EVALUATION, CONTROL, AND PREDICTION
OF DRUG DIFFUSION THROUGH
PRAMOD BHAURAO CHEMBURKAR
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
To my wife,
whose patience and help made this accomplishment possible
and to my parents,
Mr. and Mrs. Bhaurao H. Chemburkar,
whose encouragement has been a source of unending energy to me.
The author expresses his sincere gratitude to Dr. Edward R.
Garrett, Chairman of the Supervisory Committee, for his valuable
guidance in research work and help in the preparation of this manu-
script. He extends his appreciation to Dr. Oscar E. Araujo, Dr. Robert
B. Bennett and Dr. Russell E. Phares, Jr. for serving on the super-
visory committee and for their helpful suggestions during the course
of this investigation.
The author gratefully acknowledges the following companies
for the supply of chemicals and materials: Abbott Laboratories,
North Chicago, Illinois, Avisun Corporation, Philadelphia, Pennsyl-
vania, Dow Corning Center for Aid to Medical Research, Midland,
Michigan, Eastman Chemical Products, Inc., Tennessee, E.I. duPont de
Nemours & Co. (Inc.), Wilmington, Delaware, May Industries, Inc.,
Atlanta, Georgia, Syntex Laboratories, Palo Alto, California, The
Polymer Corporation, Reading, Pennsylvania, Upjohn Company, Kalamazoo,
Michigan, Vicks Division Research and Development, New York, New York.
The author wishes to extend his appreciation to Mr. Barry
Dvorchik, Mr. Arthur H. Kibbe, Miss Michele Deckle, and Mrs. Maria
Losada for help in the experimental work, Mr. George L. Perry for
preparing illustrations, and Mrs. Sharon Cooper for typing this
dissertation. He also thanks Mr. H. J. Lambert, Mr. P. J. Mehta,
Dr. H. J. Nestler and the rest of his colleagues for their suggestions
and participation in the formal and informal seminars pertaining to
The author also wishes to thank the American Institute of
Biological Sciences, Washington, D. C., for a grant to defray a part
of the expenses of the preparation of this manuscript and the College
of Pharmacy and Graduate School of the University of Florida for the
financial help during his tenure as a graduate student.
Sincere thanks are extended to Mrs. Arthur H. Kibbe and Mr.
Y. Raghunathan for their help in proofreading this manuscript.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ............................................. iii
LIST OF TABLES ............................................... viii
LIST OF FIGURES .............................................. x
LIST OF APPENDICES ........................................... xiii
INTRODUCTION ................................................. 1
HISTOR ICAL ................................................... 2
MATHEMATICS OF TRANSPORT PROCESSES ........................... 7
Materials ........................................... 13
Methods of Analysis ................................. 17
Screening of Polymer Films for the Permeability
of Drugs ............................................ 23
Diffusion Apparatus ................................. 24
Solubility Studies .................................. 29
Determination of Partition Coefficients .............. 30
Measurement of Thickness of Membranes ............... 32
Study of Permeability of Silastic Membrane to
Phosphate Buffer Salts and Hydrochloric Acid ........ 32
Preparation of Solutions ............................ 34
A Typical Steady State Diffusion Experiment ......... 35
A Typical Quasi-Steady State Diffusion Experiment ... 37
Diffusion of 4'-Aminopropiophenone Through
Silastic Membrane ................................... 38
Diffusion of 4'-Aminoacetophenone and 3'-Amino-
acetophenone ........................................ 44
Diffusion of Barbituric Acid Derivatives ............ 45
Diffusion of Phenylalkylamines ...................... 46
Diffusion of Dextromethorphan and Progesterone ...... 46
Diffusion of Drugs Through Silastic Capsules ........ 47
Screening of Permeability of Polymer Films .......... 50
TABLE OF CONTENTS (Continued)
Solubility Studies .................................. 51
Partition Coefficients............................... 51
Thickness of Membranes .............................. 52
Permeability of Silastic Membranes to Phosphate
Buffer Salts and Hydrochloric Acid Solution........... 52
Treatment of Data ................................... 53
Diffusion of 4'-Aminopropiophenone Through Silastic
Membrane ..................................... .... ... 54
Diffusion of 4'-Aminoacetophenone and 3'-Amino-
acetophenone ........................................ 61
Diffusion of Barbituric Acid Derivatives ............ 62
Diffusion of Phenylalkylamines ...................... 62
Diffusion of Dextromethorphan and Progesterone ...... 63
Diffusion of Drugs from Silastic Capsules ........... 64
Permeability of Silastic Membrane ................... 65
Fick's Law of Diffusion ............................. 66
Effect of pH on Diffusion ........................... 67
Apparent Diffusion Constants from Steady
and Quasi-steady State Diffusion Experiments ........ 67
Non-dependence of Apparent Diffusion Constants of
Drugs on Molecular Weights .......................... 68
Postulated Mechanism of Transport in Silastic
Membrane ............................................ 69
Apparent Diffusion Constants and Partition
Coefficients ........................................ 70
Apparent Diffusion Constants and Solubilities ....... 71
Effect of Ethanol on Diffusion of 4'-Amino-
propiophenone through Silastic Membrane ............. 72
Temperature Dependence of Diffusion in Silastic
Membrane ............................................ 74
Pharmaceutical Application .......................... 76
Control and Prediction of Diffusion of Drugs
through Silastic Membranes .......................... 78
SUMMARY AND CONCLUSIONS ....................................... 80
A. Tables .......................................... 84
B. Figures ......................................... 102
C. Derivations and Statistical Analyses ............ 127
TABLE OF CONTENTS (Continued)
REFERENCES ...................................... ........ 145
BIOGRAPHICAL SKETCH ........................................... 151
LIST OF TABLES
I WAVELENGTHS OF MEASURED ABSORBANCES, MOLAR
ABSORPTIVITIES A:. pKa VALUES OF C ,P:.. .,NDS .......... 85
II DIFFUSION OF 4'-AMINCPROPIOPHENONE FROM SATURATED
SOLUTION (2.36 x 10 M) IN pH 6.5 PHOSPHATE BUFFER
THROUGH 3 MIL SILASTIC MEMBRANE INTO 200 ML. of
0.12 N HCI AT 37.30 ................................. 86
III SOLUBILITIES, PARTITION COEFFICIENTS, AND APPF'PE;T
DIFFUSION CONSTANTS OF BARBITURIC ACID DERIVATIVES
AT 25.00 ............................................ 87
IV PARTITION COEFFICIENTS, SOLUBILITIES, ACTIVATION
E;:KRGIES OF DIFFUSION, AND APPARENT DIFFUSION
CONSTANTS OF AMINOALKYLPHENONES ..................... 88
V SOLUBILITY, AND APPARENT DIFFUSION CONSTANTS OF
4'-AMINOPROPIOPHENONE AS A FUNCTION OF ETHANOL
CONCENTRATION AT 25.00 .... .......................... 89
VI SPECIFIC RATES OF DIFFUSION OF 4'-AMINOPROPIOPHENONE
AS A FL..CTION OF :I:.RANE THICKNESS AT 24.900 ....... 90
VII SPECIFIC RATES OF DIFFUSION, AND APPARENT DIFFUSIC;;
CONSTANTS OF 4'-AMINOPROPIOPHENONE AS FUNCTION OF
E;.IE:.ATURE .:D CONCENTRATION ....................... 91
VI I APPARENT DIFFUSION CONSTANTS OF 4'-AMINOPROPI CPHEL;ONE
AS A FUNCTION OF pH AT 25.00 ........................ 92
IX APPARENT DIFFUSION CONSTANTS, AND SPECIFIC RATES
OF DIFFUSION OF 4'-AMINOPROPIOPHENONE AS A FUNCTION
OF ETHANOL CONCENTRATION AT 24.600 .................. 93
X CONSTANTS OF QUASI-STEADY STATE DIFFUSION OF 4'-
AMINOPROPICPZPH.:NE THROUGH SILASTIC MEMBRANE AS A
FUNCTION OF ETHANOL CO;IENTRATION AT 25.250 ......... 94
XI SPECIFIC RATES OF DIFFUSION OF ETHANOL AS A FL.:.TION
OF 4'-AMINOPROPIC.-.NONE CONCENTRATION AT 24.600 .... 95
LIST OF TABLES (Continued)
XII APPARENT ACTIVATION ENERGIES OF DIFFUSION, AND
APPARENT DIFFUSION CONSTANTS OF BARBITURIC ACID
DERIVATIVES ...................................... ... 96
XIII APPARENT DIFFUSION CONSTANTS OF BARBITAL AND
PENTOBARBITAL THROUGH SILASTIC MEMBRANE AT 37.30
AS A FUNCTION OF pH ............................... .. 97
XIV APPARENT DIFFUSION CONSTANTS, AND SPECIFIC RATES
OF DIFFUSION OF PHENYLALKYLAMINES THROUGH SILASTIC
'.oR.:ANE AT 25.00 FROM BORATE BUFFER SOLUTIONS INTO
200 ML. of pH 6.8 PHOSPHATE BUFFER .................. 98
XV APPARENT DIFFUSION CONSTNATS, AND RATES OF DIFFUSION
OF DEXTROMETHORPHAN FROM PE/,!T OIL, MINERAL OIL AND
pH 10.1 BORATE BUFFER THROUGH 3 MIL SILASTIC MEMBRANE
INTO pH 6.8 PHOSPHATE BUFFER AT 37.50 ............... 99
XVI APPARENT DIFFUSION CONSTANTS OF PROGESTERI FROM
PEA 5IT OIL, A 1~ pH 6.8 PHOSPHATE BUFFER T-:jLCH 3
MIL SILASTIC :.:EL:EANE INTO pH 6.8 PHOSPHATE BUFFER
AT 37.50 ............................................ 100
XVII APPARENT DIFFUSION CONSTANTS AND RATES OF DIFFUSION
OF DRUGS FROM SILASTIC CAPSULES AT 37.50 ............ 101
LIST OF FIGURES
1. Typical calibration curves for spectrophotometric
analyses of various drugs at their pertinent wave-
lengths .............................................. 103
2. Sketch of an assembled vial for screening polymeric
films for their permeability to drugs ................ 104
3. Sketch of diffusion cell used for steady state
diffusion ............................................ 105
4. Sketch of diffusion cell used for quasi-steady state
diffusion ............................................ 106
5. Sketch of diffusion apparatus and pump assembly for
steady state diffusion studies ....................... 107
6. Solubility of 4'-aminopropiophenone in phosphate
buffer containing various concentrations of ethanol
at 25.00 plotted as absorbance of the filtered
solutions at 307 mp versus percentage of ethanol
in phosphate buffer .................................. 108
7. Diffusion of 4'-aminopropiophenone (1.52 X 10-3 M)
from pH 6.8 phosphate buffer through Silastic
membranes of different thicknesses at 24.900 ......... 109
8. Effect of thickness of Silastic membrane on the
specific rate of diffusion (rate of diffusion/concen-
tration x area) of 4'-aminopropiophenone from
phosphate buffer solutions at 24.900 ................. 110
9. Effect of concentration of 4'-aminopropiophenone in
phosphate buffer solution on the rate of diffusion
of the drug through 3 mil Silastic membrane at
24.900, 33.600, and 41.00 ............................ 111
10. Arrhenius plot for the apparent diffusion constants
of aminoalkylphenones through 3 mil Silastic membrane
from phosphate buffer solutions ...................... 112
LIST OF FIGURES (Continued)
11. A plot of log (C2 CI/CO) versus time for quasi-
steady state diffusion of 4'-aminopropiophenone from
phosphate buffer through 5 mil Silastic membrane at
25.00 ................................................ 113
12. Apparent diffusion constants of 4'-aminopropiophenone
through 3 mil Silastic membrane at 24.900 as a
function of pH of the drug solutions ................. 114
13. Effect of ethanol on the rate of diffusion of 4'-
aminopropiophenone through 3 mil Silastic membrane
from solutions in phosphate buffer containing varying
percentages of ethanol at 25.00 ...................... 115
14. Diffusion of barbituric acid derivatives through 3
mil Silastic membrane from solutions in pH 4.7
acetate buffer at 37.30 .............................. 116
15. Diffusion of barbituric acid derivatives through 3
mil Silastic membrane from solutions in pH 4.7
acetate buffer at 37.30 .............................. 117
16. Apparent diffusion constants of barbital and pento-
barbital through 3 mil Silastic membrane as a
function of the pH of the drug solutions ............. 118
17. Arrhenius plots for the apparent diffusion constants
of barbituric acid derivatives from solutions in pH
4.7 acetate buffer through 3 mil Silastic membrane ... 119
18. Plots for the diffusion of phenylalkylamines from
solutions in borate buffer through 3 mil Silastic
membrane at 25.0 ..................................... 120
19. Plots for the diffusion of dextromethorphan from
solutions in pH 10.1 borate buffer, peanut oil, and
mineral oil through 3 mil Silastic membrane at 37.50 121
20. Plots for the diffusion of progesterone from
solutions in phosphate buffer and peanut oil through
3 mil Silastic membrane at 37.50 .................... 122
21. Plots of the apparent diffusion constants of
barbituric acid derivatives through 3 mil Silastic
membrane at 25.00 versus partition coefficients for
the compounds between acetate buffer solutions and
