the change of
the ph^pQ with time for all the powders studied. For
A-17, T-61, C-30 DB and gamma alumina the pH^p^
increased


70
investigated the effects of adsorbed aluminum ions on the
dissolution of fused silica. Using fused silica
eliminated the possibility of the ion exchange mechanism
since no alkali were present in the glass. Therefore only
network breakdown was involved. Iler's results showed
that as the amount of aluminum ions in solution increased,
the dissolution of silica decreased for a given period of
time. However, Her exposed his samples to solutions
containing a 1:1 molar ratio of aluminum to citrate. He
presumed that negatively charged aluminosilicate sites
were created. The inhibition mechanism proposed was
repel of hydroxide ions from the negatively charged sites.
The present work will show that this presumption is
incorrect. Also since Lyon's work on container glass
suggests that aluminum ion adsorption is not permanent,
this chapter considers certain conditions under which
aluminum ion adsorption may be reversible or irreversible.
The conditions considered are those which are known to
alter the hydration state of the silica surface (42).
Materials
Silica used for this investigation was commercially
obtained.* Just before use in the experiments, 10 grams
*Vitreosil, Thermal American Fused Quartz Co., Montville,
New Jersey.


46
Table IV. Calculated Values for Disturbed Layer
Thickness (t) and Soluble S2 at
Equilibrium Using Data from van Lier
etal. (ref. 22)
Silica phase
SiC>2 x 10 (moles/liter)
t (A)
Quartz
0.30
70
Cristobalite
0.50
119
Tridymite
0.70
167
Vitreous
3.88
968
Hydrated
u,
/V

^Calculations show total dissolution


45
W
f
Nf p4/3Trr^
[26]
where p is the density of the solid phase, W. and are
the initial and final total weight of powder respectively.
Since N. = then
i f
p4/37Tr? p4/3irr^
-4
A value for r. = 1,5 x 10 cm is chosen since it
range of the particle size range used by van Lier
Therefore,
[27]
is mid-
et al.
Wr -i / -.
r- = ( ".) ^~ ~>v
f i
[28]
i
For quartz the value of r^ is 1.493 x 10 ^. The
thickness t of the disturbed layer would be t = r. r^ =
7 x 10 ^ cm= 70 A.
Similar calculations for the four other silica
species were made. Table IV shows values of t and the
total concentration of soluble SO2 at equilibrium for each
species. The values of t in Table IV represent the amount
of the particle which could be dissolved away if the
layer was each of the phases studied. They also represent
the thickness which the disturbed layer must have for the
solution to become saturated with respect to each phase
considered. On this basis, the disturbed layer cannot


flow in the pressure range studied. However, streaming
potential-flow pressure relationships remain linear and
pass through the origin when electrode polarization
effects are eliminated. Therefore, zeta potential
values calculated from the Smoluchowski equation under
noncreeping flow conditions are as valid as if creeping
flow conditions existed.
Using mass balance concepts, surface charge
density error incurred by assuming that all hydrogen
or hydroxide ions which cannot be accounted for as free
ions in solution adsorb to the surface rather than form
part of complex ions in solution is calculated for five
silica phases in three electrolyte solution concentra
tions. The error increases as thermodynamic stability
of the silica decreases, as pH increases and as the total
surface area present in the experimental system decreases.
A new method is developed for determining adsorption
isotherms directly from electrophoretic measurements.
The unique feature is gathering zeta potential data as a
function of ion concentration for various solids
concentrations. The adsorption of aluminum ions onto
montmorillonite clay is presented as an experimental
example. The values of adsorption densities and equili
brium concentrations of aluminum ions in solution after
Xll


23
y = 0.009 poise, and r = 0.87 cm, the values of and
are calculated to be 0.0040 and 51, respectively.
The data of Figure 6 fit an equation of the form
given by Equation 4 if k^ and are 0.00375 and 48, re
spectively. This is in close agreement with the re
spective calculated values. Thus, the nonlinear behavior
follows what is expected from the earlier flow studies.
From Figures 4 and 6, it can be seen that the E/P
ratio remains unchanged even though solution flow
becomes nonlinear. The streaming potential is
proportional to the streaming current. The streaming
current is given by the integral over space of the product
of the charge density and velocity (projected in the net
flow direction) at every point (12), and so this integral
must be proportional to pressure. However, the charge
density distribution does not change with pressure. This
strongly suggests that the velocity of the liquid at every
point near the surface increases in direct proportion
with the pressure. This conclusion would be in agreement
with the current views that streamline flow near the
particle surfaces remains even after standing eddies
develop.


114
Figures 27 and 28 show results of zeta potential
behavior as a function of solution concentration of NaCl,
CaCl2*2H20 and AlCl^l^O for 1608 and 1613 montmoril-
lonites. The values corresponding to the data points
on each curve represent the pH which resulted from
preparation of the colloids in NaCl and CaCl22H20 solu
tions. The pH values for AlCl2*6H20 solutions were
purposely adjusted to avoid aluminum ion hydrolysis.
From the pH values for NaCl and CaC^Z^O curves it
appears that ion exchange occurred resulting in excess
hydrogen ions in solution. The zeta potential behavior
in NaCl and CaC^^^O demonstrates the fact that sodium
ions cannot penetrate the Stern layer (resulting in little
change in zeta potential values with increased NaCl
concentration) whereas calcium ions penetrate this layer
(resulting in a decrease in zeta potential to zero at
higher CaC^^^O concentrations).
Data for AlCl^^O show that aluminum ions adsorb
to 1608 and 1613 beyond the degree of charge equilivalent
ion exchange. If ion exchange occurred such that charge
neutrality persisted, then zeta potential sign reversal
would not be expected. However, sign reversal does occur
3 A
between concentrations 10 -10 M/L.
Figures 29 and 30 show the same type of data for
1613Na and 1613Ca clays as in Figures 27 and 28.


Zeta potential, (mV)
123
pH
Figure 32. Zeta potential vs. pH for 1613 montmorillonite
in 0, 10~5, 10-^ M/L A1C13-6H20 solutions.


91
Table VII. Surface Areas of A-16, A-17, T-61, C-30 DB and
Gamma Alumina Powders
Alumina
2
Specific surface area (m /g)
Surface area
range from Flock
(ref. 54)
A-16
5.26
4.00-6.50
A-17
3.02
1.5 -2.5
T-61
0.59
0.15-0.45
C-30 DB
1.4
not listed
gamma
108
not listed


136
water and measuring the pH. The condition at which
further addition of clay resulted in no pH change was
the equilibrium pH of the clay. The equilibrium pH for
these clays was between 9 and 10. Therefore, much less
acid would have to be added to bring the pH to 8.5 than
to 4.1 to create possible coagulation conditions. Future
studies should focus on long term aging of these clays
at the zero zeta potential point and measuring the degree
of coagulation with time.
Conclusions
It has been confirmed that the double layer for a
calcium clay is smaller than for a sodium clay. This is
due to the ability of calcium ions to penetrate the Stern
plane and decrease the size of the diffuse layer in
solution. Sodium ions cannot penetrate the Stern layer
creating a larger diffuse layer on the clay particle
faces.
Ion exchange phenomena were shown to control the
electrokinetic behavior of 1613Na clay in CaCl2'2H20
solutions. Atomic emission data showed that at the
solution concentration of Ca which decreased the zeta
potential by a factor of 2, the amount of Ca present in
solution was also decreased by a factor of 2. Other


65
concentration for various solids content has been
described and tested experimentally. It was found that
the experimental data fit an analytical form of the Stern
equation. However, the experimentally determined slope
was lower than the theoretical Stern slope. This was due
to the lowered effective valence of the aluminum ions
involved in ion exchange reactions.


67
sols as a function of suspension pH. It is clear from
their results that the absence of isolated silanol groups
lowered the amount of NaCl required for coagulation at low
pH's. Also, the absence of hydrogen bonded silanol groups
did not lower the amount of NaCl required for coagulation.
They concluded that isolated silanol groups were primarily
responsible for the stability of silica suspensions
calcined between temperatures 110C and 800C. Con
sequently, hydrogen bonding between silica particles
could not have been the mechanism of their coagulation.
Removal of silanol groups from the surface by heat
treatment also removes the mechanism of surface charge
development in solution. Their removal is highly
irreversible as shown by Rubio and Kitchener (34).
Removal of the charge development mechanism consequently
decreases the repulsive forces between particles to a
point which may allow close interaction of the primary
particles and ultimately their coagulation. This
coagulation would be facilitated by hydrophobic bonding.
If this is true then the results of Tschapek and
Sanchez (33) for untreated silica sol and silica sol
calcined at 800C suggest that only those silanol groups
which are not hydrogen bonded to each other can contribute
to the formation of adsorption sites for ionic species in


TABLE OF CONTENTS continued.
Page
CHAPTER
5 REVERSIBILITY OF ALUMINUM ION ADSORPTION
ON FUSED SILICA 66
Introduction 66
Materials 70
Methods 71
Results and Discussion 76
Conclusions 84
6 THE EFFECTS OF AGING ON THE ZERO POINT OF
CHARGE OF ALUMINA 85
Introduction 85
Materials and Methods 87
Results 90
Discussion 97
Conclusions 101
7 ELECTROKINETIC PROPERTIES OF
MONTMORILLONITE 103
Introduction 103
Materials 107
Methods 109
Results and Discussion Ill
Conclusions 136
APPENDIX A 140
APPENDIX B 142
BIBLIOGRAPHY 145
BIOGRAPHICAL SKETCH 15 3
v


134
present work can become hydrogen clays although probably
not as hydrogen saturated as those of Mathers et al.
Possibly, during equilibration the clays used in this
investigation initially became acid clays and with time
converted to aluminum clays. The exchangeable aluminum
could become available for adsorption to the clay
particles. A simple calculation of the amount of aluminum
which becomes exchangeable can be made since the clay
composition of 1613 is known from Table VIII. For a 0.017o
suspension, and assuming half of the aluminum becomes
exchangeable, the amount of Al in solution would be
.01 g x .06 g = .0006 g. The number of moles of
aluminum would be .0006/28 = .00002 moles/lOOcc = 2 x
-4
10 moles/liter. This should be enough Al to change the
sign of the zeta potential at pH = 6 as shown in
Figure 32 without further aluminum ion additions.
However, as Figure 25 showed, the zeta potential remained
negative at pH = 6. Therefore, conversion to an aluminum
clay probably does not occur for 1613.
If zero potential (ZZP) is required for coagulation,
then from a practical standpoint, the point at pH = 8.5
-4
in 10 M/L AlCl^'^O would be more desirable. The
reason for this is shown in Figure 37. These curves were
obtained by placing a known amount of clay into 25 ml of


Zeta potential, (mV)
52
Figure 11.
Zeta potential vs. concentration of AlCl3*6H20
for 0.00125, 0.0025, 0.025, 0.25 and 1.0%
weight percent montinorilIonite .


CHAPTER 6
THE EFFECTS OF AGING ON THE ZERO POINT OF CHARGE OF ALUMINA
Introduction
Parks (45) has compiled, a considerable amount of
information from many different authors concerning the ZPC
of several oxides and hydroxides including those of
aluminum. Knowledge of the ZPC allows prediction of the
sign of the surface charge at any pH of a solution in
contact with the oxide. Also, knowledge of the ZPC is
important for understanding coagulation processes since
minimum surface charge (minimum interparticle repulsive
forces) is necessary for maximum coagulation as predicted
by DLVO theory.
The ZPC for aluminum oxides ranges from pH = 6.0 to
9.5. The scatter in the ZPC is indicative of varying
experimental conditions and solid phase used. Excluding
adsorption of impurities, Parks (45) and others (46) have
attributed most of the scatter to varying degrees of
surface hydration. Since the ZPC of the oxide depends
on the degree of surface hydration, aging phenomena of
alumina can be studied by observing changes in the ZPC
with aging time, and, in this way, surface hydration can
85


73
Figure 16. Streaming potential apparatus modified to
maintain constant flow pressure; a) secondary
reservoir; b) primary reservoir; c) pump;
d) solution flow valve; e) cell; f) electro
meter; g) recorder; h) solution head height.


79
characteristics for untreated silica sols and silica sols
calcined at 800C.
An atomistic view of the adsorption-desorption
process may occur as follows. The pH of the solution
-4
containing 10 M aluminum ions was 4.2-4.5. In this
range the aluminum ions are mostly in the aluminic
[ [ |
(A1 ) form with some probability that some Alg(0H)2Q
species exist. This complex species has been postulated
and shown to exist by several workers (43,44). When
_3
desorption begins using a 10 M NaCl solution whose
pH = 5.5-6.0, more formation of the complex aluminum
ion species near the surface is favored. The contribu
tion of the double layer characteristics of this species
will be greater than A1 since they are quatrivalently
charged. Hence, their contribution to the double layer
characteristics will be the same for both types of
surfaces at time = 0. However, if these ions are to
remain on the surface during the desorption experiment,
they must hydrogen bond to the surface. They can hydrogen
bond only to the surface containing isolated silanol
groups since the surface containing adjacent silanol
groups cannot hydrogen bond with solution species. There
fore, during the desorption experiment those aluminum
species which cannot hydrogen bond are streamed away and
the E/P value with time decreases significantly.


68
solution. Adsorption site formation occurs by ionization
of the silanol group. This could be the reason why
isolated silanol groups stabilized their suspensions,
i.e., the ionization of the isolated silanol group
created repulsion between the particles. Only in this
way could these two sols have identical coagulation
characteristics.
The previously mentioned mechanism for sol stability
demonstrated how thermal treatment rendering the silica
surface hydrophobic created the possibility of hydro-
phobic bonding between silica particles. Rubio and
Goldfarb (35) showed that chemical treatment can also
create hydrophobic surfaces. Their results suggest that
if a number of quaternary ammonium cations attach to the
surface by means of a nitrogen atom the surface will
become hydrophobic. The greater the surface coverage
the more hydrophobic the surface becomes. They state that
in this way, aggregation of the particles may be effected
by hydrophobic bonding. This type of mechanism is sup
ported by the fact that quaternary ammonium salts have
been used as flotation collectors for quartz
particles (36).
Adsorption characteristics of species from solution
will be affected by surface hydration states of silica


72
study not only adsorption but also desorption of aluminum
ions from the solid surface. Studying desorption
characteristics yields information about the reversibility
of adsorption. This is accomplished by using an apparatus
in Figure 16. Desorption studies were accomplished by
first exposing the particles in the cell (e) to a 10 M
NaCl solution containing 10 ^ M aluminum ions. Figure 17
shows that a positive zeta potential value resulted at
this aluminum ion concentration indicating specific
adsorption of aluminum ions to the surface. The solution
_3
in reservoirs a and b was changed to a 10 M NaCl
solution containing no aluminum ions. This solution was
streamed through the cell at constant pressure [made
possible by maintaining a constant hydrostatic head in
(b)] for an extended period of time. Solution flowed
continually during the desorption experiment except when
data were taken. To obtain these data, the solution flow
valve (d) was turned off and on causing a deflection in
the electrometer (f) indicating the streaming potential
at that particular time. This operation required only a
few seconds resulting in only momentary interruption of
the desorption phenomenon. The signal from the electro
meter was recorded on a strip chart recorder (g) and the
time was noted. A special circuitry discussed in


61
N
r =
l
-,,1000 /elJJs+(\
1+^7^- exp (^)
[35]
MC
Multiplying the numerator and denominator of the right
MC ve^s+d
side of Equation 35 by -j-q-q-q exp-(^) yields
r
mc
imu Ni exp~( kT }
MC ,ve Vh ,,
T exP-(TV~)+1
[36]
^S M -cb
If y = -^r, K = exp(^), and ips = ?, then
C K exp-(vy)
r C K exp-(vy)+l
Taking the inverse of both sides and rearranging Equa
tion 37 yields
[37]
N-,
C(-
exp yv
K
[38]
Since the total number of adsorption sites (N-^) is much
greater than occupied adsorption sites, N-^ >> 1. There
fore ,
C exp(vy)
r NjK
[39]
Taking the natural log of both sides and converting to
common log,
ig(£)
log
2.303 y
[40]


59
Table VI. Values of C, T and log (C/T) for Various
Zeta Potential Values
(mV)
-T
^5
0
+5
+10 '
' +15
C
(moles/1)
3.5x
10-5
8x
10-5
2xa
10"4
5x
104
1.2x
IO-3
2.7x
10-3
r 2
(moles/cm )
3.63x
lo-n
6.75x
lo-n
1.25x
io-io
2.25x
10-10
3.7 5x
10-10
6.25x
10-10
log
(C/D
2.92
3.07
3.20
3.35
3.51
3.64


147
24. E. J. Verwey and J. Th. G. Overbeek, "Theory of the
Stability of Lyophobic Colloids," Elsevier,
Amsterdam (1948).
25. Zeta Meter Manual, Second edition, Zeta Meter, Inc.,
New York.
26. 0. Stern, "The Theory of the Electrolytic Double
Layer," Z, Electrochem. 30, 508 (1924).
27. L. T. Zhuravlev and A. V. Kiselev, "Surface Area
Determination" (Symposium Proceedings), Buttersworth,
London (1970).
28. R. K. Her in "Surface and Colloid Science"
(E. Matijevic, ed.), Vol. 6, Interscience, New York
(1973).
29. R. K. Iler, "Colloid Chemistry of Silica and
Silicates," Cornell Univ. Press, Ithaca, New York
(1955).
30. G. S. Moore and H. E. Rose, "Structure of Powdered
Quartz," Nature (London) 242, 187 (1973).
31. K. R. Lange, "The Characterization of Molecular
Water on Silica Surfaces," J. Colloid Interface
Sci. 20, 231 (1965) .
32, L. H. Allen and E. Matijevic, "Stability of
Colloidal Silica: 1. Effect of Simple Elec
trolytes," J. Colloid Interface Sci. 31, 287 (1969).
33, M. Tschapek and R. M. T. Sanchez, "The Solubility of
Silica and Quartz Suspensions," J. Colloid Interface
Sci. 54, 460 (1976) .
34 J. Rubio and J. A. Kitchener, "The Mechanism of
Adsorption of Poly(ethylene oxide) Flocculant on
Silica," J. Colloid Interface Sci. 57, 132 (1976).
35. J. Rubio and J. Goldfarb, "Stability of Colloidal
Silica in the Presence of Quaternary Ammonium
Salts," H Colloid Interface Sci. 36, 289 (1971).
36. J. Rogers, K. L. Sutherland, E. E. Wark, and
J. W. Wark, "Principles of Flotation-Paraffin Chain
Salts as Flotation Agents," Trans. AIME 169, 312
(1946).


82
nonaluminated silica. The negative zeta potential values
for concentrations greater than 3 x 10 M citrate for the
aluminated silica curve could be due to one of two pos
sible effects. Either aluminum ions were removed from
the silica surface to form aluminum citrate complexes in
solution or citrate ions adsorbed to positively charged
aluminosilicate sites. Study of Figure 19 alone does not
help to eliminate either of the possible mechanisms.
However, if the desorption characteristics of the
system are studied, one mechanism can be eliminated.
Therefore, untreated silica samples were exposed to a
3 /[
10 M NaCl solution containing 10 M citrate ions and
-5 -3
10 M aluminum ions. Then a 10 M NaCl solution void
of citrate and aluminum ions was streamed through the
cell. Figure 20 shows the results obtained. If aluminum
ions were removed from the surface to complex with citrate
ions in solution, then the E/P value should decrease to
_3
E/P value for nonaluminated sample streamed with 10 M
NaCl solution. However, the E/P increased at early
times indicating the removal of a negatively charged
specie from the surface. The only negatively charged
specie present in the system capable of specific
adsorption was citrate ions. These results indicate that
citrate ions first adsorbed to aluminosilicate sites and


Zeta potentia I, (m V)
100
Figure 24.
Zeta potential vs. pH for unwashed T-61
alumina (coarse particles) aged in water for
1, 2 and 3 days.


26
acid. They also suggest that surface roughness con
tributes to high inner layer capacities creating high
surface charges due to slight interpenetration of
potential determining and adsorbed counter ions.
An alternative hypothesis for anomalously high
surface charge densities can be investigated by examining
the mass balance equation which describes an aqueous-
oxide system in a titration experiment. The real
adsorption density (T a^) is defined as the excess moles
per unit of surface area of hydrogen over hydroxide ions
on the solid surface. The correct mass balance equation
for this value is
[(H-H) +AH-(H-H) i -C]v
real a [DJ
where (H-OH) -¡_ntial the total excess moles of hydrogen
over hydroxide species per unit volume of solution in the
system before titrant addition, AH is the moles of titrant
per unit volume of solution added to the initial system,
(H-OH) so]_ut;pon as the excess moles of free hydrogen over
hydroxide ions per unit volume in solution, V is the
solution volume, A is the total surface area of oxide
powder used in the titration and C is the excess moles
per unit volume of hydrogen over hydroxide ions which are


Zeta potent ial, (mV)
112
Figure 25. Zeta potential vs. pH for 1608 and 1613
montmorillonite.


89
The methods of determining the ZPC are numerous.
For this study, two methods were employed. First, zeta
potential as a function of pH for 0.1 weight percent
alumina suspensions in water was performed for samples
aged 0, 1, 2, 4, 8, 16, 33 and 97 days for each of the
aluminas. The pH at which the zeta potential is zero is
defined as the ZPC. The second method of determining
the ZPC is by determining the equilibrium pH of an
alumina-water system. The equilibrium pH occurs when
the addition of more powder causes no further change in
the pH of the aqueous-solid system. This occurs when
the amount of potential determining ions (H+ and OH ) in
the solution and on the surface are in equilibrium. The
ZPC and equilibrium pH should have the same value. If
not, then the difference between the two values indicates
the existence of impurities adsorbed onto the surface of
the powders.
The method used to determine zeta potentials was
microelectrophoresis using the Riddick cell. The entire
assembly (cell and electronics) are commercially
available.* Operation of the Zeta Meter and the method of
obtaining particle mobility are described in detail by
*Zeta Meter Inc., New York.


137
electrokinetic evidence showed that sodium ions in the
_|_ _j |_
clay also exchange for either H or Ca in solution
since the zeta potential of 1613Ca increased in
concentrated NaCl solutions to a value corresponding to
a sodium clay in water.
Aluminum ions were shown to specifically adsorb to
the surfaces of all the clays studied at concentrations
/ O
of AlCl^l^O solutions between 10 and 10 moles/
liter. This was shown by observing the sign reversal
of the zeta potential at these concentrations. Studies
of the clays in solutions of varying pH and constant
AlCl2*6H20 concentrations revealed a dependence of zeta
potential characteristics on the degree of hydrolysis
of the aluminum ion. Also, adsorption of aluminum ions
was found to depend on the equilibration time and pH of
the clay in water and on the method of solution prepara
tion. The presence of a complex aluminum species
4+
(A1q(0H)2q) at 7 > pH > 4 as suggested by other workers
was supported by these results. However, Auger analysis
of the surface of silica disks which had been exposed to
constant aluminum ion concentrations at varying pH values
refuted the aluminum hydroxide coating theory proposed
by other workers by showing that aluminum hydroxide did
not adsorb to the surface at high pH values.


BIBLIOGRAPHY
1. B. V. Derjaguin and L. Landau, "Theory of the
Stability of Strongly Charged Lyophobic Sols and the
Adhesion of Strong Charged Particles in Solutions of
Electrolytes," Acta Physiochim USSR 14, 633 (1941).
2. M. L. Hair, "Infrared Spectroscopy in Surface
Chemistry," Dekker, New York (1967).
3. B. Ball and D. W. Fuerstenau, "Review of the
Measurement of Streaming Potentials," Miner. Sci.
Engrg. 5, 267 (1973).
4. M. von Smoluchowski, Bull. Intern. Acad. Polon.
Sci. Classe, Sci. Math. Nat. 184~ (1903).
5. A. A. Boumans, "Streaming Currents and Turbulent
Flows in Metal Capillaries," Physica 23, 1007
(1957) .
6. S. Ergun, "Fluid Flow Through Packed Columns,"
Chem. Eng. Prog. 48, 93 (1952).
7. B. R. Bird, W. E. Stewart, and E. N. Lightfoot,
"Transport Phenomena," John Wiley and Sons,
New York (1966).
8. P. Somasundaran and G. E. Agar, "The Zero Point of
Charge of Calcite," J. Colloid Interface Sci. 24,
433 (1967).
9. G. K. Korpi and P. L. deBruyn, "Measurement of
Streaming Potentials," J. Colloid Interface Sci. 40
263 (1972).
10. L. L. Hench, H. A. Paschall, W. C. Allen, and
G. Piotrowski, "An Investigation of Bonding
Mechanisms at the Interface of a Prosthetic
Material," Army Report No. 5 (1974).
11. A. M. Gaudin and D. W. Fuerstenau, "Quartz Flotation
with Anionic Collectors," Trans. AIME 202, 68 (1955).
145


63
y
Figure 15. Log of the ratio of C and F vs. zeta potential
for montmorilIonite for the experimental data.


62
The experimental data (C and T) fit the Stern equa-
C
tion, since Figure 15 shows that a plot of log(|r) vs. y
yields a straight line. According to Equation 40 the
slope should be 1.3 if v = +3. However, the straight line
obtained has a slope of 0.8.
A plausible explanation for the discrepancy between
the theoretically determined and experimentally obtained
slope must come from examining the factors involved in
v, the valence of the adsorbed ion. Errors in any other
term lead only to shifts in the position of the line but
no change in slope. In calculating the theoretical
slope, v was assumed to be +3 for the aluminic ion.
However, for the slope to decrease to the experimentally
determined value, v must be lower than three. That is,
the effective valence of the adsorbed ion must be smaller
than 3. This is possible only if aluminum ions are in
volved in ion exchange for sodium ions (+1 valence, making
the effective v.-^, +2) or calcium ions (+2 valence, making
the effective v^, +1). Therefore the v^ would have a
value between 1 and 2. The effective valence calculated
from Figure 15 is jp-jO) = 1.8.
Energetically, the work required in bringing an
aluminum ion from solution to the surface is partially
supplied by the "negative" work involved in releasing a


cm Hg
Figure 18. Streaming potential-pressure ratio vs. stream
time for untreated, heat treated, base treated,
heat treated and base treated and non-aluminated
fused silica.


adsorption determined by this method fit an analytical
form of the Stern equation.
The desorption of aluminum ions from thermally
and/or chemically treated vitreous silica surfaces is
investigated by observing the changes of the streaming
potential-flow pressure ratio as a function of streaming
time. Aluminum ions desorb from surfaces whose treatment
has created adjacent silanol groups whereas they remain
on surfaces which contain isolated silanol groups.
Analogous to interpretations of adsorption-desorption
phenomena for fine particle systems, the mechanism of
adsorption onto coarse particle surfaces is hydrogen
bonding of the aluminum ion to isolated hydroxyl groups
on the silica surface.
The aging of alumina powder surfaces is studied by
observing changes in the zero point of charge (ZPC) with
aging time in water using electrophoresis measurements.
Alumina powders age due to the changing hydration state
of the surface. Grinding of T-61 powder from relatively
coarse to fine powder causes the ZPC to shift from
pH = 7.0 to pH = 9.5. Nonalpha phase aluminas age much
more slowly but to a greater extent than alpha phase
powders.
The surface chemistry properties of montmorillonite,
an aluminosilicate clay mineral, are investigated by
XI11


127
suitable for Auger Electron Spectroscopy (AES) and were
analyzed for aluminum on the surface.
Figure 33 shows the results of this analysis. These
data are presented as Auger peak height ratio of aluminum
to oxygen as a function of pH. The results show that a
large amount of aluminum persists on the silica surface
from pH = 5-9. Beyond pH = 9, the amount of aluminum on
the silica decreases significantly. This is direct
evidence that aluminum hydroxide does not coat the silica
surfaces at higher pH values.
The results shown in Figure 29 contradict results
of Swartzen-Allen and Matijevic (73). They found that
-4
10 M/L A1 ion did not reverse the sign of the electro
phoretic mobility for any pH > 4. As stated in the
introduction several reasons for these differences are
possible. Therefore a detailed investigation of the
effect of clay solution preparation, clay-water equili
bration, aluminum salt used, and clay-aluminum ion solu
tion aging was undertaken using 1613Mat. Comparison of
Figures 34 and 35 shows no significant difference in the
zeta potential behavior with pH at constant A1 ion con
centration when either the chloride or nitrate salt is
used. When the DSM was used, no difference in the zeta
potential behavior as a function of pH was observed


119
Comparison of Figure 29 to Figure 28 shows that 1613Na
_ C.
is truly a sodium clay, since the zeta potentials in 10
NaCl and CaCl2*2H90 for 1613Na and 1608 are approximately
twice the value for 1613 clay. Also, the shape of the
CaC^^H^O curve for 1613Na is similar to the same curve
for 1608. Both curves, however, differ from the shape of
the same curves for 1613 and 1613Ca. In fact, a simple
calculation can be made which suggests that the shapes
of the sodium clay curves for CaCl9are controlled
_j_ _l j_
by ion exchange of either Na or H in the clay for Ca
in solution. For 1608 and 1613Na, the magnitude of
change of the zeta potential from 10 ^ to 10 ^ M/L
CaC^ZF^O is a factor of 2-2.5. This is the difference
between sodium and calcium clays (1608 and 1613 or 1613Na
_| |__
and 1613Ca) when no Ca are added to the solution. The
_j
solution concentration of Ca which represents twice
the weight percent of calcium in a calcium'clay (1613)
should result in decreasing the zeta potential by a
factor of 2-2.5 if ion exchange occurs in a sodium clay -
_| |_ _
Ca ion solution system. This concentration is 10 M/L
CaCl2(4 weight percent Ca). Figure 30 also shows
that 1613Ca was converted to a sodium clay as the NaCl
concentration increased. This is shown by an increasing
negative zeta potential as the concentration of NaCl
increases.


