Water and ion flow through imperfect osmotic membranes

MISSING IMAGE

Material Information

Title:
Water and ion flow through imperfect osmotic membranes
Physical Description:
viii, 96, 1 l. : ill. ; 28 cm.
Language:
English
Creator:
Breton, Ernest Joseph, 1924-
Publisher:
s.n.
Place of Publication:
Gainesville
Publication Date:

Subjects

Subjects / Keywords:
Osmosis   ( lcsh )
Saline water conversion   ( lcsh )
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis - University of Florida.
Bibliography:
Bibliography: l. 91-94.
General Note:
Manuscript copy.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 000423978
notis - ACH2383
oclc - 11054044
System ID:
AA00003986:00001

Full Text










WATER AND ION FLOW THROUGH

IMPERFECT OSMOTIC MEMBRANES











By
ERNEST JOSEPH BRETON, 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
January, 1957














ACKNOWLEDGE NTS


The work reported in this dissertation was made

possible by the support of others. Financial support for

the study was afforded by the Office of Saline Water of

the United States Department of the Interior. This Office

has been very cooperative in every aspect of our association.

The congenial relationship which exists between

graduate students and the Chemistry Department faculty

creates a very favorable atmosphere for graduate research.

Within the department many individuals have been exceedinrly

helpful. B. J. Otte, Morris Mixson, and J. H. Mans have

gone out of their way to furnish necessary supplies. Calvin

Workinger has been of inestimable help in building the ap-

paratus that was required for this work. He displayed a

great degree of resourcefulness and ingenuity in making

the apparatuses for measuring semipermeability and resistance.

Association with my faculty adviser, C. E. Reid, has

been very stimulating and harmonious. I am especially rrate-

ful for the very wide degree of research freedom that he has

given me. He has proved to be an authoritative source of

information and help in the areas of chemistry, chemical

physics, and mathematics. In three years of close










association, I never did perceive the limit to his knowledge

in these fields. He has also stimulated me to broaden my

interest in the arts--especially music.

Much of the analysis work involved in obtaining the

data for Tables 1 and 2 was done by Hugh Wise.

Several very helpful suggestions in the des.in of

the semipermeability apparatus were made by G. R. Kulkarni.

Mary Joy Breton has been a tremendous help in pre-

parlng this dissertation. In addition to typing several

drafts and the final copy, she assisted in editing the

text.


iii
















TABLE OF CONTENTS


Pare


A"KY.' I'.LFDGMENTS . .

LIST OF TABLES . .

LIST OF FIGURES. . .

. '"'- C'rUCTION . .


* .

* S 4

* *


. vi

. vii

. 1


Chapter


I DISCOVERY . .


Review of the Literature.
Semipermeability Tests. .


a 4 4 *
* e
eeeeeee


II T!TORY FOR THE SEMIPERMTEAILITY OF C"LLULOSE
ACETATE .. . .

Review of Existing Theories .
Fronosed Mechenism .
Literature Support for Theory .

III E(rP EIrflNAL SUPPORT OF THEORY BASED UPON
:..tiIl!P:. ABILITY DETERMINATIOiNS .

Effect of Pressure on Semipermeability. .
Effect of Crystallinity on Semipermeability

IV ELECTRICAL MEASURK' iE,.; S TO SUPPOhT THEORY .

Dielectric Measurements of Cellulose
Acetate. . .
Electrical Resistance Across Cellulose
Membranes .

V MECHANISM FOR LOSS OF rE.Bf'P.:'. SEMIPEfI1 -
ABILITY . .

Loss of Semipermeability. .
Reactions Between Solute and membrane .
Mechanism of Membrane Failure .
Structural Chnnres. .


. . ii










TABLE OF COIJTENTS (Continued)


Chapter Page

VI RECOMMENDATIONS FOR AN IMPROVED inlEinRA1:E 75

VII SU3I A RY .. . 78

APPENDIX . . 81

Analytical Procedures. ... 82
Preparation of Films . 82
Determination of Dielectric Constants. ,. 84
Determination of Membrane Resistances. .. 86

BIBLIOGRAPHY .. . 91

BIOGRAPHICAL ITEMS . . 95













LIST OF TAPL:S


Table Page

1. Semipermeability of Various Membranes to
Aqueous Sodium Chloride Solutions 9

2. Semipermeability of Cellulose Acetate to
Aqueous Solutions of Various Electrolytes .. 11

3. Effect of Semipermeability on Birefringence. 72












LIST OF FIGURES


Figure Page

1. Apparatus for determining the semipermeability
of imperfect osmotic membranes. . 7

2. Effect of pressure on the semipermeability of
40% cellulose acetate membranes . 26

3. Effect of acetyl content on salt rejection 28

4. Effect of acetyl content on flow rate. 28

5. Effect of pressure on the dielectric constant
of cellulose acetate (du Pont CA-43). 31

6. Resistance versus pressure across 40% cellulose
acetate membrane. . 34

7. Resistance versus pressure for sodium chloride
across cellulose acetate membranes (du Pont
CA-h3) of known salt rejection. . 36

8. Comparison of semipermeability and dielectric
constant of cellulose acetate membranes 38

Q. Resistance versus pressure for cellophane
(du Pont PT-300). . 40

10. Resistance versus pressure for cellulose
acetate membrane (du Pont 88 CA-43) with
a known salt rejection of over 90. .... 42

11, Effect of temperature on membrane resistance
of cellophane (du Pont PT-300) .. 43

12. Resistance versus pressure for 4C 0 cellulose
acetate film. No previous cycles 4

13. Resistance versus pressure across cellulose
acetate (du Pont CA-43) 47

1L. Resistance versus pressure across 40 cellulose
acetate ..... . 51


vii











LIST OF FIGURES (Continued)


Figure Page

15. Effect of method of removin, solvent on the
durability of !7.2$ cellulose acetate
membrane . . 4

16, Effect of method of removing solvent on the
variation of salt rejection with volume. 55

17. Effect of method of removing solvent on the
variation of flow rate with time 56

18. Effect of acetyl content on durability of
cellulose acetate. .. 60

19. Effect of solute on durability of 37.""
cellulose acetate membranes. 62

20. Effect of pH on durability of 37.2'' cellulose
acetate. .. . 64

21. Unit cell of native cellulose . 69

22. Unit cell of swelled cellulose. . 6

23. Birefringence of a du Pont CA-143 membrane
that had a hiah salt rejection (over O0"). 73

2h. Birefrin7ence of a du Pont CA-l3 membrane
that had failed (5.h' salt rejection). 73

25. Apparatus for measuring membrane resistances. 87

26, Wiring diagram for determinlnr membrane re-
sistances.. . 88


viii













INTRODUCTION


As the population of the world continues to in-

crease, greater and greater demands are being made upon

the available fresh water resources. Consequently, there

are many heavily populated regions of the world in which

fresh water is becoming increasingly scarce. The problem

is especially acute in southwestern United States and in

the Mediterranean area. Fresh water needs could be more

than adeouately met if an economical process could be

found for purifying saline waters. In order to develop

a suitable process, the Congress of the United States has

set up the Office of Saline Water under the Department of

the Interior. This Office is considering every conceivable

means for removing salt from saline waters. One promising

method is based upon the principle of osmosis. At the

University of Florida, the Department of the Interior is

supporting a program to explore this particular method

for purifying sea water.

The phenomenon of osmosis has long been of scien-

tific interest. As far back as the eighteenth century it

was observed that when certain aqueous solutions were sepa-

rated from water by animal membranes, water spontaneously

passed through the membrane into the solution. The membrane









served the function of permitting the water to pass while

blocking the counter-flow of the solute. J. H. van't Hoff

(1) described these membranes as semipermeable.

Since then, osmosis has had a significant place in

textbooks on physical chemistry. It is frequently used to

illustrate the concept of reversibility. Consider an os-

motic cell in which the water is passing from the fresh

water side through a semipermeable membrane into an aqueous

salt solution. This flow of water through the membrane can

be retarded by applying pressure to the solution. The pres-

sure at which flow stops is the osmotic pressure. If the

pressure is decreased by an incremental amount, water will

again begin flowing into the solution. But if the pressure

is increased by an incremental amount, a reverse flow of

water will occur from the salt solution to the pure water

side. The term reversible is used to describe this process.

Recently it has been suggested (2) that this effect

of reverse osmosis might be practical for purifying saline

waters. Since the process would approximate reversible

conditions, the energy requirements should approach the

thermodynamic minimum required to separate salt from sea

water.

However, no membrane was known that could be used

in such a process. In fact, no synthetic organic membrane

had yet been discovered that would function effectively as










a semipermeable membrane in salt water.

This, then, is the problem with which this disser-

tation is concerned--the semipermeability of synthetic or-

ganic membranes in strong electrolytes.

In Chapter I the discovery of a membrane (cellulose

acetate) which is semipermeable in strong electrolytes is

described. This discovery prompted much curiosity about

the mechanisms involved in the transfer of water and ions

across the membrane. A theory of these mechanisms is pro-

posed in Chapter II. In the two subsequent chapters, various

tests of the theory are described.

Although the discovery of cellulose acetate as an

effective semipermeable membrane was a big step in the de-

velopment of an osmotic membrane for the purification of

saline water, yet for various reasons this membrane would

not be practical. One of the reasons is its limited life.

After several weeks of continuous use, cellulose acetate

ceases to behave as a semipermeable membrane in strong

electrolytes. In order to extend the life of the membrane

it is necessary to gain an understanding of the process

involved in film failure. This work is presented in

Chapter V.

In order to maintain continuity throughout the

text of the dissertation, experimental details have been

placed in the Appendix.













CHAPTER I


DISCOVERY


Review of the Literature


Most of the studies on osmosis reported in the

literature have dealt with systems in which the solutes

were relatively large organic molecules. Usually cello-

phane, collodion, and natural membranes were used. Very

little has been reported on osmosis of solutions of strong

electrolytes. This is due mainly to the difficulty which

has been encountered in finding a semipermeable membrane

for these systems.

Certain biological membranes are known to function

as osmotic membranes in strong electrolytes. Cooper (3),

for example, has reported that the membrane in a particular

species of seaweed found off the coast of India is semi-

permeable in sea water.

Of all the reported attempts to synthesize a perfect

osmotic membrane, the copper ferrocyanide membrane, first

discovered by M. Traube in 1864, comes closest to achieving

this goal. Using this membrane, Morse (4) perfected a

technique for measuring high osmotic pressures. Employing

a 0.5 N potassium chloride solution, this membrane was able








5
to hold an osmotic pressure of twenty atmospheres for twen-

ty days. However, the film failed rapidly in succeeding

experiments. Berkeley and Hartley (5), employing the same

membrane, measured the osmotic pressure of a wide range of

inorganic electrolytes. More recent work with this film

has been reported by Craig (6) and Mibashan (7).