LIST OF FIGURES (Continued)
22. Plot of the apparent diffusion constants of amino-
alkylphenones through 3 mil Silastic membrane at
25.00 versus the partition coefficients for the
compounds between solutions in phosphate buffer
and chloroform ....................................... 124
23. Plot of the apparent diffusion constants of
barbituric acid derivatives through 3 mil Silastic
membrane at 25.00 versus reciprocal of the solubility
of the compounds in pH 4.7 acetate buffer solutions
at 25.00 ............................................. 125
24. Plot of the apparent diffusion constants of 4'-amino-
propiophenone from solutions in phosphate buffer
containing varying percentages of ethanol versus the
reciprocal of solubility in the respective buffer
solutions ............................................ 126
LIST OF APPENDICES
C I Derivation of Sutherland and Einstein Equation ..... 128
C II Derivation of Fick's Second Law of Diffusion ....... 130
C III Partial Derivation of Equation for Calculation of
Diffusion Constants by Time-lag Method ............. 131
C IV Derivation of Equations for Quasi-steady State
Diffusion .......................................... 133
C V Berthier Method for Calculation of Diffusion
Constants .......................................... 136
C VI Statistical Analysis of Thickness Measurements
Obtained on 3 mil Silastic Membranes ............... 138
C VII Regression Analysis of Raw Data for Diffusion of
4'-Aminopropiophenone .............................. 139
C VIII Analysis of Variance of Data Collected to Show the
Reproducibility of Rates of Diffusion of 4'-Amino-
propiophenone through 3 mil Silastic Membranes ..... 140
C IX Effect of Ionic Strength on Diffusion of 4'-Amino-
propiophenone through 3 mil Silastic Membrane into
200 ml. of 0.12 N HCI at 23.00 as a Function of
lonic Strength of Drug Solution in Phosphate Buffer. 143
C X Specific Rates of Diffusion of 4'-Aminopropio-
phenone Obtained as a Function of Hydrostatic
Pressure Exerted on Silastic Membrane at 25.00...... 144
The differential transport of substances in solution through
membranes has been used for separations based on differences in
molecular weights (1-6). The selectivity of membranes to penetrants
has been used to enrich or separate mixtures of gases (7, 8, 9) based
on the basic information obtained'on the transport of gases and water
vapors through membranes (10). The models of transport processes in
the artificial membranes are being used to explain the possible
mechanisms for the passage of nutrients and drugs across a succession
of membranes in the living organism (11-15).
However, fundamental studies on the diffusion of drugs
through artificial membranes to formulate optimal dosage forms are
scarce and mostly qualitative in nature (16, 17, 18). Basic and
quantitative information is needed to predict the transference of
drugs in solution through membranes to provide proper dosage forms
based on known values of release of drugs through these membranes.
The purposes of this investigation were to determine and
quantify the basic factors that influence the diffusion of drugs
through synthetic polymeric membranes. The rates of diffusion were
to be correlated with all measurable physical and chemical para-
meters that could be used to predict the diffusivities of drugs.
Ultimately it was planned to test the predicted in vitro diffusion
of drugs from pharmaceutically proper dosage forms.
A membrane is an imperfect barrier separating two fluids,
whether gases or liquids. Membrane technology (19) and the applica-
tion of polymeric materials in the medical and health related
professions (20, 21) have been discussed in recent fine reviews.
Diffusion is defined as the tendency for molecules to migrate
from region of high concentration to a region of lower concentration
and is a direct result of molecular movement. Dialysis is a term
applied to the use of membranes for the separation of particles of
colloidal dimensions from the molecules of suspending liquid and is
a consequence of diffusion (22).
Dialysis, as observed in transport through cellophane and
animal membranes and parchment papers (23, 24, 25), is due to sieve
action. Such membranes may be considered as heterogeneous barriers
in the sense that they possess pores. The transport is generally a
measure of the probability of a solvated molecule entering and diffusing
through the pores. There is little selectivity in the separation of
two closely related molecules except when their size is approximately
that of the size of the pore (26). In general, the solvent as well
as the solute is transported; membranes which allow salt transport
are permeable to water (27).
Dialysis, where a membrane acts as a barrier to free diffusion
of a substance in an isotropic medium, is largely dependent on the
molecular weight of the diffusate and the viscosity of the solvent.
The Sutherland and Einstein equation (28) for spherical colloidal
particles is (Appendix C-1)
D = (RT/6/rn N) (4TIN/3 M v)1/3 (Eq. 1)
where D is the diffusion coefficient, R is the molar gas constant, T
is the absolute temperature, n is the viscosity of the solvent, N is
the Avogadro's number, M is the molecular weight and 7 is the partial
specific volume of the solute. An empirical relation correlating
diffusion constant, molecular weight and viscosity is (29)
D = 7.4 X 10-8 (XM)05 T/n 7 0.6 (Eq. 2)
where X is an association parameter defining the effective molecular
weight of the solvent with respect to the diffusing species (for water
X = 2.6).
Dialysis has been used for the separation of the components
of blood (26, 30), the transfer of macromolecules in an artificial
kidney (31), the fractionation of high polymers (25), the correlation
of molecular size and structure with transport rates (1-6.) and the
determination of the binding of drugs and chemicals to proteins and
macromolecules (32). Ion-exchange resins have been used extensively
for the separation of charged particles or molecules (33-37). Electro-
dialysis, where electromotive force is used as the driving force for
the separation of ionic solutes, has been used for the recovery of
salts from sea water (38, 39, 40), the recovery of acids from spent
acid solutions (41), and the partial demineralization of milk and
Many polymeric membranes such as copolymers based on poly-
oxyethylene glycols and polyethylene terephthalate act as homogeneous
barriers (26). Transport is generally dependent on the relative adsorp-
tion of the molecules diffusing to the face of the membrane and
solubility of these molecules in the membrane (26). The selectivity
of polymeric membranes such as polystyrene (43) and ethyl cellulose
(44) has been used for the enrichment of air with respect to oxygen
and attempts to make these processes industrially feasible have been
reported (45). The diffusion of gases in polymers follows the
Arrhenius equation (46) and the temperature dependence of diffusion
is given by
D = D, e -Ea/RT (Eq. 3)
where Do is a constant, and AEa is the apparent activation energy for
The methods to determine the diffusivity of water vapors
through membranes have been considered in great detail (10). The
uptake of moisture by hygroscopic substances could not be prevented
by encapsulating them in gelatin capsules (47). /The permeation of
water vapors decreased with increased chain length of the acid moiety
of cellulose ester membranes (48). The water vapor transmission
initially decreased and then increased asthe concentration of the
plasticizers in polymer was gradually increased (49). The solubility
of water and the Arrhenius parameters for the diffusion of water in
a polymer differ above and below the temperature at which the slope
of the volume-temperature curve for the polymer changes (50)--the
"glass temperature" (51).
The extensive use of polymeric materials in the pharmaceutical
industry for packaging purposes has initiated the investigation of
polymer-drug interactions. Kapadia et al. (52) showed interaction
between salicylic acid and Nylon 66 and calculated the heat of sorption
from equilibrium sorption studies at several temperatures using the
van't Hoff equation (53). The heat of sorption for this and in the
subsequent work with other weak organic acids(54, 55) was low (1-
4 Kcal./mole). The magnitude of the sorption decreased with decreases
in the polarity of the solvent. The pH-sorption studies implied inter-
action of tne unionized acids with the basic group in the polyamide.
The diffusion within the polymeric material was shown to be the rate
determining step with the heats of activation in the range of 10 20
Kcal./mole. Studies on the sorbic acid-Nylon interactions (56-60)
confirmed the results obtained with weak organic acids.
One of the possible important uses of polymeric materials in
the pharmaceutical industry is as coating material for sustained
release products (61 -64). Vinyl, acrylic and cellulosic polymers
were shown to be good for prolonged action coatings based on their
solubility in simulated gastric and intestinal fluids (62). Copolymer-
coated prednisolone tablets extended absorption of prednisolone over a
period of 10 12 hours in intact dogs and in segments of intestinal
tracts (63). Silicone rubber which has been used extensively in the
subcutaneous prosthetic devices (65) has been shown to be permeable
to steroids (16), cardiac pacers (66) and some other materials (17, 18).
Lyman and coworkers (26, 67, 68) have prepared synthetic
membranes with the express intent of endowing them with specific
characteristics which would transfer substances by an adsorption and
solubility mechanism and not by sieve action. When the weight percent
of polyoxyethylene glycol, the hydrophilic monomer in the copolyether-
ester membranes was increased, the rates of transfer of glucose and
urea increased. The magnitude of the increase in rates was different
for the two compounds and the authors postulated a different degree of
association or partitioning with the membrane (30).
MATHEMATICS OF TRANSPORT PROCESSES
The mathematics of transport processes depends upon the model
chosen for the particular transport phenomenon under consideration.
This has been discussed in detail by Tuwiner (69), Jost (70), Crank
(71) and Lakshminarayaniah (72). Higuchi and Higucn; (73) have giv '-
a theoretical analysis of diffusional movement through heterogeneous
A transport process is considered to be in the steady state
when the amount of penetrant passing through a reference point in a
membrane matrix is invariant with time. When the amount passing
.'l-ruh a reference point varies with time, the process is said to be
in the non-steady state.
When a membrane is interposed between a solution of the
penetrant and a solvent, the penetrant is transported initially by
a non-steady state process. This ag period" continues until the
amount leaving the membrane is equal to the amount entering. A steady
state results when the concentrations of the solutions on either side
of the membrane are kept constant. However, if the concentration of
the solutions are allowed to equilibrate, the amount entering and
leaving may be equal for all analytical purposes even though the rate
of permeation will be changing with time as a function of the concen-
tration gradient. This may be termed a "quasi-steady state" transport.
The rate determining factor in non-steady state transport
through the membrane is the rate of diffusion in the membrane; in steady
state transport it is the constant concentration gradient alone; and
in quasi-steady transport both the concentration gradiert and the rates
of approach to equilibrium of both extra-membrane phases are rate
determining. Fick's first law of diffusion (74) states that the
rate of diffusion is proportional to the concentration gradient,
dA/dt = DSdC/dx
where A is the amount of penetrant in moles diffusing in time t in
seconds through a membrane having a surface area of S cm.2. The
concentration gradient is dC/dx across the membrane in moles per
liter-cm. and D is the diffusion coefficient in cm.2/sec. The term
x is the distance into the membrane in cm. The concentration of the
penetrant at a particular position in a polymer at a given time is
given by Fick's second law of diffusion
dC/dt = D S d2C/dx2
where d2C/dx2 is the change in the concentration gradient as a function
of the distance x within the membrane. The derivation of Fick's second
law from Fick's first law is given in Appendix C-Il.