To My Lovely Wife, Barbara


6
conditions of zero zeta potential (ZZP) of the particle
surfaces.
To fully understand the montmorilIonite system, its
two major components, silica and alumina, were studied
independently. Chapter 7 shows that montmorillonite has
two important properties. First it specifically adsorbs
aluminum ions under certain conditions. Since silica also
has this property, a detailed study of the desorption of
this ion from silica surfaces treated with chemical and
thermal agents known to alter hydration (2) is
presented in Chapter 5. This is accomplished by using
a slightly modified streaming potential technique from
that described in Chapter 2. Information obtained from
these desorption studies gives insight to the adsorption
mechanism of aluminum ions onto silica surfaces.
The second major property of montmorillonite
described in Chapter 7 is that equilibration time and pH
in water affects its capacity to specifically adsorb
aluminum ions. This appears to be a phenomenon caused by
varying degrees of hydration of the clay particles.
Another "aging effect" of this type is described in
Chapter 6 for aluminum oxide surfaces. It is possible
that hydration of clay particles is controlled in some
manner by the hydration of alumina in the clay. Therefore


Zeta potential, (mV)
93
Zeta potential vs. pH for y-alumina after
aging for 0, 1, 8, 16, 33 and 97 days in
water.
Figure 21.


Zeta
113
pH
Figure 26. Zeta potential vs. pH for 1613Na and 1613Ca
montmorillonite.


ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE
By
JOHN MILTON HORN, JR.
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1978

To My Lovely Wife, Barbara

ACKNOWLEDGEMENTS
Deep appreciation and many thanks are extended to the
author's graduate supervisory committee which included
Dr.
G. Y.
Onoda, Jr.,
chairman;
Dr. L. L. Hench;
Dr.
R. W.
Gould; and,
Dr. D. 0.
Shah. Special thanks go to
his
advisor, Dr. G. Y.
, Onoda, Jr
., without whose many
helpful and lengthy discussions, this work would not have
been possible.
The author wishes to thank Mr. Fumio Ouchi for the
Auger data presented in Chapter 7. Also, thanks go to
Mr. Peter Curreri and Mr. Jim Adair for helpful discus
sions, and to Mr. Nick Gallantino for technical
assistance in the lab.
Finally, the author wishes to acknowledge the
National Institute of General Medical Sciences grant
#GM21056-02 and the National Science Foundation
grant #AER76-24676 for partial financial support for this
work.
in

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT xi
CHAPTER
1 INTRODUCTION 1
2 STREAMING POTENTIAL AND NONCREEPING
FLOW IN POROUS BEDS 8
Introduction 8
Materials and Methods 11
Results and Discussion 16
Conclusions 24
3 CALCULATION OF SURFACE CHARGE DENSITY ERROR
DUE TO COMPLEX SPECIES FORMATION IN
SOLUTION 25
Introduction 25
Procedure 28
Results 35
Discussion 41
Conclusions 47
4 DETERMINATION OF ADSORPTION CHARACTERISTICS
FOR FINE PARTICLE SYSTEMS FROM ELECTRO
PHORESIS MEASUREMENTS 49
Introduction 49
Materials and Methods 50
Results and Discussion 51
Conclusions 64
IV

TABLE OF CONTENTS continued.
Page
CHAPTER
5 REVERSIBILITY OF ALUMINUM ION ADSORPTION
ON FUSED SILICA 66
Introduction 66
Materials 70
Methods 71
Results and Discussion 76
Conclusions 84
6 THE EFFECTS OF AGING ON THE ZERO POINT OF
CHARGE OF ALUMINA 85
Introduction 85
Materials and Methods 87
Results 90
Discussion 97
Conclusions 101
7 ELECTROKINETIC PROPERTIES OF
MONTMORILLONITE 103
Introduction 103
Materials 107
Methods 109
Results and Discussion Ill
Conclusions 136
APPENDIX A 140
APPENDIX B 142
BIBLIOGRAPHY 145
BIOGRAPHICAL SKETCH 15 3
v

LIST OF TABLES
Table Page
I Standard Free Energy of Formation Values
at 298K (Kcal/mole) 29
II Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2SO3, and Their Equilibrium
Constants (K) 31
III Activity Coefficients for Ionic Species
at Various Solution Concentrations 32
IV Calculated Values for Disturbed Layer
Thickness (t) and Soluble SO2 at
Equilibrium Using Data from van Lier
eta 1. (ref. 22) 46
V Values of CG for Various Zeta Potentials
and Weight Percent Solids 55
VI Values of C, T and log (C/I) for Various
Zeta Potential Values 59
VII Surface Areas of A-16, A-17, T-61, C-30 DB
and Gamma Alumina Powders 91
VIII Compositions of Montmorillonite Clays 1608
and 1613 in Weight Percent 108
IXChemical Conditions for Zero Zeta
Potential (ZZP) for 1608 and 1613
Montmorillonites 125
vi

Figure
1
LIST OF FIGURES
Page
Streaming potential cell; a) PMMA tube;
b) threaded PMMA plugs; c) platinum leads
to electrodes consisting of perforated
disks with attached platinum mesh;
d) platinum electrode; e) solution flow
entrance; f) solution flow exit;
g) porous bed; h) O-ring 13
2 Streaming potential solution flow system 15
3 R-C circuitry used in streaming potential
experiments 17
4 Streaming potential vs. pressure using
the R-C measuring circuit 19
5 Streaming potential vs. pressure without
the R-C measuring circuit 21
6 Flow rate vs. pressure for fused silica 22
7 Surface charge density error (Aa) vs. pH in
10"1, 10"2) 103 M/L NaCl solution for
vitreous silica using Bolt's experimental
conditions 36
8 Surface charge density error (Aa) vs.
total surface area for amorphous silica
at pH = 7, 8, 9 and 10 37
9 Surface charge density error (Aa) vs. pH
in 10_1, 10-2, 10_3 M/L NaCl solutions
for hydrated silica using Tadros's and
Lyklema's experimental conditions 39
10 Surface charge density error (Aa) vs.
total surface area for precipitated
silica at pH = 7, 8, 9 and 10 40
vi 1

LIST OF FIGURES continued.
Figure Page
11 Zeta potential vs. concentration of
AICI36H2O for 0.00125, 0.0025, 0.025,
0.2 and 1.0% weight percent montmoril-
lonite 52
12 Apparent aluminum ion concentration C0
vs. weight percent montmorillonite for
zeta potential = 0 at pH = 4.0 56
13 Concentration of aluminum CG minus the
equilibrium concentration (C) of aluminum
in solution vs. weight percent montmoril
lonite for zeta potential = +15, +10, +5,
0, -5, -10 mV at pH = 4.0 57
14 Zeta potential and log adsorption density
(T) vs. equilibrium concentration (C) of
aluminum in solution for montmorillonite
at pH = 4.0 60
15 Log of the ratio of C and r vs. zeta
potential for montmorillonite for the
experimental data 63
16 Streaming potential apparatus modified
to maintain constant flow pressure;
a) secondary reservoir; b) primary
reservoir; c) pump; d) solution flow
valve; e) cell; f) electrometer;
g) recorder; h) solution head height 73
17 Zeta potential vs. concentration of
sodium citrate and aluminum chloride 74
18 Streaming potential-pressure ratio vs.
stream time for untreated, heat treated,
base treated heat treated and base
treated and non-aluminated fused silica 77
viii

LIST OF FIGURES continued.
Figure Page
19 Zeta potential vs. concentration of
sodium citrate for fused silica in
supporting electrolytes of 10~3 M/L
NaCl and 10-3 M/L NaCl, 10-4 m/L
A1C13 6H2O 81
20 Streaming potential-pressure ratio vs.
stream time for aluminated fused silica
for untreated, heat treated, base
treated and heat treated and base
treated surfaces 83
21 Zeta potential vs. pH for y-alumina
after aging for 0, 1, 8, 16, 33 and
97 days in water 93
22 pH of the zero point of charge (pH^pc)
vs. aging time in water for T-61, A-17 ,
C-30 DB, y, and A-16 aluminas 94
23 Solution pH vs. weight of alumina added
for A-16 alumina 96
24 Zeta potential vs. pH for unwashed T-61
alumina (coarse particles) aged in water
for 1, 2 and 3 days 100
25 Zeta potential vs. pH for 1608 and 1613
montmorillonite 112
26 Zeta potential vs. pH for 1613Na and
1613Ca montmorillonite 113
27 Zeta potential vs. log concentration of
NaCl, CaCl2*2H20 and AlCl3*6H20 solutions
for 1608 montmorillonite 115
28 Zeta potential vs. log concentration of
NaCl, CaCl2*2H20 and AlCl3*6H20 solutions
for 1613 montmorillonite 116
xx

LIST OF FIGURES continued.
Figure Page
29 Zeta potential vs. log concentration
of NaCl, CaCl2*2H20, and AlCl3*6H20
for 1613Na montmorillonite 117
30 Zeta potential vs. log concentration
of NaCl, CaCl2-2H20, and AlCl3*6H20
for 1613Ca montmorillonite 118
31 Zeta potential vs. pH for 1608 montmoril
lonite in 0, 10~5 and 10-4 M/L AlCl3*6H20
solutions 122
32 Zeta potential vs. pH for 1613 montmoril
lonite in 0, 10~5, 10~4 M/L AlCl3*6H20
solutions 123
33 Auger peak height ratio vs. pH for
vitreous silica after one hour exposure
to 10-4 M/L AlCl3*6H20 solution 128
34 Zeta potential vs. pH for 1613Mat in
10"4 M/L A1C136H2O solution. SSM-0
means single solution method, clay
plus A1 solution aging time = 0 hours.
35 Zeta potential vs. pH for 1613Mat in
10~4 M/L Al (NO3) 3 9H20 solution 130
36 Zeta potential of 1613 montmorillonite
in 10-4 M/L AlCl3*6H20 and water at pH =
6 vs. clay equilibrium time at pH = 4,
6 and 8 132
37 Solution pH vs. total weight of montmoril
lonite added to 25 ml of water 135
x

Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE
By
John Milton Horn, Jr.
March 1978
Chairman: George Y. Onoda, Jr.
Major Department: Materials Science and Engineering
The surface chemistry of montmorillonite, vitreous
silica and alumina is investigated by electrokinetic
methods of streaming potential and microelectrophoresis.
Also, improved analytical techniques are developed to
obtain these electrokinetic data and adsorption
information for these oxide materials.
Many investigators find that linear streaming
potential-flow pressure relationships do not pass through
the zero potential-zero flow pressure origin. One source
of this error, electrode polarization, is eliminated
by introducing an R.C. circuit in the system. Simulta
neous study of flow pressure versus flow rate reveals a
nonlinear curve suggesting the existence of noncreeping
xi

flow in the pressure range studied. However, streaming
potential-flow pressure relationships remain linear and
pass through the origin when electrode polarization
effects are eliminated. Therefore, zeta potential
values calculated from the Smoluchowski equation under
noncreeping flow conditions are as valid as if creeping
flow conditions existed.
Using mass balance concepts, surface charge
density error incurred by assuming that all hydrogen
or hydroxide ions which cannot be accounted for as free
ions in solution adsorb to the surface rather than form
part of complex ions in solution is calculated for five
silica phases in three electrolyte solution concentra
tions. The error increases as thermodynamic stability
of the silica decreases, as pH increases and as the total
surface area present in the experimental system decreases.
A new method is developed for determining adsorption
isotherms directly from electrophoretic measurements.
The unique feature is gathering zeta potential data as a
function of ion concentration for various solids
concentrations. The adsorption of aluminum ions onto
montmorillonite clay is presented as an experimental
example. The values of adsorption densities and equili
brium concentrations of aluminum ions in solution after
Xll

adsorption determined by this method fit an analytical
form of the Stern equation.
The desorption of aluminum ions from thermally
and/or chemically treated vitreous silica surfaces is
investigated by observing the changes of the streaming
potential-flow pressure ratio as a function of streaming
time. Aluminum ions desorb from surfaces whose treatment
has created adjacent silanol groups whereas they remain
on surfaces which contain isolated silanol groups.
Analogous to interpretations of adsorption-desorption
phenomena for fine particle systems, the mechanism of
adsorption onto coarse particle surfaces is hydrogen
bonding of the aluminum ion to isolated hydroxyl groups
on the silica surface.
The aging of alumina powder surfaces is studied by
observing changes in the zero point of charge (ZPC) with
aging time in water using electrophoresis measurements.
Alumina powders age due to the changing hydration state
of the surface. Grinding of T-61 powder from relatively
coarse to fine powder causes the ZPC to shift from
pH = 7.0 to pH = 9.5. Nonalpha phase aluminas age much
more slowly but to a greater extent than alpha phase
powders.
The surface chemistry properties of montmorillonite,
an aluminosilicate clay mineral, are investigated by
XI11

electrophoresis. Ion exchange of sodium, calcium and
hydrogen ions in solution for similar cations in the
clay controls the electrokinetic behavior of the
montmorillonite. Aluminum ions are found to specifically
adsorb from solution onto the surface. However, the
degree of aluminum ion adsorption as shown by electro
phoresis measurements depends on the equilibration time
and pH of the clay in water whereas the electrokinetic
behavior of the clay in the absence of aluminum ions in
solution is independent of equilibration time and pH
in water.
xiv

CHAPTER 1
INTRODUCTION
The surface chemistry of montmorillonite, an
aluminosilicate clay mineral, and its two major con
stituents, silica and alumina, is investigated by
electrokinetic methods of streaming potential and micro
electrophoresis. Using these techniques, many authors
have studied the changing characteristics of the elec
trical double layer surrounding the particles or colloids
as a function of adsorption of ions from solution, ion
exchange, and/or aging of hydrated surfaces. However,
proper use of these two techniques can also yield
important information about the mechanisms of these
processes. Opportunities are provided for improving
these techniques which can then be used to develop new
methods for determining adsorption properties of oxide
surfaces.
The electrokinetic parameter used to describe the
nature of the electrical double layer is the zeta
potential. Essentially, the zeta potential is the
electrical potential at the Stern plane in solution. The
Stern plane separates the diffuse (Gouy-Chapman) layer of
1

2
counter ion charge (the ions in solution which electrical
ly balance the charge on the solid surface) from the
layer of adsorbed ions (Stern layer). The magnitude of
the zeta potential is determined by the concentration of
ions in the Stern layer and/or the concentration of
ions in the diffuse layer. Zeta potentials can be
determined from streaming potentials using coarse
particles or from electrophoretic mobilities of colloids.
In the past, two practical problems of noncreeping
flow and electrode polarization have limited accurate
measurements of streaming potentials. In a streaming
potential experiment, solution is forced to flow through
a porous bed of coarse particles. A linear relationship
between the streaming potential, E, and solution flow
pressure, P, is predicted by the Smoluchowski equation
which is used to calculate zeta potentials as described
in Chapter 2. In practice a linear relationship is
usually observed. However, Chapter 2 also shows that
a nonlinear relationship exists between volumetric flow
rate and flow pressure. This suggests that noncreeping
flow exists in the pressure range used in many typical
streaming potential measurements. Since the Smoluchowski
equation assumes creeping flow, zeta potentials calculated
from streaming potential values obtained under noncreeping

3
flow conditions may be invalid. Chapter 2 shows why
streaming potential values obtained under commonly
encountered noncreeping solution flow conditions can
still be used to calculate zeta potential values using
the Smoluchowski equation.
The second problem commonly experienced in streaming
potential measurements is the nonzero intercept of the
streaming potential-solution flow pressure relationship.
Theoretically, when the flow pressure is zero, the
streaming potential must be zero. However, a finite
rest potential is commonly observed at zero pressure.
This problem may be due to electrode polarization.
Therefore, Chapter 2 presents a method for measuring
streaming potentials without the undesirable effects of
electrode polarization.
Calculation of surface charge densities (a) of the
solid oxide surface from potentiometric titrations using
potential determining ions is used by many investigators
to determine the variations of a with pH. In a
potentiometric titration, the concentration of hydrogen
ions before and after addition of a known amount of
titrant is recorded. The quantity of hydrogen or
hydroxide ions which cannot be accounted for in solution
is assumed by most investigators to be adsorbed onto the

4
solid surface. However, if dissolution of the solid
occurs, some of the ions may not adsorb to the surface,
but may be part of complex ions in solution. However,
they are not free hydrogen or hydroxide ions. Their
absence cannot be detected by pH measurement. Therefore,
absolute values of surface charge densities calculated
by most investigators are too large. For silica, surface
charge densities are negative values since the surface
is negatively charged in water above pH = 3.0. Therefore,
Chapter 3 describes the method used for aqueous silica
systems to calculate the surface charge density error due
to use of mass balance expressions which are incomplete
when hydrogen or hydroxide ions which are part of complex
ions in solution due to dissolution of the solid are
neglected.
Accurate adsorption density values which are used
to calculate surface charge densities are easily
determined for oxide systems since hydrogen and hydroxide
ions are potential determining ions. However, more time
consuming and sometimes less accurate methods must be
used to determine adsorption densities of other ions on
the oxide surface. Chapter 4 presents a new method for
determining this information from electrophoresis data.
The unique aspect of this method is the use of various

5
solids concentrations in gathering data of zeta potential
versus apparent ion concentration in solution. From
these data, adsorption densities and equilibrium
concentrations of ions in solution after adsorption can
be determined. The Stern equation is then used to
determine if these data fit the expected form for common
adsorption isotherms.
The techniques discussed in Chapters 2 and 4 are
used to study the electrokinetic properties of an
aluminosilicate system. The system chosen was montmoril-
lonite clay, which is a major component of phosphate
slimes. The slow settling of phosphate slimes due to
fine particle size clays such as montmorillonite is a
major problem in the phosphate mining industry. Dense
coagulation of clay particles would result in faster
slime settling rates and higher final sediment densities.
The degree of coagulation of the particles is partially
controlled by their electrical double layer properties.
According to DLVO theory (1), coagulation will result
when the electrical repulsive forces between particles
are small enough to allow van der Waal's forces of
attraction to cause particle coalescence. Since zeta
potentials are a measure of the degree of these electrical
forces, Chapter 7 is devoted to finding chemical

6
conditions of zero zeta potential (ZZP) of the particle
surfaces.
To fully understand the montmorilIonite system, its
two major components, silica and alumina, were studied
independently. Chapter 7 shows that montmorillonite has
two important properties. First it specifically adsorbs
aluminum ions under certain conditions. Since silica also
has this property, a detailed study of the desorption of
this ion from silica surfaces treated with chemical and
thermal agents known to alter hydration (2) is
presented in Chapter 5. This is accomplished by using
a slightly modified streaming potential technique from
that described in Chapter 2. Information obtained from
these desorption studies gives insight to the adsorption
mechanism of aluminum ions onto silica surfaces.
The second major property of montmorillonite
described in Chapter 7 is that equilibration time and pH
in water affects its capacity to specifically adsorb
aluminum ions. This appears to be a phenomenon caused by
varying degrees of hydration of the clay particles.
Another "aging effect" of this type is described in
Chapter 6 for aluminum oxide surfaces. It is possible
that hydration of clay particles is controlled in some
manner by the hydration of alumina in the clay. Therefore

7
Chapter 6 presents evidence of alumina surface hydration
by studying the change of the zero point of charge
with aging time in water.

CHAPTER 2
STREAMING POTENTIAL AND NONCREEPING FLOW IN POROUS BEDS
Introduction
The measurement of streaming potential on porous beds
is an important method for determining zeta potential (3).
The method is convenient because many materials cannot
readily be shaped as capillary tubes. A common practice
for porous beds is to use the Smoluchowski equation (4)
4Trr)XE
eP
[1]
to calculate the zeta potential (c) from the measured
streaming potential (E), the viscosity (q), the specific
conductivity (A), the dielectric constant (e), and the
driving pressure (P).
Equation 1 was originally derived for simple
capillaries assuming laminar flow conditions. Boumans (5)
has shown that the E/P ratio in simple capillaries is
smaller under turbulent flow than under laminar flow.
A change in flow behavior with increasing pressure is
also known to occur in porous beds. The relationship
between flow rate and pressure changes from linear to
nonlinear at a certain Reynolds number. Many streaming
potential measurements on porous beds, in the past, have
8

9
been made under conditions near the linear to nonlinear
transition zone. This raises questions about the
validity of using the E/P ratio to calculate the zeta
potential when the flow condition is nonlinear. The
purpose of this investigation was to determine whether
the E/P ratio changes with increasing P, as flow changes
from linear to nonlinear. If no changes occur,
Equation 1 is valid in the nonlinear region as well as
the linear region. Only a rather narrow pressure range
was investigated since this was the range of interest in
typical streaming potential measurements.
In porous beds, a Reynolds number, Re, has been
defined (6) as
Re
yp
y
[2]
where is the mean particle diameter which is equal to
400 microns for the particles used, v is the superficial
velocity found by taking the ratio of volumetric flow
rate and cross sectional area of the bed, p is the
solution density, y is the solution viscosity and e is the
bed porosity.
At Re values below around 10, flow rate and pressure
are linear (7). This is the region of creeping or Darcy
flow. At high Reynolds number, a nonlinear relationship

10
develops (the region of noncreeping or nonDarcy flow).
This change is a result of the formation of standing
eddies behind the particles. The Ergun equation (6)
describes the flow behavior over the range of interest:
(
AP P) (#) () = 150 y
L 1-e Dp(vp/y)
2T
v p
[3]
where AP is the pressure drop across the bed, L is the
bed length and all other variables as defined in Equa
tion 2.
Accurate E versus P measurements cannot be obtained
without recognizing and dealing with the experimental
problems of electrode polarization. Ball and
Fuerstenau (3) cite electrode polarization as the
probable cause for E versus P curves not passing through
the origin. Somasundaran and Agar (8) proposed that the
instantaneous change in the voltage when solution flow
is initiated is the true streaming potential. Korpi and
deBruyn (9) incorporated a recorder into their system to
aid in the measurement of the instantaneous voltage
changes during flow initiation and termination.
In the present investigation, a R-C circuit is
introduced which directly nulls the background potentials
(rest potential and electrode polarization potentials).
This greatly facilitates the measurement of the true

11
streaming potential, especially under conditions where
the background potential is large and varies with time.
A modified streaming potential apparatus is described
that is suitable for low pressure studies and for use on
materials that are considerably reactive. A new cell
design is introduced that is easier to pack, easier to
clean, and is less fragile than previous designs.
Materials and Methods
1. Materials
The bed materials used for this study were fused
silica* and an invert silicate glass (denoted as
"bioglass") whose preparation is noted elsewhere (10) and
whose composition is as follows: 45 wt. % SO2 ,
24.5 wt.7o CaO, 24.5 wt.% ^2*3, and 6.0 wt.% P2O5 This
glass has certain reactive properties which makes it an
interesting material for biological implant studies (10).
The fused silica and bioglass were ground and sieved, and
the -20+45 fraction (0.0833-0.0354 cm aperture) was used
in the experiments. The fused silica was acid washed by
conventional methods used by other investigators (11).
Due to the reactivity of bioglass, no acid washing was
*Vitreosil,Thermal American Fused Quartz Co., Montville,
New Jersey.

12
performed. Instead, bioglass was only washed with con
ductivity water.
The bed lengths were 4.1 cm and the cross sectional
2
area of the beds was 2.38 cm The bed porosity was
calculated to be 36%.
Water used to rinse the bed material and to prepare
solutions was obtained from a water deionization system*
with the following specifications: a resistivity of
1.87 x 107 ohm-cm, dissolved solids at the parts per
billion level, dissolved gases removed, and organics
removed. Since dissolved gases were removed, equilibra
tion with air was required which caused the pH to drop to
5.5-6.0 due to absorption of CO2 This is the pH range
at which the streaming potential experiments were
performed. Sodium chloride** solutions of various
concentrations were prepared with this water and used in
the experiments.
2. Apparatus
The cell for the streaming potential system is shown
schematically in Figure 1. It consists of a thick walled
'-'Continental Water Conditioning Co., Inc., Gainesville,
Florida.
**ACS reagent grade, Scientific Products, Ocala, Florida.

13
Figure 1. Streaming potential cell; a) PMMA tube;
b) threaded PMMA plugs; c) platinum leads to
electrodes consisting of perforated disks with
attached platinum mesh; d) platinum electrode;
e) solution flow entrance; f) solution flow
exit; g) porous bed; h) O-ring.

14
polymethylmethacrylate (PMMA) tube with ends tapped to
receive two threaded PMMA plugs. This design allows
easier cleaning between experiments and easier packing of
the porous bed material. The platinum leads to the
electrodes* protrude completely through the PMMA plugs
and are sealed by a press-fit using a Teflon spacer. The
platinum electrode consists of a perforated disk for
easy solution flow and an attached platinum mesh for
increased electrode surface area. Because the electrodes
are able to slide through the PMMA plug, adjustments can
be made to obtain a tight packing of the bed.
The solution flow system is shown schematically in
Figure 2. The solution reservoir is a 25 liter poly
ethylene container with a mechanical on-off flow valve.
The height of the outlet tube is varied to produce dif
ferent hydrostatic pressures across the cell. This
design allows for E versus P measurements at pressures as
low as 1.0 cm Hg. Also, flow rates can be measured at
the outlet tube exit.
The potentials from the electrodes were measured with
an electrometer.** The output from the electrometer was
*Englehard Industries, Carteret, New Jersey.
**Keithley, Model 602.

15
Figure 2. Streaming potential solution flow system.

16
passed through an R-C circuit (details of which are dis
cussed in the Results and Discussion section) into a strip
chart recorder.* E versus P curves were generated by
plotting the peak height value on the recorder cor
responding to the applied pressure.
Results and Discussion
1. Circuitry for Measuring Streaming Potential
A R-C circuit shown in Figure 3 was introduced into
the system for the purpose of directly measuring only the
true streaming potential on the recorder within a 997, ac
curacy by nulling out all background potentials. Two
different circuits can be employed by changing the switch.
When the liquid is not streaming, the switch is in
position 1. The rest potential is rapidly stored in the
capacitor, nulling the signal to the recorder. The time
constant of this circuit is 2.2 seconds and so more than
997, of the nulling occurs in 10 seconds.
After charging the capacitor with the rest potential,
the switch is turned to position 2 and flow through the
cell is initiated within two seconds. The time constant
for the new circuit is 220 seconds. Therefore, the rest
^Hewlett Packard Model 680.

17
cell
Recorder
5
with 2x10 Q
resista nee
Figure 3.
R-C circuitry used in streaming potential
experiments.

18
potential stored in the capacitor does not decay more than
17o in two seconds. The potential received by the electro
meter when flow is initiated is the sum of the rest
potential and the true streaming potential. However,
since the rest potential has already been stored in the
capacitor, the rest potential contribution of the input
voltage is nulled out and only the streaming potential is
recorded. Because of the arrangement of the resistors,
the input voltage to the recorder is 1/100 of the output
from the electrometer (which is one volt full scale).
Therefore, the recorder is kept on a 10 mV full scale
range.
Streaming can be terminated after around five seconds
of flow. Then the switch is placed in position 1. The
capacitor again charges rapidly to the rest potential and
a new measurement can be initiated.
Using the R-C circuit, streaming potential measure
ments were carried out on fused silica using water and
5 A 3
NaCl solutions of 10 10 and 10 mol/L as streaming
solutions and on bioglass using water as the streaming
solution. The E versus P curves obtained are shown in
Figure 4. Within experimental error, all curves are
observed to be linear and pass through the origin.
Similar findings have been observed for a variety of


T
Figure 4. Streaming potential vs. pressure using the R-C
measuring circuit.

20
materials and solutions without deviations in linearity
or intersection with the origin.
Without the R-C circuit, under otherwise identical
conditions, straight lines passing through the origin
are not obtained in the E versus P curves as shown in
Figure 5. This can be attributed to the fact that rest
potential contributions were not taken into account.
2. Creeping Versus Noncreeping Flow Conditions
During the E-P measurement described above, solution
flow rates were also determined. Volumetric flow rates,
Q, versus pressure are given in Figure 6. A nonlinear
relationship between Q and P was observed.
In Equation 3, v can be converted to Q, since
2
Q = vfTr where r is the radius of the bed. The Ergun
function becomes
AP = ^. Q +
[4]
where
and
P
Since e = 0.36, 0.04 cm, L = 4.1 cm, p = 1 g/cm^,

r^>
Pressure (cm. Hg)
Figure 5. Streaming potential vs. pressure without the R-C
measuring circuit.