The literature survey revealed that several syn-

thetic organic membranes have been evaluated. Trautmann

and Ambard (8) have checked the semipermeability of cello-

phane in solutions of uni-univalent and uni-bivalent salts.

When a 0.0017 molar solution was forced through cellophane,

they found that it prevented the passage of 30 per cent of

the uni-univalent salts and 75 per cent of the uni-bivalent

salts. Baumgarth (9) checked the effect of ultrasonics on

the diffusion of NaC1 through cellophane and observed that

its rate of diffusion was increased.

Dried collodion membranes have been found by North-

rop (10) to be only slightly permeable to salts of strong

acids and bases, yet highly permeable to water. Employing

various special treatments, Sollner (11), Elford (12), and

others have also worked with collodion membranes.

BurPess (13) made osmotic films from sodium algi-
nate that retarded the passage of the sodium ion.

An apparatus for ultrafiltration has been described

by Baudouin and Lewin (14) in which a cellulose acetate








6

membrane was used. However, no mention was made in the ab-

stract of filtering solutions of strong electrolytes. Re-

cently Terazawa (15) has proposed that in general crystal-

line semipermeable membranes should be impermeable to ions

of strong electrolytes. He classified copper ferrocyanide

as a crystalline membrane. Gels are another type of film

that may function as osmotic membranes. In this category

Hermans (16) has evaluated the osmotic properties of cellu-

lose xanthate gels in strong electrolytes.


Semipermeability Tests

In order to find a semipermeable membrane for

strong electrolytes, a trial-and-error approach was used--

although it was felt that the desired membrane should con-

tain some hydrophilic groups. Many types of thin films

were obtained from the leading chemical manufacturers in

the country. Some films were prepared in the laboratory.

The apparatus that was ultimately developed for measuring

the semipermeability is shown in Figure 1. It embodied the

following features: (1) Pressures up to 2000 p.s..i could

be obtained by means of compressed air or nitrogen. (2)

Stirring was accomplished by convection. One leg was

heated and the other cooled. (3) Monel was used throughout,

which reduced corrosion to a minimum. (4) The membrane was

supported by a ceramic disc which was recessed into the base









So Impres- ed__


Cool nR
Ja c ket


Heat i n Coil



380t /fo les





le rnb ra ne


Fip. 1.--Apparatus for determining the semi-
permeability of imperfect osmotic membranes.







8

plate and turned on a lathe to a flat surface. This design

permitted the testing of very thin films at high pressures.

To measure the semipermeability, a film was first

clamped into place as illustrated in Figure 1. The apparatus

was filled with salt solution and the pressure applied. Water

that passed through the membrane was collected at the bottom

and analyzed. From the concentration of salt in the appara-

tus and that in the water that had passed through the mem-

brane, the percentage of salt rejected was calculated. The

percentage of salt rejected is taken as a measure of semi-

permeability.

The results of the screening program are recorded

in Table 1. As can be seen by the table, the semipermeability

of du Pont cellulose acetate far exceeded that of any other

film tested. Consequently, further study was confined for

the most part to cellulose acetate membranes. The semi-

permeability of du Pont CA-48 cellulose acetate to various

solutions is given in Table 2.

These results were significant. They indicated that

it is possible to synthesize an organic membrane that is

semipermeable in strong electrolytes. However, the cellu-

lose acetate membranes that were tested could not be used

as ultrafilters for purifying saline waters--first, because

the rate of filtration would be too slow and second, because

their life was limited to only a few weeks.










TABLE 1

SEMIPIRMEIABILITY OF VARIOUS MEMBRANES TO
AQUEOUS SODIUM CHLORIDE SOLUTIONS


Pressure on Semipermeability,
Mb nMembrane % Rejection(a)
Tested Membrane RejectionRemarks
p.s.i. of Dissolved NaC1
by Membrane


Polyvinyl alcohol
(du Pont Elvanol
71-24)
Polyvinyl alcohol
(Baked at 145C.)

Polyvinyl alcohol
(Baked at 1450C.)

50% Polyvinyl alco-
hol, 50f phenol-
formaldehyde

75% Polyvinyl alco-
hol, 25% polytetra-
allylammonium bro-
mide

Polytetraallyl-
ammonlum bromide

Amberplex A-I anion
membrane (Rohm and
Haas)

Amberplex C-1 cation
membrane (Rohm and
Haas)
Ethyl cellulose

Nylon (Plastex
Process Co.)


100


600

780


100


100



70


700


None


26


None


Effluent
pH 11

No pH
change

No pH
change

Effluent
pH 12


Effluent
pH 12


20-35


None


700

800


None

None


800 None










TAELE 1 (Continued)


Pressure on Zemipermeability,
Membrane Hiembrane % Rejection(a)ak
Test ed Remarks
p.s.i. of Dissolved NaC1
by 1 membrane


Cellophane (du
Pont PT 300)

Rubber hydro-
chloride


Polystyrene (Dow
Chemical Co.)

Saran (Dow Chemi-
cal Cc.)

Polyethylene

Cellulose acetate
butyrate

Cellulose acetate
(Celanese S-60h)

Cellulose acetate
(du Font 88 CA-43)

Cellulose acetate
(du Pont 88 CA-b3)


L'o pH
chance


700

700



800


800

800

800


800


850


o00


No flow
through
membrane(b)


None


None

None

None


Very slow
flow rate


No flow


97.4


(a)Percent rejection was based upon the change in
chloride concentration from influent to effluent.

(b)If after 48 to 72 hours, no water was collected,
an entry of no flow was made.










TABLE 2

SENIPERrABILITY OF CELLULOSE ACETATE TO AQUEOUS
SOLUTIONS OF VARIOUS ELECTROLYTES

Semipermeability, %
membrane Pressurelectro- Concen-(b) Rejection of Dis-
onMe- lye traction solved Electrolyte
brane by Membrane
p.s *


Cellulose
acetate
Cellulose
acetate
Cellulose
acetate
Cellulose
acetate
Cellulose
acetate
Cellulose
acetate
Cellulose
acetate


Cellulose
acetate
Same film as
used for NH3
experiment


850

715

800

5oo
500

850

840


NaCI


MRCI2

CaCla


0.11 M


0.031 M

0.0054 M


NagSO, 0.018 M


uTaBr

HaP


800 H3BO0


800


830


NH3


NaC1


0.0012 M

0.0012 M

0.0011 M

0.04 M


0.17 M


Cellulose
acetate


800 Ocean
Water


(a)D


Pont CA-48 cellulose acetate membrane 0.88


mils thick was used for this series of tests.

(b)Analytical procedures are given in the Appen-


dix.


-- --- -- --







12

Before an improved membrane can be developed on a

systematic basis, some knowledge of the mechanisms in-

volved in the transfer of water and ions across this mem-

brane is needed.













CHAPTER II


THEORY FOR THE SEMIPERMEABILITY OF

CELLULOSE ACETATE


Review of Existing Theories

Of the many theories proposed for the semiperme-

ability of membranes, none adequately explains the semi-

permeability of cellulose acetate. Several of the more

significant theories will be reviewed.

Sieve mechanism.--One of the earliest that was

proposed was the sieve mechanism (1). According to this

concept, a semipermeable membrane possesses pores inter-

mediate in size between the solvent and solute molecules.

Thus the solute molecules would be blocked and the smaller

solvent molecules would be allowed to pass. However,

several semipermeable membranes have been found in which

the pore size is larger than the solute molecules. Our

results indicate that the sieve mechanism could not account

for the high semipermeability of cellulose acetate. Refer-

ring to Table 1, nylon and saran which had much lower flow

rates than cellulose acetate (and presumably smaller pores)

exhibited no semipermeability under the conditions of the

experiment. Furthermore, the difference in size between







14
polymerized water molecules and sodium or chloride ions is

not sufficient to permit the sieve mechanism to apply.

Distillation mechanism.--Another theory is based up-

on the perfect semipermeability of the surface of a solu-

tion (1). If solute molecules (such as NaC1) are not vola-

tile, then only the volatile solvent molecules can pene-

trate the surface barrier. H. L. Callendar (1) considered

that solvent molecules distilled across a semipermeable

membrane. Of necessity the walls of the capillaries

through which the solvent distills would have to be non-

wettable to the solvent or the solution. This requirement

renders the theory inapplicable to cellulose acetate, be-

cause water has a high affinity for the free hydroxyl groups

in the film (17).

Ion exchange mechanism.--In recent years a consider-

able amount of research has been devoted to ion exchange

membranes. These membranes are permselective. That is,

they have a very low resistance to the passage of either

cations or anions but not both. This selectivity is due

to the high concentration of positive or negative charges

that are incorporated into the film (18, 19, 20). Conse-

quently, these membranes would be semipermeable if they

effectively blocked the passage of either the cation or

the anion of the solute and permitted solvent molecules to

pass.







15
It is unlikely that the high degree of salt rejec-

tion that cellulose acetate exhibits could be due to an ion

exchange mechanism. Cellulose acetate with approximately

40 per cent acetyl content has a low concentration of hy-

droxyl groups.. Thus, according to the theory of Teorell

and Meyer-Sievers (19), it would behave only slightly as

an ion exchange membrane.

On the basis of the ion exchange theory, the semi-

permeability of several ion exchange membranes was deter-

mined. None of them (i.e. tetraallylammonium bromide,

Amberplex A-1 and C-l in Table 1) exhibited a high degree

of semipermeability. McKelvey, in an unpublished report

to the National Colloid Symposium in 1056, indicated simi-

lar results.

However, a series of experiments was conducted to

determine whether or not cellulose acetate actually does

behave as a permselective membrane. The results of these

experiments, which are reported in Chapter IV, indicate

that it does not.

Adsorption mechanism.--A theory of semipermeability

based upon the relative adsorption of solvent and solute

molecules by the membrane has been proposed (1). Semi-

permeability results when the solvent molecules are ad-

sorbed positively and the solute molecules negatively

adsorbed. This type of semipermeability is most pronounced









when the pore size is small and surface flow predominates

(21).

Solubility mechanism.--Closely related to the ad-

sorption theory is the solubility theory. This was first

proposed by M. L'Hermite in 1855 (1) and has today gained

wide acceptance (17). The theory proposes that semiperme-

ability is an outgrowth of the solubility of the solvent

and the insolubility of the solute in the membrane. Since

the solvent is soluble in the membrane, it can pass but

the solute cannot.