For a steady state condition, the change in the concentration
at any point in the membrane is zero.
dC/dt = 0
D S d2C/dx2 = 0
and the concentration gradient is a constant
dC/dx = K
It follows from Eqs. 4 and 8 that
dA/dt = D S K = D S (C2 C1)/X
where C2 and C1 are the invariant concentrations of the concentrated
and dilute solutions in contact with the membrane surfaces respectively
and X is the thickness of the membrane. This states that the rate of
diffusion through a membrane of thickness X and surface area S is
constant with time for a constant concentration gradient. Thus the
total amount diffused can be plotted against time to obtain a straight
line with slope of D S K from which the diffusion constant D can be
calculated when S, C2, C1 and X are known.
A more rigorous mathematical treatment considers the concen-
trations of the penetrant at the membrane surfaces or in the first
monolayer of the membrane material, instead of the concentrations in
the solutions for the concentration gradient in Eq. 9. This equation
may be written as
dA/dt = D' S (Cm2 Cml)/X (Eq. 10)
where Cm2 and Cml are the concentrations of penetrant at the two
membrane surfaces and D' is the intrinsic diffusion constant. The
concentrations are related to the activity of the penetrant in the
Cm = ami/m and Cm2 = am2/xm (Eq. 11)
where aml and am2 are the activities of the penetrant at the two surfaces
and m is the activity coefficient of the penetrant in the membrane
The relation between activities and concentration of penetrants
in the solution is given by
asl = CIYs and as2 = C2~s
where asl and as2 are the activities of the penetrant in the solutions
and s is the activity coefficient of the penetrant in the solution.
Assuming a rapid equilibration of penetrant between solutions and
membrane material at the surfaces
as1 = aml and as2 = am2 (Eq. 13)
From Eqs. 11, 12, and 13 it follows that
Cml = Ci 's/Ym and Cm2 = C2 Ys/Am (Eq. 14)
When these values of Cm1 and Cm2 are substituted in Eq. 10,
the following expression is obtained
dA/dt = (D' S s/X Ym) (C2 C1) (Eq. 15)
Since the ratio of activity coefficients is the partition
Is / m = Kp (Eq. 16)
Eq. 15 may be rewritten as
dA/dt = (D' SKp/X) (C2 C1) (Eq. 17)
The product D'Kp is the term D in the original Fick's
diffusion equation (74) and is termed the permeability constant for the
The time lag method used extensively (46, 75, 76) for the
calculation of diffusion constant in the non-steady state is based on
the premise that a finite amount of time will be needed for a penetrant
to traverse the thickness of the membrane before the attainment of a
steady state (76). After this initial lag period, when the concen-
trations on both sides of the membrane are constant, the plot of the
amount diffused versus time will be a straight line (Eq. 17) which
when extrapolated will give an intercept on the time axis.
0 = X2 / 6 D' (Eq. 18)
The mathematical basis for this method is
A = D S C2 (t X2 / 6 D)/ X (Eq. 19)
derived partially in Appendix C-Ill. This equation may be written in
terms of the partition coefficient Kp and the intrinsic diffusion
coefficient D' as (73)
A = (D' S Kp C2 / X) (t X2 / 6 D') (Eq. 20)
Thus the slope of the plot of the amount diffused versus
time, for a constant concentration gradient in the membrane, will be
D' S Kp C2/X. The Eq. 20 is the more rigorous version of Eq. 17 with
C1 = 0.
In the equilibrium diffusion experiments, the concentration
gradient across the membrane decreases to zero (77). The mathematical
expression derived for this quasi-steady state is given in Appendix C-
IV and is (78)
[XV1V2/S(V1 + V2)] In (C2 C1)/Co) = -Dt (Eq. 21)
where VI and V2 are the volumes of the solutions in the compartments
with molar concentrations CI and C2. CO is the concentration in one
compartment at t = 0. When the volumes V in the two compartments
are equal Eq. 21 simplifies to
(XV/0.869 S) log (C2 CI/Co) = -Dt (Eq. 22)
An alternate method is the method of Berthier (79) wherein
the fractional uptake has been used for the calculation of diffusion
constants (52). This method has been explained in Appendix C-V.
RILSAN (Nylon 11) (May Industries Inc., Atlanta, Ga.) is 11-amino
undecanoic acid polymer (80).
POLYPENCO (Nylon 101) (The Polymer Corporation, POLYPENCO Division,
Reading, Pa.) is a polyamide (81).
Cellulose Acetate (KODACEL A 29), Cellulose Triacetate (KODACEL TA 401),
and Cellulose Acetate Butyrate Sheets (KODACEL B 298) (Eastman Chemical
Products, Inc., Kingsport, Tennessee) are thermoplastic cellulosic
Polyethylene Type B, Mylar Polyester Type S (E.I. DuPont De Nemours
and Co., Inc., Wilmington, Delware). Mylar Polyester is polyethylene
terephthalate (83, 84).
Polypropylene (Avisun Corporation, 215, 12th Street, Philadelphia, Pa.)
Silastic Medical Grade Sheeting (H-0169, H-0293) (Dow Corning Center
for Aid to Medical Research, Midland, Michigan) is a dimethylsiloxane
The following chemicals were supplied by Abbott Laboratories, North
Amobarbital Equivalent weight.-Calculated for C11H18N203: 226.28.
Barbital Equivalent weight.--Calculated for C8H12N203: 184.20.
Butabarbital Equivalent weight.--Calculated for C10H16N203: 212.23.
Cyclobarbital Equivalent weight.--Calculated for C12H16N203: 236.26.
Diallylbarbituric Acid Equivalent weight.--Calculated for C10H12N203:
208.21. Found: 201.72.
Mephobarbital Equivalent weight.--Calculated for C13H14N203: 246.26.
Metharbital Equivalent weight.-Calculated for C9H14N203: 198.23.
Found: 200.92. *
Pentobarbital Equivalent weight.--Calculated for C11H18N203: 226.26.
Phenobarbital Equivalent weight.-Calculated for C12H12N203: 232.24.
Secobarbital Equivalent weight.--Calculated for C12H18N203: 238.29.
Thiamylal Equivalent weight.--Calculated for C12H17N202S: 254.34.
Thiopental sodium Equivalent weight.-Calculated forC11H18N202S:
242.33. Found: 246.30.
The following chemicals were supplied by Smith Kline and French
Laboratories, Philadelphia, Pa.
a-methyIphenethylamine hydrochloride Equivalent weight.--Calculated
for CgH13N.HCI: 171.5. Found: 165.3.
a-ethylphenethylamine hydrochloride Equivalent weight.--Calculated*
for C10H15N.HCI: 185.5. Found: 203.1.
2-amino-4-methyl-4-phenylpentane hydrochloride Equivalent weight.-
Calculated for C12HI9N.HCI: 213.5. Found: 205.31.
3-amino-1-phenylbutane sulfate Equivalent weight.--Calculated for
(C10H15N) *H2SO4: 198. Found: 242.13.
1-methyl-5-phenylpentylamine hydrochloride Equivalent weight.-
Calculated for C12H19N-HCl: 213.5. Found: 355.93.
The following compounds were purchased from Eastman Organic Chemicals,
Rochester, 3, New York.
4'-Aminopropiophenone Melting point 139 140.50; literature value
4'-Amir:i..: t.:then:.r, Melting point 105 1060; literature value
3'-Aminoacetophenone Melting point 97 990; literature value 990
The following compounds were supplied by The Upjohn Company, Kalamazoo,
Progesterone Melting point 128 1300; literature value 127 1310
Cortisone Melting point 230 2360; literature value 236 2400 (87).
Hydrocortisone Melting point 217 2200; literature value 217 2200
Prednisolone Melting point 239 2400; literature value 240 2410
The following compounds were purchased from National Biochemical
Corporation, Cleveland, Ohio.
Sulfathiazole Melting point 200 203; literature value 200 -
Sulfisoxazole Melting point 195 1980; literature value i940
Sulfabenzamide Melting point 181 1830; literature value 181.
Sulfadiazine Melting point 256 2570; literature value 252 -
Tetraethylthiuram disulfide (Disulfiram) (Ayerst Laboratories, Incor-
porated, New York, N. Y.) Melting point 70 720; literature value
Dextromethorphan (Vick Divisions Research and Development, Richardon-
Merrell Inc., Mt. Vernon, New York).
Peanut oil and Mineral oil used were of United States Pharmacopoecial
Methods of Analysis
All compounds used in this investigation except ethanol were
analyzed spectrophotometrically by the Cary Model 15 dual beam recording
spectrophotometer, Beckman Model DU spectrophotometer or Beckman Model
DU-2 spectrophotometer. Standard, square, silica cells of 10 mm.
light path (Sargent /# S-75730) were used. All spectrophotometric
measurements were made at 24.0 1.00.
Spectrophotometric measurement of absorbances was used for
the quantitative estimation of barbituric acid derivatives, amino-
alkylphenones, progesterone, phenylalkylamines and dextromethorphan.
The linear relationship between the absorbance of drug solutions and
their concentrations in accordance with Lambert-Beer's law (88) was
verified in all cases. A few representative calibration curves are
shown in Fig. 1.
The absorbances of solutions of barbituric acid derivatives
were measured in pH 10.1 borate buffer as the molar absorptivities
(C values) of their anionic forms are much higher than those in the
uncharged form. Absorbances of all barbituric acid derivatives except
thiamylal and thiopental were measured at a wavelength of 238 mp.
The wavelength of 238 ml is not the /max of all these compounds.
However, measurements at one wavelength were found to be convenient
and time saving. The absorbances of thiamylal and thiopental were
measured at their max of 304 mL. The wavelengths of absorbance
measurement and the E values at these wavelengths are recorded in
The absorbances of dextromethorphan, progesterone and amino-
alkylphenones were measured in pH 6.8 phosphate buffer solutions. The
wavelengths at which the absorbances were measured and the values
at these wavelengths are shown in Table I.
The phenylaklylamines-a-methylphenethylamine, a-ethyl-
phenethylamine, 1-methyl-5-phenylpentylamine, 3-amino-l-phenyl-butane
and 2-amino-4-methyl-4-phenylpentane were analyzed colorimetrically
by the method of Gettler and Sunshine (89) modified in the following
manner: A 1.00 ml. sample of phenylalkylamine (1.5 X 10-5 1.0 X
10-4 M) in pH 6.8 phosphate buffer was transferred to a 6" x 5/8"
pyrex test tube. The solution was made alkaline with .0.1 ml. solution
of 2 N NaOH and was mixed on a Vortex Jr. Mixer. Five ml. of chloro-
form (A. R. Grade) was then added and mixed on the Vortex Jr. Mixer
for about 1 minute. The solution was centrifuged at about 3,200
r.p.m. for 3 minutes. A 4 ml. pipette was inserted through the
aqueous layer into the chloroform layer blowing lightly through the
pipette to avoid entry of the aqueous solution. Four ml. of the
chloroform was removed and transferred into a test tube of similar
dimensions. Two-tenth ml. of freshly prepared methyl orange reagent
(equal volumes of saturated solutions of methyl orange and boric
acid in water) was added and mixed for about one minute. This solu-
tion was centrifuged at 3200 r.p.m. for 3 minutes and 3 ml. of the
chloroform layer was carefully pipetted into another test tube. Then
0.2 ml. of absolute ethanol containing 2% concentrated sulfuric acid
was added. The solution was mixed and then transferred to a cuvette
for the measurement of absorbance against the chloroform layer obtained
from a phosphate buffer blank treated identically. The absorbance was
measured on a Beckman DU spectrophotometer at 520 mp. A five point
calibration curve was prepared each day the samples from the diffusion
The remaining compounds cortisone, hydrocortisone, predniso-
lone, sulfadiazine, sulfathiazole, sulfisoxazole, sulfabenzamide and
disulfiram were used only in the screening part of this investigation.