Flow rate, (cc per sec.
Figure 6. Flow rate vs. pressure for fused silica.

23
y = 0.009 poise, and r = 0.87 cm, the values of and
are calculated to be 0.0040 and 51, respectively.
The data of Figure 6 fit an equation of the form
given by Equation 4 if k^ and are 0.00375 and 48, re
spectively. This is in close agreement with the re
spective calculated values. Thus, the nonlinear behavior
follows what is expected from the earlier flow studies.
From Figures 4 and 6, it can be seen that the E/P
ratio remains unchanged even though solution flow
becomes nonlinear. The streaming potential is
proportional to the streaming current. The streaming
current is given by the integral over space of the product
of the charge density and velocity (projected in the net
flow direction) at every point (12), and so this integral
must be proportional to pressure. However, the charge
density distribution does not change with pressure. This
strongly suggests that the velocity of the liquid at every
point near the surface increases in direct proportion
with the pressure. This conclusion would be in agreement
with the current views that streamline flow near the
particle surfaces remains even after standing eddies
develop.

24
Conclusions
In the porous beds that were studied, noncreeping
flow did not appreciably affect streaming potential-flow
pressure relationships at the pressures studied. This
suggests that streaming potential experiments in the
past which have been performed under similar noncreeping
flow conditions would have had the same E/P ratios as
would have been measured under creeping flow conditions.
For those cases, the calculated zeta potentials would have
been as valid under noncreeping flow as under creeping
flow conditions.
Electrode polarization effects were nulled out by
using a R-C circuit which allowed only the true streaming
potential to be recorded. This method greatly facili
tated the measurement of streaming potentials of reactive
materials such as bioglass.
Combined results of the electrode polarization and
noncreeping flow studies showed that electrode polari
zation and not noncreeping flow was the reason for E
versus P curves not passing through the origin as found
by previous investigators. This was demonstrated by the
fact that E versus P curves were linear and passed through
the origin under conditions of noncreeping flow as long
as electrode polarization effects were accounted for,
vis., by using the described R-C circuitry.

CHAPTER 3
CALCULATION OF SURFACE CHARGE DENSITY ERROR DUE TO
COMPLEX SPECIES FORMATION IN SOLUTION
Introduction
Calculations of surface charge densities are made
from adsorption density values of potential determining
ions on oxide surfaces (13-16). Potentiometric titration
of the aqueous-oxide system with potential determining
ions are used to determine adsorption density values as
a function of pH. For aqueous-silica systems, different
negative values of surface charge density at any given
pH are found depending on the phase of silica used in the
titration. Using precipitated silica, Tadros and
Lyklema (17) found that absolute values of surface charge
densities were an order of magnitude higher than those of
Bolt (14), who studied amorphous silica. Tadros and
Lyklema suggest that a gel structure exists on the
precipitated silica surface and that extension of
surface and counter ion charge inside the pores of this
gel structure causes higher charge densities. However,
Yates and Healy (18) suggest that precipitated silica does
not have a gel structure in the surface but that it
contains an incompletely condensed layer of polysilicic
25

26
acid. They also suggest that surface roughness con
tributes to high inner layer capacities creating high
surface charges due to slight interpenetration of
potential determining and adsorbed counter ions.
An alternative hypothesis for anomalously high
surface charge densities can be investigated by examining
the mass balance equation which describes an aqueous-
oxide system in a titration experiment. The real
adsorption density (T a^) is defined as the excess moles
per unit of surface area of hydrogen over hydroxide ions
on the solid surface. The correct mass balance equation
for this value is
[(H-H) +AH-(H-H) i -C]v
real a [DJ
where (H-OH) -¡_ntial the total excess moles of hydrogen
over hydroxide species per unit volume of solution in the
system before titrant addition, AH is the moles of titrant
per unit volume of solution added to the initial system,
(H-OH) so]_ut;pon as the excess moles of free hydrogen over
hydroxide ions per unit volume in solution, V is the
solution volume, A is the total surface area of oxide
powder used in the titration and C is the excess moles
per unit volume of hydrogen over hydroxide ions which are

27
part of complex species in solution due to dissolution
of the solid. Essentially Equation 5 says that those
hydrogen or hydroxide ions after titrant addition which
cannot be accounted for as free ions in solution or in
solution as part of complex species must be adsorbed to
the solid surface. Only relative values of F can
be determined if titrations are performed in one elec
trolyte concentration since the quantity (H-OH) . .
is unknown. However, by performing titrations at various
electrolyte concentrations, several adsorption density
versus pH curves can be obtained which intersect at one
point. In the absence of specific adsorption of counter
ions to the oxide surface, T is zero at this point.
Therefore, from Equation 5, (H-OH). can be
M v 'initial
determined and absolute values of r can be calculated.
real
Many workers choose to neglect the C term in Equa
tion 5 since they assume that the concentration of complex
species which form in solution is negligible compared to
the concentration of free hydrogen or hydroxide ions in
solution. However, this may not always be true especially
under conditions of a thermodynamically unstable solid
phase in solutions of high pH. If "C" is neglected, then
r
+AH-(H-OH)
it-- ]v
solution-1
apparent
A

28
where r is the adsorption density calculated from
clU p cl L 011 L
potentiometric titration data by most investigators. If
Equation 6 is subtracted from Equation 5, then
at = r i r
real apparent
VC
' A
[7]
C will be the amount of error incurred in the adsorption
density value when these hydrogen or hydroxide ions
assumed to be free in solution actually compose part of
complex solution species. Therefore, this chapter serves
to determine C in terms of experimental parameters so
that r i r and, therefore, surface charge
density error o a can be calculated for the
V631 cippcilT0n L
silica water system.
Procedure
1. Formation of Complex Species
It is possible to calculate the equilibrium solu
bility of silica from thermodynamic information. All
that is required is knowledge of the free energy of
formation of the solid and soluble species at the
appropriate temperature. Table I shows the standard free
energy of formation for five solid silica phases, a sodium
silicate, a neutral aqueous silica species and two aqueous
complex ionic species. Also shown are the values for

29
Table I. Standard Free Energy of Formation
Values at 298K (Kcal/mole)
N
Species
AGU (Kcal/mole)
1
^*"^2 (quartz)
-192.4
2
S^2 (cristobalite)
-192.1
3
^^2 (trydymite)
-191.9
4
^^2 (vitreous)
-190.9
5
^^2 (hydrated)
-187.8
H2Sl03(aq)
-242.0
HSOt N
3 (aq)
-228.36
Si0o, s
3 (aq)
-212.0
2 3 (c)
-341.0
Na+. s
(aq)
-62.6
H2(aq)
-56.69
H~t *
(aq)
0
oh; v
(aq)
-37.6
Sources: Pourbaix, ref. 14 and
Dickerson, Gray and Haight,
ref. 20.

30
H2(aq) Na+(aq)> H+(aq) and 0H~(aq) (1920>- Etlua-
tions are written describing the formation of the neutral
soluble silica species (I^SiO^) Since five different
silica phases are considered, five equations yielding
five equilibrium constants (K[N] where N = 1-5) are
obtained. The reactions and values for their equilibrium
constants are shown in Table II.
The reactions for the formation of the various ions
from H2SiC>3 species are shown in Equations 8-10. Their
equilibrium constants are calculated from data in Table I.
H2Si03 = HSiO + H+ KA = 1.01 x 10~10 [8]
H2Si03 = Si03 + 2H+ KB = 9.92 x 1023 [9]
2Na+ + H2Si03 Na2Si03 + 2H+, KC = 2.81 x 1027 [10]
Activity coefficients for the various ions were
incorporated into the final calculations of surface
charge density error. The values for these coefficients
were obtained from calculations by Klotz (21) using the
Debye-Hueckel theory for strong electrolytes. Table III
shows the values for ions used in the calculations at
several ionic strengths. The values for HSi03 and Si03
were not directly available from Klotz (21), but were
chosen for ions of similar size as HSi03 and SiC>3.

31
Table II. Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2S03, and Their Equilibrium
Constants (K)
Reaction
K[NJ (where N=l-5 from Table I)
Sl02(quartz)+H20=H2Si03
K 1 = [H2Si03] = 6.31 x 10"6
Sl02(crist.)+H20=H2Si03
K 2 = [H2Si03] = 1.05 x 10"5
Sl02(trid.) +H20=H2Si03
K 3 = [H2Si03] = 1.47 x 10-5
S102(vit.) +H20=H2Si03
K 4 = [H2Si03] = 7.49 x 10"5
S102(hyd.) +H2O-H2Si03
K 5 = [H2Si03] = 1.47 x 10-2

32
Table III. Activity Coefficients for Ionic Species
at Various Solution Concentrations
Solution
Concentration
(moles/liter)
Species
T
n
0
01
001
HSiO^
0
.750
0
898
0
964
Si03
0
. 360
0
660
0
.867
H+
0
830
0
914
0
967
Na+
0
.770
0
901
0
.964
Source: Klotz, ref. 21

33
The excess of hydrogen of hydroxide species which
is part complex species is directly related to the
concentration of each complex species in solution. From
Equations 8-10 it can be seen that for every HSiO^ ion
formed, the excess of hydrogen over hydroxide is -1.
Accordingly, for SiO^ the excess is -2 and for Na2Si03
it is -2. Therefore,
C = -QHSiO"] + 2 [SiO^] + 2[Na2Si03]) [11]
2. Development of the Working Equation for Aa
By writing the equilibrium constant expressions for
Equations 8-10 and substituting K[N] from Table II for
[l^SiOg], the equilibrium concentration of each complex
species can be written in terms of experimental
parameters. That is,
[HSi03]
[SiO]
KA K[N]
+ .
YHSi03(YH+)[H ]
KB K[N]
YSiO~ yH+ ^
[12]
[13]
[Na2Si03]
KC
K[N] Y[+
~2
[Na
+ .2
YH+
[hV
[14]
Now, C in Equation 7 can be expressed in terms of experi
mental parameters:

34
AT =
V(_
A''
KA K[N]
Y
T+
+
2KB K[N]
+ 7?
HSi
io: yh+ ^ yso0 yh+ ^
2KC K[N] [Na+]2
+ 2"' r+'2 }
Yh+ [H ]
[15]
Adsorption densities can be converted to surface charge
densities by the relation
a = 106Fr [16]
where F is Faraday's constant (9.65 x 10^ coul/mole) and
10 is the number of microcoulombs per coulomb. There
fore ,
Ao = 106 F J(C) [17]
where C is the term in parentheses in Equation
-3 -2 -1
Calculations of Aa were made for 10 10 and 10
M/L NaCl solutions for the five different silica species
for a pH range of 7-10 in increments of 0.1 pH units.
Since the calculations are numerous, a computer program
was written and is appended. The program is written
specifically for the silica system. However, as the
appendix shows, the program can be generalized for any
material as long as the free energy of formation data for
the various reacting species are known.

35
Results
Figure 7 shows the calculated values for Aa as a
function of pH for 10 ^ 10 10 ^ M/L NaCl solutions
for vitreous silica using Bolt's (14) experimental
conditions of surface area and solution volume. It can
be seen that as the ionic strength of the electrolyte
solution increases, the absolute value of Aa increases
for any given pH value. This trend is consistent with
the experimental surface charge densities measured by
Bolt.
Comparison of the calculated values with Bolt's
absolute values in Table I of his paper (14) for amorphous
silica shows that little error is incurred in neglecting
complex species formation in solution. This is not an
unexpected result since Bolt used a high surface area
to volume ratio in his experiments. Therefore, Bolt's
data represents a as well as a since Aa is
r real apparent
negligible for the pH range studied.
Using Bolt's data it is possible to calculate at
what value of total surface area significant error would
have occurred for any given pH value. Figure 8 shows
Aa as a function of total surface area for pH = 7, 8, 9,
5 2
10. For pH 9 and total surface area of 10 cm 10% error
7 2
would have occurred. Bolt used 5.4 x 10 cm total

microcoul
36
pH
Surface charge density error (Act)
102, 10"3 M/L NaCl solution for
silica using Bolt's experimental
vs. pH in
vitreous
conditions.
Figure 7.

37
p
Total surface area (cm )
Figure 8. Surface charge density error (AT) vs. total
surface area for amorphous silica at pH = 7,
8, 9 and 10.

38
surface area in his studies. Therefore, even at pH = 10,
less than 1% error should be expected based on his
experimental values of surface charge density. This
calculation quantitatively demonstrates the importance of
the experimental condition of surface area in a
titration experiment if the formation of complex
species is to be neglected. To achieve greatest accuracy
a high surface area to volume ratio must be used.
Tadros and Lyklema (17) studied the surface charge
density as a function of pH for precipitated silica.
2
The absolute values range from 15 to 200 micro coul/cm
in the pH range 7-10. Figure 9 shows that values of
2
Aa calculated in the present work reach 30 micro coul/cm
which suggests that at pH = 10, 10-15% error is incurred
by Tadros and Lyklema by neglecting the complex species
6 2
formation. Tadros and Lyklema used 8 x 10 cm surface
2
area in their experiments (20g of 40 m /g per 100 cc).
Figure 10 shows a plot of Ao versus total surface area.
At a surface area of 8 x 10 for pH = 10, 10% error should
be expected based on their value of a. Tadros and
8 2
Lyklema therefore should have used 10 cm of total sur
face area to achieve YL error or less at pH = 10 if they
wanted to neglect the complex species formation.
From data such as presented in Figures 8 and 10,
the amount of error in surface charge density incurred as

microcoul
39
pH
Figure 9. Surface charge density error (Aa) vs. pH in 10 ,
10"2, io-3 m/L NaCl solutions for hydrated
silica using Tadros's and Lyklema's experi
mental conditions.

40
Total
surface area,
(cm2)
Surface charge density error (Act) vs total
surface area for precipitated silica at
pH = 7, 8, 9 and 10.
Figure 10.

41
a function of pH for any given surface area used can be
determined. The error becomes more significant as the
pH increases and surface area decreases. This should be
expected since soluble complex species form in greater
quantities at higher pH's due to increased ionization of
the neutral soluble silica species.
According to the calculations, based on the data
of Tadros and Lyklema, significant error should be
2
expected by using 2g of 40 m /g material in 100 cc of
solution. However, these authors claim no significant
difference in the amount of OH/g SO2 adsorbed when 2, 10
or 20 grams of solid were used in the same volume of
solution. To observe this, they must have used much
less volume than 100 cc, which was the solution
volume V used in the calculation in the present work.
However, no indication of solids concentration was given.
Discussion
These calculations demonstrate the importance of
knowing the amount of each phase present in an aqueous
silica system. In an experiment using the same total
surface area for all five phases less error in surface
charge density will occur for quartz than any other solid
silica phase. The order of least to most significant

42
error is the same as the most to least thermodynamically
stable. The calculations show that in a mixed phase
solid of 99.99% quartz and 0.01% hydrated silica, the
amount of soluble complex species formed from the hydrated
phase will be the same as the amount formed from the
quartz.
One occurrance of a mixed phase solid has been
addressed by van Lier et al. (22). In their quartz
solubility studies, they confirmed the existence of a
disturbed layer on ground quartz particles by dissolution
O
studies at high pH values. The thickness of 300 A which
they calculated from their results agreed well with
Gibb et al. (23). Van Lier et al. found that abnormally
high solubilities in water and high pH solutions of
quartz are obtained if the disturbed layer is not re
moved. However, removal of this layer yielded normal
solubility data. This suggests that the layer is not
quartz but probably a more thermodynamically unstable
phase.
Van Lier et al. were not able to identify the
disturbed layer phase. Using their experimental condi
tions and the free energy of formation values in Table I
of the present work, it is possible to calculate the
theoretical thickness of the layer assuming the silica
phase is each of the five solid phases considered.

A3
To make the thickness calculations, dissolution
reactions must be written. These are
Si02 + OH" = HSiOg
Si02 + 2OH"
SiOg + h2o
[18]
[19]
Using quartz as an example, the standard free energy of
reaction for Equation 18 is 1640 cal/mole and for Equa
tion 19 it is -1100 cal/mole assuming the data in
Table I are correct for quartz phase. Neglecting activity
coefficient, the equilibrium constant expression for each
equation is
K
18
,-AGs
exP (~rt~") =
[HSi03]
Si02J|OHJ
K
19
= exp (
-AG\ [H20][Si02]
[Si02][OH]2
RT
-) =
[20]
[21]
K^g is calculated to be 0.061 and K^g is 6.53. Since K^g
and K-^g are known, the concentrations of HSiOg and SiOg
can be calculated using pH = 12.30 from experimental
conditions of van Lier et al. These values are [HSiOg] =
1.2 x 10 3 M/L and [SiOg] = 2.6 x 10 3 M/L. The equi
valent concentrations of Si02 are 9.4 x 10"^ M/L and
_3
2.05 x 10 M/L. Therefore the total concentration of
quartz (SiOg) present in solution after dissolution is
2.99 x 10 3 M/L or 0.18 g/L. Since van Lier et al. used

44
0.030 liters, the final weight of powder present in the
system after dissolution was 0.3854g-0.0054g = 0.3800
grams, where 0.3854 is the initial weight of powder used
by van Lier et al. before dissolution.
To calculate the thickness of the disturbed layer,
two assumptions are needed. First, the particles are
assumed to be spheres. Secondly, the number of particles
in the system remains constant. The total initial volume
V. of particles in the system is related to the initial
particle radius r^ by
V. = N. 4/3irr3
i i i
[22]
where INF is the initial number of particles. The same
equation can be written relating the final total volume
and final particle radius r^ after dissolution. That
is ,
Vf = Nf 4/3rr3
[23]
where N is the final number of particles after dis
solution. Since
[24]
then
N.p 4/3rr3
i i
[25]
and

45
W
f
Nf p4/3Trr^
[26]
where p is the density of the solid phase, W. and are
the initial and final total weight of powder respectively.
Since N. = then
i f
p4/37Tr? p4/3irr^
-4
A value for r. = 1,5 x 10 cm is chosen since it
range of the particle size range used by van Lier
Therefore,
[27]
is mid-
et al.
Wr -i / -.
r- = ( ".) ^~ ~>v
f i
[28]
i
For quartz the value of r^ is 1.493 x 10 ^. The
thickness t of the disturbed layer would be t = r. r^ =
7 x 10 ^ cm= 70 A.
Similar calculations for the four other silica
species were made. Table IV shows values of t and the
total concentration of soluble SO2 at equilibrium for each
species. The values of t in Table IV represent the amount
of the particle which could be dissolved away if the
layer was each of the phases studied. They also represent
the thickness which the disturbed layer must have for the
solution to become saturated with respect to each phase
considered. On this basis, the disturbed layer cannot

46
Table IV. Calculated Values for Disturbed Layer
Thickness (t) and Soluble S2 at
Equilibrium Using Data from van Lier
etal. (ref. 22)
Silica phase
SiC>2 x 10 (moles/liter)
t (A)
Quartz
0.30
70
Cristobalite
0.50
119
Tridymite
0.70
167
Vitreous
3.88
968
Hydrated
u,
/V

^Calculations show total dissolution

47
have a quartz, cristobalite or tridymite structure since
the values of the thickness calculated are lower than
O
300 A. That is, the solution becomes saturated with
respect to quartz, cristobalite or tridymite when 70, 119
O
or 169 A of thickness are dissolved away. However, the
disturbed layer could have either an amorphous or hydrated
structure since saturation cannot occur in a system
containing silica particles with a disturbed layer thick
ness of 300 A.
Conclusions
It has been shown that care must be taken in
adsorption density measurements using potentiometric
titrations to use high surface area powder to solution
volume ratio systems if one is to neglect complex species
formation. Guidelines for such experimental conditions
have been presented by using experimental data presently
available in the literature for the basis of the calcula
tions. It is important to note that more experimental
error will be incurred by not considering complex species
formation for less stable silica phases especially at
higher pH values where the ionization of the neutral
soluble silica species occurs in greater quantity.
These calculations have also shown that as little
as 0.017, hydrated phase present in quartz forms equivalent

48
concentration of complex species as all of the quartz
itself. Hence the importance of careful consideration
of the solid phase under study cannot be over emphasized.
The thickness of the disturbed layer on quartz has
been calculated using data on the solubility of quartz
and thermodynamic data for solid silica species. The
calculations show that the layer cannot be quartz,
cristobalite or tridymite.

CHAPTER 4
DETERMINATION OF ADSORPTION CHARACTERISTICS FOR FINE
PARTICLE SYSTEMS FROM ELECTROPHORESIS MEASUREMENTS
Introduction
Different types of adsorption information can be
obtained for fine particle-aqueous systems depending on
the type of experimental method used. The method com
monly used to calculate adsorption densities of oxide
systems from potentiometric titration using potential
determining ions to determine the zero point of charge of
the surface was described in Chapter 3. To determine
adsorption densities accurately for ions other than
potential determining ions, other techniques such as
solution depletion (as measured by atomic adsorption or
emission) must be used. These measurements require
synthesis of calibration curves which essentially doubles
the amount of work involved in determining an entire
adsorption isotherm.
The unique feature in the method presented in this
chapter for determining adsorption isotherms from electro
phoresis measurements is the use of various solids
contents. By obtaining zeta potential behavior as a
function of apparent ion concentrations in solutions for
49

50
different solids concentrations, adsorption densities
and equilibrium solution concentration of the ions can be
calculated. The montmorillonite clay-aluminum ion solu
tion system is used to obtain these experimental data.
These data are then plotted on the basis of their
relationship in the Stern equation (24) to determine if
they fit an analytical form of this equation.
Materials and Methods
Montmorillonite 1613 was used as received from the
supplier.* A stock suspension of 270 solids was prepared
by ultrasonic dispersion of the clay in water and was
equilibrated for one week at ambient temperature. After
equilibration, 0.5, 0.05, 0.005 and 0.0025% suspensions
were prepared by diluting the stock suspensions with
water described in Chapter 2.
-1 -2
Aluminum chloride solutions of 2 x 10 2 x 10
2 x 10~3, 2 x 10"4, 2 x 105 and 2 x 10-6 M/L were
prepared from one molar stock solution. Fifty ml of each
solution was adjusted to pH = 4.0. This was mixed with
50 ml of clay suspension previously adjusted to pH = 4.0.
This mixture constituted the electrophoresis solution.
*Georgia Kaolin, Inc., Elizabeth, N.J.

51
This procedure was carried out for each of the solids
suspensions listed above. Therefore, electrokinetic data
as a function of aluminum ion concentration for various
solids content could be obtained.
Electrokinetic data were generated using microelectro
phoresis using the Riddick cell. The methods of
determining electrophoretic mobilities and calculating
zeta potentials are discussed elsewhere (25).
All chemicals used in this study were certified
reagent grade materials.
Results and Discussion
1. Calculation of C and r
Figure 11 shows the results of zeta potential as a
function of aluminum ion concentration CQ for various
solids contents. This concentration is the number of
moles of aluminum ions added to the system divided by
the solution volume. It is not the actual concentration
of aluminum ions in solution since adsorption occurs onto
the montmorillonite particle surfaces. Therefore, it is
the apparent concentration if no adsorption takes place.
It can be seen that zeta potential-concentration curves
for various solids contents differ except for very dilute
suspensions. This indicates that, at higher solids

Zeta potential, (mV)
52
Figure 11.
Zeta potential vs. concentration of AlCl3*6H20
for 0.00125, 0.0025, 0.025, 0.25 and 1.0%
weight percent montinorilIonite .

53
contents, the equilibrium concentration, C, of aluminum
ions in solution is significantly lower due to adsorption
onto the clay particle surfaces.
A mass balance equation can be written which de
scribes the system at equilibrium as follows:
C = C + ^-(W ) [29]
where Cq is the apparent concentration of aluminum ions in
solution if no adsorption occurs, C is the concentration
of aluminum ions in solution after adsorption to the
particle surface, T is the adsorption density of aluminum
ions on the clay surface, VT is the solution volume, W
is the weight of solids in the system, and A is the
specific surface area of the clay, Wg/V^ can be replaced
by the weight percent by the following argument: weight
W
g
percent (W7o) of solid is defined as ^ x 100 where =
s WL
weight liquid and W = weight of solid. If W, >> W then
S s
w
g
W?0 = y x 100. Since where = density of
L
W
liquid, when W7o = x 100. Since p^ = 1.00/cc, the
L L
right side of Equation 29 must be multiplied by a factor

54
1000 cc/liter. Therefore
Cq = C + 10pL AF(W%) [30]
and the units of each term in Equation 30 are moles/liter.
If enough solid is present in the system, most of the
ions in solution will adsorb to the surface, and
C << 10p^AFW7o; therefore Equation 30 becomes
Co = 10pLAF(W7o) [31]
Taking the log of both sides of Equation 31 yields
log CQ = log(lOpAr) + log(W%) [32]
Table V lists the values of C for zeta potentials =
15, 10, 5, 0, -5, -10 mV.
A plot of concentration Cq as a function of weight
percent using data at zeta potential equal to zero mV
from Figure 11 is shown in Figure 12. For high solid
contents the experimental points approach a straight line
whose slope is 1.0. At low solids contents, the points
approach a line of zero slope. The Cq value corresponding
to this line is C. Rearrangement of Equation 30 and
taking logarithm yields
log(Co-C) = log(10pLAr) + log(W%) [33]
Therefore if Equation 32 correctly describes the system
under concentration, then plots of Cq-C versus W70 should
yield straight lines for each zeta potential. As
Figure 13 shows, straight lines are obtained.

55
Table V. Values of CQ for Various Zeta Potentials
and Weight Percent Solids
wt. %
C (mV)
0.00125
0.0025
0.025
0.25
1.0
-10
3.5x10-5
4.5x10-5
1.1x10-^
7 xlO-4
2.9x10-3
-5
8.5xl05
9.3xl0~5
2.lxlO-4
1.3xl0-3
5.4xl0"3
0
2.2xl0~4
2.3xl0~4
4.5xl0'4
2.3xl0'3
_2
10
+5
5 xlO-4
5.Ixl0~4
8.8xl0-4
5.4xl0"3
1.8xl0-2
+10
1,2xl0-3
1.2xl0~3
1.8xl0-3
8,5xl0"3
3 xlO"2
+15
2.7xl0-3
2.8xl0"3
3.6xl0-3
1.3xl0"2
5 xlO"2

Concentration
56
Weight percent solids
Figure 12. Apparent aluminum ion concentration CQ vs.
weight percent montmorillonite for zeta
potential = 0 at pH = 4.0.

57
-2
Weight percent solids
Figure 13. Concentration of aluminum C0 minus the
equilibrium concentration (C) of aluminum in
solution vs. weight percent montmorilIonite
for zeta potential = +15, +10, +5, 0, -5,
-10 mV at pH = 4.0.

58
From Equation 33, at 1% solids, C-C = 10pTAF.
O l_i
Therefore, the adsorption density, r, can be determined if
the specific surface area of the solid is known. For
montmorillonite, a generally accepted theoretical specific
2
surface area of 800 m /g was used in the calculations.
C
Table VI lists the values of C, r and log(p-) for each
zeta potential considered. A plot of C and r vs. C is
shown in Figure 14.
2. Application of Experimental Data to the Stern
Equation
Stern (26) has derived an adsorption isotherm
2
relating the surface charge/cm (a) to the Stern potential
(ip ) The Stern equation is
b
1 =
N^ve
1+:
Mn
exp (-
veip +tj>
[34]
TdT
-)
where is the surface charge/cmZ associated with the
adsorbed ionic layer, N-^ is the number of adsorption sites
2
per cm at surface, v is the valence of the ion adsorbed,
e is the charge of the electron, N is Avogadro's number,
M is the molecular weight of the solvent, n is the number
3
of ions per cm far from the surface, \ps is the potential
at the Stern plane, of the adsorbed ion, k is Boltzmann's constant and T is
o-
1 MC
temperature Since T = and n = qqq-
then

59
Table VI. Values of C, T and log (C/T) for Various
Zeta Potential Values
(mV)
-T
^5
0
+5
+10 '
' +15
C
(moles/1)
3.5x
10-5
8x
10-5
2xa
10"4
5x
104
1.2x
IO-3
2.7x
10-3
r 2
(moles/cm )
3.63x
lo-n
6.75x
lo-n
1.25x
io-io
2.25x
10-10
3.7 5x
10-10
6.25x
10-10
log
(C/D
2.92
3.07
3.20
3.35
3.51
3.64

Zeta potential, (mV)
60
Concentration c, (m/|)
Figure 14.
Zeta potential and log adsorption density. (T)
vs. equilibrium concentration (C) of aluminum
in solution for montmorillonite at pH = 4.0.
Log adsorption density

61
N
r =
l
-,,1000 /elJJs+(\
1+^7^- exp (^)
[35]
MC
Multiplying the numerator and denominator of the right
MC ve^s+d
side of Equation 35 by -j-q-q-q exp-(^) yields
r
mc
imu Ni exp~( kT }
MC ,ve Vh ,,
T exP-(TV~)+1
[36]
^S M -cb
If y = -^r, K = exp(^), and ips = ?, then
C K exp-(vy)
r C K exp-(vy)+l
Taking the inverse of both sides and rearranging Equa
tion 37 yields
[37]
N-,
C(-
exp yv
K
[38]
Since the total number of adsorption sites (N-^) is much
greater than occupied adsorption sites, N-^ >> 1. There
fore ,
C exp(vy)
r NjK
[39]
Taking the natural log of both sides and converting to
common log,
ig(£)
log
2.303 y
[40]

62
The experimental data (C and T) fit the Stern equa-
C
tion, since Figure 15 shows that a plot of log(|r) vs. y
yields a straight line. According to Equation 40 the
slope should be 1.3 if v = +3. However, the straight line
obtained has a slope of 0.8.
A plausible explanation for the discrepancy between
the theoretically determined and experimentally obtained
slope must come from examining the factors involved in
v, the valence of the adsorbed ion. Errors in any other
term lead only to shifts in the position of the line but
no change in slope. In calculating the theoretical
slope, v was assumed to be +3 for the aluminic ion.
However, for the slope to decrease to the experimentally
determined value, v must be lower than three. That is,
the effective valence of the adsorbed ion must be smaller
than 3. This is possible only if aluminum ions are in
volved in ion exchange for sodium ions (+1 valence, making
the effective v.-^, +2) or calcium ions (+2 valence, making
the effective v^, +1). Therefore the v^ would have a
value between 1 and 2. The effective valence calculated
from Figure 15 is jp-jO) = 1.8.
Energetically, the work required in bringing an
aluminum ion from solution to the surface is partially
supplied by the "negative" work involved in releasing a

63
y
Figure 15. Log of the ratio of C and F vs. zeta potential
for montmorilIonite for the experimental data.