In their most Reneral sense, the solubility and

adsorption theories would certainly cover the observed be-

havior of cellulose acetate. However, this question is

then raised: Why is the solubility or adsorption of water

in cellulose acetate so much greater than that of strong

electrolytes? If it could be attributed merely to the

presence of hydrophilic groups, then cellophane and poly-

vinyl alcohol, which have a very high concentration of

hydroxyl groups, should have a greater degree of salt re-

jection than cellulose acetate does. But as indicated in

Table 1, they do not. Therefore, a clearer understanding

of the mechanisms involved in the transfer of water and ions

across cellulose acetate is needed.











Proposed Mechanism

To formulate a theory which would explain the semi-

permeability of cellulose acetate, it was necessary to find

out what unique combination of properties of cellulose ace-

tate caused this phenomenon. Fortunately a great amount of

work has been done on cellulose chemistry. This work is

well summarized in Volume V of High Polymers (17), and per-

tinent parts of it will be briefly described here before

the proposed mechanism is presented.

Cellulose type membranes are made up of a network
0
of lone polymers averaging 15,000 A in length and 5 to
0
20 A in width. These polymers have for the most part a

random orientation in the membrane. As an analogy, Soll-

ner (18) uses a pile of straw to describe their packing.

'.'hen polymer chains run parallel to each other in an or-

derly manner, crystalline regions are set up. In these

regions the polymers are held together by hydrogen bonds

and van der Waal forces and assume definite spaclnr-s. Qa

the other hand, when the chains lie in a disorderly manner,

the spaces between polymers are much greater. These are

called amorphous regions. The formation of these crystal-

line regions is a dynamic process. Mark, in High Polymers

(17), says that the individual chains are subject to

Brownian motion. During a given period within a given








18

volume, segments of polymer chains will become parallel to

each other and assume a high degree of order. Later these

crystalline regions will disappear as a result of Brownian

motion. Thus the system as a whole has an average degree

of order intermediate between complete crystallinity and

complete amorphousness.

Compression of a membrane results in a complicated

rearrangement of the polymer network. During compression

the system goes through many transition states because of

the irregular shape of the polymers and the variety of

forces that come into play between individual polymer

chains. The consolidation of the three-dimensional lat-

tice thus becomes a complicated multi-step process of

settling down to an arrangement of highest compactness

and stability.

This concept of the internal structure of membranes

applies to all types of cellulose films. Baker (22) has

shown, however, that cellulose acetate is unique in that

it possesses a very high degree of crystallinity. That

is, the internal forces are numerous enough and strong

enough to restrict Brownian motion and hold a large per-

centage of the polymer chains in highly ordered regions.

Of necessity the spaclnps in the amorphous regions are

much smaller than they would be if there were little or

no crystallinity. When water molecules are introduced








19

into this structure, they are concentrated in the amorphous

regions (17).

It is now postulated that these water molecules,

through hydrogen bonding, cross-link the polymer chains and

fill the voids in the amorphous regions with bound water.

The extent of cross-linking depends upon the size of the

voids. When they are large, as in the case of cellophane,

the structure is weakly cross-linked. When they are small,

as in the case of cellulose acetate, the cross-linking is

extensive.

Furthermore, when there is a low degree of cross-

linking, the voids are only partially filled with bound

water leaving holes through which ions can pass. When the

network is highly cross-linked there is little or no free

space. All of the water is hydrogen bound to the polymer

network either directly or through other water molecules.

Brownian motion of the cellulose chains would thereby be

restricted.

In a structure such as this, it is proposed that

two different mechanisms of diffusion occur. Ions and

molecules that cannot enter into hydrogen bonding are

transported across the membrane by hole-type diffusion.

That is, their diffusion depends upon the concentration

of holes in the membrane. Cross-linking cellulose polymers

with water reduces the probability of hole formation,











thereby reducing diffusion of this type.

Those ions and molecules that can combine with the

membrane through hydrogen bonding and can fit into the

bound water structure are presumed to be transported across

the membrane by alignment-type diffusion. Their diffusion

depends upon the establishment of hydrogen bonds with the

membrane. After these molecules combine with one side of

the membrane, they migrate across by transferring from one

hydrogen bonding site to another and are finally discharged

from the other side.

Based upon these concepts (i.e. bound water, hole-

and alignment-types of diffusion), it can now be deduced

that the semipermeability of cellulose acetate is lue to

this situation: that within the membrane strong elec-

trolytes diffuse very slowly compared to the rate of

diffusion of water. In cellulose acetate the high degree

of crystallinity reduces Brownian motion and pore size

sufficiently to permit bound water to fill the voids

which greatly reduces the probability of hole formation.

Hence the rate of diffusion of sodium chloride is very low.

If the membrane were compressed, more cross-linking would

occur, the bound water would become more stabilized, and

the rate of diffusion would become even less.

On the other hand, the diffusion rate of water

through cellulose acetate is relatively hih since it is










transported by a different mechanism. Comoression of the

membrane would not be expected to reduce appreciably align-

ment-type diffusion of water. Therefore, compression of

the structure should increase the semipermeability of the

membrane.

The hydrogen bonding mechanism does not exclude the

sieve mechanism. The size of the holes that are continually

appearing and disappearing plays a secondary role. The rates

of diffusion of solutes that cannot enter into hydrogen

bonding depend upon pore size. However, in the case of

those ions and molecules that can enter into hydrogen

bonding, the permeability depends upon their ability to

fit into the ice-like structure of the membrane.


Literature Support for Theory

Cellulose acetate provides excellent conditions for

hydrogen bonding because there are large numbers of oxygen

atoms in each polymer. That water is bound to cellulose

acetate polymers is amply supported in the literature.

Gruner (23), Petitpas (24), and others (17) have discussed

chemical type bonding between water and cellulose polymers.

It has been reported by Davidson (25) that the density of

the sorbed water along the boundaries of crystalline regions

is 5 to 7 per cent higher than water at atmospheric pressure.

This corresponds to a pressure of 2000 kg./cm.2







22

If water were passing through cellulose acetate by

combining with the bound water within the membrane, its re-

duction in entropy should be comparable to the conversion of

liquid water to ice. Stamm and Loughborough (26) report

that the first water sorbed by cellulose acetate has a TA S

value equivalent to the heat of fusion of water indicating

that the water does have orientation (and hence bondinp)

comparable to that of ice. Neale (27) has also reported

that the bonding between cellulose and water is comparable

to that of ice.

The work of Baumgarth (9) on ultrasonics surgests

that strong electrolytes migrate in cellophane by hole-

type diffusion. He observed, that the rate of diffusion

of NaC1 in cellophane was increased two-fold by the action

of ultrasonics. The ultrasonic acoustical radiation in-

creased the vibration of the cellulose polymers and this

mil-ht be expected to increase the concentration of holes

in the polymer. Consequently, hole-type diffusion would

be increased.

Henser (29) makes the point that the hydroscopicity

of cellulose derivatives depends upon the geometrical ar-

rangement of the cellulose molecules. This indicates that

any molecules that penetrated cellulose acetate membrane

by hydrogen bonding would have to be sterically compatible

with the water-cellulose acetate structure.














CHAPTER III


EXPERIMENTAL SUPPORT OF THEORY BASED UPON

SEMIPFRr'lARILITY DETERMINATIONS


Although there is abundant evidence to substantiate

indirectly the theory presented in the previous section,

little experimental work has yet been reported to support

it directly. Consequently, a series of experiments was

designed to test this concept. These experiments fall

into two broad categories. The first group was based

upon the determination of the semipermeability of cellu-

lose acetate under various conditions. They are reported

in this chapter. The second groun of experiments was

based upon the measurement of the electrical properties

of cellulose acetate under various conditions. These ex-

periments are reported in the following chapter.

The hydrogen bonding concept is compatible with the

results contained in Table 2. To facilitate an explanation,

the semipermeability will be expressed as:

Semlperme-,-., Rate of diffusion for solvent molecules (1)
ability Rate of diffusion for solute molecules

The relationship between the rate of diffusion of

salt molecules and the rates of diffusion of the individual








24

ions is not clear., However, the following general relation-

ship would be expected. An increase in the diffusion rate

of either one or both ions of a salt would result in an

increase in the diffusion rate of the salt.

In the foregoing expression, perfect semipermeability

corresponds to infinity, whereas, when semipermeability is

expressed in terms of per cent salt rejection, perfect semi-

permeability corresponds to 100 per cent salt rejection.

Inasmuch as neither the cations nor the anions of

NaC1 and NaBr can enter into hydrogen bonding, these salts

should be effectively rejected, and they are. The very high

rejection of ':gC12 and CaC1, may be attributed to the large

size of the hydrated magnesium and calcium ions which would

reduce their rate of diffusion.

Since the fluoride ion can enter into hydrogen

bonding, this ion should have a greater rate of diffusion

than the chloride ion. Sodium fluoride, therefore, should

have a lower semipermeability than NaC1, and this was found

to be true.

A very low salt rejection would be expected for

those solutes that can enter into the water-cellulose acetate

structure through hydrogen bonding. Ammonia is an example

of this type of solute. As indicated in Table 2, the semi-

permeability for NH3 was only 30 per cent--about a third of

that for NaCI.









Apparently the H3BO molecule can enter into the

water-cellulose acetate structure with considerable ease,

because its salt rejection was 46 per cent.

The mechanism presented in Chapter II is also com-

patible with the results presented in Table 1. Polyvinyl

alcohol and cellulose both have large concentrations of

oxygen atoms that permit cross-linking via hydrogen

bonding. However, the degree of crystallinity of these

two polymers is low; consequently, there is much Brownian

movement and the concentration of holes is relatively large.

For this reason these membranes have a low degree of semi-

permeability.

For this same reason, cellulose acetate butyrate

and ethyl cellulose exhibit no semipermeability. Spence

(30) has shown that because of the plasticizing effect of
the butyrate group, cellulose acetate butyrate has a much

lower decree of crystallinity than cellulose acetate.


Effect of Pressure on Semipermeability

When the semipermeability of cellulose acetate

(expressed as per cent salt rejection) was determined at

various pressures, the relationship depicted in Figure 2

was obtained. An explanation for this -henomenon has al-

ready been suggested. As pressure is applied, the Brownian

movement is restricted and the "pile of straw" is compressed.

















0
-0
O

%0
-0
o


4>
H


0

-0 a
*






O a
P r 0



;



H 0




-0 -4 o
o E
S me








-0 4
0
So-











So a
0 TB


'.0 Vh0

O





SH


S00 L0
o< 4cd








27

Much more cross-linking by water then becomes possible and

the membrane becomes a more impenetrable barrier to sodium

chloride.