Spectrophotometric methods were used only for their qualitative
analysis and no atTempt was made to verify the absorbance concentra-
Since the Xmax and c values of all steroids were independent
of the pH of the solution, the spectra of their solutions were obtained
without any adjustment of pH.
The solutions of sulfadiazine, sulfathiazole, sulfisoxazole
and sulfabenzamide were made alkaline with NaOH solution before
obtaining their U.V. spectra on the Cary spectrophotometer. The \ max
and the 6 values of these compounds are given in Table 1.
Disulfiram was qualitatively analyzed by a method involving
chelation with copper ion (90). Five ml. of 0.02 M CuSO4 solution in
pH 6.5 phosphate buffer was added to 10 ml. of solution of disulfiram
in phosphate buffer. The solution was mixed on Vortex Jr. Mixer. The
copper complex formed was extracted into 5 ml. of ethylene dichloride
with vigorous mixing on the Vortex Jr. Mixer for 1 minute. The
ethylene dichloride layer was separated and its absorbance was measured
against the ethylene dichioride layer obtained from identical treat-
ment of a phosphate buffer blank. The disulfiram-copper complex showed
two peaks at wavelengths of 272 and 285 mu.
Ethanol was quantitatively determined using vapor phase
chromatography. An F & M Model 700 Gas Chromatograph with flame
ionization detector was used with a 4' x 1/4" o.d. stainless steel
column packed with 2~:' Carbowax 20 M. on 60-80 mesh Chromosorb W,
worked isothermally at 50.00. The temperature of the detector was 250.00,
and that of injection port was 150.00. The carrier gas, helium, was
used at a pressure of 30 psig. while hydrogen and air for the flame
were used at 10 and 40 psig. respectively. Samples of five microliters
of aqueous ethanol were injected without any prior treatment. The
peak heights were measured and the concentration of the ethanol in
the sample was obtained from a calibration curve prepared with ethanol
solutions of known concentrations.
Determination of pKa.-The pKa is the negative logarithm of
the acid dissociation constant. The pKa's of barbituric acid deri-
vatives, dextromethorphan and phenylalkylamines were determined by
potentiometric titration. The pKa of 4'-aminppropiophenone was
The potentiometric titrations were performed with a Sargent
Model D automatic titrator. The titrator was equipped with a syringe
with a titrant capacity of 2.5 ml. The pH scale of the titrator was
standardized with two of the pH 4.0, 7.0 and 10.0 standard Beckman
buffer solutions (Beckman Instrument, Inc., Fullerton, California)
bounding the pH range to be titrated. The accuracy of the pH measure-
ment was 0.05 pH unit. All titrations were performed at 24.0 1.00.
The barbituric acid derivatives were dissolved in 2 ml. of
0.04 N NaOH and the solution was diiuted with freshly boiled distill ed
water. A 20 ml. a iquot of this solution was titrated with 0.1 N HC!04
on the titrator.
The acid salts of phenylalkylamines were dissolved in
distilled water to contain about 5 milliequivalent of accurately
weighed compound in 30 ml. of distilled water. The solutions were
titrated on the titrator against 0.1 N NaOH.
A weighed quantity of dextromethorphan was dissolved in
2 ml. of 0.01 N HCI and the solution was diluted to 40 ml. This
solution was titrated against 0.1 N NaOH. In all these titrations
an equal volume of the blank solution was titrated under identical
conditions. The calculated equivalent weights are given in the
section on materials.
The pKa of a compound was determined from these titration
curves by the method of Parke and Davis (91) wherein the difference
between the volumes of the titrant for attaining the same pH for the
sample and blank solutions was plotted against the pH. The pH value
corresponding to the midpoint of the resultant sigmoidal curve was
the half neutralization point or the pKa of the compound.
The pKa of 4'-aminopropiophenone was determined spectrophoto-
metrically. Hundred ml. of 8.56 X 10-5 M solution was used. The pH
of the solution was measured on the Beckman Expanded Scale pH meter
standardized with pH 4.0 and 7.0 standard Beckman buffer solutions.
An ultraviolet spectrum of this solution was obtained against a
distilled water blank on the Cary Recording spectrophotometer. The
sample solution in the cuvette was returned to the bulk solution. A
drop of concentrated HCI was added and the solution was stirred on a
magnetic stirrer. The pH of the solution was measured and a U.V.
spectrum was obtained again. Thus, U.V. spectra of the solutions were
obtained for several pH-values. The absorbance of th. sc .ons at a
wavelength of 307 mp was measured from recorded spectra and plotted
against the pH of the solutions. The pH value corresponding to the
midpoint of the resultant sigmoidal curve was the half-neutralization
point or pKa of 4'-aminopropiophenone. The pKa values of the compounds
are reported in Table I.
D :- r~-.i-', --I f 6 -.-A weighed amount of barbituric
acid derivative was dissolved in 2 ml. of 0.04 N NaOH solution and
diluted to a total volume of 25 ml. with freshly boiled distilled
water. Twenty ml. of the solution was pipetted into a 30 ml. beaker
and titrated against 0.10509 N HCI04 on the Sargent Model D automatic
titrator. The molarity of the compound present in the 20 ml. solution
was calculated from the volume and normality of HC104 used in the
titration. The remaining 5 ml. of the solution was diluted with pH
10.1 borate buffer solution to obtain 5 different concentrations of
the solution whose absorbances were then measured on The Beckman DU
spectrophotometer at wavelength of 238 mu for all the barbituric acid
derivatives except thiamylai and thiopental whose absorbances were
measured at 304 mi. The absorbances so obtained were then plotted
against the molarity of the solution and E values were calculated from
the slope by the equation (88)
S= Absorbance / Concentration (Eq. 23)
The compounds 4'-aminopropiophenone, 4'-aminoacetophenone, 3'-amino-
acetophenone, progesterone, dextromethorphan were weighed accurately
into a volumetric flask. These were dissolved in distilled water and
the absorbances of the adequately diluted soluTions were measured.
The C values were then calculated by Eq. 23. The C values are reported
in *- .'. '; I .
Screening of Polymer Films for the Permeability To Drugs
The polymer films Rilsan, Polypenco, cellulose acetate,
cellulose triacetate, cellulose acetate butyrate, polyethylene, Mylar
polyester, polypropylene and Silastic were used in thicknesses of 5,
5, 5, 3, 3, 3, 3, 5, and 3 mil respectively.
The membranes were washed in running tap water and then with
distilled water. These membranes were then dried and cut into small
pieces ("1 x -").
Saturated solutions of 4'-aminopropiophenone, 4'-aminoaceto-
phenone, 3'-aminoacetophenone, sulfadiazine, sulfathiazole, sulfabenza-
mide, sulfisoxazole, disulfiram, barbital, phenobarbital, progesterone,
cortisone, prednisolone in 0.1 N HCI, 0.1 N NaOH, pH 6.8 phosphate
buffer, propylene glycol, peanut oil, ethanol, mineral oil, ethylene
glycol and polyethylene glycol 200 were used.
Glass serum vials of 10 ml. capacity wiTh rubber stoppers and
aluminum caps were thoroughly cleaned and dried. A hole was bored
into the rubber stopper with a # 3 cork-borer. About 2 ml. of a
saturated solution of a drug under study was delivered into the serum
vial without touching its lip. The vial was stoppered with a rubber
stopper with a hole in it. A small piece of the membrane under study
was laid flat on the stopper to cover it completely and an aluminum
cap was then crimped on the rubber stopper so as to sandwich the
membrane between the two. The sketch of an assembled vial is shown
in Fig. 2. The detachable circular disc in the aluminum cap was
removed to expose the membrane. The serum vial was then inverted
and placed into a two oz. ointment jar conTaining about 10 ml. of pH 6.8
phospha e buffer. The jars were capped, properly labelled and set
aside for at least one week.
After this time the phosphate buffer solution in each jar
was qualitatively analyzed on a Cary Model 15 recording spectrophoto-
meter. In the cases of the barbituric acid derivatives and sulfonamides,
the phosphate buffer solutions from the jar were made alkaline before
reading Them on the spectrophotometer. Disulfiram was chelated with
copper and the chelate was extracted into ethylene dichloride. The
spectrum of the ethylene dichloride solution was then obtained on the
A drug was said to have permeated through a membrane when the
peaks at wavelengths corresponding to the Xmax, values of the drug
were observed in the spectrum. Experiments with positive results were
repeated to avoid the acceptance of leaking vials as evidence for
permeation. Solvents without drugs were put in the vials to test
their permeability through the membranes and their effect on the
rubber stopper and the membranes.
The diffusion apparatus consisted of diffusion cells, beakers,
Durrum 12-Channel Dial-A-Pump, and a stirring device.
Diffusion cells.--Two types of diffusion cells were designed
to study the steady state and quasi-steady state diffusion.
The steady state diffusion cell designed was a modification
of the cell used by Lyman et al. (68). An exploded schematic view of
the diffusion cell is shown in Fig. 3. It consisted of two stainless
steel ;iates (2 1/2" x 2" x 1/8"), a glass T joint, two silicone
rubber gaskets (2.57 cm. i.d. x 2.95 cm. o.d. x 0.16 cm. thick) and
a set of four stainless steel; nuts and bolts.
Each of the stainless steel plates had a hole in the center,
2.57 cm. in diameter. Around this hole there was a border 0.43 cm.
in width from the perimeter of the hole and recessed 2 mm. into one
face of the stainless steel plate. The silicone gaskets fit into the
recessed borders around the holes. The two plates had four holes in
four corners for the stainless steel bolts.
The glass T joint consisted of a 3 cm. long hollow glass
cylinder 2.60 cm. in diameter joined in its center at right angles on
one side to an 11 cm. long glass tube 0.8 cm. in diameter to form a
hollow T shaped device. The two annular edges of the glass cylinder
fitted into the recessed borders against the gaskets in the plates.
A piece of membrane under study was positioned between the
recessed border of each of the plates and a silicone rubber gasket.
The open ends of the glass cylinder were then fitted into the recessed
borders on the face of the silicone rubber gaskets. The whole
assembly was held in position by a set of four nuts and bolts
passing through the holes in the corners of the plates. The volume
of the assembled cell was approximately 22 ml. and the tot-i area of
the two membranes available for diffusion was 10.4 cm.2.
The quasi-steady state diffusion cell (see Fig. 4) consisted
of two stainless steel plates (2 ,/2" x 2 1/2" x 1/8"). Eac of these
plates was welded to one end of a stainless steel tube (1 1/2" in
diameter and 7" long) at an angle of 45. Each plate had a hole in
the center (3.1 cm. in diameter) with a circular border 0.8 cm. in
width from the perimeter of the hole and recessed 2 mm. into the face
of.the plate. The plates had four holes in the four corners for the
stainless steel bolts. Two silicone rubber ga(ets (3.1 cm. i.d. x
3.8 cm. o.d. x 3 mm. thick) fitted into the recessed border around the
holes in the plates. A membrane under study was clamped between the
two silicone rubber gaskets placed in the recessed borders in the
plates. The plates .were then clamped together with four stainless
steel nuts and bolts. The diffusion cell when assembled formed a V
shaped device and held 150 ml. of solution in each of the tubes. The
area of the membrane available for the diffusion was 7.55 cm.2
Beakers.--A 200 ml. or 400 ml. Pyrex beaker was used with
each steady state diffusion cell. A plastic cover was fabricated for
each beaker from a disposable polyethylene Petri dish. The rim of the
Petri dish was sawed off at one position to accommodate the beak of the
beaker. A hole, 1/2" in diameter, was burned into the center of the
cover with a hot glass rod. Another hole of the same size was made
in the cover about 1" away from the center.