64
sodium or calcium ion from the surface to the solution.
Therefore, v in the Stern equation is related not just
to the valence of the adsorbed ion, but also to the net
work involved in bringing the ion to the surface during
ion exchange reactions.
The intercept of the straight line with the y-axis
in Figure 15 yields the specific adsorption potential if
N-^ is known. However, since the slope can vary due to
ion exchange processes, the specific adsorption potential
will also vary with the type and degree of ion exchange
reaction.
The data treated in this chapter cover only one
pH value. Future work involving the entire pH range
should give more insight into ion exchange reactions of
aluminum for other ions in the clay. Also, the influence
on the Stern slope by aluminum ion hydrolysis as the pH
increases can be investigated. However, this type of
study would have to be performed using a material other
than montmorillonite so that the contribution by ion
exchange to Stern slope characteristics is minimal.
Conclusions
A new method for determining adsorption isotherms
from electrokinetic data of zeta potential vs. ion

65
concentration for various solids content has been
described and tested experimentally. It was found that
the experimental data fit an analytical form of the Stern
equation. However, the experimentally determined slope
was lower than the theoretical Stern slope. This was due
to the lowered effective valence of the aluminum ions
involved in ion exchange reactions.

CHAPTER 5
REVERSIBILITY OF ALUMINUM ION ADSORPTION ON FUSED SILICA
Introduction
Considerable interest has been generated in the past
ten years in the coagulation and flocculation behavior of
silica sols. It has been shown that of utmost importance
in this behavior is the hydration state of the surface.
It is now generally accepted that the surfaces of silica
sols are covered by silanol groups with a density of 5 OH/
O
100 A (27), whereas a quartz surface is relatively inert,
being covered by siloxane groups (28,29). However, some
silanol groups have been shown to exist on quartz,
especially on ground or milled material (22,30).
Lange (31) found that the amount of water hydrogen bonded
to precipitated silicas was about 387o of the total pos
sible adsorption capacity. This meant that 62% of the
silanol groups were hydrogen bonded to each other. Using
an explanation involving ion exchange of hydrogen ions
for metallic ions, Allen and Matijevic (32) found that
the hydrophilic nature of silica decreased in the presence
of simple electrolytes.
Recently, Tschapek and Sanchez (33) studied the
amount of NaCl required to coagulate different silica
66

67
sols as a function of suspension pH. It is clear from
their results that the absence of isolated silanol groups
lowered the amount of NaCl required for coagulation at low
pH's. Also, the absence of hydrogen bonded silanol groups
did not lower the amount of NaCl required for coagulation.
They concluded that isolated silanol groups were primarily
responsible for the stability of silica suspensions
calcined between temperatures 110C and 800C. Con
sequently, hydrogen bonding between silica particles
could not have been the mechanism of their coagulation.
Removal of silanol groups from the surface by heat
treatment also removes the mechanism of surface charge
development in solution. Their removal is highly
irreversible as shown by Rubio and Kitchener (34).
Removal of the charge development mechanism consequently
decreases the repulsive forces between particles to a
point which may allow close interaction of the primary
particles and ultimately their coagulation. This
coagulation would be facilitated by hydrophobic bonding.
If this is true then the results of Tschapek and
Sanchez (33) for untreated silica sol and silica sol
calcined at 800C suggest that only those silanol groups
which are not hydrogen bonded to each other can contribute
to the formation of adsorption sites for ionic species in

68
solution. Adsorption site formation occurs by ionization
of the silanol group. This could be the reason why
isolated silanol groups stabilized their suspensions,
i.e., the ionization of the isolated silanol group
created repulsion between the particles. Only in this
way could these two sols have identical coagulation
characteristics.
The previously mentioned mechanism for sol stability
demonstrated how thermal treatment rendering the silica
surface hydrophobic created the possibility of hydro-
phobic bonding between silica particles. Rubio and
Goldfarb (35) showed that chemical treatment can also
create hydrophobic surfaces. Their results suggest that
if a number of quaternary ammonium cations attach to the
surface by means of a nitrogen atom the surface will
become hydrophobic. The greater the surface coverage
the more hydrophobic the surface becomes. They state that
in this way, aggregation of the particles may be effected
by hydrophobic bonding. This type of mechanism is sup
ported by the fact that quaternary ammonium salts have
been used as flotation collectors for quartz
particles (36).
Adsorption characteristics of species from solution
will be affected by surface hydration states of silica

69
since surface hydroxyls act as primary adsorption sites
for polar molecules (37). Conventional methods used to
study these phenomena require the use of finely divided
materials. In this chapter, a technique is introduced
which allows investigations of silica material which has
a size range of 300-800 microns. The results obtained
using this technique are interpreted in the same manner
as those obtained when fine material is studied.
Adsorption of aluminum ions from solutions by silica
surfaces is of particular interest to glass corrosion
studies. It has been shown that aluminum ions decrease
corrosion of silica when they are present either in the
glass itself (38) or in the corrosion medium in contact
with the glass (39) Weyl (40) proposed that a require
ment for corrosion inhibition for the latter case is that
aluminum ions adsorb to the surface and not form a more
soluble compound than the glass itself. Lyon (41) found
that rinsing of a container glass with aluminum ion
solutions and subsequently rinsing with water did not
inhibit alkali extraction from the glass which suggests
that aluminum ion adsorption may not always be permanent.
Aluminum ion adsorption to a glass surface may
theoretically affect either ion exchange or network
breakdown. In his glass solubility work, Her

70
investigated the effects of adsorbed aluminum ions on the
dissolution of fused silica. Using fused silica
eliminated the possibility of the ion exchange mechanism
since no alkali were present in the glass. Therefore only
network breakdown was involved. Iler's results showed
that as the amount of aluminum ions in solution increased,
the dissolution of silica decreased for a given period of
time. However, Her exposed his samples to solutions
containing a 1:1 molar ratio of aluminum to citrate. He
presumed that negatively charged aluminosilicate sites
were created. The inhibition mechanism proposed was
repel of hydroxide ions from the negatively charged sites.
The present work will show that this presumption is
incorrect. Also since Lyon's work on container glass
suggests that aluminum ion adsorption is not permanent,
this chapter considers certain conditions under which
aluminum ion adsorption may be reversible or irreversible.
The conditions considered are those which are known to
alter the hydration state of the silica surface (42).
Materials
Silica used for this investigation was commercially
obtained.* Just before use in the experiments, 10 grams
*Vitreosil, Thermal American Fused Quartz Co., Montville,
New Jersey.

71
of a -20+45 mesh fraction (833-350 micron) was boiled in
100 ml concentrated hydrochloric acid until no discolora
tion of the acid was observed. The sample was then rinsed
with conductivity water until all chloride ions were
removed.
All solutions used in these investigations were
prepared from Certified ACS reagent grade chemicals.
Water used to prepare the solutions was obtained from a
deionization system previously described in Chapter 2.
Methods
Electrokinetic theory can be applied to study
adsorption of electrolytes near the solid surface (12).
Theory predicts the existence of an electrical double
layer near the surface consisting of ions present in the
solution. The double layer contains an immobile (Stern
Layer) and mobile portion (Guoy-Chapman diffuse layer).
The electrical potential at the plane separating these
two layers is the zeta potential.
Usually zeta potential values are calculated from
the Smoluchowski equation (Equation 1, Chapter 2). As
shown in Chapter 2, to obtain zeta potential information
streaming potential experiments are performed. However,
modified streaming potential techniques can be used to

72
study not only adsorption but also desorption of aluminum
ions from the solid surface. Studying desorption
characteristics yields information about the reversibility
of adsorption. This is accomplished by using an apparatus
in Figure 16. Desorption studies were accomplished by
first exposing the particles in the cell (e) to a 10 M
NaCl solution containing 10 ^ M aluminum ions. Figure 17
shows that a positive zeta potential value resulted at
this aluminum ion concentration indicating specific
adsorption of aluminum ions to the surface. The solution
_3
in reservoirs a and b was changed to a 10 M NaCl
solution containing no aluminum ions. This solution was
streamed through the cell at constant pressure [made
possible by maintaining a constant hydrostatic head in
(b)] for an extended period of time. Solution flowed
continually during the desorption experiment except when
data were taken. To obtain these data, the solution flow
valve (d) was turned off and on causing a deflection in
the electrometer (f) indicating the streaming potential
at that particular time. This operation required only a
few seconds resulting in only momentary interruption of
the desorption phenomenon. The signal from the electro
meter was recorded on a strip chart recorder (g) and the
time was noted. A special circuitry discussed in

73
Figure 16. Streaming potential apparatus modified to
maintain constant flow pressure; a) secondary
reservoir; b) primary reservoir; c) pump;
d) solution flow valve; e) cell; f) electro
meter; g) recorder; h) solution head height.

Figure 17. Zeta potential vs. concentration of sodium
citrate and aluminum chloride.

75
Chapter 2 was used to eliminate undesirable effects of
electrode polarization.
_3
By using 10 M NaCl as a supporting electrolyte for
adsorption of aluminum ions, no significant change in
E/P due to varying solution conductivity as predicted by
Equation 1 should occur during the desorption experiment.
Also since desorption studies were performed under
conditions of constant pressure, no change in the
streaming potential due to varying pressures as predicted
by Equation 1 should be expected. Therefore, during the
desorption studies, the change in E/P should only be due
to change in the zeta potential (O due to removal of
aluminum ions from the silica surface.
Desorption characteristics of several types of
silica surfaces were studied by this technique. One set
of samples remained untreated. A second set of samples
was heat treated at 800C for 8 hrs. in vacuum.'1' A third
set was treated with 1 M NaOH solution for 24 hrs. at
22C. Finally, a combined thermal and chemical treatment
of the glass particles was performed using the agents
described above.
*Centorr hot press vacuum chamber, 10-* Torr.

76
Results and Discussion
Figure 18 shows that major differences exist in the
desorption behavior of aluminum ions from silica after
various surface treatments. It can be seen that most of
the aluminum ions desorb from the untreated silica.
However, some aluminum ions remain on the surface even
after 60 minutes of streaming as shown by the fact that
the E/P value does not reach the E/P value for non-
_3
aluminated silica streamed only with 10 M NaCl solution.
Chemical treatment of the silica particles in one molar
NaOH solution caused most of the aluminum ions to desorb
after approximately 80 minutes of streaming.
For heat treated silica desorption of aluminum ions
was much less than for untreated or chemically treated
silica. Only a small amount of aluminum ions appeared to
reversibly absorb to the surface.
Chemical treatment of the heat treated samples with
one molar NaOH solution reestablished the reversible
adsorption capacity of the silica. It can be seen that
almost complete desorption of aluminum ions occurred since
the E/P value approaches the value for nonaluminated
_3
silica streamed in 10 M NaCl solutions.
It should be noted that data for nonaluminated
_3
silica samples streamed in 10 M NaCl solution were in
dependent of the various surface treatments.

cm Hg
Figure 18. Streaming potential-pressure ratio vs. stream
time for untreated, heat treated, base treated,
heat treated and base treated and non-aluminated
fused silica.

78
The major difference between untreated and heat
treated silica samples is that the former is dominated
by adjacent silanol groups while the latter contains
isolated silanol groups (2). The data show that aluminum
ions reversibly adsorb to a silica surface containing
adjacent silanol groups and irreversibly adsorb to a
surface containing isolated silanol groups. However,
since initial E/P values for the two samples were the
same as seen in Figure 18 at stream time equal to zero,
there appears to be little difference in the amount of
aluminum initially adsorbed. If only those silanol
groups which are not hydrogen bonded to each other can
become adsorption sites, as suggested earlier, then only
those which are not hydrogen bonded to each other can
be true specific adsorption sites for aluminum ions. When
aluminum ions adsorb, the double layer characteristics
change as indicated by changes in zeta potential. If
zeta potential characteristics change in the same way (as
indicated at stream time = 0 for untreated and heat
treated silica) for these two surfaces having different
hydration states, then the aluminum ion adsorption site
must be present in the same amount on both surfaces. This
trend is supported by the results of Tschapek and
Sanchez (33) which showed identical coagulation

79
characteristics for untreated silica sols and silica sols
calcined at 800C.
An atomistic view of the adsorption-desorption
process may occur as follows. The pH of the solution
-4
containing 10 M aluminum ions was 4.2-4.5. In this
range the aluminum ions are mostly in the aluminic
[ [ |
(A1 ) form with some probability that some Alg(0H)2Q
species exist. This complex species has been postulated
and shown to exist by several workers (43,44). When
_3
desorption begins using a 10 M NaCl solution whose
pH = 5.5-6.0, more formation of the complex aluminum
ion species near the surface is favored. The contribu
tion of the double layer characteristics of this species
will be greater than A1 since they are quatrivalently
charged. Hence, their contribution to the double layer
characteristics will be the same for both types of
surfaces at time = 0. However, if these ions are to
remain on the surface during the desorption experiment,
they must hydrogen bond to the surface. They can hydrogen
bond only to the surface containing isolated silanol
groups since the surface containing adjacent silanol
groups cannot hydrogen bond with solution species. There
fore, during the desorption experiment those aluminum
species which cannot hydrogen bond are streamed away and
the E/P value with time decreases significantly.

80
Similar interpretations of adsorption from solution
of poly(ethylene oxide) (PEO), a neutral species, onto
silica has been reported by Rubio and Kitchener (34).
Their results clearly show that isolated silanol groups
provide the best adsorption sites for PEO. Adsorption
occurs by hydrogen bonding of the ether oxygen with the
hydrogen of the silanol group. They found that complete
dehydroxylation by heat treatment rendered the surface
incapable of adsorbing PEO. Also, the hydrated surface
could not adsorb PEO since its hydrogen bonding capacity
was exhausted in its effort to hydrogen bond with another
silanol group on the surface.
Her (39) was not able to show whether or not
citrate ions subsequently adsorbed to negatively charged
aluminosilicate sites. In fact, he assumed they probably
did not since adsorption between two negatively charged
species should not be expected. However, since
adsorption of aluminum ions onto silica creates positively
charged sites as shown by the results in Figure 16,
citrate ion adsorption may be possible.
Figure 19 shows that the zeta potential is reversed
back to a negative value when aluminated silica is
exposed to increased concentrations of citrate ion
solutions. Also shown is the curve for citrate on

Zeta potential,(mV)
Figure 19. Zeta potential vs. concentration of sodium
citrate for fused silica in supporting electro
lytes of IO-3 M/L NaCl and 10~3 M/L NaCl,
10-4 M/L A1C136H2O.

82
nonaluminated silica. The negative zeta potential values
for concentrations greater than 3 x 10 M citrate for the
aluminated silica curve could be due to one of two pos
sible effects. Either aluminum ions were removed from
the silica surface to form aluminum citrate complexes in
solution or citrate ions adsorbed to positively charged
aluminosilicate sites. Study of Figure 19 alone does not
help to eliminate either of the possible mechanisms.
However, if the desorption characteristics of the
system are studied, one mechanism can be eliminated.
Therefore, untreated silica samples were exposed to a
3 /[
10 M NaCl solution containing 10 M citrate ions and
-5 -3
10 M aluminum ions. Then a 10 M NaCl solution void
of citrate and aluminum ions was streamed through the
cell. Figure 20 shows the results obtained. If aluminum
ions were removed from the surface to complex with citrate
ions in solution, then the E/P value should decrease to
_3
E/P value for nonaluminated sample streamed with 10 M
NaCl solution. However, the E/P increased at early
times indicating the removal of a negatively charged
specie from the surface. The only negatively charged
specie present in the system capable of specific
adsorption was citrate ions. These results indicate that
citrate ions first adsorbed to aluminosilicate sites and

cm
Figure 20. Streaming potential-pressure ratio vs. stream
time for aluminated fused silica for untreated,
heat treated, base treated and heat treated and
base treated surfaces.

84
_3
then desorbed upon streaming the particles with 10 M
NaCl solution. At longer streaming times a slight
decrease in E/P occurred indicating removal of aluminum
ions from the silica surface.
'i
Conclusions
A method for studying the desorption behavior of
large particles has been presented. This method yielded
results which can be interpreted in terms of the hydration
state of the surface of the silica particles.
Aluminum ions reversibly adsorbed to surfaces con
taining adjacent silanol groups and they irreversibly
adsorbed to surfaces containing isolated silanol groups.
The mechanism of irreversible adsorption proposed for
aluminum ions was hydrogen bonding of the complex aluminum
species to the isolated silanol surface groups.
Contrary to Iler's results (39) aluminum ions adsorb
to fused silica forming positively charged rather than
negatively charged sites. Also, aluminum ion adsorption
was shown to activate the silica surface for specific
adsorption of citrate ions.

CHAPTER 6
THE EFFECTS OF AGING ON THE ZERO POINT OF CHARGE OF ALUMINA
Introduction
Parks (45) has compiled, a considerable amount of
information from many different authors concerning the ZPC
of several oxides and hydroxides including those of
aluminum. Knowledge of the ZPC allows prediction of the
sign of the surface charge at any pH of a solution in
contact with the oxide. Also, knowledge of the ZPC is
important for understanding coagulation processes since
minimum surface charge (minimum interparticle repulsive
forces) is necessary for maximum coagulation as predicted
by DLVO theory.
The ZPC for aluminum oxides ranges from pH = 6.0 to
9.5. The scatter in the ZPC is indicative of varying
experimental conditions and solid phase used. Excluding
adsorption of impurities, Parks (45) and others (46) have
attributed most of the scatter to varying degrees of
surface hydration. Since the ZPC of the oxide depends
on the degree of surface hydration, aging phenomena of
alumina can be studied by observing changes in the ZPC
with aging time, and, in this way, surface hydration can
85

86
be studied. Robinson et al. (47), O'Connor et al. (46),
Johansen et al. (48,49), and Schuylenborgh et al.(50-53)
all agree that treatment which leads to bulk or surface
dehydration results in a more acid ZPC than for oxides
which are hydrated. Those treatments which dehydrate the
)
surface (e.g. heat) lower the ZPC whereas treatments
which increase hydration of the surface (e.g. grinding)
increase the ZPC.
Most of the ZPC information on aluminum oxide com
piled by Parks was obtained on either naturally occurring
minerals or on synthetic materials prepared in the
author's laboratories. However, little ZPC information
can be found for commercially prepared aluminas.
Information of this nature could be beneficial to both
suppliers and users of commercial aluminas particularly
if the powders are subjected to aqueous environments
during processing. Parks (45) has shown that very small
levels of adsorbed impurities such as phosphate and
sulfate as well as certain organics greatly affect the
ZPC of the oxide. If the impurities were undesirable,
they could easily be detected by measuring the ZPC of
the oxide. Appropriate steps could then be taken to
eliminate the impurities during processing.

87
Materials and Methods
In this chapter, four commercial aluminas* were
investigated. Three of the powders, A-16, A-17, and T-61
are thoroughly described by Flock (54). A-16 alumina is
a reactive powder of uniform phase and high bulk purity.
I
There is present a certain amount of nonalpha phase
2
material. Typical surface areas range from 4.0-6.5 m /g.
The average particle is 0.6 microns. A-17 alumina is a
transition reactive powder in that it is a mixture of
reactive and nonreactive agglomerates. Typical surface
2
areas are 1.5-2.5 m /g. The particle size is around 1.5
microns. Nonalpha phases are absent in this powder.
T-61 is a tabular-nonreactive powder of uniform phase
and high bulk purity. Tabular aluminas are massive low
shrinkage forms which have been sintered without added
permanent binders (55). Like A-17, nonalpha phases are
absent in this material. A fourth powder designated as
C-30 DB is a hydrated alumina. This particular powder
was not discussed by Flock. This powder was investigated
in its as-received form and after heating to 500C for
24 hours in a Vycor crucible in air. This heat treatment
is known to cause formation of gamma phase alumina from
Bayer alumina trihydrate (56).
Aluminum Company of America, Houston, Texas.

88
Surface areas of each powder were determined using
a surface area-pore volume analyzer.* The results of
these measurements are compared to those stated by
Flock (54). This author classifies surface area as
"useful" (when surface area is produced only by physical
adsorption of ^ molecule due to van der Waal's forces)
and "nonuseful" (when the surface area is produced by the
reactive chemical surface of nonalpha agglomerates). A
third type of surface area is produced by reactive
alumina agglomerates. This is attributed to weak elec
trostatic charges due to incomplete phase conversion,
i.e. lattice mismatch and/or localized cation defect
structures. Since they are alpha phase, the higher
surface areas are "useful" and correlate with increased
thermal reactivity, i.e. lower firing temperatures.
Flock has used the petrographic microscope to identify
the existence of these phases in alpha aluminas.
In this chapter further evidence of the existence of
nonalpha phases is given by studying the change in ZPC
with aging time. The advantage of using a chemical method
along with an optical method is that not only crystallo
graphic differences but also chemical purity of the
powders can be studied.
*Quantachrome Corp., Greenvale, N.Y.

89
The methods of determining the ZPC are numerous.
For this study, two methods were employed. First, zeta
potential as a function of pH for 0.1 weight percent
alumina suspensions in water was performed for samples
aged 0, 1, 2, 4, 8, 16, 33 and 97 days for each of the
aluminas. The pH at which the zeta potential is zero is
defined as the ZPC. The second method of determining
the ZPC is by determining the equilibrium pH of an
alumina-water system. The equilibrium pH occurs when
the addition of more powder causes no further change in
the pH of the aqueous-solid system. This occurs when
the amount of potential determining ions (H+ and OH ) in
the solution and on the surface are in equilibrium. The
ZPC and equilibrium pH should have the same value. If
not, then the difference between the two values indicates
the existence of impurities adsorbed onto the surface of
the powders.
The method used to determine zeta potentials was
microelectrophoresis using the Riddick cell. The entire
assembly (cell and electronics) are commercially
available.* Operation of the Zeta Meter and the method of
obtaining particle mobility are described in detail by
*Zeta Meter Inc., New York.

90
the manufacturer (25). The average mobility of 10-20
particles was used to obtain the zeta potential calculated
from the Helmholtz-Smoluchowski equation.
The equilibrium pH values were determined by placing
a known amount of powder into a polypropylene bottle
containing 100 ml 1^0. The pH was allowed to equilibrate
and was recorded. This was continued until addition of
powder caused no further pH change.
All pH measurements* were performed using a glass
combination electrode. When required, solution pH's
were adjusted by dropwise addition of 0.1 or 0.0 M/L
standard solutions of HC1 or NaOH. Water used to prepare
the alumina suspensions and acid and base solutions is
described in Chapter 2.
Results
1. Surface Area
Table VII lists the values of surface area measured
in the present work. Also shown are the ranges cor
responding to the powders described by Flock (54).
Alumina A-16 falls within the range whereas A-17 and T-61
fall outside the range of Flock's values. For A-17, more
*Brinkman Model 512 pH meter.

91
Table VII. Surface Areas of A-16, A-17, T-61, C-30 DB and
Gamma Alumina Powders
Alumina
2
Specific surface area (m /g)
Surface area
range from Flock
(ref. 54)
A-16
5.26
4.00-6.50
A-17
3.02
1.5 -2.5
T-61
0.59
0.15-0.45
C-30 DB
1.4
not listed
gamma
108
not listed

92
nonalpha phase material may have caused the slightly
higher surface area determined in the present work. The
T-61 used in the present investigation probably does not
contain nonalpha phase material. Instead, the difference
may be due to the degree of grinding. This might be
expected since the sample used in the present work came
from a batch whose agglomerate size was designated as
-325 mesh. The data described by Flock probably came
from powder which had better particle size characteriza
tion and consequently from narrower particle size ranges.
Table VII also shows that the surface area of C-30 DB
increased 1000 fold when the powder was heated to 500C
for 24 hours. The high surface area of the heated powder
is strong evidence for the conversion to a transition
phase. X-ray diffraction data of the powder before and
after heat treatment showed that the heat treated powder
was gamma alumina.
2. ZPC Determinations
Figure 21 is an example of the type of data obtained
when zeta potential as a function of aging time in water
is studied. Figure 22 shows the results of the change of
the ph^pQ with time for all the powders studied. For
A-17, T-61, C-30 DB and gamma alumina the pH^p^
increased

Zeta potential, (mV)
93
Zeta potential vs. pH for y-alumina after
aging for 0, 1, 8, 16, 33 and 97 days in
water.
Figure 21.

zpc
Figure 22. pH of the zero point of charge (pHgpc) vs. aging
time in water for T-61, A-17, C-30 DB, y, and A-16
aluminas.

95
with time. However, for A-16 the ZPC decreased with time.
The degree of aging is most striking for C-30 DB which has
a ZPC of pH = 6.5 at time zero and a value of 9.8 after
30 days. This alumina probably contains several dif
ferent phases (as indicated by the X-ray diffraction
pattern) which can hydrate upon exposure to water. Gamma
alumina has a ZPC value of 8.9 at time zero and a value
of 10.1 after 30 days. Not much aging was observed for
T-61. Within 2 days a ZPC of 9.6 was achieved. The same
is true of A-17. The most peculiar behavior was shown by
A-16. Most workers find that during aging, the ph^pQ
increases. However, most workers acid-wash their powders
to remove surface impurities. Since it was desirable to
study as-received powders in the present work, no acid
washing prior to ZPC experiments was performed for these
particular powders. It appears that A-16 alumina contains
considerable amount of adsorbed impurity. As indicated
earlier, not much impurity is needed to shift the ZPC
by several pH units (45). However, this data alone is
not conclusive. Figure 23 shows that the equilibrium pH
for A-16 is equal to 8.2. This is significantly different
from the pH^p^. The values of equilibrium pH for the
other powders were 9.5, 9.2, 7.4 and 9.4 for gamma, T-61,
C-30 DB and A-17, respectively. Except for C-30 DB, these
equilibrium pH values are close to pH ZPC.