Effect of Crystallinity on
Semipermeability

Spence (30) has also shown that as the acetyl con-

tent of cellulose acetate is increased, the degree of

crystallinity likewise increases. Varying the acetyl con-

tent then becomes a means of varying the crystallinity.

Increasing the degree of crystallinity would have

a two-fold effect according to the hydrogen-bonding mecha-

nism. First, as explained in Chapter II, the semiperme-

ability should increase. Second, the flow rate of water

through the membrane should decrease since the percentage

of amorphous regions (through which the water passes) is

decreased.

Variations of salt rejection and flow rate with

acetyl content have been determined. They are presented

in Figures 3 and 4 respectively. The procedure used in

preparing the membranes is given in the Appendix.

The role of crystallinity as depicted above is

borne out by the data. Up to an acetyl content of 40 per

cent there was a continual increase in the semipermeability

and a decrease in the penetration of water.










100





2 96





o 92
*-I



cO







842.









ri


0.5
o










34 36 38 40 2 44
Acetyl content, per cent
Fig. 3 .--F"ffect of Acetyl Content on Flow Rate
a













34 36 38 40 42 44

Acetyl content, per cent
Fig. 3I..-Eff.ect of Acetyl Content on Flow Rate














CHAPTER IV


ELECTRICAL MEASUREMENTS TO

SUPPORT THEORY


In the previous chapter semipermeability-type ex-

periments were described which support the hydroren bonding

concept of semipermeability. However, these experiments

support only indirectly the crux of the theory--namely

that hydrogen bonding sets up an ice-like structure im-

penetrable to strong electrolytes. More direct evidence

was needed--first, to establish the existence of the bound

water regions and second, to show that two different types

of diffusion are involved in the transfer of water and

sodium chloride ions across cellulose acetate.


Dielectric Measurements of
Cellulose Acetate

Evidence for the existence of the water-cellulose

acetate structure could be obtained by showing that com-

pression of the membrane which sets up the structure and

increases the amount of bound water is accompanied by an

increase in hydrogen bonding. In order to detect an in-

crease of hydrogen bonding, some observable property which

depended upon hydrogen bonds was needed. The dielectric








30

constant meets this requirement. It has been established

that hydrogen bonding markedly increases the dielectric

constant of some liquids. Pauling (31) has explained this

effect on the basis that hydrogen bonding increases the

dipole moment which in turn increases the dielectric con-

stant.

Latimer (32) on the other hand explains this effect

on the basis of atomic polarization. He presents arguments

to show that the polarization of hydrpgen atoms involved in

hydrogen bonding causes a large contribution to the dielec-

tric constant. Furthermore, using Raman spectra, he shows

that the largest contribution to the dielectric constant is

made when the axis of the hydrogen bond is parallel to the

field intensity.

If Latimer's concept is correct, the dielectric

constant of hydrated cellulose acetate should increase with

pressure since the amount of bound water is also increasing.

If no additional hydrogen bonding were occurring when cellu-

lose acetate is compressed, the dielectric constant of hy-

drated cellulose acetate should remain constant.

The relationship between the increase in dielectric

constant and pressure for cellulose acetate is given in

Figure 5. Experimental details are included in the Appen-

dix. As shown, there was a decided increase in dielectric

constant with pressure. This large increase in dielectric




















o 4.)
0


0
-0 0
0




r







o o
0 04 0
I o i
0.A 0

S-0 -1






0 0



0
04
-0

*H \ C O



I i I _





(amssoad oyxtqdsouq) 33 (eanssead ass osq-9) T9 V
;u3q\uo 01.1Q10TT UT es|veaouI








32
constant is due not only to the formation of hydrogen bonds,

but also to the large probability that they are formed in a

direction parallel to the electric field intensity.

It is felt that these results furnish direct evi-

dence of the existence of bound water regions in cellulose

acetate.


Electrical Resistance Across
Cellulose Membrane

Hole-type and alignment-type diffusion in cellulose

acetate.--Electrical resistance measurements were used to

demonstrate that two different types of diffusion are occur-

ring in cellulose acetate. In electrolytic conduction the

current is carried by anions and cations in solution. Both

electrical resistance and rate of diffusion depend upon the

mechanical resistance that these ions encounter during mi-

gration. Therefore the electrical resistance across a mem-

brane can serve as a quantitative index of their diffusion

rates. Diffusion rate and electrical resistance, of course,

vary inversely with each other. In subsequent sections the

term membrane resistance refers to the electrical resistance

across the membrane.

The membrane resistance of a specific ion can be

determined by placing an appropriate ion exchange membrane

on each side of the semipermeable membrane. The procedure

is given in the Appendix. The membrane resistance of water








33
molecules was approximated by determining the resistance of

hydronium ions (HO ).

The electrical resistances for Na+, Cl-, NaC1, and

H0,O across cellulose acetate as a function of pressure is

plotted in Figure 6. At atmospheric pressure the resistances

of Na+, Cl-, and NaC1 are much higher than the resistance of

HO+ Further, as the membrane is compressed, the resistance

of H0+ remains relatively constant whereas the resistances

of Na+, Cl-, and NaCl rapidly increase.

Considered in terms of diffusion, this indicates

that at atmospheric pressure the diffusion rates of Na+ and

Cl" which cannot enter into hydrogen bonding were very low

and became even lower when the membrane was compressed. In

contrast, the diffusion rate of H0+ which can enter into

hydrogen bonding was relatively high at atmospheric pressure

and was only slightly reduced when the membrane was com-

pressed.

It is considered that these results furnish strong

direct evidence to show that those ions and molecules that

cannot combine with cellulose acetate diffuse by a different

mechanism than those ions and molecules that can combine with

the membrane through hydrogen bonding.

Relationship between membrane resistance and semi-

permeability.--In Chapter ITT a qualitative expression was

aiven relating to semipermeability, the diffusion rate





















9


8
NaCl

7-

06







0
03

2








Fig. 6 .--Resistance versus pressure
across celluloseacetatemembrane.
a 1 + +
a(Na followed)

0
0 400 800 1200 160


Fig. 6 .--Resistance versus pressure
g ^ /




0,
















across l^Of' cellulose acetate membrane.


wed)


0









of solvent molecules, and the diffusion rate of solute

molecules. Bearing in mind the inverse relationship be-

tween diffusion rate and membrane resistance, this expres-

sion may be written as:


Sem i perme- vMembrane resistance for solute molecules (2)
ability Membrane resistance for solvent molecules

That semipermeability does vary with the resistance

of the solute is borne out by a comparison of Figures 2 and

6. The rate of increase of semipermeability with pressure

siven in Figure 2 corresponds to the rate of increase in

resistance of the solute given in Figure 6. It can also be

deduced from Figure 6 that in the case of an ionized solute,

the semipermeability depends largely upon the ion with the

higher membrane resistance. Note that the increase in

semipermeability with pressure corresponds more closely to

the increase in Cl- resistance with pressure than with that

of either Na+ or NaCl.

To provide further evidence that semipermeability is

related to membrane resistance, the resistances of membranes

with high and low semipermeabilities were determined. It

can be seen from Figure 7, in which these runs are com-

pared, that the membrane with a high semipermeability also

had high electrical resistance, and the membrane with a

low semipermeability had a low resistance that did not in-

crease with pressure.


















5-
Over c00e salt rejection


O
11
0


4)
+3

(a

0



o





S0





S 0 o00 800 1200 1600

Pressure on membrane, p.s.i.

Fir. 7 .--Resistance versus pressure for
sodium chloride across cellulose acetate mem-
branes(du Pont CA-43) of known salt rejections.







37

Effect of bound water on hole-type diffusion.--In a

previous section dielectric constants were used to show

that compression of cellulose acetate sets up regions of

bound water. In the two preceding sections membrane re-

sistances were used to show that compression of cellulose

acetate causes the rate of hole-type diffusion to decrease.

A comparison of dielectric constants and membrane resistances

as a function of pressure illustrates that the formation of

bound water regions restricts hole-type diffusion. Such a

comparison is made in Figure 8. The lower curve in Figure

8 represents the increase in hydrogen bonding with pressure.

The close resemblance of this lower curve to the resistance

curve of C1- indicates very strongly that the hydrogen-

bound water-cellulose acetate structure is responsible for

the high membrane resistance or low diffusion rate of strong

electrolytes.

The shift of the resistance versus pressure curve

to the left was probably the result of a hysteresis effect

as the pressure was reduced.

Semipermeability of cellophane.--It was mentioned

in Chapter III that the low degree of semiermeability of

cellophane is attributable to its low degree of crystal-

linity. A certain amount of energy is required to overcome

the Brownian motion of the cellulose polymers and to com-

press the system so that a cross-linked water-cellulose




















Aq
8 Resistance of Cl
18- a a

o o




-O
o 16- -










I I-- ---
4 6 */- P
















0 2000 00
S 4/ i C
T 12- -3 40J40
0 43-



0 0



6- e









Pressure on membrane, p.s.i.
Fir. 8.--Comparison of semipermeability and
dielectric constant of cellulose acetate membranes.
4J 2 g7



0 2000 4000 6000
Pressure on membrane, p.s.i.

Fir. 8.--Comparlson of semlpermeability and
dielectric constant of cellulose acetate membranes.








3

structure can be set up. Crystallinity supplies energy re-

quired for the formation of the desired structure. Cellu-

lose acetate has a high degree of crystallinity, so only

little additional pressure is needed to set up the bound

water structure. With cellophane, however, much additional

pressure is needed to accomplish the same result.

To learn whether or not the semipermeability of

cellophane increases at higher pressures, the membrane re-

sistances of H+, Na Cl-, and NaC1 were determined up to

500 p.s.i. These results are presented in Figure 0. The

membrane resistance of H.O+ remained relatively constant.

The resistance of Cl~, however, increased rapidly. There-

fore the semipermeability of cellophane should increase

with pressure.

The semipermeability of cellophane was determined

up to 1500 p.s.i. The salt rejection of cellophane did

increase from about 8 per cent at 600 p.s.i. up to 14 per

cent at 1500 p.s.i. (33).

Effect of temperature.--Another type of experiment

was conducted to determine if hydrogen bonding was connected

with membrane resistance. It is recognized that hydrogen

bonds are very sensitive to temperature. At lower tempera-

tures the hvdroen bonding is greatly Increasei. For

example, at hC. water is more polymerized than at room

temperature.


