Durrum 12-Channel Dial-A-Pump.--This pump (Durrum Instrument
Corporation, 925, E. Meadow Drive, Palo Alto, California) was used for
pumping a solution from a reservoir into the diffusion cell and back.
The pumped fluids entered the pumping unit by way of plug-in flexible
plastic tubing, received their pumping action by a flat pressure pate
vertically cycling against a series of independently, adjustable backing
blocks, and emerged by way of another plug-in connection similar to
input. The resilient pumping tubes used were amber latex surgical
tubing (Rubber Latex Products, Inc., Cuyaholga Falls, Ohio) 5/1"" o.d.
x 3/16" i.d. Each channel used two pieces of vinyl :;L.ng (Becton,
Dickinson and Company, Rutherford, N. J.) 3/16" o.d. x 1/16" i.d.
One piece of the tubing carried the solution from a container through
the plug-in connection into the pumping tube. The exit was connected
via a plug-in connection to another piece of tubing that delivered the
solution from the pumping tube to its destination.
Stirring device.--A thermostated shaker bath (American Instru-
ment Company, Silver Spring, Maryland and Eberbach Corporation, Ann
Arbor, Michigan) shook the beakers and agitated the solutions in the
beakersbesides keeping the temperature of the solutions in the diffu-
sion cell assembly constant within 0.50. The shaking rate of the
ban was 108 strokes per minute with each stroke traveling a distance
of 1 1/2".
Two other stirring devices tried were Mag-Jet stirrers
(Will Scientific, Atlanta, Georgia, Catalog # 25212) and nitrogen
bubbling. The Mag-Jet stirrer kept underneath a beaker was operated
by water pressure and rotated a magnetic stirring rod in the beaker
and another magnetic stirring rod in the diffusion cell kept in the
beaker. The speed of the Mag-Jet stirrer decreased slowly to zero in
5 to 6 hours probably due to deposition of salts in the small clearance
between the rotor and its metal casing.
Nitrogen under pressure was bubbled through the solutions in
the diffusion cell and the beaker, using narrow bore glass tubing.
It was observed that the solutions were stirred only at the spot of
the bubbling. The bubbling of the gas was, therefore, not useful for
stirring a large volume of the solution in the beaker. Also the
bubbling of the gas over a long period of time through the solution
caused evaporation of the solution. The volume of the solution in the
beaker was ai .'s cri-icai because the amount of drug diffusing through
the membrane was calculated from the concentration and the volume of
the beaker solution. When the solution used in the diffusion cell
was a saturated solution, the bubbling of the gas was used as a
stirring device. The volume of the solution in the diffusion cell
was not critical and the evaporation of the saturated solution did
not in any way affect the concentration of the drug in the solution.
The small volume of the saturated solution in the cell could there-
fore be well agitated by nitrogen gas bubbling method.
The Durrum 12-Channel Dial-A-Pump pumped the solution of a
drug in and out of the steady state diffusion cell and was observed
to be a good method of agitating the solution in the diffusion cell
(see Fig. 5). The drug solution from a reservoir was pumped into
the cell through one channel at the rate of 25-30 ml. per minute.
The end of the delivery tube from this channel reached the diametric
center of the horizontal portion of the glass T joint of the assembled
diffusion cell. Another tube carried the solution from the cell to
the channel which pumped the solution at the rate of 35-40 ml. per
minute. The end of the tube carrying the solution from the cell to
the pumping tube, was adjusted in the stem of the cell at the level
of the solution in the beaker. Since the channel carrying the solution
out of the cell pumped the solution at a rate faster than the other
channel pumping the solution into the cell, the level of the solution
in the cell was maintained at the level of the solution in the beaker.
The pulsating action of the pump kept the solution in the cell well
agitated besides renewing its contents every few minutes because of
the high turnover of the solution. The thermostated shaker bath held
the diffusion assembly rigidly in a metal rack fixed to the shaker
tray and agitated the solution in the beaker maintaining the tempera-
ture of the solutions constant.
The solutions in the arms of the quasi-steady state diffusion
cell were agitated by the thermostated shaker bath in which the cell
was wired to the shaker tray.
Saturated solutions of barbituric acid derivatives in pH 4.7
acetate buffer (p = 0.1) were prepared at 50.00. These solutions were
then allowed to cool to 25.00 in the thermostated shaker bath and to
equilibrate at that temperature for 48 hours in presence of excess
solids. Similarly saturated solutions of aminoalkylphenones were
prepared in pH 6.8 phosphate buffer (4 = 0.3) at 50.0 and equilibrated
to 37.50 in the thermostated shaker bath. The saturated solutions of
4'-aminopropiophenone in pH 6.8 phosphate buffer containing 0, 10, 20,
30 and 40% ethanol were prepared by vigorously shaking the solutions
at room temperature in presence of excess solids and allowed to equili-
brate at 25.00 in the thermostated shaker bath for 48 hours.
The solutions were filtered by suction through electrode
isolation tubes (E. H. Sargent & Co., 4647 West Foster Avenue, Chicago,
Illinois, Catalog # S-30417). The electrode isolation tube is a tube
fitted with a finely porous fritted glass membrane and is 125 mm. long
and 13 mm. in diameter in its lower section, 10 mm. in diameter in
upper section. Aliquots of the filtered solutions of barbituric acid
derivatives were appropriately diluted with pH 10.1 borate buffer.
Aliquots of filtered solutions of aminoalkylphenones were appropriately
diluted with pH 6.8 phosphate buffer. The absorbances of these diluted
solutions were measured on a spectrophotometer at the pertinent wave-
lengths reported in Table I. The solubilities of these compounds in
respective buffer solutions were then calculated from these absorbance
values and the knowledge of their E values reported in Table I.
The solubility values of 4'-aminopropiophenone in phosphate
buffer containing 0, 10, 20, 30 and 40% ethanol were plotted against
the concentration of ethanol. in the phosphate buffer solutions as
shown ;n Fig. 6. This curve was used as a calibration curve to obtain
the solubility of 4'-aminopropiophenone in phosphate buf.ier containing
7.5, 15, 22.5, 30 and 37.5% ethanol.
Determination of Partition Coefficients
The coefficients of partition between the solutions of
barbituric acid derivatives in acetate buffer, aminoalkylphenones in
phosphate buffer, and an organic liquid were determined at room tempera-
ture. The organic liquids used were chloroform, mineral oil, cyclo-
hexane and silicone liquid 200.
The aqueous media used for the preparation of the solutions
of these compounds and the organic liquids used for the partitioning
were saturated with respect to each other by shaking them together in
large quantity and then separating them, first by separatory funnel
and then by centrifugation at 3200 r.p.m. for five minutes.
An approximate 5 x 10-4 M solution of a barbituric acid
derivative was prepared in pH 4.7 acetate buffer presaturated with
chloroform. One ml. of this solution was diluted with 10 ml. of pH
10.1 borate buffer and its absorbance was measured on Beckman DU
spectrophotometer at wavelength reported in Table J. The blank
solution for the absorbance measurement consisted of acetate buffer
medium presaturated with chloroform and diluted 10 times with pH 10.1
borate buffer. The pH of the diluted solution was observed to be 10.1.
Generally the concentration of the compound in the solution used was
such that the diluted solution would have an absorbance between 0.5
and 0.7. Five ml. of the solution in acetate buffer was transferred
to a 10 ml. glass vial. Five ml. of chloroform presaturated with acetate
buffer was added to it. The vial was closed with a rubber stopper and
sealed with an aluminum cap. The solution was mixed on a Vortex Jr.
Mixer for 3 minutes. The solution was then centrifuged at 3200 r.p.m.
for about 3 minutes. About 3 ml. of the aqueous layer was withdrawn
from the vial with a glass syringe and a needle and transferred to a
test tube. One ml. of this solution was then diluted with 10 ml. of
pH 10.1 borate buffer and its absorbance was measured against an
acetate buffer blank treated identically. The difference between the
absorbances of the aqueous solution before and after the partitioning
represented the concentration of the substance partitioned into the
chloroform layer. The ratio of the difference in absorbances to the
absorbance of the aqueous layer after partitioning was the partition
coefficient for the barbituric acid derivative between its solution
in acetate buffer and chloroform.
The same procedure was used for studying the partitioning of
barbituric acid derivatives between their solutions in acetate buffer
and the organic phases, cyclohexane and silicone liquid 200. The
amounts partitioned into these organic phases were very small and gave
small differences between the absorbances of the acetate buffer layer
before and after the partitioning with these organic liquids. A small
error in the absorbance measurement therefore introduced large error
in the calculation of the partition coefficient. Hence these values
were not used in further investigation and have not been reported.
The coefficients of partition of aminoalkyiphenones 4'-
aminopropiophenone, 4'-aminoacetophenone, and 3'-amiroacetophenone -
between their solutions in pH 6.8 phosphate buffer and chloroform
were determined by the method described for barbituric acid derivatives.
Measurement of Thickness of Membranes
The silastic membranes were available in four different
labelled thicknesses of 3, 5, 10 and 20 mil. The actual thickness
of each membrane was measured with a micrometer screw capable of
measuring a minimum thickness of 0.1 mil. A clean sheet of paper
was cut into a rectangle measuring 2 1/2" x 5". This was folded into
a square of 2 1/2" x 2 1/2". The paper was marked lightly with a
pencil in a square pattern of 1" x 1" from number 1 to 9. The thick-
ness of the paper at each number was measured with the micrometer
screw. A membrane was carefully placed flat between the folded paper
and the thickness was measured at each number again. The thickness
of the membrane was then obtained by difference. The measurements
were made on seven different pieces of the similarly labelled thick-
nesses of the membrane.
Study of Permeability of Silastic Membrane to Phosphate
Buffer Salts and Hydrochloric Acid
A steady state diffusion cell was assembled with 5 mil. thick
silastic membrane in position. It was filled with about 20 ml. of
distilled water. The cell was kept in a 400 ml. beaker containing
about 250 ml. of pH 6.5 phosphate buffer (L = 1.2). A sample of
distilled water from the cell was tested after 15 hours for the
presence of P04- by the ammonium molybdate test (92).
Another diffusion cell filled with 20 ml. distilled water
was kept in a beaker containing 250 ml. of 0.1 N HCI solution. The
distilled water inside the cell was tested for the presence of chloride
ions after 11 hours by the silver nitrate test (93). The pH of the
distilled water was also measured.
Two-tenths of a ml. of 0.6 M and 5 ml. of 4.80 x 10-4
phosphate buffer solutions were diluted to 250 ml. with distilled
water to obtain 4.80 x 10-4 M and 9.60 x 10-6 M phosphate buffer
solutions respectively. Ammonium molybdate reagent (92) was prepared
by dissolving 1.59 g. of molybdic acid in 3.5 ml. of concentrated
ammonia solution diluted with 3.5 ml. of water. This solution was
slowly added to a mixture of 8 ml. of concentrated nitric acid and
10 ml. of distilled water. Three ml. of this ammonium molybdate
reagent was added to 3 ml. of sample solution for testing the presence
of phosphate salts.
The silver nitrate solution used for testing the presence of
chloride ions was prepared by dissolving 2 g. of silver nitrate in
20 ml. of distilled water. The sample to be tested for the presence
of chloride ions was acidified with a drop of concentrated nitric acid
and an equal volume of 10% silver nitrate solution was added. A
solution of 4 x 10-5 N HCI was used as a standard solution for the test.