96
Weight of powder added, (grams)
Figure 23. Solution pH vs. weight of alumina added for
A-16 alumina.

97
The discrepancy in the equilibrium pH value and
pHzpc fr C-30 DB indicates a positive ion impurity
whereas for A-16 it indicates a negative ion impurity.
Positive ion impurities create a more basic ZPC and
negative ion impurities create a more acidic ZPC.
Sources of positive ion impurities are most likely iron
for C-30 DB whereas sources of negative ion impurities
are probably phosphate or sulphate for A-16.
Discussion
O'Connor et al. (46) give a detailed account of the
sequence of events during hydration and dehydration for
alumina. From an atomistic viewpoint, the alumina lattice
consists of oxygen ions arranged in a hexagonal close
packing with aluminum ions occupying two-thirds of the
available octahedral holes. Each aluminum ion is sur
rounded by six oxygens and each oxygen by four aluminum
ions. Coordination at a freshly fractured surface will
be incomplete but must eventually be satisfied. Aluminum
ions adsorb hydroxyl ions from solution while dangling
oxygen ions bonded to surface aluminum ions adsorb
hydrogen ions. Subsequently, surface hydroxyl groups
ionize to produce the surface charge. O'Connor et al.
(46) also state that it is likely that more severely

98
damaged surfaces undergo a more extensive hydration of the
type A^O^ + 3H2O = 2Al(0H)g, yielding a surface layer of
gibbsite. For less severely damaged surfaces, a less
complete reaction probably occurs, yielding a surface
corresponding to A10-0H. Polarization of the hydroxyl
groups by the aluminum cation determines the degree of
their dissociation in water. The hydroxyl group in
A10-0H should be more highly polarized since the 0:A1 =
2:1. In A10-0H, therefore, the OH group is slightly
acidic relative to the OH group in AlCOH)^-
O'Connor et al. (46) found that freshly crushed
powder always had a positive zeta potential in water.
This suggests that the ZPC for alumina is greater than
the pH of the water. However, heating the alumina to high
temperatures eventually reversed the sign of the zeta
potential to a negative value. They attributed this
change to dehydration resulting in a surface approximately
A10-0H or y-alumina. If the alumina was placed in water
for a certain period of time, the zeta potential again
became positive indicating rehydration. These phenomena
were observed for samples heated to temperatures <1000C.
For powders heated >1000C, rehydration is very slow as
shown by Robinson etal. (47). O'Connor et al. (46)
suggest that heating to temperatures >1000C creates

99
y-A^O^ followed by recrystallization to a-A^O- on
the surface. This surface can rehydrate only to a surface
resembling A10-0H which is slightly acidic as shown
previously. In water, the zeta potential would be
slightly negative indicating a pH^p^, < pH ^0.
The results in the present work indicate that the
zeta potential in water at pH = 7 for A-17, T-61,
y-A^O^ is always positive since the ZPC is never lower
than pH = 8.0. For A-17 and y-A^O^ this result is
expected. However, for T-61, one might expect the ZPC
to be significantly lower, e.g. pH = 6-7 since this
alumina has been sintered at temperatures >1000C during
its fabrication process. This may be the case for un
ground T-61 as shown in Figure 24. These data were
obtained by the streaming potential method for course
particles described in Chapter 2. The amount of grinding
required to obtain particle sizes of 300-800 microns
(-20+45 mesh) is considerably less than is required for
obtaining particles less than 40 microns. Therefore
grinding effects should not be significant in this case.
The lowered ZPC is not caused by impurity since an acid
washed sample produced similar results. It can be seen
that the ZPC occurs at pH = 7.0 and does not change during
three days exposure to water which is direct evidence that

Zeta potentia I, (m V)
100
Figure 24.
Zeta potential vs. pH for unwashed T-61
alumina (coarse particles) aged in water for
1, 2 and 3 days.

101
grinding creates a disturbed layer on fine T-61 powder.
This directly influences the hydration and thereby
influences its ZPC characteristics.
Since all the aluminas used in this investigation
have been ground, they all probably rehydrate to form
a gibbsite-type surface layer. This would explain why
all have ZPC's of around 9.5. A-16 and C-30 DB are
exceptions but their behavior has been explained by
control of the ZPC by adsorbed impurities on the basis
of the mismatch of their pH^p^ and pH equilibrium values.
Conclusions
The surface areas of the fine alumina samples
investigated were in the classified ranges stated by
Flock (54) .
Using interpretations suggested by O'Connor et al.
(46), ZPC behavior of alumina powders was a function of
the degree of hydration. It was shown that grinding of
T-61 produces markedly different ZPC behavior from
relatively unground materials suggesting that grinding
influences the hydration behavior of the powder.
It is suggested that ZPC determinations can be
used as a method of investigating the presence of small
amounts of adsorbed impurities on commercial powder

102
surfaces.
phenomena:
This can be accomplished by observing
1) irregular aging of A-16 powder in
ZPC
two
water,
and 2) mismatch of pH
and pH equilibrium.

CHAPTER 7
ELECTROKINETIC PROPERTIES OF MONTMORILLONITE
Introduction
The electrokinetic properties of montmorillonite
clays are of interest because of their possible relation
to coagulation and dispersion. According to the DLVO
theory, coagulation may occur when the zeta potential is
small enough to allow London-van der Waal forces of
attraction to become effective.
The origin of charge on the face of a clay particle
is well known (57). It arises from a charge imbalance
in the clay lattice due to isomorphous substitution of
a positively charged ion for an ion of higher positive
charge. This results in a net negative charge in the
particle which is neutralized by counter ions such as
sodium and calcium. Their position in the clay structure,
their exchangeability for ions in solution, and their
hydration have been the subject of numerous articles (58-
67). All of these ions are not tightly held by the clay
particle since the negative charge on the particle is not
highly localized. Therefore, these ions can be exchanged
for one another or with hydrogen ions when the clays are
103

104
exposed to aqueous environments. The exchange phenomenon
gives rise to the cation exchange capacity (CEC) of the
clay. Clays such as montmorillonite with a high degree
of isomorphous substitution have high CEC's.
The neutralizing counter ions in a clay-liquid
system form an electrical double layer on the particle
faces. Sodium-saturated montmorillonites have a more
negative zeta potential than calcium montmorillonites
since calcium ions penetrate the Stern layer to a higher
degree than sodium ions.
Early work by Freundlich et al. (68) showed sign
reversal of the zeta potential for montmorillonite when
contaminated with small amounts of thorium and aluminum
chlorides. Additions of small amounts of potassium and
sodium hydroxide resulted in more negative zeta
potentials. Oakes and Burcik (69) were unsure whether
these effects were due entirely to either adsorption in
the Stern layer or ion exchange processes. These authors
also studied electrokinetic characteristics of a Wyoming
bentonite which had equilibrated in water for several
months. They found that calcium ions produced a less
negative zeta potential rapidly but at a decreasing rate
as more salt was added. They compared their results to
those of Freundlich et al. (68) and found good qualitative

105
agreement. Ho and Handy (70) also found similar results
for lime treated bentonites. Oakes and Burcik (68) found
that calcium added as either the nitrate or chloride salt
produced similar results. This phenomenon has been
reported by other investigators for other materials such
as silica (71).
Packham (72) studied the coagulation of dispersed
montmorillonite with various hydrolyzing salts. The
amount of coagulant required to halve the turbidity was
a constant low value below pH = 7.5 for montmorillonite.
Other clays such as kaolinite and halloysite displayed
a minimum in this value between pH = 7-8. Electrokinetic
data for montmorillonite were not presented. However, data
for kaolinite which showed the change in electrophoretic
mobility with pH at constant aluminum ion concentration
display striking similarities to data reported in the
present work. This suggests that the solution chemistry
of aluminum (i.e. Al ion hydrolysis) controls the electro-
kinetic properties in this particular type of investi
gation .
Recently, Swartzen-Allen and Matijevic (73) have
studied the electrokinetic properties of sodium montmoril
lonite as a function of pH, aluminum ion concentration
and the effect of aluminum ion hydrolysis. They found

106
that aluminum ions at pH < 4 did not reverse the sign
of the electrophoretic mobility. However, they did not
present data for aluminum ion concentrations greater
than 10 ^ M/L. Also they concluded that aluminum ion
hydrolysis reduced the ability of this ion to decrease
the negative mobility of the clay particles.
The results of Swartzen-Allen and Matijevic (71)
contradict the results presented in this chapter and
apparently those of Freundlich et al. (68). Several
possible explanations exist. First, solution preparation
of the electrophoresis sample may have been different.
Dissimilar results may occur when these solutions are
prepared by a single solution method (SSM) or by a double
solution method (DSM). Also, equilibration time of
clays in water before electrophoresis solutions are
prepared may be important. Considerable evidence exists
which demonstrates hydration and aging effects in clays
on many different properties (58,59,74-77). This may
also affect the ability of the clays to adsorb aluminum
ions from solution. Therefore, this chapter considers
conditions under which aluminum ions may or may not
reverse the sign of the zeta potential of sodium and
calcium montmorillonite.

107
Materials
Montmorillonite clays of various compositions were
obtained from a commercial supplier.* Table VIII lists
the composition of two as-received montmorillonites used
in the present work. It can be seen that, relative to
each other, 1608 is more nearly a sodium clay and 1613
is more nearly a calcium clay. Also, 1613 has much less
iron than 1608. Because of this, 1613 was chosen for
sodium or calcium saturation. These saturated clays
were designated 1613Na and 1613Ca. Their preparation
consisted of conditioning as-received 1613 clay in 1 M
NaCl or CaC^^F^O for one week with a clay-solution
ratio of 20 ml/gram clay. The clays were rinsed in
water to remove excess salt. For 1613Na only two rinses
could be achieved since the clay remained suspended even
after centrifugation** at 1750 rpm for 20 minutes after
the second rinse. For 16130a multiple rinses (at least
10) were possible. After rinsing the clays were dried in
an oven at 90C. After drying, both clays were ground and
used in electrokinetic experiments. A second sodium clay
was prepared by the method described by Swartzen-Allen and
Matijevic (73) and was designated as 1613Mat.
-'Georgia Kaolin, Inc., Elizabeth, N.J.
**Damon/IEC HN S-II centrifuge.

Table VIII. Compositions of Montmorillonite Clays 1608
and 1613 in Weight Percent
Clay
SiC>2
Ai23
Fe23
Ti02
MgO
CaO
k2o
Na20
1608
51.2
24.8
3.39
0.19
3.48
0.67
0.20
5.40
1613
60.8
21.5
0.9
0.21
3.62
2.21
0.04
1.30
Source: Georgia Kaolin, Inc., Elizabeth, N.J.

109
As-received 1613 montmorillonite was chosen for the
Na and Ca saturations because of its low iron content.
It was desirable to confirm that calcium clays have a
smaller double layer than sodium clays. Iron is a known
coagulant for silica sols because of its ability to
specifically adsorb to the surface (78). Since specific
adsorption of iron to the clay surface might also be
expected and would strongly influence the size of the
double layer, the lower iron containing clay was chosen
for saturation.
All salts used in these studies were ACS certified
reagent grade materials. Water used to prepare solutions
is described in Chapter 2.
Methods
1. Solution Preparations
a. Clay suspensions 1608, 1613, 1613Na, 1613Ca
For 1608, 1613, 1613Na and 1613Ca, stock suspensions
of one gram per liter were prepared by ultrasonic disper
sion for one hour. The suspensions were allowed to
equilibrate one week before use in the electrokinetic
experiments.
b^ Clay suspension 1613Mat
Suspensions of 1616Mat were prepared in the same manner
as Swartzen-Allen and Matijevic (73). This consisted of

110
ultrasonically dispersing one gram of clay in 1000 ml
water. However, no equilibration time was allowed
(Swartzen-Allen and Matijevic did not indicate whether
or not their suspensions were equilibrated).
c_. Electrophoresis solutions Single Solution
Method (SSM) and Double Solution Method (DSM)
To study the electrokinetic behavior of the various
montmorillonites as a function of pH in constant A1
concentration solutions, electrophoresis solutions were
prepared by two methods. The single solution method (SSM)
consisted of 10 ml (0.1%) clay slurry, 10 ml of lOx the
desired final A1 concentration and water for a final
volume of 100 ml. The pH was then adjusted by dropwise
addition of acid or base (0.1 or 0.01 M HC1, HNO^, NaOH).
The double solution method (DSM) consisted of preparing
separately 50 ml clay slurry (0.02%), prepared from stock
suspension, and 50 ml of twice the final desired aluminum
concentration at the proper pH. The two solutions were
then mixed and the pH remeasured. The pH values drifted
only slightly (0.2-0.3 pH units) in the pH range 6-9. At
the acid and basic extremes no drift was observed. The
solutions were then used in electrophoresis measurements.
The methods used for determining zeta potentials from
electrophoresis measurements have been described in
Chapter 6.

Ill
The pH measurements for all solutions were performed
on a pH meter* employing a glass combination electrode.
Results and Discussion
Figure 25 shows how the zeta potential varies with
solution pH for 1608 and 1613. It can be seen that both
clays have negative zeta potentials throughout the pH
range studied. This behavior differs from nonclay oxides
such as silica and alumina which display a zero point of
charge (ZPC) when similar measurements are performed.
This result shows that H+ and OH are not potential
determining ions for 1608 and 1613 montmorillonite clays.
Figure 25 also shows that 1608 has larger negative
zeta potentials than 1613 for pH 3-11. This suggests
that sodium montmorillonite has a larger double layer
than calcium montmorillonite. However the data are
not conclusive, since 1608 and 1613 differ in their gross
chemical composition. Therefore, zeta potential as a
function of pH was studied for 1613Na and 1613Ca.
Figure 26 shows conclusively that based on electro-
kinetic information, sodium montmorillonite has a more
negative zeta potential (larger double layer) than calcium
montmorillonite.
*Brinkman Model 512.

Zeta potent ial, (mV)
112
Figure 25. Zeta potential vs. pH for 1608 and 1613
montmorillonite.

Zeta
113
pH
Figure 26. Zeta potential vs. pH for 1613Na and 1613Ca
montmorillonite.

114
Figures 27 and 28 show results of zeta potential
behavior as a function of solution concentration of NaCl,
CaCl2*2H20 and AlCl^l^O for 1608 and 1613 montmoril-
lonites. The values corresponding to the data points
on each curve represent the pH which resulted from
preparation of the colloids in NaCl and CaCl22H20 solu
tions. The pH values for AlCl2*6H20 solutions were
purposely adjusted to avoid aluminum ion hydrolysis.
From the pH values for NaCl and CaC^Z^O curves it
appears that ion exchange occurred resulting in excess
hydrogen ions in solution. The zeta potential behavior
in NaCl and CaC^^^O demonstrates the fact that sodium
ions cannot penetrate the Stern layer (resulting in little
change in zeta potential values with increased NaCl
concentration) whereas calcium ions penetrate this layer
(resulting in a decrease in zeta potential to zero at
higher CaC^^^O concentrations).
Data for AlCl^^O show that aluminum ions adsorb
to 1608 and 1613 beyond the degree of charge equilivalent
ion exchange. If ion exchange occurred such that charge
neutrality persisted, then zeta potential sign reversal
would not be expected. However, sign reversal does occur
3 A
between concentrations 10 -10 M/L.
Figures 29 and 30 show the same type of data for
1613Na and 1613Ca clays as in Figures 27 and 28.

Zeta potentiaI, (mV)
Ionite.
115

Zeta potential,(mV)
Log concentration,
(moles
/
)
liter
Figure 28. Zeta potential vs. log concentration of NaCl,
CaCl2'2H20 and AlCl3*6H20 solutions for 1613 montmoril-
lonite.
116

Zeta potential, (mV)
117

Zeta potential, (mV)
Figure 30. Zeta potential vs. log concentration of NaCl,
CaCl2,2H?0) and AlClg-HpO for 1613Ca montmorillonite.
118

119
Comparison of Figure 29 to Figure 28 shows that 1613Na
_ C.
is truly a sodium clay, since the zeta potentials in 10
NaCl and CaCl2*2H90 for 1613Na and 1608 are approximately
twice the value for 1613 clay. Also, the shape of the
CaC^^H^O curve for 1613Na is similar to the same curve
for 1608. Both curves, however, differ from the shape of
the same curves for 1613 and 1613Ca. In fact, a simple
calculation can be made which suggests that the shapes
of the sodium clay curves for CaCl9are controlled
_j_ _l j_
by ion exchange of either Na or H in the clay for Ca
in solution. For 1608 and 1613Na, the magnitude of
change of the zeta potential from 10 ^ to 10 ^ M/L
CaC^ZF^O is a factor of 2-2.5. This is the difference
between sodium and calcium clays (1608 and 1613 or 1613Na
_| |__
and 1613Ca) when no Ca are added to the solution. The
_j
solution concentration of Ca which represents twice
the weight percent of calcium in a calcium'clay (1613)
should result in decreasing the zeta potential by a
factor of 2-2.5 if ion exchange occurs in a sodium clay -
_| |_ _
Ca ion solution system. This concentration is 10 M/L
CaCl2(4 weight percent Ca). Figure 30 also shows
that 1613Ca was converted to a sodium clay as the NaCl
concentration increased. This is shown by an increasing
negative zeta potential as the concentration of NaCl
increases.

120
The zeta potential at high NaCl concentrations for
the calcium clays approaches the zeta potential value of
sodium clay in water. Bar-On et al. (79) have shown that
electrophoretic mobilities of sodium montmorillonite are
higher than calcium clays. In fact, their values of
mobility corresponding to a clay with 0% exchangeable
sodium are precisely the values for 1613Ca and the value
for a clay with 1007, exchangeable sodium is the same for
1613Na clay. These findings suggest that double layer
characteristics are controlled by ion exchange phenomena.
_J
To test the hypothesis that ion exchange of Ca in
solution for Na~*~ or in the clay controls the electro-
kinetic behavior, a solution depletion experiment was
_ _j |_
performed. A solution of 10 M/L Ca was analyzed
before and after containing 0.01 weight percent 1613Na
montmorillonite for one hour. Afterwards, the clay
suspension was filtered through a millipore syringe
employing 0.22 micron pore size filter paper. The
_j |_
filtrate was analyzed for Ca in an atomic emission
spectrophotometer.* A standard curve was prepared by
_j
analyzing 0-10 ppm standard solutions of Ca Analysis
_|
of the filtrate revealed exactly half as much Ca than a
*Heath Model EU 703.

121
-4 ++
solution of 10 M/L Ca which had contained no clay.
_| |_
This evidence confirms that ion exchange of Ca in
solution for H+ or Na~*~ in the clay controls the electro-
kinetic characteristics of this clay.
Of the three salts studied, aluminum ions decreased
the zeta potential (i.e. repulsive forces between
particles) to zero at the lowest concentration as pre
dicted by the Schulze-Hardy rule. However, as shown in
Figures 27-30 the pH range for the A1 studies was
restricted to 3-4. Therefore, zeta potential character
istics of 1608 and 1613 clays from pH = 3-10 in constant
aluminum ion concentrations (10 10 ^ M/L) were studied.
The samples for electrophoresis were prepared by the
double solution method (DSM).
Figures 31 and 32 show the results for 1608 and 1613
respectively. For 1608 10 M/L AlCl^^O does not
reverse the sign of the zeta potential for any pH in the
range studied. However, 10 ^ M/L A1C1^6H20 reversed the
sign from negative to positive at pH = 4.1 and back to
negative at pH = 8.5. For 1613, 10 M/L AlCl26H20
reversed the sign from negative to positive at pH = 5.6
and back to negative at pH = 7.1. In 10 M/L AlCl2-6H20
for 1613, the sign reversed from negative to positive at
pH = 4.1 and back to negative at 8.2-8.3. Also shown in

Zeta potential, (mV)
122
Figure 31. Zeta potential vs. pH for 1608 montmoril-
lonite in 0, 10~5 and 10-,+ M/L AlC^'f^O
solutions.

Zeta potential, (mV)
123
pH
Figure 32. Zeta potential vs. pH for 1613 montmorillonite
in 0, 10~5, 10-^ M/L A1C13-6H20 solutions.

124
these figures is the zeta potential behavior vs. pH when
no aluminum is present. Table IX summarizes the chemical
conditions of zero zeta potential (ZZP) for these
clays.
These results indicate a remarkable change in zeta
potential behavior with change in the hydrolysis of the
aluminum ions. At pH < 3-4 the aluminum is in the form
+3
of the aluminio (Al ) ion. Matijevic and Stryker (80) and
Matijevic (81) have suggested that from pH = 4 to 7 the
aluminum is present as a complex AlgiOH)^ species. The
results presented in this work lend support to this view
since the existence of such a species would be expected
to increase the positive mobility of the clay particles
because of the higher charge on the ion.
As the pH increases from 8 to 10 two possible
mechanisms exist for sign reversal back to a negative
value in Figures 31 and 32. Most workers (82,83) suggest
that aluminum hydroxide precipitate coats the particles.
The measured zeta potential behavior would be dominated
by aluminum hydroxide rather than the uncoated particle.
A possible alternative is that aluminum hydroxide does not
coat the particles but returns into solution as either
AIO2 or A1(0H)^ as the pH increases from 8. The measured
zeta potential behavior would then be characteristic of

125
Table IX. Chemical Conditions for Zero Zeta
Potential (ZZP) for 1608 and 1613
MontmorilIonites
Clay
[aici3-6h2oJ m/l
PH
1608
1
O
i1
4.1
1608
l
o
11
8.5
1613
10~4
4.1
1613
10-4
8.5
1613
m
i
o
!1
5.6
1613
10 5
7.1

126
the clay particles rather than aluminum hydroxide.
Figures 31 and 32 alone suggest that the second alterna
tive may be correct since all the curves converge at
higher pH values. Also, the value of the zeta potential
at this point is the same value for sodium clay when no
aluminum ions were introduced. This convergence would not
be expected if aluminum hydroxide was coating the
particles since different initial concentrations of
aluminum would result in varying degrees of coating.
However these results are only indirect support of the
noncoating hypothesis since they do not show any chemical
evidence of aluminum hydroxide not coating the surfaces
of the particles at higher pH values.
The coating hypothesis has arisen from work on
materials other than clay such as silica and titania (83).
To exclude this hypothesis the following experiment was
performed. Fused silica disks (1 x 1 x 0.5 cm) were
cleaned by boiling in a 0.1 M HCl solution. The disks
were then rinsed with conductivity water and placed in
-4
10 M/L AlCl^'^O solutions whose pH values were
adjusted with HCl or NaOH. After one hour the disks
were removed from solution. Excess solution was removed
by placing the edge of the sample against adsorbent paper.
The samples were then mounted on a specimen holder

127
suitable for Auger Electron Spectroscopy (AES) and were
analyzed for aluminum on the surface.
Figure 33 shows the results of this analysis. These
data are presented as Auger peak height ratio of aluminum
to oxygen as a function of pH. The results show that a
large amount of aluminum persists on the silica surface
from pH = 5-9. Beyond pH = 9, the amount of aluminum on
the silica decreases significantly. This is direct
evidence that aluminum hydroxide does not coat the silica
surfaces at higher pH values.
The results shown in Figure 29 contradict results
of Swartzen-Allen and Matijevic (73). They found that
-4
10 M/L A1 ion did not reverse the sign of the electro
phoretic mobility for any pH > 4. As stated in the
introduction several reasons for these differences are
possible. Therefore a detailed investigation of the
effect of clay solution preparation, clay-water equili
bration, aluminum salt used, and clay-aluminum ion solu
tion aging was undertaken using 1613Mat. Comparison of
Figures 34 and 35 shows no significant difference in the
zeta potential behavior with pH at constant A1 ion con
centration when either the chloride or nitrate salt is
used. When the DSM was used, no difference in the zeta
potential behavior as a function of pH was observed

Auger peak height ratio ,
pH
Figure 33.
Auger peak height ratio vs. pH for vitreous
silica after one hour exposure to 10-f M/L
A1C3*6H20 solution.
128

pH
Figure 34. Zeta potential vs. pH for 1613Mat in 10-1^ M/L
AlCl3*6H20 solution. SSM-0 means single solu
tion method, clay plus A1 solution aging time =
0 hours.
129

80
60
40
20
0
20
40
:0
1613 Mat
I0"4 AI(N03)3 -9H20 (m/,)
SSM -0
o DSM -0
SSM 20
a DSM 20
_i I l 1 I I I I
3.0 40 5.0 60 70 8.0 9.0 10.0 11.0
pH
Figure 35. Zeta potential vs. pH for 1613Mat in 10"^ M/L
Al(NO3)39H20 solution.
130

131
between clays that were unaged or aged for 20 hours in
aluminum ion solutions at any pH value. If the SSM is
used, aging of the system is observed. Also, the zeta
potential sign is not reversed for any of the solutions.
In fact, Figure 34 shows that unaged, unequilibrated
1613Na and 1613Ca have negative values of zeta potential
at pH = 6. Also freshly prepared suspensions of 1613
(as-received clay) containing 10 ^ AlCl^^O at pH = 6
prepared by the DSM display a negative zeta potential
value. The only difference between this solution and the
solutions used to obtain previous results depicted in
Figure 32 at pH = 6 is that the stock clay suspension for
which the results in Figure 32 were obtained was equili
brated for at least a week before electrophoresis solu
tions were prepared. This prompted an investigation into
the effects of 1613 clay equilibration time in water on
the zeta potential before addition of aluminum ions.
These results are shown in Figure 36. First, zeta
potential is independent of equilibration time in water
when no aluminum ions are added. However, significant
changes occur when the aluminum ions are added after the
clay is equilibrated in water at pH = 6 for a certain
period of time. For 1613 equilibrated 18 hours in water
the zeta potential changed from -20 mV to +18 mV when

Figure 36. Zeta potential of 1613 montmorillonite in
10-4 M/L AlCl3*6H20 and water at pH = 6 vs.
clay equilibration time at pH = 4, 6 and 8.
132

133
10 ^ AlCl^'l^O was added at pH = 6, whereas little change
in the zeta potential is observed after aluminum ions
are added at clay equilibration time = 0 hours. Equili
bration at various pH values also affected the results of
adding aluminum ions to the solution. However, zeta
potential measured at pH = 6 is independent of equili
brium pH of the clay in water when no aluminum ions are
added. Therefore, not only equilibration time but also
equilibration pH affected the adsorption capacity of the
clay for aluminum ions. These results suggest that
variations in the hydration of the clay affect the
adsorption capacity for aluminum ions.
Mathers et al. (84) have studied the effect of acid
and heat treatment of clays on structure and cation
exchange properties of montmorillonites. They found
that hydrogen montmorillonite converted to an aluminum
montmorillonite when stored moist at 30C. That is,
aluminum ions from lattice position became exchangeable
cations with time. The clays used by these authors were
purposely converted to hydrogen clays by acid treatment.
However, it is known that both sodium and calcium clays
when suspended in water increase the pH indicating ion
exchange of these cations for hydrogen ions in solution.
In this way, the sodium and calcium clays used in the

134
present work can become hydrogen clays although probably
not as hydrogen saturated as those of Mathers et al.
Possibly, during equilibration the clays used in this
investigation initially became acid clays and with time
converted to aluminum clays. The exchangeable aluminum
could become available for adsorption to the clay
particles. A simple calculation of the amount of aluminum
which becomes exchangeable can be made since the clay
composition of 1613 is known from Table VIII. For a 0.017o
suspension, and assuming half of the aluminum becomes
exchangeable, the amount of Al in solution would be
.01 g x .06 g = .0006 g. The number of moles of
aluminum would be .0006/28 = .00002 moles/lOOcc = 2 x
-4
10 moles/liter. This should be enough Al to change the
sign of the zeta potential at pH = 6 as shown in
Figure 32 without further aluminum ion additions.
However, as Figure 25 showed, the zeta potential remained
negative at pH = 6. Therefore, conversion to an aluminum
clay probably does not occur for 1613.
If zero potential (ZZP) is required for coagulation,
then from a practical standpoint, the point at pH = 8.5
-4
in 10 M/L AlCl^'^O would be more desirable. The
reason for this is shown in Figure 37. These curves were
obtained by placing a known amount of clay into 25 ml of

135
Figure 37. Solution pH vs. total weight of montmorillonite
added to 25 ml of water.

136
water and measuring the pH. The condition at which
further addition of clay resulted in no pH change was
the equilibrium pH of the clay. The equilibrium pH for
these clays was between 9 and 10. Therefore, much less
acid would have to be added to bring the pH to 8.5 than
to 4.1 to create possible coagulation conditions. Future
studies should focus on long term aging of these clays
at the zero zeta potential point and measuring the degree
of coagulation with time.
Conclusions
It has been confirmed that the double layer for a
calcium clay is smaller than for a sodium clay. This is
due to the ability of calcium ions to penetrate the Stern
plane and decrease the size of the diffuse layer in
solution. Sodium ions cannot penetrate the Stern layer
creating a larger diffuse layer on the clay particle
faces.
Ion exchange phenomena were shown to control the
electrokinetic behavior of 1613Na clay in CaCl2'2H20
solutions. Atomic emission data showed that at the
solution concentration of Ca which decreased the zeta
potential by a factor of 2, the amount of Ca present in
solution was also decreased by a factor of 2. Other

137
electrokinetic evidence showed that sodium ions in the
_|_ _j |_
clay also exchange for either H or Ca in solution
since the zeta potential of 1613Ca increased in
concentrated NaCl solutions to a value corresponding to
a sodium clay in water.
Aluminum ions were shown to specifically adsorb to
the surfaces of all the clays studied at concentrations
/ O
of AlCl^l^O solutions between 10 and 10 moles/
liter. This was shown by observing the sign reversal
of the zeta potential at these concentrations. Studies
of the clays in solutions of varying pH and constant
AlCl2*6H20 concentrations revealed a dependence of zeta
potential characteristics on the degree of hydrolysis
of the aluminum ion. Also, adsorption of aluminum ions
was found to depend on the equilibration time and pH of
the clay in water and on the method of solution prepara
tion. The presence of a complex aluminum species
4+
(A1q(0H)2q) at 7 > pH > 4 as suggested by other workers
was supported by these results. However, Auger analysis
of the surface of silica disks which had been exposed to
constant aluminum ion concentrations at varying pH values
refuted the aluminum hydroxide coating theory proposed
by other workers by showing that aluminum hydroxide did
not adsorb to the surface at high pH values.

138
A necessary coagulation condition is the reduction of
electrical repulsive forces between particles. Further
studies involving aging these clays at the chemical
conditions of zero zeta potential will be necessary to
determine if these conditions are sufficient for dense
coagulation of primary clay particles.