Na C
9 NaC1

II (
o 8-
r-
Cl
e 7
4J5


O





3 /






"/ J i..
o 2+ 4

2








Fig. 9.--Resistance versus pressure for cello-
phane (du Pont PT-300).
phane (du Pont PT-300).









Consequently, increasing the temperature should

decrease the semipermeability by breaking up bound water

regions and, conversely, decreasing the temperature should

increase the semipermeability. The former is illustrated

in Figure 10. The temperature was elevated by increasing

the passage of current through the membrane. By intro-

ducing four times as much heat into the membrane, the

change in electrical resistance between 0 and 60n p.s.i.

was decreased from 3600 ohms to 350 ohms.

To show the effect of decreasing the temperature,

resistance versus pressure determinations for cellophane

are given in Figure 11. The temperature was lowered by

packing the entire resistance cell in ice. Lowering the

temperature had two effects. The overall resistance was

increased and the rate of change of resistance with pressure

was also increased. This effect of temperature on membrane

resistance shows in another way that hydrogen bonding sets

up regions that restrict the diffusion of strong electrolytes.

Effect of ions in the water-cellulose acetate struc-

ture.--In the course of determining membrane resistances, an

unexpected phenomenon was observed. This is illustrated in

Figure 12. A cellulose acetate membrane was soaked in water,

placed in the resistance cell, and compressed to 7500 P.s.I.

After compression, the current was turned on and resistance

measurements were made periodically as the pressure was


















_Q
2.8 ~ 12 x 10 calories developed per
a minute due to passage of current

I _
0
NaCl
o 2.6





2.





S2.2

in
o
0
50 x 10" calories developed
20 er minute due to passage of
2.0 current

0


S18
0 400 800 1200 1600
Pressure on membrane, p.s.i.

Fig. 10.--Resistance versus pressure for
cellulose acetate membrane (du Pont 88 CA-f3)
with a known salt rejection of over 00'.














Cell packed in ice
\^/


2.5

o


2 2.0

4)


Q6
01.5





m 1.0
0
a

do.5

0
03


NaC1


2000 40C0 6000 8000
Pressure on membrane, p.s.,.

Fi. 11.--Zffect of temperature on membrane re-
sistance of cellophane (du Pont PT-300).


Cell at room temperature






















o 18- CI
111

o 16-

x


12 -
0

, 12
O

4
a 10
CCI











0
*1-4
0)

0 2000 4000 6000 8000
Pressure on membrane, p.s.i.

Pig. 12 .--Resistance versus pressure for
40% cellulose acetate film. No previous
cycles.









lowered to 0 p.s.i. The upper curve was obtained. But

when the resistance measurements were made as the pressure

was increased, the lower curve was obtained.

It is felt that this effect may be caused by sodium

ions lodged in the membrane. To allow chloride ions to be

electrically forced through the membrane, electrical neu-

trality within the membrane must be maintained. This is

accomplished by the leakage of sodium ions through the

high resistance anion exchange membrane into the cellulose

acetate membrane. Once inside the cellulose acetate membrane,

the sodium ions are relatively immobile because of the very

high cation resistance of the anion exchange membranes. It

is considered likely that the presence of these sodium ions

disrupts the formation of bound water regions.

Now consider Fi1ure 12 in the light of this explana-

tion. When the membrane was first compressed to 7500 p.s.l.

no sodium ions were present so the bound water resIons

readily formed. The membrane resistance was then able to

reach 96,000 ohms. On the other hand, starting resistance

measurements at atmospheric pressure introduced large num-

bers of sodium ions into the structure before the bound

water regions were completely developed. These sodium ions

then disrupted the continued growth of the bound water

regions when the pressure was Increased. Consequently at

7500 p.s.i, the membrane resistance had increased to only

37,000 ohms.








46

It appears that the water-cellulose acetate struc-

ture can be broken down at sufficient pressures. Referring

again to Pigure 12, it can be seen that the membrane re-

sistance begins to decrease at pressures above 4000 p.s.i.

In Figure 13 the effect is more pronounced. A du Pont CA-

43 cellulose acetate membrane was used. The breaks in the

Na+ and Cl- curves at 3000 p.s.i, are attributed to the

weakening of the water-cellulose acetate structure by

sodium and chloride ions. Beyond 3000 p.s.1. the weakened

structure begins to fail. On the other hand, the membrane

resistance of NaC1, which was determined last, continued to

increase up to 5500 p.s.i.

The difference between the resistance curves of

NaCl and its ions might be explained in the following way:

In the determination of the membrane resistance of NaC1,

both ions are free to migrate out of the membrane into the

solution. Since both ions are mobile, the formation of

the water-cellulose acetate structure would not be inhibited

or weakened. In the measurement of the resistances of C1l

or Na+, either Na+ or Cl" is immobilized within the membrane.

These immobile ions may then weaken the water-cellulose

acetate structure.

It should be mentioned that the shapes of the re-

sistance versus pressure curves for du Pont cellulose ace-

tate varied widely from sample to sample. This difference









47









0
It
o 7.0
r-
Na CI




6.0-
O








42
o

a C
.0
0

rI
->
co


S300 2000 4000 6000
Pressure on membrane, p.s.I.

'ir. 13.--F.eslstance versus pressure across
cellulose acetate (du Pont CA-43).








h8

between the resistance curves of NaC1 and its ions was in

general repeated. The erratic behavior of the commercial

films could be caused by the presence of plasticizers.

Good reproducibility was obtained with the cellulose

acetate membranes prepared in this laboratory--no plasti-

cizers were used.

Effect of pressure on transference numbers.--Origi-

nally the intention was to test the proposed mechanism of

semipermeability by determining the transference numbers

of various ions across cellulose acetate. Transference

numbers at elevated pressures were needed. The apDaratus

that was used for measuring membrane resistances was origi-

nally designed for measuring transference numbers. Employ-

ing the Hittorf method, the transference numbers for Na+ and

Cl1 across cellulose acetate were determined at pressures

up to 4000 p.s.i. No reproducibility or correlation of

results could be obtained. The difficulty was traced to

the heating effect of the current passing through the mem-

brane. To reduce the current sufficiently to eliminate

temperature effects would have made the experiment prohibi-

tively lonr. It was then decided to employ membrane re-

sistances for testing the theory.

It is still of interest to observe what effect

pressure has on the transference numbers of Na+ and Cl"

in cellophane and cellulose acetate. Basically, trans-

ference numbers depend upon the relative electrical







49
resistances of the ions in the system. For example, in an

aqueous sodium chloride solution, the electrical resistance

for sodium ions is greater than that of chloride ions.

When the solution is conducting electricity, the migration

of sodium ions accounts for only 38.5 per cent of the cur-

rent. The transference number of Na+ (tNa+) is then 0.385

and that of Cl" is 0.615. If the resistance for chloride

ions increased, its transference number would decrease.

Thus it seems that the resistance-pressure rela-

tionships which have been described previously should serve

as a basis for predicting (qualitatively) the variation of

transference numbers with pressure.

From these resistance versus pressure measurements

it appears that transference numbers within cellophane and

cellulose acetate are dependent among other things upon the

water-cellulose structure. To illustrate, in Figure 9 at

6000 p.s.i., the resistance of Cl- is very high compared to

Na+; therefore, the transference number of Cl" (tC1-) across

cellophane at this pressure is very much smaller than tNa+.

As the pressure on the membrane is decreased below 1000

p.s.i., the resistance of Cl- decreases very rapidly which

in turn results in a sharp increase in tC1-. Neale (34)

has found that at atmospheric pressure the transference

number of Cl- across cellophane is of the order of 0.4 in a

C.1 N NaC1 solution.







50

It might be concluded, then, that the formation of

the water-cellulose structure causes the transference num-
+ +
ber of Na to increase. The hydrated Na must be able to

fit into the water-cellulose structure with greater ease

than Cl can.

The transference number of sodium ions across

cellulose acetate that had not previously been compressed

is less than the transference number of chloride ions.

Note from Figure 14 that for this membrane the resistance

to Na is greater than that to Cl1. Yet after measuring

the membrane resistance for H0O+ up to 1600 p.s.i, the

situation is reversed. The transference number of Na+ is

now greater than that of Cl-. Referring back to Figure 6,

the resistance of Na is seen to be appreciably lower than

that of Cl". The effect of hydrogen ions is to set up the

water-cellulose acetate structure. This increase in tNa+

caused by the passage of H30+ through the membrane indicates

again that sodium ions can preferentially diffuse through

the water-cellulose-type structure.

The significance of this work is two-fold. First,

it shows that the semipermeability of cellulose acetate is

not caused by an ion exchange mechanism. Had the semiperme-

ability been due to an ion exchange reaction, there would

have been a very large difference in resistance between Na+

and C"-. Second, it illustrates the dependence of trans-

ference numbers on the internal structure of a membrane.





















o

a
41 8





6
0


0
i-l








0
a




0




0
a,



0

I 0
S


+
Na


0 400 800 1200 1600
Pressure on membrane, p.s.,i

Fir. 14.--Resistance versus pressure across 60f
cellulose acetcte. Resistances of l'a and Cl" were deter-
mined across separate unused membranes.














CHAPTER V


MECHANISM FOR LOSS OF MEMBRANE

Si-.MIPERMILABILITY

In Chapter I mention was made of the fact that

cellulose acetate lost its semipermeability after a

limited period of time. On the basis of the bound water

theory of semipermeability and the results of a series of

experiments, a mechanism for membrane failure is proposed.

The experiments that are first described illustrate

the nature of the film failure. The next set of experiments

shows that chemical reactions between the solute and mem-

brane are involved. And the final group of experiments

shows the effects of these chemical reactions on the

crystallinity and polymer orientation in the membrane.


Loss of Semipermeability

In order to study the nature of film failure, cellu-

lose acetate films containing 37 per cent combined acetyl

content were employed. By usinr cellulose acetate films

with a low acetyl content, the length of the durability

tests was reduced. The concentration of all solutes was

approximately 0.08 U NaCl. The apruratus and procedure

described in Chapter I were used.









Plots of semipermeability versus time and volume

of water passing through the membrane are presented in

Figures 15 and 16 respectively. In Figure 17 the relation-

ship between time and the rate of diffusion of water through

the membrane is given. Two methods of film preparation were

employed. Both membranes were cast from the same acetone

solution, and the solvent was allowed to evaporate at room

temperature. In one case, however, the remaining traces of

solvent were removed by forcing water through the film and

in the other this was accomplished by heating. These two

methods of film preparation are compared in Figures 15, 16,

and 17.