Preparation of Solutions
The solutions of aminoalkylphenones were prepared in pH 6.8
phosphate buffer. A weighed amount of a compound (about 2.5 g.) was
dissolved in a known volume (about 100 ml.) of 0.4 N HCl. A known
aliquot of this solution (usually 20 ml.) was then diluted with pH
6.8 phosphate buffer (p = 0.3) to 2 liters.
Aliquots of 4'-aminopropiophenone solution in 0.4 N HCI were
diluted to 2 liters with acetate buffers and phosphate buffer of
different pH values and hydrochloric acid solutions of different
normalities. These solutions were then used to study the effect of
pH on the diffusion of 4'-aminopropiophenone through Silastic membrane.
The solutions of barbituric acid derivatives amobarbital,
barbital, cyclobarbital, diallylbarbituric acid, mephobarbital,
metharbital, pentobarbital, phenobarbital, secobarbital, thiamylal
and thiopental were prepared in pH 4.7 acetate buffer. A known
amount (about 0.5 g. in the case of mephobarbital, thiamylal and
thiopental and 1.0 g. in the case of the remainder of the barbituric
acid derivatives) was dissolved in 20 ml. of 0.1 N NaOH and then
diluted with pH 4.7 acetate buffer (p = 0.1) to 2 liters. Similarly
2 liters of the solutions of barbital and pentobarbital were also
prepared in acetate, phosphate and borate buffers of different pH
values to study the effect of pH on the rate of diffusion of these
barbituric acid derivatives.
Solutions of a-methylphenethylamine hydrochloride, 2-amino-
4-phenylpentane hydrochloride, 3-amino-1-phenylbutane sulfate, 1-
meTnyi-5-pheny!pentylamine hydrochloride and a-ethyiphenethyiamine
hydrochloride were prepared in borate buffer. A known amount (about
0.5 g.) of a drug was dissolved in 20 ml. of distilled water and
diluted with 0.1 N NaOH to 2 liters. The pH of the solution was then
adjusted with boric acid solution to a value close to the pKa value
of the drug. (The pKa values of these compounds are given in Table I).
The pH values of the final solutions of the compounds in the order
listed above were 8.93, 9.45, 9.40, 9.60 and 9.28 respectively.
A saturated solution of dextromethorphan was prepared in pH
10.1 borate buffer by equilibrating the solution at the temperature
of study in presence of excess of undissolved dextromethorphan for
about 12 hours. The saturated solution of progesterone was prepared
in the same way in pH 6.8 phosphate buffer solution.
A Typical Steady State Diffusion Experiment
A strip of Silastic membrane was washed with water several
times to remove the sodium bicarbonate dusting powder from its surfaces.
The membrane was finally washed with distilled water and driuc in air.
It was then cut into 2" x 2" pieces. A steady state diffusion cell was
assembled and two such pieces of Silastic membrane were fixed in
position between the stainless steel plates and silicone gaskets.
(See Fig. 5.)
The diffusion cell was filled with the buffer solution used
for the preparation of the solution of the drug under study. The
level of this solution in the stem of the glass T was kept about 1 1/2"
above the horizontal portion of the T joint. The cell was the.
placed in a 200 or. 400 ml. capacity beaker. The beaker containeG
either 120 or 200 ml. of 0.1 N HCI, borate buffer or phosphate buffer
solution. The solvents chosen depended on the drug under study. The
beaker was then covered with a piece of Parafilm (American Can Company,
Neenah, Wisconsin) with a hole in it for the stem of the diffusion cell.
The Parafilm was then taped to the sides of the beaker. The weaker
was covered with a plastic Petri dish with two holes in it. The stem
of the diffusion cell was inserted through one hole anc the other hole
was used to remove the samples from the solution in the beaker. The
Petri dish cover was taped to the sides of the beaker. The Petri
d sh cover held the diffusion cell immobile in th,, .,,
the widening of the hole in the Parafilm by the stem of the diffusion
cell, and thus minimized the evaporation or contamination of -he
solution in the beaker.
The beaker with the diffusion cell was fitted rigidly in the
metal rack resting in the shaker bath. This assembly was allowed to
equilibrate at the temperature of the bath for about 10 hours. A two
liter solution of the drug under study was also allowed to equilibrate
in another constant temperature bath maintained at The same temperature.
This solution was used as reservoir of the drug solution for the
The pump was started with the ends of all four vinyl tubing
from the two channels dipping in the drug solution. The pumping was
continued until the tubes were full with the drug solution. After
thermal equilibration a hole was burned into the Parafilm by passing
a hot glass rod through the opening in the Petri dish cover. A
sample of the solution from The beaker was removed and analyzed
spectrophotometrica ly. The buffer solution in the diffusion cell
was withdrawn in-;-o suc; *.-. flask.
At zero time 20 ml. of the drug solution was pipetted i>to
the diffusion cell. The delivery and the suction ends of the tubes
from the pump were positioned in the diffusion cell as described
earlier. The pump was then started.
Samples were removed from the solution in the beaker a;T
regular intervals of time. In all cases, excepting the phenyialkyl-
amines, these samples were analyzed immediately with the spectro-
photometer. Samples of the drug solution from the reservoir were
taken at various intervals during the course of the diffusion
experiment. These samples were adequately diluted and analyzed
spectrophotometrically. The temperature of the bath was monitored
throughout the course of an experiment.
At the end of an experiment, the volume and pH of the solution
in the beaker and the pH of the solution in the diffusion cell were
measured. The pH values did not change throughout the course of an
experiment for all studies.
A Typical Quasi-steady State Diffusion Experiment
A clean piece of Silastic membrane was clamped into positi.
in the V-shaped quasi-steady state (see Fig. 4) diffusion cell. Each
of the arms of the diffusion cell was filled with 100 ml. of phosphate
buffer solution. The cell was then kept in a thermostated shaker bath
for equilibration for about 8 10 hours. After this the solutions
from both arms were withdrawn into a suction flask. Into one arm of
the cell 50 to 100 ml. of phosphate buffer was added and at zero time
an equal volume of the drug solution in phosphate buffer ,,s added to
the other arm. The open ends of the arms were covered wirh Parafilm.
At regular intervals of time, samples from both the arms of
the diffusion cell were removed and analyzed for the drug spectro-
photometrically. The experiment was continued until the difference
between the absorbance measurements became very small. At the end
of the experiment the volume and pH of the solutions in both arms
Diffusion of 4'-Aminopropiophenone Through
The solution of 4'-aminopropiophenone (PAPP) prepared in pH
6.8 phosphate buffer was used for both the steady state and quasi-
steady state diffusion work. The solutions used on the desorption
side of the membrane were 0.12 N HCI for the steady state diffusion
and pH 6.5 phosphate buffer for-the quasi-steady state diffusion
The steady state diffusion of PAPP from its saturated
solutions through Silastic membrane was studied to determine the
reproducibility of the results. The effects on diffusion of PAPP
of ionic strength, concentrations of PAPP solutions, thickness of
Silastic membranes and the hydrostatic pressure exerted on the
membrane were studied.
Reproducibility of results.--The diffusion of PAPP from its
saturated solution in pH 6.5 phosphate buffer through 3 mil thick
Silastic membrane into 200 ml. of 0.12 N HCI at 37.50 was studied.
The cell solution containing excess undissolved PAPP was agitated by
bubbling nitrogen gas under pressure through it. The whole assembly
was shaken in a thermostated shaker bath. Samples (1 mi.) of the
beaker solution were removed every hour for about 8 10 hours. Each
was diluted with 4 ml. of pH 6.8 phosphate buffer (p = 0.3) and the
absorbance was measured at 307 mp, on the Beckman DU spectrophotometer.
Five separate experiments were performed on each of the five days.
Typical raw data for one such day are given in Table II.
Effect of hydrostatic pressure.--Solutions of PAPP in pH 6.8
phosphate buffer having same concentration were circulated into four
diffusion cells. The suction tubes of the channels returning the
solutions from the diffusion cells to the reservoir were adjusted at
four different levels (see Fig. 5) in the stem of the diffusion cell.
The levels of the tubes were 0, 1/2", 1" and 3" above the level of the
solution in the beaker. The higher rate of pumping the solution from
the cell to the reservoir over that of pumping the solution from the
reservoir to the cell, kept the level in the cell constant at the
level of the suction tube orifice. The rate of diffusion was monitored
by spectrophotometric analysis of the samples from the beaker.
Effect of ionic strength.-A 100 ml. solution of 0.4 N HCI
containing 2.5 g. of PAPP was prepared. Twenty ml. aliquots of this
solution were added with 170, 330, 500 and 670 ml. of pH 6.8 phosphate
buffer with an ionic strength of p = 1.2. The solutions were then
diluted with distilled water to 2 liters to obtain solutions of
PAPP of the same concentrations in phosphate buffer but with ionic
strengths of 0.102, 0.198, 0.300 and 0.402 respectively. The pH
values of these solutions were 6.69, 6.73, 6.75 and 6.78 respectively.
These solutions were then circulated in four diffusion cells to study
the diffuso.c of PAPP through Silastic membrane into 200 ml. of 0.12
N HCI at 23.00.
Effect of thickness of membrane.--Silastic membrane was
available in four different thicknesses of 3, 5, 10 and 20 mil. The
diffusion of PAPP through these four thicknesses was studied at 24.900.
A solution of PAPP in pH 6.8 phosphate buffer from one reservoir
containing about 8 liters of the solution was circulated through four
cells, each one fitted with a Silastic membrane of different thickness.
The samples of 0.12 N HCI from the beakers were analyzed as a function
of time to monitor the rates of diffusion of PAPP.
Effect of temperature.-The diffusion of PAPP from its solution
in pH 6.8 phosphate buffer through 3 mil thick Silastic membrane into
200 ml. of 0.12 N HCI was studied at seven different temperatures:
24.750, 24.900, 30.400, 31.250, 33.600, 37.500 and 41.00. At each
temperature the diffusion was studied at four. or more concentrations
of PAPP in the phosphate buffer solutions. The same diffusion cells
were used without changing the membranes to avoid any variation in
membrane thickness and in area of the membrane available for diffusion.
The temperature of the bath (American Instrument Company) was monitored
throughout the diffusion experiment and was observed to hold constant
Quasi-steady state diffusion of PAPP.--The diffusion of PAPP
from its solution in pH 6.8 phosphate buffer through 5 mil thick
Silastic membrane into an equal volume of pH 6.8 phosphate buffer was
studied at 25.00. The volumes of phosphate buffer solutions used -
PAPP solution and phosphate buffer without any drug in it in the two
experiments were 50 and 100 ml.
Effect of pH on diffusion of PAPP.-The solutions of PAPP
were prepared in 0.001 N, 0.01 N and 0.1 N HCI and pH 3.48, 4.38 and
5.47 acetate buffer and pH 6.70 phosphate buffer. The steady state
diffusion of PAPP from these solutions through 3 mil thick Silastic
membrane in 200 ml. of 0.12 N HCI was studied at 25.00. The pH of
the solutions were noted before and after the diffusion experiments
and were observed to be unchanged. The absorbance of PAPP in the HCI
solution in the beaker was measured as a function of time after 1:5
dilution with phosphate buffer. The absorbances of the solutions in
the reservoirs, used for the circulation into the diffusion cells, were
measured after appropriate dilutions with phosphate buffer.
Effect of ethanol on the rate of diffusion of PAPP.--The
effect of ethanol on the steady state diffusion of PAPP through 3 mil
thick Silastic membrane was studied at 25.00. In one set of experiments
identical percentages of ethanol were present in the solutions of PAPP
in pH 6.8 phosphate buffer as well as in the 0.12 N HCI on the desorp-
tion side of the membrane. In another set of experiments ethanol was
present in the phosphate buffer solution of PAPP but no ethanol was
present in the HCI solution. In the third set of experiments ethanol
was present only in the HCI solution on the desorption side of the
membrane but no ehtanol was present in the phosphate buffer solutions
of PAPP inside the diffusion cells.