APPENDIX A
COMPUTER PROGRAM FOR CALCULATING THE SURFACE CHARGE
DENSITY ERROR VALUE IN CHAPTER 3

V PENS
[1] KA+-1.0177 10
[ 2 ] KE-*-% 9 2/. 2 3
[3] KC<-2 81 3/7 2 7
[4] /O 6.31 /7~ 6 1 0 577" 3 1.47/7 5 7.9 4/7 S 0.0147
[ 5 ] P<7+-9 65 0 0 00 000 0
[ 6 1 17/1 '
[ 7 ] 77,1 <-
rol (7,7'
r o i <7/mi
[ i o ;i an'
cm <7/mi
r 1 2 ] (7/7/1'
[13] <7/7/Ml
[14] (](?+-0
[15] /C/M/i/ t( (7//X (7/1 )
[1G] A'M/f/? (<7/? x (<7//* 2 ) )
[17] K Z<-(KC*( Gi'A *?.)):( GU 2 )
[18] ;/<-o
[19] 1/'
[20] i/^n
[21] SA'
[22] /7/M1
[23] /'
[24] /mi
[25] 77/ 7 <77/ x 1/
[26] / <77/ ?
[27] '7'
[28] y<-n
[29] PI!'
[30] P/M]
[31] X1
[32] Ml
[33] ITA<-YpX
[34] RETURN: N<-H +1
[35] //<-l 0* PI!
[36] Q <- ( V / ) x ( ( 2 x KY x K [ N ] ( // 2 ) ) + ( XX x K [ // ]://) + ( 2 x X % x 77 [ // ] x ( /// 2 ;
[37] QQ+-QQ ,i>
[38] -*-(//< 5) /RETURN
[39] ??l<-?r?[l + i7]
[40] (Of? 2<- [41] 00 3 --(7(7 [ ( ( 2x7 ) + l ) + i 7]
[42] [43] (>0 5 <-(?(,? [ ( ( 4 x 7 ) + 1 ) + i 7 ]
[44] <7i?f?l^f7(71 x/T
[45] <7(7 (7 2 -*-(7f? 2 xP(7
[46] CQQ3+QQ3*FC
[47] [48] <7 [49] 'DORE'
7
:(//* 2 )) )
140

APPENDIX B
LIST OF SYMBOLS USED IN THE COMPUTER PROGRAM
IN APPENDIX A

KA = Equilibrium constant for formation of HSiO^ from
f^SiO^
KB = Equilibrium constant for formation of SiO^ from
l^SiO^
KC = Equilibrium constant for formation of Na2Si02 from
f^S iO^
K = Equilibrium concentration for the soluble species for
each solid phase considered
FC = Product of Faraday's constant (96,500 coul/mole) and
the number of microcoulombs per coulomb (10^)
GA = Activity coefficient of HSiO^
GB = Activity coefficient of SiO^
GH = Activity coefficient of H+
GNA = Activity coefficient of Na
KX = Equilibrium concentration of HSiO^ from the ioniza
tion of f^SiO^
KY = Equilibrium concentration of SiO^ from the ionization
of H^SiO^
KZ = Equilibrium concentration of Na2Si02 from the ioniza
tion of F^SiO^
t 1~1
N = "N" character of vector K
V = Volume of solution in liters
2
SA = Specific surface area in cm /g
W = Weight of powder in grams
142

143
SAT = Total surface area
Y = Number of pH unit divisions (example: for pH 7-10,
Y = 31 for pH 7.0, 7.1, 7.2, etc. through 10.0)
PH = pH
X = Solution molarity (mols/liter)
NA-^-YpX is a vector of Na concentration "Y" characters in
length
H = H+ ion concentration in moles/liter
Q = Difference between the real surface charge density and
apparent surface charge density (areaxaapp)
QQ = Vector of Q's (the first QQ = 0 as defined in line 14
of the program)
QQl = First "Y" number of characters in vector QQ
QQ2 = Second "Y" number of characters in vector QQ
QQ3 = Third "Y" number of characters in vector QQ
QQ4 = Fourth "Y" number of characters in vector QQ
QQ5 = Fifth "Y" number of characters in vector QQ
CQQ1 = Product of QQl and FC
CQQ2 = Product of QQ2 and FC
CQQ3 = Product of QQ3 and FC
C0Q4 = Product of QQ4 and FC
CQQ5 = Product of QQ5 and FC
"Done" is printed when calculations are completed

144
To generalize the program for any material, "K" in
line [4] should be made a variable so that the vector can
have the same length as the number of different solid
phases being considered. Also, the number of individual
physical constants such as equilibrium constants for
the formation of the ionic species, and the activity
coefficients of the ions will vary as the number of ionic
species formed varies. Finally, since the number of
characters in "K" will vary, "N<5" statement in line [38]
should read N the vector K. Therefore, since K is a variable, Z will
be a variable.

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BIOGRAPHICAL SKETCH
John Milton Horn, Jr., was born October 23, 1951, in
Jacksonville, Florida. He received his early childhood
as well as high school education in Jacksonville Beach,
Florida. After graduating from high school, he entered
Stetson University in Deland, Florida, where he earned
a Bachelor of Science degree in Biology. While
matriculating at Stetson, he played intercollegiate
soccer and played violin in the Stetson University
Symphony Orchestra. He met Barbara Helen Furr, of
Fort Pierce, Florida, in September 1970, and they were
married June 15, 1974. He received his Master of Science
degree in August, 1976, from the Department of Materials
Science and Engineering at the University of Florida,
and has been pursuing a Doctor of Philosophy degree in
the same department since that time. He is a member of
Sigma Nu social fraternity. Professional societies
include American Institute of Mining, Metallurgical and
Petroleum Engineers (AIME) and the American Ceramic
Society. Honorary memberships include Keramos and
Alpha Sigma Mu.
153

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
George Y. Onoda, JpTT1-Chairman
Associate Professor of
Materials Science and
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
/
Ji;
Larry L < Hen'cn
Profssor and Head of Ceramics
Division
Materials Science and
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
a
Robert W. Gould
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
U- Q ,qrU^. U
Dinesh 0. Shah
Professor of Chemical
Engineering

This dissertation was submitted to the Graduate Faculty of
the College of Engineering and to the Graduate Council,
and was accepted as partial fulfillment of the require
ments for the degree of Doctor of Philosophy.
March 1978
Dean, College of Engineering
Dean, Graduate School

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109
As-received 1613 montmorillonite was chosen for the
Na and Ca saturations because of its low iron content.
It was desirable to confirm that calcium clays have a
smaller double layer than sodium clays. Iron is a known
coagulant for silica sols because of its ability to
specifically adsorb to the surface (78). Since specific
adsorption of iron to the clay surface might also be
expected and would strongly influence the size of the
double layer, the lower iron containing clay was chosen
for saturation.
All salts used in these studies were ACS certified
reagent grade materials. Water used to prepare solutions
is described in Chapter 2.
Methods
1. Solution Preparations
a. Clay suspensions 1608, 1613, 1613Na, 1613Ca
For 1608, 1613, 1613Na and 1613Ca, stock suspensions
of one gram per liter were prepared by ultrasonic disper
sion for one hour. The suspensions were allowed to
equilibrate one week before use in the electrokinetic
experiments.
b^ Clay suspension 1613Mat
Suspensions of 1616Mat were prepared in the same manner
as Swartzen-Allen and Matijevic (73). This consisted of


55
Table V. Values of CQ for Various Zeta Potentials
and Weight Percent Solids
wt. %
C (mV)
0.00125
0.0025
0.025
0.25
1.0
-10
3.5x10-5
4.5x10-5
1.1x10-^
7 xlO-4
2.9x10-3
-5
8.5xl05
9.3xl0~5
2.lxlO-4
1.3xl0-3
5.4xl0"3
0
2.2xl0~4
2.3xl0~4
4.5xl0'4
2.3xl0'3
_2
10
+5
5 xlO-4
5.Ixl0~4
8.8xl0-4
5.4xl0"3
1.8xl0-2
+10
1,2xl0-3
1.2xl0~3
1.8xl0-3
8,5xl0"3
3 xlO"2
+15
2.7xl0-3
2.8xl0"3
3.6xl0-3
1.3xl0"2
5 xlO"2


CHAPTER 5
REVERSIBILITY OF ALUMINUM ION ADSORPTION ON FUSED SILICA
Introduction
Considerable interest has been generated in the past
ten years in the coagulation and flocculation behavior of
silica sols. It has been shown that of utmost importance
in this behavior is the hydration state of the surface.
It is now generally accepted that the surfaces of silica
sols are covered by silanol groups with a density of 5 OH/
O
100 A (27), whereas a quartz surface is relatively inert,
being covered by siloxane groups (28,29). However, some
silanol groups have been shown to exist on quartz,
especially on ground or milled material (22,30).
Lange (31) found that the amount of water hydrogen bonded
to precipitated silicas was about 387o of the total pos
sible adsorption capacity. This meant that 62% of the
silanol groups were hydrogen bonded to each other. Using
an explanation involving ion exchange of hydrogen ions
for metallic ions, Allen and Matijevic (32) found that
the hydrophilic nature of silica decreased in the presence
of simple electrolytes.
Recently, Tschapek and Sanchez (33) studied the
amount of NaCl required to coagulate different silica
66


12
performed. Instead, bioglass was only washed with con
ductivity water.
The bed lengths were 4.1 cm and the cross sectional
2
area of the beds was 2.38 cm The bed porosity was
calculated to be 36%.
Water used to rinse the bed material and to prepare
solutions was obtained from a water deionization system*
with the following specifications: a resistivity of
1.87 x 107 ohm-cm, dissolved solids at the parts per
billion level, dissolved gases removed, and organics
removed. Since dissolved gases were removed, equilibra
tion with air was required which caused the pH to drop to
5.5-6.0 due to absorption of CO2 This is the pH range
at which the streaming potential experiments were
performed. Sodium chloride** solutions of various
concentrations were prepared with this water and used in
the experiments.
2. Apparatus
The cell for the streaming potential system is shown
schematically in Figure 1. It consists of a thick walled
'-'Continental Water Conditioning Co., Inc., Gainesville,
Florida.
**ACS reagent grade, Scientific Products, Ocala, Florida.


148
37. A. V. Kiselev, "The Structure and Properties of
Porous Materials" (D. H. Everett and F. S. Stone,
eds.), p. 195, Buttersworth, London (1958).
38. S. K. Dubrovo, "Reaction of Vitreous Silicates and
Sodium Aluminosilicates with Aqueous Solutions;
Part III. Influence of Silica and Alumina Content in
the Composition of Sodium Silicates and the Corrosion
Ability in Acids," Izv. Akad. Nauk SSR Otd. Khim.
Nauk 2, 244 (1954).
39. R. K. Her, "Effect of Adsorbed Alumina on the
Solubility of Amorphous Silica in Water," J. Colloid
Interface Sci. 43, 399 (1973).
40. W. A. Weyl, "Some Practical Aspects of the Surface
Chemistry of Glass, IV," Glass Industry 28, 408
(1947) .
41. K. C. Lyon, "Effect of Rinsing on the Chemical
Durability of a Container Glass," J. Amer. Ceram.
Soc. 32, 46 (1949).
42. M. L. Hair and W. Hertl, "Acidity of Surface
Hydroxyl Groups," H Phys, Chem, 74, 91 (1970).
43. E. Matijevic and L. J. Stryker, "Coagulation and
Reversal of Charge of Lyophobic Colloids by
Hydrolyzed Metal Ions; III. Aluminum Sulfate,"
J. Colloid Interface Sci. 22, 68 (1966).
44. E. Matijevic in "Principles and Applications of
Water Chemistry" (S. D. Faust and J. V. Hunter,
eds.), p. 238, John Wiley and Sons, New York (1967).
45. G. A. Parks, "The Isoelectric Points of Solid
Oxides and Aqueous Hydroxo-Complex Systems," Chem.
Rev, 65, 177 (1965).
46. P. J. O'Connor, P. G. Johansen, and A. S. Buchanan,
"Electrokinetic Properties and Surface Reactions of
Corundum," Trans. Faraday Soc. 52, 229 (1956).
47. M. Robinson, J. A. Pask, and D. W. Fuerstenau,
"Surface Charge of AI2O3 and MgO in Aqueous Media,"
J. Amer. Ceram. Soc. 47, 516 (1964).


32
Table III. Activity Coefficients for Ionic Species
at Various Solution Concentrations
Solution
Concentration
(moles/liter)
Species
T
n
0
01
001
HSiO^
0
.750
0
898
0
964
Si03
0
. 360
0
660
0
.867
H+
0
830
0
914
0
967
Na+
0
.770
0
901
0
.964
Source: Klotz, ref. 21


29
Table I. Standard Free Energy of Formation
Values at 298K (Kcal/mole)
N
Species
AGU (Kcal/mole)
1
^*"^2 (quartz)
-192.4
2
S^2 (cristobalite)
-192.1
3
^^2 (trydymite)
-191.9
4
^^2 (vitreous)
-190.9
5
^^2 (hydrated)
-187.8
H2Sl03(aq)
-242.0
HSOt N
3 (aq)
-228.36
Si0o, s
3 (aq)
-212.0
2 3 (c)
-341.0
Na+. s
(aq)
-62.6
H2(aq)
-56.69
H~t *
(aq)
0
oh; v
(aq)
-37.6
Sources: Pourbaix, ref. 14 and
Dickerson, Gray and Haight,
ref. 20.


78
The major difference between untreated and heat
treated silica samples is that the former is dominated
by adjacent silanol groups while the latter contains
isolated silanol groups (2). The data show that aluminum
ions reversibly adsorb to a silica surface containing
adjacent silanol groups and irreversibly adsorb to a
surface containing isolated silanol groups. However,
since initial E/P values for the two samples were the
same as seen in Figure 18 at stream time equal to zero,
there appears to be little difference in the amount of
aluminum initially adsorbed. If only those silanol
groups which are not hydrogen bonded to each other can
become adsorption sites, as suggested earlier, then only
those which are not hydrogen bonded to each other can
be true specific adsorption sites for aluminum ions. When
aluminum ions adsorb, the double layer characteristics
change as indicated by changes in zeta potential. If
zeta potential characteristics change in the same way (as
indicated at stream time = 0 for untreated and heat
treated silica) for these two surfaces having different
hydration states, then the aluminum ion adsorption site
must be present in the same amount on both surfaces. This
trend is supported by the results of Tschapek and
Sanchez (33) which showed identical coagulation


LIST OF FIGURES continued.
Figure Page
11 Zeta potential vs. concentration of
AICI36H2O for 0.00125, 0.0025, 0.025,
0.2 and 1.0% weight percent montmoril-
lonite 52
12 Apparent aluminum ion concentration C0
vs. weight percent montmorillonite for
zeta potential = 0 at pH = 4.0 56
13 Concentration of aluminum CG minus the
equilibrium concentration (C) of aluminum
in solution vs. weight percent montmoril
lonite for zeta potential = +15, +10, +5,
0, -5, -10 mV at pH = 4.0 57
14 Zeta potential and log adsorption density
(T) vs. equilibrium concentration (C) of
aluminum in solution for montmorillonite
at pH = 4.0 60
15 Log of the ratio of C and r vs. zeta
potential for montmorillonite for the
experimental data 63
16 Streaming potential apparatus modified
to maintain constant flow pressure;
a) secondary reservoir; b) primary
reservoir; c) pump; d) solution flow
valve; e) cell; f) electrometer;
g) recorder; h) solution head height 73
17 Zeta potential vs. concentration of
sodium citrate and aluminum chloride 74
18 Streaming potential-pressure ratio vs.
stream time for untreated, heat treated,
base treated heat treated and base
treated and non-aluminated fused silica 77
viii


51
This procedure was carried out for each of the solids
suspensions listed above. Therefore, electrokinetic data
as a function of aluminum ion concentration for various
solids content could be obtained.
Electrokinetic data were generated using microelectro
phoresis using the Riddick cell. The methods of
determining electrophoretic mobilities and calculating
zeta potentials are discussed elsewhere (25).
All chemicals used in this study were certified
reagent grade materials.
Results and Discussion
1. Calculation of C and r
Figure 11 shows the results of zeta potential as a
function of aluminum ion concentration CQ for various
solids contents. This concentration is the number of
moles of aluminum ions added to the system divided by
the solution volume. It is not the actual concentration
of aluminum ions in solution since adsorption occurs onto
the montmorillonite particle surfaces. Therefore, it is
the apparent concentration if no adsorption takes place.
It can be seen that zeta potential-concentration curves
for various solids contents differ except for very dilute
suspensions. This indicates that, at higher solids


86
be studied. Robinson et al. (47), O'Connor et al. (46),
Johansen et al. (48,49), and Schuylenborgh et al.(50-53)
all agree that treatment which leads to bulk or surface
dehydration results in a more acid ZPC than for oxides
which are hydrated. Those treatments which dehydrate the
)
surface (e.g. heat) lower the ZPC whereas treatments
which increase hydration of the surface (e.g. grinding)
increase the ZPC.
Most of the ZPC information on aluminum oxide com
piled by Parks was obtained on either naturally occurring
minerals or on synthetic materials prepared in the
author's laboratories. However, little ZPC information
can be found for commercially prepared aluminas.
Information of this nature could be beneficial to both
suppliers and users of commercial aluminas particularly
if the powders are subjected to aqueous environments
during processing. Parks (45) has shown that very small
levels of adsorbed impurities such as phosphate and
sulfate as well as certain organics greatly affect the
ZPC of the oxide. If the impurities were undesirable,
they could easily be detected by measuring the ZPC of
the oxide. Appropriate steps could then be taken to
eliminate the impurities during processing.


146
'f, 12. H. L. Kruyt, "Colloid Science," Vol. I, Elsevier,
Amsterdam (1952).
13. G. W. Sears, "Determination of Specific Surface Area
of Colloidal Silica by Titration with Sodium
Hydroxide," Anal. Chem. 28, 1981 (1956).
14., G. H. Bolt, "Determination of the Charge Density of
Silica Sols," J. Phys. Chem. 61, 1166 (1957).
15. W. M. Heston, R. K. Her, and G. W. Sears, "The
Adsorption of Hydroxyl Ions from Aqueous Solution on
the Surface of Amorphous Silica," J. Phys. Chem. 64,
147 (1960).
16. H. C. Li and P. L. deBruyn, "Electrokinetic and
Adsorption Studies on Quartz," Surface Sci. 5,
203 (1966).
17. Th. F. Tadros and J. Lyklema, "Adsorption of
Potential Determining Ions at the Silica-Aqueous
Electrolyte Interface and the Role of Some Cations,"
J. Electroanal. Chem. 17, 267 (1968).
18. D. E. Yates and T. W. Healy, "The Structure of the
Silica/Electrolyte Interface," J. Colloid Interface
Sci. 55, 9 (1976).
19. M. Pourbaix, "Atlas of Electrochemical Equilibria in
Aqueous Solutions," Pergamon Press, New York (1966).
20. R. E. Dickerson, H. B. Gray, and G. P. Haight,
"Chemical Principles," W. A. Benjamin, Inc.,
New York (1970).
21. I. M. Klotz, "Chemical Thermodynamics," Prentice-
Hall, New Jersey (1950).
22. J. A. van Lier, P. L. deBruyn, and J. Th. G. Overbeek,
"The Solubility of Quartz," J. Phys. Chem. 64, 1675
(1960).
23. J. Gibb, P. D. Ritchie, and J. W. Sharpe, "Physico-
Chemical Studies on Dusts: IV. Electron-Optical
Examination of Finely Ground Silica," J. Appl. Chem.
(London) 3, 182 (1953).


ACKNOWLEDGEMENTS
Deep appreciation and many thanks are extended to the
author's graduate supervisory committee which included
Dr.
G. Y.
Onoda, Jr.,
chairman;
Dr. L. L. Hench;
Dr.
R. W.
Gould; and,
Dr. D. 0.
Shah. Special thanks go to
his
advisor, Dr. G. Y.
, Onoda, Jr
., without whose many
helpful and lengthy discussions, this work would not have
been possible.
The author wishes to thank Mr. Fumio Ouchi for the
Auger data presented in Chapter 7. Also, thanks go to
Mr. Peter Curreri and Mr. Jim Adair for helpful discus
sions, and to Mr. Nick Gallantino for technical
assistance in the lab.
Finally, the author wishes to acknowledge the
National Institute of General Medical Sciences grant
#GM21056-02 and the National Science Foundation
grant #AER76-24676 for partial financial support for this
work.
in


42
error is the same as the most to least thermodynamically
stable. The calculations show that in a mixed phase
solid of 99.99% quartz and 0.01% hydrated silica, the
amount of soluble complex species formed from the hydrated
phase will be the same as the amount formed from the
quartz.
One occurrance of a mixed phase solid has been
addressed by van Lier et al. (22). In their quartz
solubility studies, they confirmed the existence of a
disturbed layer on ground quartz particles by dissolution
O
studies at high pH values. The thickness of 300 A which
they calculated from their results agreed well with
Gibb et al. (23). Van Lier et al. found that abnormally
high solubilities in water and high pH solutions of
quartz are obtained if the disturbed layer is not re
moved. However, removal of this layer yielded normal
solubility data. This suggests that the layer is not
quartz but probably a more thermodynamically unstable
phase.
Van Lier et al. were not able to identify the
disturbed layer phase. Using their experimental condi
tions and the free energy of formation values in Table I
of the present work, it is possible to calculate the
theoretical thickness of the layer assuming the silica
phase is each of the five solid phases considered.


ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE
By
JOHN MILTON HORN, JR.
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1978


75
Chapter 2 was used to eliminate undesirable effects of
electrode polarization.
_3
By using 10 M NaCl as a supporting electrolyte for
adsorption of aluminum ions, no significant change in
E/P due to varying solution conductivity as predicted by
Equation 1 should occur during the desorption experiment.
Also since desorption studies were performed under
conditions of constant pressure, no change in the
streaming potential due to varying pressures as predicted
by Equation 1 should be expected. Therefore, during the
desorption studies, the change in E/P should only be due
to change in the zeta potential (O due to removal of
aluminum ions from the silica surface.
Desorption characteristics of several types of
silica surfaces were studied by this technique. One set
of samples remained untreated. A second set of samples
was heat treated at 800C for 8 hrs. in vacuum.'1' A third
set was treated with 1 M NaOH solution for 24 hrs. at
22C. Finally, a combined thermal and chemical treatment
of the glass particles was performed using the agents
described above.
*Centorr hot press vacuum chamber, 10-* Torr.


electrophoresis. Ion exchange of sodium, calcium and
hydrogen ions in solution for similar cations in the
clay controls the electrokinetic behavior of the
montmorillonite. Aluminum ions are found to specifically
adsorb from solution onto the surface. However, the
degree of aluminum ion adsorption as shown by electro
phoresis measurements depends on the equilibration time
and pH of the clay in water whereas the electrokinetic
behavior of the clay in the absence of aluminum ions in
solution is independent of equilibration time and pH
in water.
xiv


13
Figure 1. Streaming potential cell; a) PMMA tube;
b) threaded PMMA plugs; c) platinum leads to
electrodes consisting of perforated disks with
attached platinum mesh; d) platinum electrode;
e) solution flow entrance; f) solution flow
exit; g) porous bed; h) O-ring.


Figure 17. Zeta potential vs. concentration of sodium
citrate and aluminum chloride.


TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT xi
CHAPTER
1 INTRODUCTION 1
2 STREAMING POTENTIAL AND NONCREEPING
FLOW IN POROUS BEDS 8
Introduction 8
Materials and Methods 11
Results and Discussion 16
Conclusions 24
3 CALCULATION OF SURFACE CHARGE DENSITY ERROR
DUE TO COMPLEX SPECIES FORMATION IN
SOLUTION 25
Introduction 25
Procedure 28
Results 35
Discussion 41
Conclusions 47
4 DETERMINATION OF ADSORPTION CHARACTERISTICS
FOR FINE PARTICLE SYSTEMS FROM ELECTRO
PHORESIS MEASUREMENTS 49
Introduction 49
Materials and Methods 50
Results and Discussion 51
Conclusions 64
IV


48
concentration of complex species as all of the quartz
itself. Hence the importance of careful consideration
of the solid phase under study cannot be over emphasized.
The thickness of the disturbed layer on quartz has
been calculated using data on the solubility of quartz
and thermodynamic data for solid silica species. The
calculations show that the layer cannot be quartz,
cristobalite or tridymite.


microcoul
36
pH
Surface charge density error (Act)
102, 10"3 M/L NaCl solution for
silica using Bolt's experimental
vs. pH in
vitreous
conditions.
Figure 7.


microcoul
39
pH
Figure 9. Surface charge density error (Aa) vs. pH in 10 ,
10"2, io-3 m/L NaCl solutions for hydrated
silica using Tadros's and Lyklema's experi
mental conditions.