Several observations can be made from the data

illustrated in these figures. First, the results indicate

that not all of the amorphous regions are available for

supporting a network of bound water. This is shown in

Figure 17. Removal of the solvent by water extraction

increased the diffusion rate of water seven-fold over

that of the heat-dried film.

This difference in diffusion rates mliht be ex-

plained in the following way: When the solvent is removed

by heating, the cellulose acetate polymers lying in non-

crystalline regions apparently become bound together by

intermolecular forces which prevent them from being used

for maintaining oriented water; whereas, when the solvent






































































O 0 0
0 CO 0

O uoT4oe


D) ;3






0 ,
0

@0 0 0
t> -0 0



4'-)
O 8C-
. c


:>

*I 0
> 4 EH 0




co1 i-











0 > P 0
H 0 O
0 07 (D

OO OH



He
00
0

o 0 0

-3 C\ H
fru


IfelI* IMs
[-














Pf-
4.1
4)

0

0~ 0~
H
-P a
"A 0
Cd

frrp

.0
CM





000

-4) II-

0 c
0 ~0 rHj$:
o 0 0

4.33
r:1 Or-I C0
OvF







r-4 0O 4)41
0 0
0 Sb 4


-d

0 0 r-4
43 > 'o 1f
0 W 43\ 00


0
cof
,0 0



H 0





043
EO 43 0









Cd





0 -:


o0000 0
0O ;r
HT4 I
~~r 'U Trfl!IV
















0
Prd
+ *0




0 0
o
10
-- O








0 c0
0 0
0+ 0
0 0 go


O .4 rt)0
H0 o 00







0 0 0 0
S3 a) C
\ 3a peas








> 0 Q


o cM
0 0 0 0
o I Cd "(D o 0
E-4 rEr,



S000

-0 0 4) 4)


0 *I 0

r- *4

0 *-4O
*8 cu a
0 4 0


r+ 0
0CH



0 co
0 0 0 0 0 0



494ta mold^








57
is removed by water extraction, water has an opportunity to

occupy a much greater proportion of the amorphous reglons.

Some of the decrease in flow rate caused by heating could

be attributed to an increase in crystallinity; however, it

is felt that obstruction of the amorphous regions is the

primary cause for this loss in flow rate. Heating, as Mark

has pointed out (35), causes irreversible packing which

could block off some of the amorphous regions within the

membrane.

Second, the water-cellulose structure apparently

persists until long after the solute break-through has

occurred. A break in the diffusion rate of water repre-

sents the beginning of extensive swelling of the structure.

For the heat-treated membrane, the salt break-through oc-

curred at 52 hours (Figure 15). Yet the break in diffusion

rate of water for this same membrane occurred at 110 hours

(Figure 17). The lag between solute and solvent break-

through has been observed in varying degrees. It was more

pronounced with heat-treated films.

Third, intermolecular forces stabilize water-

cellulose structure. It is to be expected that the film

prepared by removing the solvent through heating should be

more resistant to failure since additional intermolecular

forces between the polymers lying in the amorphous regions

are set up. That this is a factor is indicated by the










longer life of this film. Note from Figure 17 that the

heat-treated film lasted 100 hours before a break in the

flow rate, as compared to fifty hours for the water-

extracted film. Also, from Figure 15 it is seen that the

stabilized film tended to maintain salt rejection for a

little longer period of time. The two films reached 80

per cent salt rejection in approximately the same time.

The film containing the greater amount of bound water,

however, treated seven times as much water because the

flow rate was seven times higher.

Fourth, film failure seems to be associated with a

structural chance rather than the development of cracks.

From Figure 17 it is seen that after film failure, the rate

of diffusion of water climbs up to a limiting value and

levels off. This suggests that the completely failed film

has a definite structure. If large breaks or cracks de-

veloped in the membrane, the flow rate would be expected

to climb continuously with time. To investigate this

point further, the semipermeability of completely failed

films to fluorescein was checked. The fluorescein molecules

were effectively blocked by the membrane indicating that the

pores in the failed film were smaller than these molecules.


Reactions Between Solute and Membrane

Based upon the work that has been done on swelling

agents for cellulose, it appears that swelling is a factor








59

in the failure of cellulose acetate to act as a semiperme-

able membrane. Neale (17) has developed a theory for the

swelling of cellulose. He has shown that the hydroxyl

groups of cellulose may be regarded as weak monobasic acids.

These active hydrogens can then enter into exchange reac-

tions with cations such as lithium and sodium. Accompanying

the exchange reaction is a swelling effect.

When cellulose acetate undergoes swelling, the

probability of hole formation increases so the diffusion

rate of solute ions likewise increases. This relationship

between swelling and hole-type diffusion of strong elec-

trolytes is very important. Nicoll, Cox, and Conaway say

in High Polymers (17) that swelling is absolutely necessary

before alkali can penetrate into cellulose, and presumably

this applies to other electrolytes.

Effect of acetylation.--The swelling of cellulose

acetate depends upon the concentration of hydroxyl groups.

As the degree of acetylation is increased, the number of

active hydrogen atoms is reduced. Accordingly, membrane

durability should be dependent upon the degree of acetyla-

tion.

In Figure 18 the durability of a film of 37 per cent

acetyl content is compared to that of a film of 43 per cent

acetyl content. Probably several factors contribute to the

longer durability of cellulose triacetate (43 per cent





















0
r(

r-i




0

4.
4-1
*<-


















S 0
0





0
ri










I)
Cd
Od a






















'-
0J


<.-rq
m-

- 0 I


B -.
L--


f^


u' 'uoT^oae ^-[IS








61

acetyl content). One would be that cellulose triacetate is

more highly cross-linked by crystalline regions which would

make it more resistant to swelling. Another reason is that

the triacetate has fewer active hydrogens, so exchange

(hence swelling) reactions with sodium ions would be limited,

thereby permitting a longer period of semipermeability.

Effect of solute on durability.--If swelling is a

factor in film failure, the life of the film should depend

upon the type of solute in contact with it; i.e. solutes

that have a high reactivity with the hydrogen should shorten

film life. A series of tests was made to determine the

effect of various solutes on the durability of the film.

Avain, films of 37 per cent combined acetyl content were

used. The concentration of the solute in all runs was

approximately 0.08 N.

In Figure 19 the durability of cellulose acetate

in various solutions is illustrated. When dextrose was the

solute, the film showed no sipns of failure even after 300

hours of operation. When MgC1, was the solute, the film

also lasted 300 hours with no appreciable drop in its semi-

permeability. Neither of these solutes would be expected to

react with the acidic hydrogens in cellulose acetate. On

the other hand, when NaC1 was used as a solute, the film

began to fail at 40 hours. Swelling, therefore, seems to

be a factor in film failure, since the ions which cause















0

a0 -
CO 0

N 0


0
I H
O rt
















.r-4
N )0


O ri



ou o



-0 o









CC

O H






0


I
-
80 u0 0 0 0\ 0 0
H/r









rapid failure are swelling agents.

To obtain additional evidence to determine if the

solute causes film failure, a run was made using pure

water. After nine months of continual passage of water

at 600 p.s.i., a membrane containing 37 per cent acetyl

content exhibited no signs of failure. That is, there was

no break or increase in the diffusion rate of water through

the membrane.

If the reactivity of the acidic hydrogen atoms were

reduced, then the life of the film should be extended. This

can be done by lowering the pH. If the pH is lowered too

far, however, the ester groups within the membrane may be

hydrolyzed. As a compromise, a pH of 4 was selected. Hy-

drochloric acid was used to lower the pH. From the com-

parison of runs at high and low pH's included in Figure

19, it appears that the presence of hydrogen ions did

retard the NaCl break-throuph. On the other hand, the

rejection of chloride ions tended to level off at 60 per

cent at the lower pH. This could be caused by the higher

rate of diffusion of HC1.

A comparison of flow rates illustrates more clearly

the effect of reducing the reactivity of the active hydro-

gen. Figure 20 gives a comparison of the flow rates through

cellulose acetate at pH's of 4.0 and 5.5. The flow rate at

the lower pH remained essentially constant up to 180 hours,































pH 5.5


p11 14.0
I p I.o


1 I I


80 120
Time, hours


160


Fil. 20.--.ffect of pH on durability
of 37.2r' cellulose acetate.


~_










indicating no extensive change in the internal structures

up to this time. Yet the NaC1 solution at the higher pH

caused a break in the flow rate after 87 hours. This dif-

ference, coupled with the fact that the flow rate at pH 4

was appreciably lower than at pH 5, indicates that the

hydronium ions stabilized the bound water regions, possibly

by reducing the reactivity of the active hydrogens in the

membrane.


Mechanism of Membrane Failure

Although swelling is associated with membrane

failure, hydrolysis is unquestionably responsible for

extensive break-up of the water-cellulose acetate struc-

ture. Using infrared snectra, Kuppers (36) has shown that

accompanying the decrease in semipermeability of cellulose

acetate is an increase in the concentration of hydroxyl

groups. The increase in hydroxyl groups is at the expense

of carbonyl groups indicating that extensive hydrolysis is

occurring.

The relationship between swelling and hydrolysis

has not been established. It is assumed that swelling

precedes hydrolysis. Initially, swelling results in the

penetration of the solute into the membrane. The solute

in turn causes hydrolysis. Since the product of the hv-

drolysis of cellulose acetate is more free hydroxyl groups,








66

Neale-type swelling continues. Concurrently, swelling and

hydrolysis probably continue until the film is converted

to cellulose.

The structure of the failed film must be different

from other cellulose membranes since the diffusion rate of

water through the failed films was very much lower. Further-

more, the failed films were mechanically weak and difficult

to handle.

The lac between the solute break-through and the

solvent break-through described earlier can possibly be

explained as follows: Solutes such as NaCl penetrate

cellulose acetate membranes by hole-type diffusion, whereas

water diffuses by alignment. When swelling and hydrolysis

commence, the probability of hole formation increases which

increases the diffusion rate of NaC1. Yet the original

structure is not altered enough to change the rate of

diffusion of H20. As hydrolysis proceeds, the particular

crystalline structure, which stabilizes the hydrogen-bound

water structure, is continuously being altered. Even

though the structure is continuously expanding, the bound

water regions continue to fill up the pores within the

membrane. A point is finally reached, however, at which

water within the pores ceases to be extensively associated

with the pore walls. Then the entropy of activation for

diffusion of water is greatly reduced which causes the rate










of diffusion to increase.


Structural Changes

The structural arrangement of polymers has an

important bearing on the semipermeability of cellulose

acetate. Cellophane, it will be recalled, had a low de-

gree of semipermeability because the internal structure

was too open or expanded. It seemed of interest, therefore,

to establish a correlation between the changes in internal

structure of cellulose acetate and film failure. To do

this work X-ray diffraction and optical studies were used.