Six flasks containing 500 ml. of pH 6.8 phosphate buffer
(, = 1.2) and 500 ml. of distilled water were added with 160, 320, 480,
640, 800 and 960 ml. of 95% ethanol and the volume in each flask was
made up to 2 liters with distilled water. A solution of 3 g. of PAPP
in 140 ml. of 0.3 N HCI was prepared. Twenty ml. of this solution was
transferred to each of the six flasks containing ethanolic phosphate
buffer solutions to obtain solutions of PAPP in phosphate buffer
containing 7.5, 15.0, 22.5, 30.0, 37.5 and 45.0% V/V ethanol. Six
250 ml. volumetric flasks containing 15 ml. of 2 N HCI were added
with 20, 40, 60, 80, 100 and 120 ml. of 95% ethanol and the volumes
of the solutions were made up to 250 ml. to obtain 0.12 N HCI con-
taining 7.5, 15.0, 22.5, 30.0, 37.5 and 45.0% V/V ethanol. Two
hundred ml. of these ethanolic solutions was used in the diffusion
experiment on the desorption side of the diffusion cells. The phos-
phate buffer solutions of PAPP containing corresponding concentration'
of ethanol were circulated into these diffusion cells.
In another set of experiments the 0.12 N HCI used on the
desorption side of the diffusion cell contained 0, 10, 20, 30, 40 and
50% ethanol prepared as described before. The PAPP solution in the
bulk volume of 8.5 liters was used for circulation into all six
diffusion cells and did not contain any ethanol.
In the third set of experiments the diffusion of PAPP from
its solution in phosphate buffer containing 18% ethanol through 3 mil
Silastic membrane into 200 ml. of 0.12 N HCI containing no ethanol was
studied. The concentration of PAPP in the ethanolic phosphate buffer
solutions was varied from 1.87 x 10-3 M to 3.28 x 10-3 M.
The diffusion of PAPP from its saturated solutions in O,
10, 20 and 30% ethanolic solution in water through 3 mil thick Silastic
membrane into 0.1 N HCI containing 0, 10, 20 and 30% ethanol was
studied at 24.250.
The effect of ethanol on the quasi-steady state diffusion
of PAPP was studied at 25.250. Five hundred ml. solutions of PAPP in
0, 10, 20 and 30% ethanolic phosphate buffer were circulated in steady
state diffusion cells fitted with 2.30 mil thick Silastic membrane
(Fig. 5 ). The diffusion of PAPP from each of these solutions into
120 ml. of 0, 10, 20 and 30% ethanolic phosphate buffer was studied.
The diffusion was monitored by measuring the absorbance of the samples
of the external solution from the beaker at 307 mp as a function of
time. The samples were returned to the beakers. The loss of solutions
from the beakers due to evaporation and repeated sampling was less than 2%.
Effect of ethanol on Silastic membrane.--Three steady state
diffusion cells were filled with 0, 10 and 30% ethanolic phosphate
buffer solutions and kept in 0.12 N HCI solutions containing the same
concentrations of ethanol. After 15 hours the cells were emptied and
washed with water, and the steady state diffusion of PAPP from one
solution of PAPP in phosphate buffer circulated in all three cells was
studied in absence of any ethanol, inside or outside the cell.
Diffusion of ethanol through Silastic membrane.--The steady
state diffusion of ethanol through 3 mil Silastic membrane into 200
ml. of 0.12 N HCI was studied from absolute ethanol, from phosphate
buffer solutions of PAPP containing 18% ethanol and from phosphate
buffer solutions containing 10, 20, 30 40 and 50% ethanol.
Absolute ethanol was circulated into a diffusion cell fitted
with a 3 mil Silastic membrane. About 0.5 ml. samples of 0.12 N HCI
were removed at hourly intervals and 5 microliters of these samples
were injected into a gas chromatograph for vapor phase analysis of
ethanol. The percent ethanol present in the samples were calculated
from a calibration curve.
The diffusion of ethanol from 18% ethanolic phosphate
buffer solution containing increasing concentrations of PAPP from
1.87 x 10-3 M to 3.28 x 10-3 M through 3 mil Silastic membrane was
studied at 24.600.
The diffusion of ethanol from phosphate buffer solutions
containing 10, 20, 30, 40 and 50% ethanol through a 3 mil thick
Silastic membrane into 200 ml. of 0.12 N HCI was studied at 24.600.
Effect of ethanol, propyl alcohol, isopropyl alcohol and
t-butyl alcohol on diffusion of PAPP.-Three hundred grams of isopropyl
alcohol, propyl alcohol and t-butyl alcohol were weighed and diluted
with pH 6.8 phosphate buffer to 2000 ml. Similarly 240 ml. of ethanol
was diluted to 2 liters with phosphate buffer. To each solution 20
ml. of a solution of 3.0 g. of PAPP in 120 ml. of 0.4 N HCI was added.
The solutions were mixed and used for circulation in the steady state
diffusion cells fitted with 3 mil Silastic membrane. The concentra-
tion of PAPP diffusing into 200 ml. of 0.12 N HCI was measured as a
function of time.
Diffusion of 4'-Aminoacetophenone and 3'-Aminoacetophenone.
The diffusion of these two drugs through a 3 mil Silastic
membrane from their solutions in pH 6.8 phosphate buffer into 120 ml.
of 0.12 N HCI was studied at 25.00 and 37.50. The concentrations of
the drugs diffusing into HCI were measured spectrophotometrically after
1:5 dilutions with pH 6.8 phosphate buffer.
Diffusion of Barbituric Acid Derivatives
The steady state diffusions of barbituric acid derivatives -
amobarbital, barbital, butabarbital, cyclobarbital, diallylbarbituric
acid, mephobarbital, metharbital, pentobarbital, phenobarbital,
secobarbital, thiopental and thiamylal through 3 mil Silas-i.
membrane from their solutions in pH 4.7 acetate buffer into pH 10.1
borate buffer were studied at 25.00 and 37.50.
Weighed amounts of barbituric acid derivatives were dissolved
in small volumes of 1 N NaOH and immediately diluted with pH 4.7
acetate buffer solution to 2 liters to obtain solutions which were
approximately 5 x 10-3 M in thiamylal, thiopental and mephobarbital
and 5 x 10-3 M in the remaining barbituric acid derivatives. These
were used as reservoir solutions and circulated through the diffusion
cells fitted with 3 mil Silastic membranes. The solutions used on the
desorption side of the membrane in the beakers were 200 ml. of pH 10.1
borate buffer. The samples of borate buffer from the beaker were
removed at regular intervals of time. Their absorbances were measured
on the Beckman DU spectrophotometer without any treatment. After
measuring their absorbances, the samples were returned to the beakers.
The steady state diffusion of barbital and pentobarbital
through 3 mil Silastic membrane was also studied from their solutions
in acetate, phosphate and borate buffer solutions of different pH
values at 37.50. Solutions of barbital in pH 4.70 acetate buffer,
pH 6.65, 7.30, 7.60 and 8.12 phosphate buffer and pH 9.15, 9.75 and
10.55 borate buffer solutions were used. Solutions of pentobarbital
in pH 4.60 acetate buffer, pH 6.99, 7.57, 7.90 and 8.18 phosphate
buffer and 8.65, 9.05 and 9.53 borate buffer solutions were used.
Diffusion of Phenylalkylamines
The solutions of a-methylphenethylamine hydrochloride, 2-amino-
4-methyl-4-phenylpentane hydrochloride, 3-amino-1-phenylbutane sulfate,
1-methyl-5-phenylpentylamine hydrochloride and a-ethylphenethylamine
hydrochloride in pH 8.93, 9.45, 9.40, 9.60 and 9.28 borate buffer
respectively were used for the diffusion work. The steady state
diffusion of these substances through 3 mil Silastic membrane into
120 ml. of pH 6.8 phosphate buffer was studied at 25.00. Samples of
the phosphate buffer solutions (1 ml.) were removed every 15 minutes
and stored in a refrigerator. All the samples were analyzed simultane-
ously with the standard solutions for the preparation of the calibra-
Diffusion of Dextromethorphan and Progesterone
The steady state diffusion of dextromethorphan from solutions
in peanut oil, mineral oil and pH 10.1 borate buffer through 3 mil
Silastic membrane into pH 6.8 phosphate buffer was studied at 37.50.
Similarly steady state diffusion of progesterone from a solution in
peanut oil and a saturated solution in phosphate buffer through 3 mil
Silastic membrane into pH 6.8 phosphate buffer was studied at 37.50.
The solutions of the two compounds in peanut oil were prepared
by dissolving an accurately weighed amount of each drug in an accurately
weighed amount of peanut oil. The density of the peanut oil was
determined by weighing 10 ml. of peanut oil in a 10 ml. volumetric
flask. The volumes of the peanut oil used for the preparation of the
solutions were then calculated from the knowledge of the weights and
the density of the peanut oil. The molarities of dextromethorphan
and progesterone in the peanut oil solutions were then calculated.
The diffusion of dextromethorphan was also studied from its
saturated solutions in peanut oil and mineral oil. The concentration
of dextromethorphan in peanut oil was determined by diluting a
weighed amount of the filtered solution with chloroform and measuring
the absorbance of the solution at 277 mp against a chloroform blank
containing an equivalent amount of peanut oil. The E value of
dextromethorphan in peanut oil diluted with chloroform was determined
the same way using the solution of dextromethorphan in peanut oil
containing a known weight of the drug. The E value of dextromethorphan
in mineral oil was determined by dissolving an accurately weighed
quantity of dextromethorphan in mineral oil and measuring its absorb-
ance against a mineral oil blank. The saturated solution of the drug
in the mineral oil was filtered. Its absorbance was measured and the
concentration in the solution was calculated from the knowledge of
the E value.
Diffusion of Drugs Through Silastic Capsules
Silastic capsules (Dow Corning Center for Aid to Medical
Research) were small Silastic pouches prepared by sealing pieces of
tubes (about 6 mm. in diameter x about 2.4 cm. in length) at both ends.
These were cleaned externally with water and dried in air.
To determine the area available for diffusion five capsules
were cut open at their sealed ends. The resultant tubes were cut
again on one side to obtain flat sheets of the capsule material. The
thickness of this capsule material was measured with a micrometer
screw as described previously. The material was then cut into small
pieces and inserted into 10 ml. graduated cylinder containing 5 ml.
of water. The volume of water displaced by the material from the five
capsules was measured. The average volume of Silastic material from
one capsule was then calculated from the volume of the water displaced
by material from five capsules. The area of the Silastic membrane of
one capsule was then obtained by dividing the volume of the Silastic
material from one capsule by the thickness of the Silastic material.
Saturated solutions of 4'-aminopropiophenone in phosphate
buffer, solutions ofbarbital, pentobarbital and phenobarbital in
acetate buffer and solutions of dextromethorphan in borate buffer,
peanut oil, and mineral oil were prepared. The Silastic capsules
were cut open at one corner and were filled with these saturated
solutions. The hole in each of the capsules was sealed by forcing a
portion of Silastic medical grade adhesive type A (Dow Corning Center
for Aid to Medical Research) into and around the hole. The capsules
were set aside for one day and were tested for leaks by pressing them
lightly between fingers. Similarly capsules containing solvents
without drugs were also prepared.
The capsules containing barbital, pentobarbital and pheno-
barbital were washed with water and put in 50 ml. of pH 10.1 borate
buffer in glass vials. These vials were preequilibrated at 37.50 for
about 10 hours. Similarly capsules of 4'-aminopropiophenone and
dextromethorphan were put in glass vials containing 50 ml. of 0.12 N
HCI and pH 6.8 phosphate buffer respectively. The samples from the
vials were assayed spectrophotometrically for the respective compounds.