131
between clays that were unaged or aged for 20 hours in
aluminum ion solutions at any pH value. If the SSM is
used, aging of the system is observed. Also, the zeta
potential sign is not reversed for any of the solutions.
In fact, Figure 34 shows that unaged, unequilibrated
1613Na and 1613Ca have negative values of zeta potential
at pH = 6. Also freshly prepared suspensions of 1613
(as-received clay) containing 10 ^ AlCl^^O at pH = 6
prepared by the DSM display a negative zeta potential
value. The only difference between this solution and the
solutions used to obtain previous results depicted in
Figure 32 at pH = 6 is that the stock clay suspension for
which the results in Figure 32 were obtained was equili
brated for at least a week before electrophoresis solu
tions were prepared. This prompted an investigation into
the effects of 1613 clay equilibration time in water on
the zeta potential before addition of aluminum ions.
These results are shown in Figure 36. First, zeta
potential is independent of equilibration time in water
when no aluminum ions are added. However, significant
changes occur when the aluminum ions are added after the
clay is equilibrated in water at pH = 6 for a certain
period of time. For 1613 equilibrated 18 hours in water
the zeta potential changed from -20 mV to +18 mV when


2
counter ion charge (the ions in solution which electrical
ly balance the charge on the solid surface) from the
layer of adsorbed ions (Stern layer). The magnitude of
the zeta potential is determined by the concentration of
ions in the Stern layer and/or the concentration of
ions in the diffuse layer. Zeta potentials can be
determined from streaming potentials using coarse
particles or from electrophoretic mobilities of colloids.
In the past, two practical problems of noncreeping
flow and electrode polarization have limited accurate
measurements of streaming potentials. In a streaming
potential experiment, solution is forced to flow through
a porous bed of coarse particles. A linear relationship
between the streaming potential, E, and solution flow
pressure, P, is predicted by the Smoluchowski equation
which is used to calculate zeta potentials as described
in Chapter 2. In practice a linear relationship is
usually observed. However, Chapter 2 also shows that
a nonlinear relationship exists between volumetric flow
rate and flow pressure. This suggests that noncreeping
flow exists in the pressure range used in many typical
streaming potential measurements. Since the Smoluchowski
equation assumes creeping flow, zeta potentials calculated
from streaming potential values obtained under noncreeping


15
Figure 2. Streaming potential solution flow system.


4
solid surface. However, if dissolution of the solid
occurs, some of the ions may not adsorb to the surface,
but may be part of complex ions in solution. However,
they are not free hydrogen or hydroxide ions. Their
absence cannot be detected by pH measurement. Therefore,
absolute values of surface charge densities calculated
by most investigators are too large. For silica, surface
charge densities are negative values since the surface
is negatively charged in water above pH = 3.0. Therefore,
Chapter 3 describes the method used for aqueous silica
systems to calculate the surface charge density error due
to use of mass balance expressions which are incomplete
when hydrogen or hydroxide ions which are part of complex
ions in solution due to dissolution of the solid are
neglected.
Accurate adsorption density values which are used
to calculate surface charge densities are easily
determined for oxide systems since hydrogen and hydroxide
ions are potential determining ions. However, more time
consuming and sometimes less accurate methods must be
used to determine adsorption densities of other ions on
the oxide surface. Chapter 4 presents a new method for
determining this information from electrophoresis data.
The unique aspect of this method is the use of various


KA = Equilibrium constant for formation of HSiO^ from
f^SiO^
KB = Equilibrium constant for formation of SiO^ from
l^SiO^
KC = Equilibrium constant for formation of Na2Si02 from
f^S iO^
K = Equilibrium concentration for the soluble species for
each solid phase considered
FC = Product of Faraday's constant (96,500 coul/mole) and
the number of microcoulombs per coulomb (10^)
GA = Activity coefficient of HSiO^
GB = Activity coefficient of SiO^
GH = Activity coefficient of H+
GNA = Activity coefficient of Na
KX = Equilibrium concentration of HSiO^ from the ioniza
tion of f^SiO^
KY = Equilibrium concentration of SiO^ from the ionization
of H^SiO^
KZ = Equilibrium concentration of Na2Si02 from the ioniza
tion of F^SiO^
t 1~1
N = "N" character of vector K
V = Volume of solution in liters
2
SA = Specific surface area in cm /g
W = Weight of powder in grams
142


33
The excess of hydrogen of hydroxide species which
is part complex species is directly related to the
concentration of each complex species in solution. From
Equations 8-10 it can be seen that for every HSiO^ ion
formed, the excess of hydrogen over hydroxide is -1.
Accordingly, for SiO^ the excess is -2 and for Na2Si03
it is -2. Therefore,
C = -QHSiO"] + 2 [SiO^] + 2[Na2Si03]) [11]
2. Development of the Working Equation for Aa
By writing the equilibrium constant expressions for
Equations 8-10 and substituting K[N] from Table II for
[l^SiOg], the equilibrium concentration of each complex
species can be written in terms of experimental
parameters. That is,
[HSi03]
[SiO]
KA K[N]
+ .
YHSi03(YH+)[H ]
KB K[N]
YSiO~ yH+ ^
[12]
[13]
[Na2Si03]
KC
K[N] Y[+
~2
[Na
+ .2
YH+
[hV
[14]
Now, C in Equation 7 can be expressed in terms of experi
mental parameters:


Figure
1
LIST OF FIGURES
Page
Streaming potential cell; a) PMMA tube;
b) threaded PMMA plugs; c) platinum leads
to electrodes consisting of perforated
disks with attached platinum mesh;
d) platinum electrode; e) solution flow
entrance; f) solution flow exit;
g) porous bed; h) O-ring 13
2 Streaming potential solution flow system 15
3 R-C circuitry used in streaming potential
experiments 17
4 Streaming potential vs. pressure using
the R-C measuring circuit 19
5 Streaming potential vs. pressure without
the R-C measuring circuit 21
6 Flow rate vs. pressure for fused silica 22
7 Surface charge density error (Aa) vs. pH in
10"1, 10"2) 103 M/L NaCl solution for
vitreous silica using Bolt's experimental
conditions 36
8 Surface charge density error (Aa) vs.
total surface area for amorphous silica
at pH = 7, 8, 9 and 10 37
9 Surface charge density error (Aa) vs. pH
in 10_1, 10-2, 10_3 M/L NaCl solutions
for hydrated silica using Tadros's and
Lyklema's experimental conditions 39
10 Surface charge density error (Aa) vs.
total surface area for precipitated
silica at pH = 7, 8, 9 and 10 40
vi 1


150
59. L. Heller and Z. H. Kalman, "An Approximate
Determination of the Position of Some Cations in
Dehydroxylated Montmorillonite," Clay Minerals
Bulletin 4, 213 (1961).
60. P. Cloos and R. D. Laura, "Adsorption of EDA on
Montmorillonite Saturated with Different Cations.
II. Hydrogen- and EDA-Montmorillonite Protonation and
Hydrogen Bonding," Clays and Clay Minerals 20, 259
(1972) .
61. R. D. Laura and P. Cloos, "Adsorption of EDA on
Montmorillonite Saturated with Different Cations.
III. Na-, K- and Li-Montmorillonite Ion Exchange,
Protonation, Coordination and Hydrogen Bonding,"
Clays and Clay Minerals 23, 61 (1975).
62. G. Ertem, "Irreversible Collapse of Montmorillonite,"
Clays and Clay Minerals 20, 199 (1972).
63. H. Kodama and M. Schnitzer, "Effects of Interlayer
Cations on the Adsorption of a Soil Humic Compound
by Montmorillonite," Soil Sci. 106, 73 (1968).
64. A. H. Weir, "Potassium Retention in Montmoril-
lonites," Clay Minerals 6, 17 (1963).
65. M. B. McBride, "Exchangeable Cation and Solvent
Effects Upon the Interlamellar Environment of
Smectites: Esr Spin Probe Studies," Clays and
Clay Minerals 25, 205 (1977).
66. I. Ravina and P. R. Low, "Change of b-Dimension
with Swelling of Montmorillonite," Clays and Clay
Minerals 25, 201 (1977).
67. T. M. Lai and M. M. Mortland, "Self Diffusion of
Exchangeable Cations in Bentonite," Clays and Clay
Minerals 9, 229 (1960).
68. H. Freundlich, 0. Schmidt, and G. Lindau, "Uber die
Thixotropic von Bentonite Suspensionen," Kolloid-
Beihefte 36, 43 (1932).
69. D. T. Oakes and E. J. Burcik, "Electrokinetic
Phenomena in Colloidal Clays," Clays and Clay
Minerals 4, 225 (1955).


58
From Equation 33, at 1% solids, C-C = 10pTAF.
O l_i
Therefore, the adsorption density, r, can be determined if
the specific surface area of the solid is known. For
montmorillonite, a generally accepted theoretical specific
2
surface area of 800 m /g was used in the calculations.
C
Table VI lists the values of C, r and log(p-) for each
zeta potential considered. A plot of C and r vs. C is
shown in Figure 14.
2. Application of Experimental Data to the Stern
Equation
Stern (26) has derived an adsorption isotherm
2
relating the surface charge/cm (a) to the Stern potential
(ip ) The Stern equation is
b
1 =
N^ve
1+:
Mn
exp (-
veip +tj>
[34]
TdT
-)
where is the surface charge/cmZ associated with the
adsorbed ionic layer, N-^ is the number of adsorption sites
2
per cm at surface, v is the valence of the ion adsorbed,
e is the charge of the electron, N is Avogadro's number,
M is the molecular weight of the solvent, n is the number
3
of ions per cm far from the surface, \ps is the potential
at the Stern plane, of the adsorbed ion, k is Boltzmann's constant and T is
o-
1 MC
temperature Since T = and n = qqq-
then


17
cell
Recorder
5
with 2x10 Q
resista nee
Figure 3.
R-C circuitry used in streaming potential
experiments.


CHAPTER 1
INTRODUCTION
The surface chemistry of montmorillonite, an
aluminosilicate clay mineral, and its two major con
stituents, silica and alumina, is investigated by
electrokinetic methods of streaming potential and micro
electrophoresis. Using these techniques, many authors
have studied the changing characteristics of the elec
trical double layer surrounding the particles or colloids
as a function of adsorption of ions from solution, ion
exchange, and/or aging of hydrated surfaces. However,
proper use of these two techniques can also yield
important information about the mechanisms of these
processes. Opportunities are provided for improving
these techniques which can then be used to develop new
methods for determining adsorption properties of oxide
surfaces.
The electrokinetic parameter used to describe the
nature of the electrical double layer is the zeta
potential. Essentially, the zeta potential is the
electrical potential at the Stern plane in solution. The
Stern plane separates the diffuse (Gouy-Chapman) layer of
1


CHAPTER 3
CALCULATION OF SURFACE CHARGE DENSITY ERROR DUE TO
COMPLEX SPECIES FORMATION IN SOLUTION
Introduction
Calculations of surface charge densities are made
from adsorption density values of potential determining
ions on oxide surfaces (13-16). Potentiometric titration
of the aqueous-oxide system with potential determining
ions are used to determine adsorption density values as
a function of pH. For aqueous-silica systems, different
negative values of surface charge density at any given
pH are found depending on the phase of silica used in the
titration. Using precipitated silica, Tadros and
Lyklema (17) found that absolute values of surface charge
densities were an order of magnitude higher than those of
Bolt (14), who studied amorphous silica. Tadros and
Lyklema suggest that a gel structure exists on the
precipitated silica surface and that extension of
surface and counter ion charge inside the pores of this
gel structure causes higher charge densities. However,
Yates and Healy (18) suggest that precipitated silica does
not have a gel structure in the surface but that it
contains an incompletely condensed layer of polysilicic
25



T
Figure 4. Streaming potential vs. pressure using the R-C
measuring circuit.


LIST OF FIGURES continued.
Figure Page
19 Zeta potential vs. concentration of
sodium citrate for fused silica in
supporting electrolytes of 10~3 M/L
NaCl and 10-3 M/L NaCl, 10-4 m/L
A1C13 6H2O 81
20 Streaming potential-pressure ratio vs.
stream time for aluminated fused silica
for untreated, heat treated, base
treated and heat treated and base
treated surfaces 83
21 Zeta potential vs. pH for y-alumina
after aging for 0, 1, 8, 16, 33 and
97 days in water 93
22 pH of the zero point of charge (pH^pc)
vs. aging time in water for T-61, A-17 ,
C-30 DB, y, and A-16 aluminas 94
23 Solution pH vs. weight of alumina added
for A-16 alumina 96
24 Zeta potential vs. pH for unwashed T-61
alumina (coarse particles) aged in water
for 1, 2 and 3 days 100
25 Zeta potential vs. pH for 1608 and 1613
montmorillonite 112
26 Zeta potential vs. pH for 1613Na and
1613Ca montmorillonite 113
27 Zeta potential vs. log concentration of
NaCl, CaCl2*2H20 and AlCl3*6H20 solutions
for 1608 montmorillonite 115
28 Zeta potential vs. log concentration of
NaCl, CaCl2*2H20 and AlCl3*6H20 solutions
for 1613 montmorillonite 116
xx


14
polymethylmethacrylate (PMMA) tube with ends tapped to
receive two threaded PMMA plugs. This design allows
easier cleaning between experiments and easier packing of
the porous bed material. The platinum leads to the
electrodes* protrude completely through the PMMA plugs
and are sealed by a press-fit using a Teflon spacer. The
platinum electrode consists of a perforated disk for
easy solution flow and an attached platinum mesh for
increased electrode surface area. Because the electrodes
are able to slide through the PMMA plug, adjustments can
be made to obtain a tight packing of the bed.
The solution flow system is shown schematically in
Figure 2. The solution reservoir is a 25 liter poly
ethylene container with a mechanical on-off flow valve.
The height of the outlet tube is varied to produce dif
ferent hydrostatic pressures across the cell. This
design allows for E versus P measurements at pressures as
low as 1.0 cm Hg. Also, flow rates can be measured at
the outlet tube exit.
The potentials from the electrodes were measured with
an electrometer.** The output from the electrometer was
*Englehard Industries, Carteret, New Jersey.
**Keithley, Model 602.


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
George Y. Onoda, JpTT1-Chairman
Associate Professor of
Materials Science and
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
/
Ji;
Larry L < Hen'cn
Profssor and Head of Ceramics
Division
Materials Science and
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
a
Robert W. Gould
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
U- Q ,qrU^. U
Dinesh 0. Shah
Professor of Chemical
Engineering


3
flow conditions may be invalid. Chapter 2 shows why
streaming potential values obtained under commonly
encountered noncreeping solution flow conditions can
still be used to calculate zeta potential values using
the Smoluchowski equation.
The second problem commonly experienced in streaming
potential measurements is the nonzero intercept of the
streaming potential-solution flow pressure relationship.
Theoretically, when the flow pressure is zero, the
streaming potential must be zero. However, a finite
rest potential is commonly observed at zero pressure.
This problem may be due to electrode polarization.
Therefore, Chapter 2 presents a method for measuring
streaming potentials without the undesirable effects of
electrode polarization.
Calculation of surface charge densities (a) of the
solid oxide surface from potentiometric titrations using
potential determining ions is used by many investigators
to determine the variations of a with pH. In a
potentiometric titration, the concentration of hydrogen
ions before and after addition of a known amount of
titrant is recorded. The quantity of hydrogen or
hydroxide ions which cannot be accounted for in solution
is assumed by most investigators to be adsorbed onto the


37
p
Total surface area (cm )
Figure 8. Surface charge density error (AT) vs. total
surface area for amorphous silica at pH = 7,
8, 9 and 10.


87
Materials and Methods
In this chapter, four commercial aluminas* were
investigated. Three of the powders, A-16, A-17, and T-61
are thoroughly described by Flock (54). A-16 alumina is
a reactive powder of uniform phase and high bulk purity.
I
There is present a certain amount of nonalpha phase
2
material. Typical surface areas range from 4.0-6.5 m /g.
The average particle is 0.6 microns. A-17 alumina is a
transition reactive powder in that it is a mixture of
reactive and nonreactive agglomerates. Typical surface
2
areas are 1.5-2.5 m /g. The particle size is around 1.5
microns. Nonalpha phases are absent in this powder.
T-61 is a tabular-nonreactive powder of uniform phase
and high bulk purity. Tabular aluminas are massive low
shrinkage forms which have been sintered without added
permanent binders (55). Like A-17, nonalpha phases are
absent in this material. A fourth powder designated as
C-30 DB is a hydrated alumina. This particular powder
was not discussed by Flock. This powder was investigated
in its as-received form and after heating to 500C for
24 hours in a Vycor crucible in air. This heat treatment
is known to cause formation of gamma phase alumina from
Bayer alumina trihydrate (56).
Aluminum Company of America, Houston, Texas.


44
0.030 liters, the final weight of powder present in the
system after dissolution was 0.3854g-0.0054g = 0.3800
grams, where 0.3854 is the initial weight of powder used
by van Lier et al. before dissolution.
To calculate the thickness of the disturbed layer,
two assumptions are needed. First, the particles are
assumed to be spheres. Secondly, the number of particles
in the system remains constant. The total initial volume
V. of particles in the system is related to the initial
particle radius r^ by
V. = N. 4/3irr3
i i i
[22]
where INF is the initial number of particles. The same
equation can be written relating the final total volume
and final particle radius r^ after dissolution. That
is ,
Vf = Nf 4/3rr3
[23]
where N is the final number of particles after dis
solution. Since
[24]
then
N.p 4/3rr3
i i
[25]
and


54
1000 cc/liter. Therefore
Cq = C + 10pL AF(W%) [30]
and the units of each term in Equation 30 are moles/liter.
If enough solid is present in the system, most of the
ions in solution will adsorb to the surface, and
C << 10p^AFW7o; therefore Equation 30 becomes
Co = 10pLAF(W7o) [31]
Taking the log of both sides of Equation 31 yields
log CQ = log(lOpAr) + log(W%) [32]
Table V lists the values of C for zeta potentials =
15, 10, 5, 0, -5, -10 mV.
A plot of concentration Cq as a function of weight
percent using data at zeta potential equal to zero mV
from Figure 11 is shown in Figure 12. For high solid
contents the experimental points approach a straight line
whose slope is 1.0. At low solids contents, the points
approach a line of zero slope. The Cq value corresponding
to this line is C. Rearrangement of Equation 30 and
taking logarithm yields
log(Co-C) = log(10pLAr) + log(W%) [33]
Therefore if Equation 32 correctly describes the system
under concentration, then plots of Cq-C versus W70 should
yield straight lines for each zeta potential. As
Figure 13 shows, straight lines are obtained.


41
a function of pH for any given surface area used can be
determined. The error becomes more significant as the
pH increases and surface area decreases. This should be
expected since soluble complex species form in greater
quantities at higher pH's due to increased ionization of
the neutral soluble silica species.
According to the calculations, based on the data
of Tadros and Lyklema, significant error should be
2
expected by using 2g of 40 m /g material in 100 cc of
solution. However, these authors claim no significant
difference in the amount of OH/g SO2 adsorbed when 2, 10
or 20 grams of solid were used in the same volume of
solution. To observe this, they must have used much
less volume than 100 cc, which was the solution
volume V used in the calculation in the present work.
However, no indication of solids concentration was given.
Discussion
These calculations demonstrate the importance of
knowing the amount of each phase present in an aqueous
silica system. In an experiment using the same total
surface area for all five phases less error in surface
charge density will occur for quartz than any other solid
silica phase. The order of least to most significant


53
contents, the equilibrium concentration, C, of aluminum
ions in solution is significantly lower due to adsorption
onto the clay particle surfaces.
A mass balance equation can be written which de
scribes the system at equilibrium as follows:
C = C + ^-(W ) [29]
where Cq is the apparent concentration of aluminum ions in
solution if no adsorption occurs, C is the concentration
of aluminum ions in solution after adsorption to the
particle surface, T is the adsorption density of aluminum
ions on the clay surface, VT is the solution volume, W
is the weight of solids in the system, and A is the
specific surface area of the clay, Wg/V^ can be replaced
by the weight percent by the following argument: weight
W
g
percent (W7o) of solid is defined as ^ x 100 where =
s WL
weight liquid and W = weight of solid. If W, >> W then
S s
w
g
W?0 = y x 100. Since where = density of
L
W
liquid, when W7o = x 100. Since p^ = 1.00/cc, the
L L
right side of Equation 29 must be multiplied by a factor


Concentration
56
Weight percent solids
Figure 12. Apparent aluminum ion concentration CQ vs.
weight percent montmorillonite for zeta
potential = 0 at pH = 4.0.


144
To generalize the program for any material, "K" in
line [4] should be made a variable so that the vector can
have the same length as the number of different solid
phases being considered. Also, the number of individual
physical constants such as equilibrium constants for
the formation of the ionic species, and the activity
coefficients of the ions will vary as the number of ionic
species formed varies. Finally, since the number of
characters in "K" will vary, "N<5" statement in line [38]
should read N the vector K. Therefore, since K is a variable, Z will
be a variable.


Zeta potential, (mV)
117


18
potential stored in the capacitor does not decay more than
17o in two seconds. The potential received by the electro
meter when flow is initiated is the sum of the rest
potential and the true streaming potential. However,
since the rest potential has already been stored in the
capacitor, the rest potential contribution of the input
voltage is nulled out and only the streaming potential is
recorded. Because of the arrangement of the resistors,
the input voltage to the recorder is 1/100 of the output
from the electrometer (which is one volt full scale).
Therefore, the recorder is kept on a 10 mV full scale
range.
Streaming can be terminated after around five seconds
of flow. Then the switch is placed in position 1. The
capacitor again charges rapidly to the rest potential and
a new measurement can be initiated.
Using the R-C circuit, streaming potential measure
ments were carried out on fused silica using water and
5 A 3
NaCl solutions of 10 10 and 10 mol/L as streaming
solutions and on bioglass using water as the streaming
solution. The E versus P curves obtained are shown in
Figure 4. Within experimental error, all curves are
observed to be linear and pass through the origin.
Similar findings have been observed for a variety of


A3
To make the thickness calculations, dissolution
reactions must be written. These are
Si02 + OH" = HSiOg
Si02 + 2OH"
SiOg + h2o
[18]
[19]
Using quartz as an example, the standard free energy of
reaction for Equation 18 is 1640 cal/mole and for Equa
tion 19 it is -1100 cal/mole assuming the data in
Table I are correct for quartz phase. Neglecting activity
coefficient, the equilibrium constant expression for each
equation is
K
18
,-AGs
exP (~rt~") =
[HSi03]
Si02J|OHJ
K
19
= exp (
-AG\ [H20][Si02]
[Si02][OH]2
RT
-) =
[20]
[21]
K^g is calculated to be 0.061 and K^g is 6.53. Since K^g
and K-^g are known, the concentrations of HSiOg and SiOg
can be calculated using pH = 12.30 from experimental
conditions of van Lier et al. These values are [HSiOg] =
1.2 x 10 3 M/L and [SiOg] = 2.6 x 10 3 M/L. The equi
valent concentrations of Si02 are 9.4 x 10"^ M/L and
_3
2.05 x 10 M/L. Therefore the total concentration of
quartz (SiOg) present in solution after dissolution is
2.99 x 10 3 M/L or 0.18 g/L. Since van Lier et al. used


Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
ELECTROKINETIC PROPERTIES OF SILICA, ALUMINA,
AND MONTMORILLONITE
By
John Milton Horn, Jr.
March 1978
Chairman: George Y. Onoda, Jr.
Major Department: Materials Science and Engineering
The surface chemistry of montmorillonite, vitreous
silica and alumina is investigated by electrokinetic
methods of streaming potential and microelectrophoresis.
Also, improved analytical techniques are developed to
obtain these electrokinetic data and adsorption
information for these oxide materials.
Many investigators find that linear streaming
potential-flow pressure relationships do not pass through
the zero potential-zero flow pressure origin. One source
of this error, electrode polarization, is eliminated
by introducing an R.C. circuit in the system. Simulta
neous study of flow pressure versus flow rate reveals a
nonlinear curve suggesting the existence of noncreeping
xi


zpc
Figure 22. pH of the zero point of charge (pHgpc) vs. aging
time in water for T-61, A-17, C-30 DB, y, and A-16
aluminas.


CHAPTER 4
DETERMINATION OF ADSORPTION CHARACTERISTICS FOR FINE
PARTICLE SYSTEMS FROM ELECTROPHORESIS MEASUREMENTS
Introduction
Different types of adsorption information can be
obtained for fine particle-aqueous systems depending on
the type of experimental method used. The method com
monly used to calculate adsorption densities of oxide
systems from potentiometric titration using potential
determining ions to determine the zero point of charge of
the surface was described in Chapter 3. To determine
adsorption densities accurately for ions other than
potential determining ions, other techniques such as
solution depletion (as measured by atomic adsorption or
emission) must be used. These measurements require
synthesis of calibration curves which essentially doubles
the amount of work involved in determining an entire
adsorption isotherm.
The unique feature in the method presented in this
chapter for determining adsorption isotherms from electro
phoresis measurements is the use of various solids
contents. By obtaining zeta potential behavior as a
function of apparent ion concentrations in solutions for
49


105
agreement. Ho and Handy (70) also found similar results
for lime treated bentonites. Oakes and Burcik (68) found
that calcium added as either the nitrate or chloride salt
produced similar results. This phenomenon has been
reported by other investigators for other materials such
as silica (71).
Packham (72) studied the coagulation of dispersed
montmorillonite with various hydrolyzing salts. The
amount of coagulant required to halve the turbidity was
a constant low value below pH = 7.5 for montmorillonite.
Other clays such as kaolinite and halloysite displayed
a minimum in this value between pH = 7-8. Electrokinetic
data for montmorillonite were not presented. However, data
for kaolinite which showed the change in electrophoretic
mobility with pH at constant aluminum ion concentration
display striking similarities to data reported in the
present work. This suggests that the solution chemistry
of aluminum (i.e. Al ion hydrolysis) controls the electro-
kinetic properties in this particular type of investi
gation .
Recently, Swartzen-Allen and Matijevic (73) have
studied the electrokinetic properties of sodium montmoril
lonite as a function of pH, aluminum ion concentration
and the effect of aluminum ion hydrolysis. They found


143
SAT = Total surface area
Y = Number of pH unit divisions (example: for pH 7-10,
Y = 31 for pH 7.0, 7.1, 7.2, etc. through 10.0)
PH = pH
X = Solution molarity (mols/liter)
NA-^-YpX is a vector of Na concentration "Y" characters in
length
H = H+ ion concentration in moles/liter
Q = Difference between the real surface charge density and
apparent surface charge density (areaxaapp)
QQ = Vector of Q's (the first QQ = 0 as defined in line 14
of the program)
QQl = First "Y" number of characters in vector QQ
QQ2 = Second "Y" number of characters in vector QQ
QQ3 = Third "Y" number of characters in vector QQ
QQ4 = Fourth "Y" number of characters in vector QQ
QQ5 = Fifth "Y" number of characters in vector QQ
CQQ1 = Product of QQl and FC
CQQ2 = Product of QQ2 and FC
CQQ3 = Product of QQ3 and FC
C0Q4 = Product of QQ4 and FC
CQQ5 = Product of QQ5 and FC
"Done" is printed when calculations are completed


102
surfaces.
phenomena:
This can be accomplished by observing
1) irregular aging of A-16 powder in
ZPC
two
water,
and 2) mismatch of pH
and pH equilibrium.


124
these figures is the zeta potential behavior vs. pH when
no aluminum is present. Table IX summarizes the chemical
conditions of zero zeta potential (ZZP) for these
clays.
These results indicate a remarkable change in zeta
potential behavior with change in the hydrolysis of the
aluminum ions. At pH < 3-4 the aluminum is in the form
+3
of the aluminio (Al ) ion. Matijevic and Stryker (80) and
Matijevic (81) have suggested that from pH = 4 to 7 the
aluminum is present as a complex AlgiOH)^ species. The
results presented in this work lend support to this view
since the existence of such a species would be expected
to increase the positive mobility of the clay particles
because of the higher charge on the ion.
As the pH increases from 8 to 10 two possible
mechanisms exist for sign reversal back to a negative
value in Figures 31 and 32. Most workers (82,83) suggest
that aluminum hydroxide precipitate coats the particles.
The measured zeta potential behavior would be dominated
by aluminum hydroxide rather than the uncoated particle.
A possible alternative is that aluminum hydroxide does not
coat the particles but returns into solution as either
AIO2 or A1(0H)^ as the pH increases from 8. The measured
zeta potential behavior would then be characteristic of


125
Table IX. Chemical Conditions for Zero Zeta
Potential (ZZP) for 1608 and 1613
MontmorilIonites
Clay
[aici3-6h2oJ m/l
PH
1608
1
O
i1
4.1
1608
l
o
11
8.5
1613
10~4
4.1
1613
10-4
8.5
1613
m
i
o
!1
5.6
1613
10 5
7.1


101
grinding creates a disturbed layer on fine T-61 powder.
This directly influences the hydration and thereby
influences its ZPC characteristics.
Since all the aluminas used in this investigation
have been ground, they all probably rehydrate to form
a gibbsite-type surface layer. This would explain why
all have ZPC's of around 9.5. A-16 and C-30 DB are
exceptions but their behavior has been explained by
control of the ZPC by adsorbed impurities on the basis
of the mismatch of their pH^p^ and pH equilibrium values.
Conclusions
The surface areas of the fine alumina samples
investigated were in the classified ranges stated by
Flock (54) .
Using interpretations suggested by O'Connor et al.
(46), ZPC behavior of alumina powders was a function of
the degree of hydration. It was shown that grinding of
T-61 produces markedly different ZPC behavior from
relatively unground materials suggesting that grinding
influences the hydration behavior of the powder.
It is suggested that ZPC determinations can be
used as a method of investigating the presence of small
amounts of adsorbed impurities on commercial powder


35
Results
Figure 7 shows the calculated values for Aa as a
function of pH for 10 ^ 10 10 ^ M/L NaCl solutions
for vitreous silica using Bolt's (14) experimental
conditions of surface area and solution volume. It can
be seen that as the ionic strength of the electrolyte
solution increases, the absolute value of Aa increases
for any given pH value. This trend is consistent with
the experimental surface charge densities measured by
Bolt.
Comparison of the calculated values with Bolt's
absolute values in Table I of his paper (14) for amorphous
silica shows that little error is incurred in neglecting
complex species formation in solution. This is not an
unexpected result since Bolt used a high surface area
to volume ratio in his experiments. Therefore, Bolt's
data represents a as well as a since Aa is
r real apparent
negligible for the pH range studied.
Using Bolt's data it is possible to calculate at
what value of total surface area significant error would
have occurred for any given pH value. Figure 8 shows
Aa as a function of total surface area for pH = 7, 8, 9,
5 2
10. For pH 9 and total surface area of 10 cm 10% error
7 2
would have occurred. Bolt used 5.4 x 10 cm total


71
of a -20+45 mesh fraction (833-350 micron) was boiled in
100 ml concentrated hydrochloric acid until no discolora
tion of the acid was observed. The sample was then rinsed
with conductivity water until all chloride ions were
removed.
All solutions used in these investigations were
prepared from Certified ACS reagent grade chemicals.
Water used to prepare the solutions was obtained from a
deionization system previously described in Chapter 2.
Methods
Electrokinetic theory can be applied to study
adsorption of electrolytes near the solid surface (12).
Theory predicts the existence of an electrical double
layer near the surface consisting of ions present in the
solution. The double layer contains an immobile (Stern
Layer) and mobile portion (Guoy-Chapman diffuse layer).
The electrical potential at the plane separating these
two layers is the zeta potential.
Usually zeta potential values are calculated from
the Smoluchowski equation (Equation 1, Chapter 2). As
shown in Chapter 2, to obtain zeta potential information
streaming potential experiments are performed. However,
modified streaming potential techniques can be used to


CHAPTER 7
ELECTROKINETIC PROPERTIES OF MONTMORILLONITE
Introduction
The electrokinetic properties of montmorillonite
clays are of interest because of their possible relation
to coagulation and dispersion. According to the DLVO
theory, coagulation may occur when the zeta potential is
small enough to allow London-van der Waal forces of
attraction to become effective.
The origin of charge on the face of a clay particle
is well known (57). It arises from a charge imbalance
in the clay lattice due to isomorphous substitution of
a positively charged ion for an ion of higher positive
charge. This results in a net negative charge in the
particle which is neutralized by counter ions such as
sodium and calcium. Their position in the clay structure,
their exchangeability for ions in solution, and their
hydration have been the subject of numerous articles (58-
67). All of these ions are not tightly held by the clay
particle since the negative charge on the particle is not
highly localized. Therefore, these ions can be exchanged
for one another or with hydrogen ions when the clays are
103


96
Weight of powder added, (grams)
Figure 23. Solution pH vs. weight of alumina added for
A-16 alumina.


110
ultrasonically dispersing one gram of clay in 1000 ml
water. However, no equilibration time was allowed
(Swartzen-Allen and Matijevic did not indicate whether
or not their suspensions were equilibrated).
c_. Electrophoresis solutions Single Solution
Method (SSM) and Double Solution Method (DSM)
To study the electrokinetic behavior of the various
montmorillonites as a function of pH in constant A1
concentration solutions, electrophoresis solutions were
prepared by two methods. The single solution method (SSM)
consisted of 10 ml (0.1%) clay slurry, 10 ml of lOx the
desired final A1 concentration and water for a final
volume of 100 ml. The pH was then adjusted by dropwise
addition of acid or base (0.1 or 0.01 M HC1, HNO^, NaOH).
The double solution method (DSM) consisted of preparing
separately 50 ml clay slurry (0.02%), prepared from stock
suspension, and 50 ml of twice the final desired aluminum
concentration at the proper pH. The two solutions were
then mixed and the pH remeasured. The pH values drifted
only slightly (0.2-0.3 pH units) in the pH range 6-9. At
the acid and basic extremes no drift was observed. The
solutions were then used in electrophoresis measurements.
The methods used for determining zeta potentials from
electrophoresis measurements have been described in
Chapter 6.