X-ray studies.--X-ray diffraction spectra were made

of two du Pont 88 CA-43 membranes. One had a salt rejec-

tion of ol,7 per cent and the other a salt rejection of

c.h per cent. No bands appeared for the film that had

94.7 per cent salt rejection. A broad band appeared at
o
approximately 4 A for the failed film. This latter X-

ray pattern was duplicated by using a failed film from an

earlier experiment.
0
The band which appeared at 4.4 A for the failed

film ties in very well with the existing data and concepts.

In connection with mercerization, a great deal of work has

been done on lattice extension due to swelling. In High

Polymers (17) the following concept is presented: Two

types of swelling are possible--inter- and intra-micellular











swelling. Intra-micellular swelling is caused by sodium

hydroxide, hydrazine, etc. attacking the crystalline regions

of the polymer. Inter-micellular swelling is brought about

by swelling between the micelles--the crystalline regions

remaining unaffected. In the case of native cellulose,

intramicellular swelling occurs in a direction normal to

the 101 plane of the unit cell. The 101 planes are held

together by hydrogen bonding. Looking down along the "b"

axis with the cellulose chains coming out of the page, the

unit cell of native cellulose is pictured in Figure 21.

When a swelling agent enters the unit cell, the hydrogen

bonds between the 101 planes are broken which causes an

increase in the 101 spacings. As shown in Figure 22, in-

creasing the 101 spacing flattens out the parallelogram.
0
This flattening effect decreases the 101 spacings to 1;.4 A.

Depending upon the swelling acent, the 101 spacings may
0 o
increase from 6 up to 16 A. For NaOH it is 7.4 A.

This mechanism for the swelling of native cellulose

appears to apnly to the swelling of cellulose acetate mem-

branes. The band which appeared at 4.4 A for the failed

cellulose acetate corresponds to spacinps between the 101

planes in mercerized cellulose (Figure 22).

The fact that only the failed film diffracted the

X-rays surgests that during failure the polymeric chains

oriented into a plane parallel to the surface of the film.














\~



/ \

/ N
/


N\
\. \ /


?ig. 21.--Unit Cell of Native Cellulose (17)


-S u r fa c e


-- #- K' r y
O- Fi l/ -" -"-


/0/

~~%.


Fig. 22.--Unit Cell of Swelled Cellulose (17)


I -l


/I \








70
This rotated the 101 planes into diffracting position. In

Figure 22 the angle that the X-rays made with the surface

of the film is inserted to show why only the 101 plane

caused diffraction.

Microscopic studies.--In order to study the effects

of polymer orientation, the same two films used in the X-

ray studies were also studied under the polarizing micro-

scope. The polarizing microscope is extensively employed

in the field of mineralogy for determining the degree of

crystallinity of minerals. Drechsel (37), Spence (30),

McNally (38), and others have employed the technique for

studying the crystallinity of cellulose and its derivatives.

The polarizing microscope permits one to obtain conveniently

a quantitative index of the degree of crystallinity in a

membrane and to determine the direction of orientation of

the polymers.

When a beam of polarized light is passed through

most crystalline substances, it will be broken into two

parts--a fast ray and a slow ray. The difference in ve-

locity between these two rays (which is called the bire-

frinrence) is governed by the degree of crystallinity in

polymeric materials. The greater the degree of crystal-

linity, the greater the birefringence. The direction of

vibration of the slow ray in cellulose acetate corresponds

to the orientation of the polymer chains (37).








71
A Leitz Berek compensator was used with an American

Optical polarizing microscope to measure the birefringence.

Actually, the Berek compensator measures the retardation.

That is, it measures the amount the slow ray is retarded

behind the fast ray. This retardation produces interference

colors which appear in the microscope. The higher the order

of the color, the higher the retardation and, hence, the

higher the crystallinity. The retardation is used to cal-

culate the birefringence with the following formula (39):


n' n" = -
d

n' n" = difference in refractive index between
fast and slow rays called birefringence

D = retardation

d = thickness of sample in mx


First the birefringence was determined normal to

the surface of the film. The results are recorded in Table

3.

These data indicate that both compression of the

membrane and hydrolysis cause orientation of the polymers.

Probably the number of unit cells was not increased, but

rather those that were present were changed from a random

orientation to an ordered arrangement.

The orientation in the film as received from the











TABLE 3

EFFECT CF SEMIPERMEABILITY ON BIRFFPINGFNCE


Salt Birefringence
Film Rejection

88-CA-h3 dry As received (unused) P.00019
88-CA-h3 wet C..7 o. 0.0046
88-CA-i3 dry 9h. 7 0. 0C046
88-CA-43 wet 5.4 0.0022
88-CA-43 dry 5.h 0.0030


manufacturer was in the direction in which the film had been

rolled.

An edge view of the film was obtained by cutting

strips of the films 108 microns wide with two razor blades

clamped together. By bending the strips, they could be

made to stand on edge on the microscope stage. We were

unable to measure the retardation with the Berek compen-

sator because of interference from light which did not pass

through the film. Attempts were made to block out the ex-

traneous light, but none were successful. However, the re-

tardation was estimated on the basis of the interference

colors. Ropers (40) gives a color chart that is very useful

for this purpose.

In Figures 23 and 24 the interference colors that

appeared for the high semipermeability and low semipermeabili-
ty cellulose acetate films are given. A magnification of










Edae view


P Slow ray
Orientation of
cellulose acetate
b polymers


S Corresponds to a birefringence of 0.0007

Fit. 23.--Birofringence of a du Font CA-43
membrane that had a high salt rejection (over 0,).


Edre view


Sb~ Slow ray
Orientation of
cellulose poly-
mers

Corresponds to a birefringence of 0.0074

Corresponds to a birefrinrence of 0.0060

Pij. 24.--Birefrinsence of a du Pont CA-43 membrane
that had failed (5.-' salt rejection).










X 860 was used. These observations indicate that film

failure is associated with extensive orientation of the

cellulose polymers into the plane of the membrane.

In view of previous work it is quite reasonable to

expect this orientation. McNally (38) and Sissons (41)

have shown that polymers will orient themselves to relieve

stresses. In the case of cellulose acetate when the films

bezin to swell and hydrolyze, the chains become free to

rotate. They then rotate into the plane of the film to

relieve the stresses.

Based on the interference colors obtained from an

edge view of a failed film, it appears that there is a

slightly greater degree of orientation at the surfaces of

the film than in the central regions. In the interior

there may not be as much freedom of rotation. Apr-arently

the orientation occurs at approximately the same rate

throughout the film rather than starting on the salt water

side and working its way through.














CHAPTER VI


RECOMMENDATIONS FOR AN IMPROVED

MEMBRANE

In the last analysis, the use of semipermeable

membranes for purifying sea water will defend upon the

economics of the process. However, as a first step a

semipermeable membrane with a very high flow rate will

be needed. On the basis of the investigation described

in this dissertation, it now is possible to propose some

requirements for such a membrane.

According to this present study, the semiperme-

ability of cellulose acetate is caused by regions of bound

water within the membrane. The polymer network serves as

a skeleton for setting up the bound water regions. These

regions, it will be recalled, permit relatively rapid

diffusion of water molecules while greatly retarding

hole-type diffusion of sodium chloride. The problem, then,

becomes one of designing a skeleton that will maintain a

large amount of bound water.

To attain this objective, it apneers that several

factors in the design of the polymer.network will need to

be considered. First, the polymers that are used should










contain large numbers of sites for hydrogen bonding.

Second, sufficient cross-linking is needed to re-

strict the Brownian motion of the polymers that make up

the network. It is postulated that the bound water regions

can grow only when the kinetic energy of the polymers and

water molecules are reduced enough to permit extensive

association of the water with the polymer network. This

beinp the case, the optimum degree of cross-l1nkinr would

be related to the variables that affected the Brownian

movement of the system. Pressure was found to restrict

Brownian motion. Consequently less cross-linking would

be needed at high pressures, say 3000 p.s.i., than at low

pressures. Temperature is another variable that affects

Brownian motion. Hence, if saline water is being purified

at, say, 50C., less cross-linking is required than at

higher temperatures.

Third, the cells or channels within the polymer

skeleton should be of optimum size for setting up bound

water regions. The size is dependent upon how far a chain

of hydrogen-bound water molecules can span without rupturing.

At low temperatures--around the freezing point of water--

hydrogen bonds are strong; so, relatively larpe cells can

be used without increasing the probability of hole formation.

Fourth, the polymers should be cross-linked suf-

ficiently to render them insoluble in water. In all










probability the membrane will be insolubilized when it is

cross-linked enough to restrict Brownian motion.

Fifth, the polymer used for constructing the skele-

ton should be inert to chemical attack by sodium chloride

or any other solute in the system.

Cellulose meets in part the requisites for a semi-

permeable membrane--it has sites for hydrogen bonding and

sufficient cross-linking through crystallinity to set up

bound water in its pores. However, only a small effective

volume of the film is available for bound water. The rest

is taken up with crystalline regions. It seems that the

effective volume for bound water could be increased by

employing cross-linking agents smaller than the crystal-

line regions of cellulose acetate.

Cellulose acetate has the further limitation of

being subject to hydrolysis. Therefore, it is probable

that ultimately some other type of semipermeable membrane

will be developed for purifying saline water.













CHAPTER VII


SUMMARY

The primary objective of this investigation has

been to explain why cellulose acetate behaves as a semi-

permeable membrane in saline water. To explain this

phenomenon, two different mechanisms for the transfer of

water and ions through cellulose acetate membranes were

formulated.

Those ions and molecules that cannot enter into

hydrogen bonding with the membrane are transferred by

hole-type diffusion. The rate of diffusion appears to be

governed by a water-cellulose acetate structure. The re-

action between water and the cellulose acetate polymers to

form bound water regions is induced by compressing the mem-

brane. As pressure is applied on the membrane, more bound

water is produced which causes the rate of hole-type dif-

fusion to decrease.

On the other hand, those ions and molecules that can

associate with the membrane through hydrogen bonding actually

combine with the membrane and are transported through it by

alignment-type diffusion. The formation of the water-

cellulose acetate structure does not appreciably diminish


78











the diffusion rate of water through the membrane.

Cellulose acetate begins to behave as an effective

semipermeable membrane in saline water when it is compressed

sufficiently to retard proatly the diffusion of NaCI.

Several types of experiments were conducted to sup-

port these hypotheses. The most important evidence was ob-

tained from resistance experiments. The resistance of

specific ions was measured across cellulose acetate at

various pressures. It was observed that the rate of dif-

fusion of those ions that cannot combine with the membrane

actually does decrease as the membrane is compressed. The

rate of diffusion of H.O+ that can enter into hydrogen

bonding with cellulose acetate is much hi-her and is not

appreciably reduced as the membrane is compressed. These

resistance-pressure relationships are correlated with the

semipermeability of the cellulose acetate.

On the basis of resistance-pressure relationships

for cellophane, it appears that at sufficient pressures

this membrane also should become semipermeable in strong

electrolytes.

A secondary objective of this investigation has

been to explain why cellulose acetate ceases to behave as

a semipermeable membrane after a limited period of use.

It was found that film failure is due to a chemical








80

reaction between the solute and cellulose acetate. X-ray

diffraction and birefringence measurements were employed

to show that extensive structural chances are brought about

by this reaction between the solute and the membrane.

On the basis of this investigation it is considered

that an improved semipermeable membrane can be obtained

first, by using a polymer that is inert to chemical attack

by sodium chloride, and, second, by increasing the amount

of bound water regions within the membrane.

























APPENDIX




Analytical Procedures

Preparation of Films

Determination of Dielectric
Constants

Determination of Membrane
Resistances











Analytical Procedures


Solute

NaCI

MgC1l

CaC1,

NaBr

Ocean Water

NH3

NaF

Na SO,


HBO,


Analytical Procedure

Mohr titration for chloride ion

Mohr titration for chloride ion

'ohr titration for chloride ion

Mohr titration for bromide ion

Mohr titration for halides

Titration with standard HC1

Conductivity

Back titrated sulfate with barium chloride
using a versenate indicator

Complexed HBBO3 with mannitol and made an
acid base titration with KOH


During the early phase of this study, it was suprested

that the reduction in chlorides was due to their adsorption

rather than to the semipermeability of the membrane. To see

if this were true the per cent salt rejections were calcu-

lated on the basis of conductivity measurements. For the

most part the semloermeabilities calculated from conduc-

tivity measurements agreed with those calculated from chemi-

cal analysis.


Preparation of Films

Films were made by dissolvin' cellulose acetate in

acetone and casting the solution on a suitable surface with









83

a doctor blade. The doctor blade could be rotated so that

films of solution 6 or 8 mils thick could be spread. The

solvent evaporated, leaving a film of cellulose acetate.

Several difficulties arose usinr this method. Be-

cause of the high humidity in the atmosphere, water would

condense on the surface of the film during evaporation of

the solvent. To avoid this difficulty the films were cast

in a controlled atmosphere. This was accomplished by con-

structini a dry box which would permit the manipulation of

the doctor blade by means of wires and also permit the

casting of the cellulose acetate solution without admitting

any outside air into the box. The removal of the solvent

was controlled by passing a stream of dry air through the

box. As a further precaution moisture was removed from the

acetone with anhydrous sodium sulfate.

Since the films that were used were only 5 to 7

microns thick, the handling of them was a serious problem.

Two methods were employed. A film could either be cast

directly on cellophane or cast on .lass and picked up by

laying a sheet of wet cellophane on top of it. The cello-

phane then acted as a backing when the films were placed

in the apparatus.

Above an acetyl content of 41 per cent, cellulose

acetate becomes Insoluble in acetone. Chloroform con-

taining 5 per cent ethanol was used to dissolve cellulose











acetates with high acetyl contents.


Determination of Dielectric Constants

The dielectric constant of du Pont CA-43 was

measured by placing the film between two brass plates ap-

proximately one inch in diameter and measurlnp the capacity

with a heterodyne beat apparatus. The instrument that was

used had an error of less than one ,,- (42). Pressure

was applied to the film with a 10,OCCt-pound hydraulic press.

The brass plates were insulated from the Dress.

As pressure was applied to the film, changes in di-

electric constant could have been due to four factors:

(1) an increase caused by structural changes within the

film, (2) a decrease caused by water squeezing out of the

film, (3) an increase caused by better contact between con-

denser plates and the membrane, and (4) a change resulting

from a change in the dielectric constant of water itself.

The change in dielectric constant due to structural

and dimensional changes was corrected for by measuring the

capacity of a dry film as a function of pressure. The nA

versus p.s.1. curve for dry CA-43 also serves as a correc-

tion for any increase in dielectric constant due to better

contact between the brass plates and the membrane. The

change in dielectric constant for dry CA-43 is included in

Figure 5.









To insure that the change in dielectric constant

was not caused by a difference in water content, the film

was first subjected to the pressure at which the capaci-

tance was to be measured. After the squeezed-out water had

been removed, the condenser formed by the brass plates and

the film was wrapped with cellophane tape to prevent evapo-

ration losses. Then the pressure was reduced to atmospheric

pressure. The dielectric constant was determined at atmos-

pheric pressure and the pressure was arain increased for

the determination at the desired pressure. The difference

between the dielectric constant at atmospheric pressure and

at the desired pressure was used for the ordinate in Fi ures

5 and 8.

The following expression from Weissberger (43) gives

the relationship for calculating the dielectric constant

from the capacitance:


= d C
0.0o05 A

C = dielectric constant

C = capacitance in micromicrofarads

d = distance between plates in centimeters

A = area of plates in square centimeters

The dielectric constant determinations were made at

a frequency of 250 KC.










It is felt that the procedure employed pave the

necessary accuracy. The dielectric constant of the dried

sample of du Pont cellulose acetate was found to be 3.1

to 3.4 which lies within the range of dielectric constants

listed in Modern Plastica for cellulose acetate ("4). Be-

sides, the change in dielectric constant was needed instead

of the actual value.


Determination of Membrane Resistances

A drawing of the cell used for measuring resistances

is shown in Fir'ure 25. The circular carborundum whetstones

serve two purposes. First, they compress the film uniformly

and second, they act as electrolyte chambers. When saturated

they hold between 4 and 5 ml of solution apiece. With this

apparatus, compressive stresses of over 8000 p.s.i. can be

applied to a film. To eliminate evaporation losses, the

sides of the carborundum stones were covered with wide

elastic bands and wrapped with "Presstite" Insulation Tape.

This tape, which is pliable like putty, was pressed to cover

all cracks.

Figure 26 is a schematic of the electrical circuit

used for measuring the resistance across the cell. As indi-

cated in the figure, the voltage drop across the cell was

measured directly. The current was determined by measuring

the voltage drop across a known resistance and using Ohm's

law. The double pole, double throw switch was used for































Silver Anode


*'1


*....

.~1
t


Membrane Being Tested


Carborundum Whetstone


Filter Paper


Silver, Silver Chloride
Uatnode
Lucite Insulator


Solid Brass Cylinder


Hydraulic Press


Scale: 1" = It


Fig. 25.--Apparatus for measuring membrane
resistances.


,








88














Resistance cell












T-nown
resistance



Beckman
Model G
pH Meter




Fig. 26.--.Wiring diagram for determining
membrane resistances.










applying the potential drop across either the known re-

sistance or across the cell to the Beckman Model G pH

meter. The voltage was determined within an accuracy of

0.1 millivolt.

In the selection of electrodes, two factors were

considered. First, the products of the electrode reactions

had to be non-gaseous. Second, the products of the elec-

trode reactions should not introduce interfering ions into

the system.

To measure the membrane resistance of chloride ions,

discs of Amberplex anion permeable membranes in the chloride

form were placed on both sides of the cellulose membrane.

The cell was compressed to the maximum pressure at which

resistances were to be measured, and the excess solution

that squeezed out was removed. After wrapping the cell

with "Presstite" tape, resistance measurements were made

as the pressure was reduced. Two to three minutes were

allowed at each pressure before measuring the resistance.

A weighted lever was used to apply pressures up to 1600

p.s.I. For pressures up to 8000 p.s.l., a hydraulic

oress was used.

The same procedure was employed for measuring the

membrane resistance of Na+. However, an Amberplex cation

nermeable membrane in the sodium form was used on each side

of the cellulose membrane. The membrane resistance of H30+










was determined by using the cation permeable membrane in

the acid form.

To measure the membrane resistance of Na K1, no ion

exchange membranes were used.

The carborundum blocks were saturated with a C.018 N

NaCI solution for the Cl1, Na+, and 1,aCl determinations.

The blocks were saturated with 0.018 N HC1 for the H30+

resistance determinations.

The membrane resistance was obtained from the

following expressions:

membranee Total cell Resistance of cell without
resistance (resistance) the cellulose membrane

Cell resistance = Voltage across cell
Current passing through the cell


Current passing = Voltage across known resistance
through the cell Known resistance


The units of membrane resistance are ohms for the

total film area (7.1 square centimeters).

The resistance across the ion exchanPe membranes

is very low and does not vary significantly with pressure.













BIBLIOGRAPHY


(1) Samuel Glasstone, "Textbook of Physical Chemistry,"
2nd ed., D. Van Nostrand Co., Jew York, 1'6.
1320 p.

(2) "Development of Synthetic Osmotic Membrane for Use in
Desaltina Saline Waters," Research Proposal for the
IT. S. Department of the Interior, February 19, 1953.

(3) P. F. Cooper and S. A. Pasha, J. Indian Botan. Soc.,
14, 237-55 (1935). C. A. 3, 2602 flr36).
(4) H. 1. Morse, "The Osmotic Pressure of Aqueous Solu-
tions," Publ. Carneire Inst. Washineton, No. 108
(1014). 222 p.

(5) Earl of Berkeley and E. G. J. Hartley, Proc. Roy Soc.
(London) A 92, i77 (l146).

(6) R. A. Craig and F. J. Hartunp, Trans. Faraday Soc.
8, 96t-9 (1052).

(7) A. Mibashan, Rec. Tray. chim. 70, 318-24 (1951).
c. A. 45, 60olo0 (lT5).

(8) S. Trautnann and L. Ambard, J. Chim. Phys., 4I,
220-5 (lc?2). C, A. L6, 8q35f I152).
(9) Franz Paumnarth, ".rzth Forsch" ., 525-30 (1L9);
"Chem. Zentr." (150) II, 1594. CA. 46, 10791d
(1W52).
(10) John H. Northrop, J. of Gen. Physiol., 11, 233 (1928).

(11) Karl Sollner and Harry P. Grepor, J. Am. Chem. Soc.,
61, 346 (1WL5).
(12) W. J. Elford, Brit. J. E.xr. Pathol., 10, 126 (1929).

(13) Laurie L. Burgess, J. Am, Chem. Soc., 56, 4l4-419 (1"31:).