84
_3
then desorbed upon streaming the particles with 10 M
NaCl solution. At longer streaming times a slight
decrease in E/P occurred indicating removal of aluminum
ions from the silica surface.
'i
Conclusions
A method for studying the desorption behavior of
large particles has been presented. This method yielded
results which can be interpreted in terms of the hydration
state of the surface of the silica particles.
Aluminum ions reversibly adsorbed to surfaces con
taining adjacent silanol groups and they irreversibly
adsorbed to surfaces containing isolated silanol groups.
The mechanism of irreversible adsorption proposed for
aluminum ions was hydrogen bonding of the complex aluminum
species to the isolated silanol surface groups.
Contrary to Iler's results (39) aluminum ions adsorb
to fused silica forming positively charged rather than
negatively charged sites. Also, aluminum ion adsorption
was shown to activate the silica surface for specific
adsorption of citrate ions.


106
that aluminum ions at pH < 4 did not reverse the sign
of the electrophoretic mobility. However, they did not
present data for aluminum ion concentrations greater
than 10 ^ M/L. Also they concluded that aluminum ion
hydrolysis reduced the ability of this ion to decrease
the negative mobility of the clay particles.
The results of Swartzen-Allen and Matijevic (71)
contradict the results presented in this chapter and
apparently those of Freundlich et al. (68). Several
possible explanations exist. First, solution preparation
of the electrophoresis sample may have been different.
Dissimilar results may occur when these solutions are
prepared by a single solution method (SSM) or by a double
solution method (DSM). Also, equilibration time of
clays in water before electrophoresis solutions are
prepared may be important. Considerable evidence exists
which demonstrates hydration and aging effects in clays
on many different properties (58,59,74-77). This may
also affect the ability of the clays to adsorb aluminum
ions from solution. Therefore, this chapter considers
conditions under which aluminum ions may or may not
reverse the sign of the zeta potential of sodium and
calcium montmorillonite.


50
different solids concentrations, adsorption densities
and equilibrium solution concentration of the ions can be
calculated. The montmorillonite clay-aluminum ion solu
tion system is used to obtain these experimental data.
These data are then plotted on the basis of their
relationship in the Stern equation (24) to determine if
they fit an analytical form of this equation.
Materials and Methods
Montmorillonite 1613 was used as received from the
supplier.* A stock suspension of 270 solids was prepared
by ultrasonic dispersion of the clay in water and was
equilibrated for one week at ambient temperature. After
equilibration, 0.5, 0.05, 0.005 and 0.0025% suspensions
were prepared by diluting the stock suspensions with
water described in Chapter 2.
-1 -2
Aluminum chloride solutions of 2 x 10 2 x 10
2 x 10~3, 2 x 10"4, 2 x 105 and 2 x 10-6 M/L were
prepared from one molar stock solution. Fifty ml of each
solution was adjusted to pH = 4.0. This was mixed with
50 ml of clay suspension previously adjusted to pH = 4.0.
This mixture constituted the electrophoresis solution.
*Georgia Kaolin, Inc., Elizabeth, N.J.


This dissertation was submitted to the Graduate Faculty of
the College of Engineering and to the Graduate Council,
and was accepted as partial fulfillment of the require
ments for the degree of Doctor of Philosophy.
March 1978
Dean, College of Engineering
Dean, Graduate School


47
have a quartz, cristobalite or tridymite structure since
the values of the thickness calculated are lower than
O
300 A. That is, the solution becomes saturated with
respect to quartz, cristobalite or tridymite when 70, 119
O
or 169 A of thickness are dissolved away. However, the
disturbed layer could have either an amorphous or hydrated
structure since saturation cannot occur in a system
containing silica particles with a disturbed layer thick
ness of 300 A.
Conclusions
It has been shown that care must be taken in
adsorption density measurements using potentiometric
titrations to use high surface area powder to solution
volume ratio systems if one is to neglect complex species
formation. Guidelines for such experimental conditions
have been presented by using experimental data presently
available in the literature for the basis of the calcula
tions. It is important to note that more experimental
error will be incurred by not considering complex species
formation for less stable silica phases especially at
higher pH values where the ionization of the neutral
soluble silica species occurs in greater quantity.
These calculations have also shown that as little
as 0.017, hydrated phase present in quartz forms equivalent


76
Results and Discussion
Figure 18 shows that major differences exist in the
desorption behavior of aluminum ions from silica after
various surface treatments. It can be seen that most of
the aluminum ions desorb from the untreated silica.
However, some aluminum ions remain on the surface even
after 60 minutes of streaming as shown by the fact that
the E/P value does not reach the E/P value for non-
_3
aluminated silica streamed only with 10 M NaCl solution.
Chemical treatment of the silica particles in one molar
NaOH solution caused most of the aluminum ions to desorb
after approximately 80 minutes of streaming.
For heat treated silica desorption of aluminum ions
was much less than for untreated or chemically treated
silica. Only a small amount of aluminum ions appeared to
reversibly absorb to the surface.
Chemical treatment of the heat treated samples with
one molar NaOH solution reestablished the reversible
adsorption capacity of the silica. It can be seen that
almost complete desorption of aluminum ions occurred since
the E/P value approaches the value for nonaluminated
_3
silica streamed in 10 M NaCl solutions.
It should be noted that data for nonaluminated
_3
silica samples streamed in 10 M NaCl solution were in
dependent of the various surface treatments.


BIOGRAPHICAL SKETCH
John Milton Horn, Jr., was born October 23, 1951, in
Jacksonville, Florida. He received his early childhood
as well as high school education in Jacksonville Beach,
Florida. After graduating from high school, he entered
Stetson University in Deland, Florida, where he earned
a Bachelor of Science degree in Biology. While
matriculating at Stetson, he played intercollegiate
soccer and played violin in the Stetson University
Symphony Orchestra. He met Barbara Helen Furr, of
Fort Pierce, Florida, in September 1970, and they were
married June 15, 1974. He received his Master of Science
degree in August, 1976, from the Department of Materials
Science and Engineering at the University of Florida,
and has been pursuing a Doctor of Philosophy degree in
the same department since that time. He is a member of
Sigma Nu social fraternity. Professional societies
include American Institute of Mining, Metallurgical and
Petroleum Engineers (AIME) and the American Ceramic
Society. Honorary memberships include Keramos and
Alpha Sigma Mu.
153


95
with time. However, for A-16 the ZPC decreased with time.
The degree of aging is most striking for C-30 DB which has
a ZPC of pH = 6.5 at time zero and a value of 9.8 after
30 days. This alumina probably contains several dif
ferent phases (as indicated by the X-ray diffraction
pattern) which can hydrate upon exposure to water. Gamma
alumina has a ZPC value of 8.9 at time zero and a value
of 10.1 after 30 days. Not much aging was observed for
T-61. Within 2 days a ZPC of 9.6 was achieved. The same
is true of A-17. The most peculiar behavior was shown by
A-16. Most workers find that during aging, the ph^pQ
increases. However, most workers acid-wash their powders
to remove surface impurities. Since it was desirable to
study as-received powders in the present work, no acid
washing prior to ZPC experiments was performed for these
particular powders. It appears that A-16 alumina contains
considerable amount of adsorbed impurity. As indicated
earlier, not much impurity is needed to shift the ZPC
by several pH units (45). However, this data alone is
not conclusive. Figure 23 shows that the equilibrium pH
for A-16 is equal to 8.2. This is significantly different
from the pH^p^. The values of equilibrium pH for the
other powders were 9.5, 9.2, 7.4 and 9.4 for gamma, T-61,
C-30 DB and A-17, respectively. Except for C-30 DB, these
equilibrium pH values are close to pH ZPC.


Zeta potential, (mV)
122
Figure 31. Zeta potential vs. pH for 1608 montmoril-
lonite in 0, 10~5 and 10-,+ M/L AlC^'f^O
solutions.


34
AT =
V(_
A''
KA K[N]
Y
T+
+
2KB K[N]
+ 7?
HSi
io: yh+ ^ yso0 yh+ ^
2KC K[N] [Na+]2
+ 2"' r+'2 }
Yh+ [H ]
[15]
Adsorption densities can be converted to surface charge
densities by the relation
a = 106Fr [16]
where F is Faraday's constant (9.65 x 10^ coul/mole) and
10 is the number of microcoulombs per coulomb. There
fore ,
Ao = 106 F J(C) [17]
where C is the term in parentheses in Equation
-3 -2 -1
Calculations of Aa were made for 10 10 and 10
M/L NaCl solutions for the five different silica species
for a pH range of 7-10 in increments of 0.1 pH units.
Since the calculations are numerous, a computer program
was written and is appended. The program is written
specifically for the silica system. However, as the
appendix shows, the program can be generalized for any
material as long as the free energy of formation data for
the various reacting species are known.


30
H2(aq) Na+(aq)> H+(aq) and 0H~(aq) (1920>- Etlua-
tions are written describing the formation of the neutral
soluble silica species (I^SiO^) Since five different
silica phases are considered, five equations yielding
five equilibrium constants (K[N] where N = 1-5) are
obtained. The reactions and values for their equilibrium
constants are shown in Table II.
The reactions for the formation of the various ions
from H2SiC>3 species are shown in Equations 8-10. Their
equilibrium constants are calculated from data in Table I.
H2Si03 = HSiO + H+ KA = 1.01 x 10~10 [8]
H2Si03 = Si03 + 2H+ KB = 9.92 x 1023 [9]
2Na+ + H2Si03 Na2Si03 + 2H+, KC = 2.81 x 1027 [10]
Activity coefficients for the various ions were
incorporated into the final calculations of surface
charge density error. The values for these coefficients
were obtained from calculations by Klotz (21) using the
Debye-Hueckel theory for strong electrolytes. Table III
shows the values for ions used in the calculations at
several ionic strengths. The values for HSi03 and Si03
were not directly available from Klotz (21), but were
chosen for ions of similar size as HSi03 and SiC>3.


Flow rate, (cc per sec.
Figure 6. Flow rate vs. pressure for fused silica.


120
The zeta potential at high NaCl concentrations for
the calcium clays approaches the zeta potential value of
sodium clay in water. Bar-On et al. (79) have shown that
electrophoretic mobilities of sodium montmorillonite are
higher than calcium clays. In fact, their values of
mobility corresponding to a clay with 0% exchangeable
sodium are precisely the values for 1613Ca and the value
for a clay with 1007, exchangeable sodium is the same for
1613Na clay. These findings suggest that double layer
characteristics are controlled by ion exchange phenomena.
_J
To test the hypothesis that ion exchange of Ca in
solution for Na~*~ or in the clay controls the electro-
kinetic behavior, a solution depletion experiment was
_ _j |_
performed. A solution of 10 M/L Ca was analyzed
before and after containing 0.01 weight percent 1613Na
montmorillonite for one hour. Afterwards, the clay
suspension was filtered through a millipore syringe
employing 0.22 micron pore size filter paper. The
_j |_
filtrate was analyzed for Ca in an atomic emission
spectrophotometer.* A standard curve was prepared by
_j
analyzing 0-10 ppm standard solutions of Ca Analysis
_|
of the filtrate revealed exactly half as much Ca than a
*Heath Model EU 703.


152
81. E. Matijevic, "Principles and Applications of Water
Chemistry"(S. D. Faust and J. V. Hunter, eds.),
Wiley, New York, 238 (1967).
82. E. Matijevic, F. S. Mangravite, Jr., and
E. A. Cassell, "Stability of Colloidal Silica:
IV. The Silica Alumina System," J. Colloid Interface
Sci. 35, 560 (1971).
83. R. 0. James and T. W. Healy, "Adsorption of
Hydrolyzable Metal Ions at the Oxide-Water Interface:
II. Charge Reversal of Si02 and Ti02 Colloids by
Adsorbed Co(II), La(III) and Th(IV) as Model
Systems," J. Colloid Interface Sci. 40, 53 (1972).
84. A. C. Mathers, S. B. Weed, and N. T. Coleman, "Effect
of Acid and Heat Treatment on Montmorillonites,"
Clays and Clay Minerals 3, 403 (1954).


Zeta potential,(mV)
Figure 19. Zeta potential vs. concentration of sodium
citrate for fused silica in supporting electro
lytes of IO-3 M/L NaCl and 10~3 M/L NaCl,
10-4 M/L A1C136H2O.


cm
Figure 20. Streaming potential-pressure ratio vs. stream
time for aluminated fused silica for untreated,
heat treated, base treated and heat treated and
base treated surfaces.


CHAPTER 2
STREAMING POTENTIAL AND NONCREEPING FLOW IN POROUS BEDS
Introduction
The measurement of streaming potential on porous beds
is an important method for determining zeta potential (3).
The method is convenient because many materials cannot
readily be shaped as capillary tubes. A common practice
for porous beds is to use the Smoluchowski equation (4)
4Trr)XE
eP
[1]
to calculate the zeta potential (c) from the measured
streaming potential (E), the viscosity (q), the specific
conductivity (A), the dielectric constant (e), and the
driving pressure (P).
Equation 1 was originally derived for simple
capillaries assuming laminar flow conditions. Boumans (5)
has shown that the E/P ratio in simple capillaries is
smaller under turbulent flow than under laminar flow.
A change in flow behavior with increasing pressure is
also known to occur in porous beds. The relationship
between flow rate and pressure changes from linear to
nonlinear at a certain Reynolds number. Many streaming
potential measurements on porous beds, in the past, have
8


27
part of complex species in solution due to dissolution
of the solid. Essentially Equation 5 says that those
hydrogen or hydroxide ions after titrant addition which
cannot be accounted for as free ions in solution or in
solution as part of complex species must be adsorbed to
the solid surface. Only relative values of F can
be determined if titrations are performed in one elec
trolyte concentration since the quantity (H-OH) . .
is unknown. However, by performing titrations at various
electrolyte concentrations, several adsorption density
versus pH curves can be obtained which intersect at one
point. In the absence of specific adsorption of counter
ions to the oxide surface, T is zero at this point.
Therefore, from Equation 5, (H-OH). can be
M v 'initial
determined and absolute values of r can be calculated.
real
Many workers choose to neglect the C term in Equa
tion 5 since they assume that the concentration of complex
species which form in solution is negligible compared to
the concentration of free hydrogen or hydroxide ions in
solution. However, this may not always be true especially
under conditions of a thermodynamically unstable solid
phase in solutions of high pH. If "C" is neglected, then
r
+AH-(H-OH)
it-- ]v
solution-1
apparent
A


Zeta potential, (mV)
60
Concentration c, (m/|)
Figure 14.
Zeta potential and log adsorption density. (T)
vs. equilibrium concentration (C) of aluminum
in solution for montmorillonite at pH = 4.0.
Log adsorption density


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99
y-A^O^ followed by recrystallization to a-A^O- on
the surface. This surface can rehydrate only to a surface
resembling A10-0H which is slightly acidic as shown
previously. In water, the zeta potential would be
slightly negative indicating a pH^p^, < pH ^0.
The results in the present work indicate that the
zeta potential in water at pH = 7 for A-17, T-61,
y-A^O^ is always positive since the ZPC is never lower
than pH = 8.0. For A-17 and y-A^O^ this result is
expected. However, for T-61, one might expect the ZPC
to be significantly lower, e.g. pH = 6-7 since this
alumina has been sintered at temperatures >1000C during
its fabrication process. This may be the case for un
ground T-61 as shown in Figure 24. These data were
obtained by the streaming potential method for course
particles described in Chapter 2. The amount of grinding
required to obtain particle sizes of 300-800 microns
(-20+45 mesh) is considerably less than is required for
obtaining particles less than 40 microns. Therefore
grinding effects should not be significant in this case.
The lowered ZPC is not caused by impurity since an acid
washed sample produced similar results. It can be seen
that the ZPC occurs at pH = 7.0 and does not change during
three days exposure to water which is direct evidence that


Ill
The pH measurements for all solutions were performed
on a pH meter* employing a glass combination electrode.
Results and Discussion
Figure 25 shows how the zeta potential varies with
solution pH for 1608 and 1613. It can be seen that both
clays have negative zeta potentials throughout the pH
range studied. This behavior differs from nonclay oxides
such as silica and alumina which display a zero point of
charge (ZPC) when similar measurements are performed.
This result shows that H+ and OH are not potential
determining ions for 1608 and 1613 montmorillonite clays.
Figure 25 also shows that 1608 has larger negative
zeta potentials than 1613 for pH 3-11. This suggests
that sodium montmorillonite has a larger double layer
than calcium montmorillonite. However the data are
not conclusive, since 1608 and 1613 differ in their gross
chemical composition. Therefore, zeta potential as a
function of pH was studied for 1613Na and 1613Ca.
Figure 26 shows conclusively that based on electro-
kinetic information, sodium montmorillonite has a more
negative zeta potential (larger double layer) than calcium
montmorillonite.
*Brinkman Model 512.


Table VIII. Compositions of Montmorillonite Clays 1608
and 1613 in Weight Percent
Clay
SiC>2
Ai23
Fe23
Ti02
MgO
CaO
k2o
Na20
1608
51.2
24.8
3.39
0.19
3.48
0.67
0.20
5.40
1613
60.8
21.5
0.9
0.21
3.62
2.21
0.04
1.30
Source: Georgia Kaolin, Inc., Elizabeth, N.J.


24
Conclusions
In the porous beds that were studied, noncreeping
flow did not appreciably affect streaming potential-flow
pressure relationships at the pressures studied. This
suggests that streaming potential experiments in the
past which have been performed under similar noncreeping
flow conditions would have had the same E/P ratios as
would have been measured under creeping flow conditions.
For those cases, the calculated zeta potentials would have
been as valid under noncreeping flow as under creeping
flow conditions.
Electrode polarization effects were nulled out by
using a R-C circuit which allowed only the true streaming
potential to be recorded. This method greatly facili
tated the measurement of streaming potentials of reactive
materials such as bioglass.
Combined results of the electrode polarization and
noncreeping flow studies showed that electrode polari
zation and not noncreeping flow was the reason for E
versus P curves not passing through the origin as found
by previous investigators. This was demonstrated by the
fact that E versus P curves were linear and passed through
the origin under conditions of noncreeping flow as long
as electrode polarization effects were accounted for,
vis., by using the described R-C circuitry.


90
the manufacturer (25). The average mobility of 10-20
particles was used to obtain the zeta potential calculated
from the Helmholtz-Smoluchowski equation.
The equilibrium pH values were determined by placing
a known amount of powder into a polypropylene bottle
containing 100 ml 1^0. The pH was allowed to equilibrate
and was recorded. This was continued until addition of
powder caused no further pH change.
All pH measurements* were performed using a glass
combination electrode. When required, solution pH's
were adjusted by dropwise addition of 0.1 or 0.0 M/L
standard solutions of HC1 or NaOH. Water used to prepare
the alumina suspensions and acid and base solutions is
described in Chapter 2.
Results
1. Surface Area
Table VII lists the values of surface area measured
in the present work. Also shown are the ranges cor
responding to the powders described by Flock (54).
Alumina A-16 falls within the range whereas A-17 and T-61
fall outside the range of Flock's values. For A-17, more
*Brinkman Model 512 pH meter.


149
48.
49.
50.
51.
52.
53.
54.
55 .
56.
57.
58.
P. G. Johansen and A. S. Buchanan, "An
the Microelectrophoresis Method to the
Surface Properties of Soluble Oxides,"
Chem. 10, 398 (1957).
Application of
Study of the
Aust. J.
P. G. Johansen and A. S. Buchanan, "An Electrokinetic
Study by the Streaming Potential Method of Ion
Exchange at Oxide Mineral Surfaces," Aust. J. Chem.
10, 392 (1957).
J. Schuylenborgh and A. M. H. Sanger, "Electrokinetic
Behavior of Fe and A1 Hydroxides and Oxides,"
Rev. Tran. Chem. 68, 999 (1949).
J. Schuylenborgh, "The Electrokinetic Behavior of
Freshly Ground a- and y-Al(0H)3," Rec. Trav. Chim.
70, 985 (1951).
J. Schuylenborgh, P. L. Arens, and J. G. J. Kok,
"Electrokinetic Behavior of Freshly Prepared y- and
a-FeOOH," Rec. Trav. Chim. 67, 1557 (1950).
J. Schuylenborgh, "The Electrokinetic Behavior of
the Sesquioxide Hydrates and its Bearing on the
Genesis of Clay Minerals," Trans. Int. Congr. Soil
Sci. 4th Congr. Amsterdam I 89-92 ; IV 63-4 (1950) .
W. A. Flock, "Bayer Processed Aluminas," in "Ceramic
Processing Before Firing"(G. Y. Onoda, Jr., and
L. L. Hench, eds.), John Wiley and Sons, New York
(1978).
W. H. Gitzen, "Alumina as a Ceramic Material,"
American Ceramic Society Publication, 20 (1970).
H. C. Stumpf, A. S. Russell, J. W. Newsome, and
C. M. Tucker, "Thermal Transformations of Aluminas
and Alumina Hydrates," Ind. Eng. Chem. 42, 1398
(1950).
R. E. Grimm, "Clay Mineralogy,""Ion Exchange and
Sorption," McGraw-Hill Book Co., New York, 183-
195 (1968).
N. Lahav and E. Bresler, "Exchangeable Cation-
Structural Parameter Relationships in Montmoril-
lonites," Clays and Clay Minerals 21, 249 (1973).


LIST OF TABLES
Table Page
I Standard Free Energy of Formation Values
at 298K (Kcal/mole) 29
II Reactions Depicting Formation of the
Neutral Soluble Silicate Species,
H2SO3, and Their Equilibrium
Constants (K) 31
III Activity Coefficients for Ionic Species
at Various Solution Concentrations 32
IV Calculated Values for Disturbed Layer
Thickness (t) and Soluble SO2 at
Equilibrium Using Data from van Lier
eta 1. (ref. 22) 46
V Values of CG for Various Zeta Potentials
and Weight Percent Solids 55
VI Values of C, T and log (C/I) for Various
Zeta Potential Values 59
VII Surface Areas of A-16, A-17, T-61, C-30 DB
and Gamma Alumina Powders 91
VIII Compositions of Montmorillonite Clays 1608
and 1613 in Weight Percent 108
IXChemical Conditions for Zero Zeta
Potential (ZZP) for 1608 and 1613
Montmorillonites 125
vi


5
solids concentrations in gathering data of zeta potential
versus apparent ion concentration in solution. From
these data, adsorption densities and equilibrium
concentrations of ions in solution after adsorption can
be determined. The Stern equation is then used to
determine if these data fit the expected form for common
adsorption isotherms.
The techniques discussed in Chapters 2 and 4 are
used to study the electrokinetic properties of an
aluminosilicate system. The system chosen was montmoril-
lonite clay, which is a major component of phosphate
slimes. The slow settling of phosphate slimes due to
fine particle size clays such as montmorillonite is a
major problem in the phosphate mining industry. Dense
coagulation of clay particles would result in faster
slime settling rates and higher final sediment densities.
The degree of coagulation of the particles is partially
controlled by their electrical double layer properties.
According to DLVO theory (1), coagulation will result
when the electrical repulsive forces between particles
are small enough to allow van der Waal's forces of
attraction to cause particle coalescence. Since zeta
potentials are a measure of the degree of these electrical
forces, Chapter 7 is devoted to finding chemical


20
materials and solutions without deviations in linearity
or intersection with the origin.
Without the R-C circuit, under otherwise identical
conditions, straight lines passing through the origin
are not obtained in the E versus P curves as shown in
Figure 5. This can be attributed to the fact that rest
potential contributions were not taken into account.
2. Creeping Versus Noncreeping Flow Conditions
During the E-P measurement described above, solution
flow rates were also determined. Volumetric flow rates,
Q, versus pressure are given in Figure 6. A nonlinear
relationship between Q and P was observed.
In Equation 3, v can be converted to Q, since
2
Q = vfTr where r is the radius of the bed. The Ergun
function becomes
AP = ^. Q +
[4]
where
and
P
Since e = 0.36, 0.04 cm, L = 4.1 cm, p = 1 g/cm^,


138
A necessary coagulation condition is the reduction of
electrical repulsive forces between particles. Further
studies involving aging these clays at the chemical
conditions of zero zeta potential will be necessary to
determine if these conditions are sufficient for dense
coagulation of primary clay particles.


Auger peak height ratio ,
pH
Figure 33.
Auger peak height ratio vs. pH for vitreous
silica after one hour exposure to 10-f M/L
A1C3*6H20 solution.
128


69
since surface hydroxyls act as primary adsorption sites
for polar molecules (37). Conventional methods used to
study these phenomena require the use of finely divided
materials. In this chapter, a technique is introduced
which allows investigations of silica material which has
a size range of 300-800 microns. The results obtained
using this technique are interpreted in the same manner
as those obtained when fine material is studied.
Adsorption of aluminum ions from solutions by silica
surfaces is of particular interest to glass corrosion
studies. It has been shown that aluminum ions decrease
corrosion of silica when they are present either in the
glass itself (38) or in the corrosion medium in contact
with the glass (39) Weyl (40) proposed that a require
ment for corrosion inhibition for the latter case is that
aluminum ions adsorb to the surface and not form a more
soluble compound than the glass itself. Lyon (41) found
that rinsing of a container glass with aluminum ion
solutions and subsequently rinsing with water did not
inhibit alkali extraction from the glass which suggests
that aluminum ion adsorption may not always be permanent.
Aluminum ion adsorption to a glass surface may
theoretically affect either ion exchange or network
breakdown. In his glass solubility work, Her


Zeta potential, (mV)
Figure 30. Zeta potential vs. log concentration of NaCl,
CaCl2,2H?0) and AlClg-HpO for 1613Ca montmorillonite.
118


APPENDIX A
COMPUTER PROGRAM FOR CALCULATING THE SURFACE CHARGE
DENSITY ERROR VALUE IN CHAPTER 3


126
the clay particles rather than aluminum hydroxide.
Figures 31 and 32 alone suggest that the second alterna
tive may be correct since all the curves converge at
higher pH values. Also, the value of the zeta potential
at this point is the same value for sodium clay when no
aluminum ions were introduced. This convergence would not
be expected if aluminum hydroxide was coating the
particles since different initial concentrations of
aluminum would result in varying degrees of coating.
However these results are only indirect support of the
noncoating hypothesis since they do not show any chemical
evidence of aluminum hydroxide not coating the surfaces
of the particles at higher pH values.
The coating hypothesis has arisen from work on
materials other than clay such as silica and titania (83).
To exclude this hypothesis the following experiment was
performed. Fused silica disks (1 x 1 x 0.5 cm) were
cleaned by boiling in a 0.1 M HCl solution. The disks
were then rinsed with conductivity water and placed in
-4
10 M/L AlCl^'^O solutions whose pH values were
adjusted with HCl or NaOH. After one hour the disks
were removed from solution. Excess solution was removed
by placing the edge of the sample against adsorbent paper.
The samples were then mounted on a specimen holder


Figure 36. Zeta potential of 1613 montmorillonite in
10-4 M/L AlCl3*6H20 and water at pH = 6 vs.
clay equilibration time at pH = 4, 6 and 8.
132


16
passed through an R-C circuit (details of which are dis
cussed in the Results and Discussion section) into a strip
chart recorder.* E versus P curves were generated by
plotting the peak height value on the recorder cor
responding to the applied pressure.
Results and Discussion
1. Circuitry for Measuring Streaming Potential
A R-C circuit shown in Figure 3 was introduced into
the system for the purpose of directly measuring only the
true streaming potential on the recorder within a 997, ac
curacy by nulling out all background potentials. Two
different circuits can be employed by changing the switch.
When the liquid is not streaming, the switch is in
position 1. The rest potential is rapidly stored in the
capacitor, nulling the signal to the recorder. The time
constant of this circuit is 2.2 seconds and so more than
997, of the nulling occurs in 10 seconds.
After charging the capacitor with the rest potential,
the switch is turned to position 2 and flow through the
cell is initiated within two seconds. The time constant
for the new circuit is 220 seconds. Therefore, the rest
^Hewlett Packard Model 680.


135
Figure 37. Solution pH vs. total weight of montmorillonite
added to 25 ml of water.