Screening of Permeability of Polymer Films
The spectrophotometric cruves obtained on the Cary recording
spectrophotometer were checked for the peaks characterizing the com-
pounds under study. This method of analysis was sensitive to 1 x 10-4
M concentration in all cases and much more sensitive in the case of
steroids (Table I).
Polyethylene membrane is permeable to4'-aminoacetophenone in
aqueous solutions and in peanut oil, and to 4'-aminopropiophenone in
ethylene glycol. Both compounds permeate Rilsan membrane from ethanolic
solutions. Polypenco membrane is permeated by 4'-aminopropiophenone
and 3'-aminoacetophenone from their solutions in 0.1 N NaOH and ethanol.
Polypropylene is nonpermeable to all the drugs tested. Substances
were leached by phosphate buffer from cellulose triacetate and cellulose
butyrate membranes and interfered with the spectrophotometric analyses
of the drugs. After compensating for the absorbances of these inter-
fering substances, it was concluded that 4'-aminopropiophenone permeated
cellulose triacetate from solutions in propylene glycol, ethanol and
polyethylene glycol 200. Positive results were recorded also for barbital
in polyethylene glycol 200 and sulfabenzamide in ethanol. Mylar poly-
ester film was available only in small quantity and was impermeable to
all the drugs tested with the available films. Silastic membrane was
permeable to 4'-aminopropiophenone from solutions in all the solvents
tested except mineral oil, to 3'-aminoacetophenone from all solvents
except HCI and mineral oil, to progesterone from ethanol, to barbital
from ethanol and ethylene glycol and to phenobarbital from peanut oil,
ethanol, polyethylene glycol 200, phosphate buffer and 0.1 N HCI.
Since the purpose of this screening was to search for a membrane
which would allow permeation of a majority of compounds in measurable
quantities only Silastic membrane was investigated further.
The solubilities of barbituric acid derivatives in pH 4.7
acetate buffer at 25.00 are reported in Table III. The solubilities
of aminoalkylphenones in pH 6.8 phosphate buffer at 37.50 are given
in Table IV. The solubility of 4'-aminopropiophenone in phosphate
buffer containing various percentages of ethanol were interpolated
from Fig. 6 showing the relation between solubility and percent
ethanol. The solubility values are given in Table V.
The solubilities of phenylalkylamines in 0.1 N NaOH could
not be determined because the insoluble liquid amines formed an
emulsion which could not be broken even after extended periods of
centrifugation. Clear solutions of these compounds for the deter-
mination of the solubilities were not obtainable.
The partition coefficients for barbituric acid derivatives
between their solutions in acetate buffer and chloroform presaturated
with each other are reported in Table Ill. The partition coefficients
for 4'-aminopropiophenone, 4'-aminoacetophenone, and 3'-aminoaceto-
phenone between their solutions in pH 6.8 phosphate buffer and chloro-
form presaturated with each other at 25.00 are given in Table IV. The
partition coefficient between phosphate buffer solutions of 4'-amino-
propiophenone containing ethanol and organic solvents were not
obtained because of the high solubility of ethanol in most organic
Thickness of Membranes
The results obtained from the measurements of the membrane
thickness are recorded in Table VI. The results were analyzed
statistically (94) as shown in Appendix C VI for 3 mil thick
Silastic membranes. The coefficients of variation (94) and the limits
within which the mean thicknesses vary have been reported at 95%
confidence levels (94) in Table VI.
Permeability of Silastic Membranes to
Phosphate Buffer Salts and Hydrochloric Acid Solution
The ammonium molybdate reagent solution (92) gave a yellow
colored solution when mixed with an equal volume of 4.8 x 10-4 M
solution of phosphate buffer. A faintly yellow colored solution was
obtained with 9.60 x 10-6 M phosphate buffer solution. The sample
from the diffusion cell kept for 15 hours in a beaker containing
phosphate buffer, did not yield a precipitate or a colored solution
with this reagent. It was therefore concluded that phosphate buffer
salts do .not diffuse significantly through Silastic membrane.
A white turbid solution was obtained when 4 x 10-5 M solution
of HCI was acidified with nitric acid and an equal volume of 10%
silver nitrate solution was added. A sample from the diffusion cell
kept for 11 hours in a beaker containing 0.1 N HCI did not yield a
turbid solution when treated similarly. Also, the 6.45 pH of the
solution in the cell was constant for the time interval. It was
therefore concluded that HCI does not diffuse significantly through
Treatment of Data
Table II shows the raw data obtained on the steady state
diffusion of 4'-aminopropiophenone from solutions in pH 6.5 phosphate
buffer through 3 mil Silastic membrane into 200 ml. of 0.12 N HCI
at 37.3. This and other such data were plotted according to the
integrated form of Eq. 9
A = D S K t = D S (C2 CO) t / X (Eq. 24)
where A is the amount in moles diffused in t seconds through a
membrane of X cm. thickness and S cm.2 area. K is the concentration
gradient (C2 C1) / X. D is the apparent diffusion constant in
cm.2/sec. and C2 is the concentration of the compound in the cell.
C1 is the concentration of the compound on the desorption side of
the membrane. In the steady state diffusion experiments the con-
centration C1 was maintained zero by keeping the diffused species
charged in the solution in the beaker. Thus 0.12 N HCI in the case
of 4'-aminopropiophenone and other aminoalkylphenones, pH 10.1 borate
buffer in the case of barbituric acid derivatives and pH 6.8 phosphate
buffer in the case of phenylalkylamines and dextromethorphan kept the
respective diffusates in the charged form. Charged species do not
diffuse through Silastic membrane. Hence the concentration C1 in the
beaker was always zero with respect to the diffusing species.
When the concentration of the solution in the beaker was
plotted against time in hours, the slope of the plot was
slope = D S C2 / X V = dC/dt (Eq. 25)
where V was the volume of the solution in the beaker. Since all the
terms in Eq. 25 are known except the apparent diffusion constant D,
it can be calculated from the value of the slope.
The quasi-steady state diffusion data were treated according
to Eq. 21. Since the volumes Vi and V2 used in the two arms of the
quasi-steady state diffusion cell were equal, Eq. 22
(X V / 0.869 S) log (C2 C1 / CO) = -D t (Eq. 22)
was used. The plot of -log (C2 Cl / CO) against time yields a slope
of 0.869 S D / X V from which D, the apparent diffusion constant, can
Diffusion of 4'-Aminopropiophenone Through Silastic Membrane
Reproducibility of results.-The regression analysis of the
raw data on the diffusion of 4'-aminopropiophenone (PAPP) in the
replicate five runs of Table II is given in Appendix C VII. The
intercepts obtained by regression analysis were tested by the "student"
t-test (94, 95, 96) to ascertain the validity of the hypothesis that
the intercepts were zero. In all five cases the hypothesis that the
intercepts were zero could not be rejected at the 95% confidence
level. Hence it was concluded that the plots of the concentration
of the diffused material versus'time go through the origin. The
coefficient of variation for values among the slopes was 6.98%.
The slopes of the concentration versus time plots for five
replicate diffusion experiments on each of five days at 37.50 are
noted in Appendix C VIII. The concentration of PAPP in the saturated
solutions used for the experiments was kept constant at 2.35 x 10-3 M.
The slopes were analyzed statistically (94, 95, 96) (Appendix C VIII)
to test the hypothesis that there was no significant difference
between the slopes on the same day and between the slopes on different
days. An analysis of variance table was constructed and by the "F"
test it was concluded that the slopes obtained within and among days
were statistically equal. The coefficient of variation for means of
the slopes on different days was 6.09%.
lonic strength effect.-The data obtained from the steady
state diffusion experiments for the concentrations of PAPP diffusing
through Silastic membrane from phosphate buffer solutions with ionic
strengths of 0.102, 0.198, 0.300 and 0.402 were plotted against time.
The slopes of these plots were normalized by dividing by the concen-
trations of the diffusing solutions. The mean of these resultant
specific rates of diffusion was (7.13 0.23) x 10-2 (Appendix C IX).
The coefficient of variation was 3.24%. This value of the coefficient
of variation is well within the value of 6.98% calculated for the
variation in slopes from five replicates of an experiment on one day.
This indicates that the ionic strength has no significant effect on
the diffusion of PAPP through Silastic membrane.
Effect of hydrostatic pressure.--The values of the slopes of
the concentration of PAPP diffused versus time in the steady state
diffusion experiments were normalized with respect to the concentra-
tion of the drug as shown in Appendix C X. The values of the
specific rates of diffusion so obtained were very close to each other.
The coefficient of variation was 1.45%. This value being less than
the coefficient of variation calculated for replicates of a diffusion
experiment on one day, it was concluded that the hydrostatic pres-
sures exerted by the liquids within the limits of the levels studied,
have no significant effect on the rate of diffusion of PAPP through
Effect of thickness of membrane on the specific rate of
diffusion.--The plots of the concentration of PAPP diffused in steady
state diffusion experiment through Silastic membrane versus time as
a function of membrane thickness are shown in Fig. 7. Equation 25
may be rewritten as
D / X = Slope x V / S C2 (Eq. 26)
Thus the slopes of the plots of concentration of diffused drug versus
time may be normalized as
D / X = slope x 0.200 / 10.40 x 1.517 x 10-3 x 3600
where 0.200 liter is the volume of the solution, 10.40 cm.2 is the
area, 1.517 x 103 is the molar concentration of PAPP and 3600 is the
factor for converting hours to seconds. The normalized values of the
slopes,specific rates of diffusion, are given in Table VI. When
these specific rates of diffusion are plotted against the reciprocal
of thickness of the membranes, a straight line is obtained as shown
in Fig. 8. The slope of this line, 3.76 0.19 x 10-10 cm.2/sec.
is the apparent diffusion constant D for the diffusion of PAPP through
Silastic membrane at 24.900.
Effect of concentration on the rate of diffusion.--The slopes
of the plots of concentration of PAPP diffused through Silastic
membrane versus time are listed in Table VII as a function of concen-
tration and temperature. These rates of steady state diffusion were
plotted against the respective concentrations at each temperature.
Fig. 9 shows such plots at three temperatures. The slopes of these
plots are the specific rates of diffusion of PAPP through Silastic
membrane and are equal to D S / X V and are given in Table VII. The
apparent diffusion constants calculated from these values are also
given in Table VII. These values were calculated using the equation
D = slope x X x V / S = slope x 7.52 x 10-3 x 0.200 / 10.40 x 3600
where 0.200 liter is the volume V, 10.40 cm.2 is the area S, 7.52 x
10-3 cm. is the thickness X and 3600 is the factor for converting
hours to seconds.
Effect of temperature on the rate of diffusion.--The effect
of temperature on the rate of steady state diffusion of PAPP through
3 mil thick Silastic membrane is shown in Fig. 9. The apparent
diffusion constants at seven temperatures are recorded in Table VII.
Figure 10 shows the plot of logarithm of the apparent diffusion
constants versus the reciprocal of the absolute temperatures. From
the slope of this plot the activation energy of diffusion AEa can be
calculated using the equation
log D = log DO 6Ea / 2.303 R T (Eq. 29)
&Ea for PAPP diffusion through Silastic membrane was calculated to be
Quasi-steady state diffusion.-The plots of -log (C2 CI/CO)
versus time for quasi-steady state diffusion of PAPP through 5 mil
Silastic membrane are shown in Fig. 11. The apparent diffusion con-
stants were calculated from the slopes of these plots to be 3.93 x 10-10
and 3.94 x 10-10 cm.2/sec. for 50 ml. and 100 ml. volumes of the
solutions in the arms of the diffusion cell at 25.00. These values
are within the estimated interval for diffusion constants 3.76 .19 x
10-10 cm.2/sec. obtained from steady state diffusion experiments.
Effect of pH on the rates of diffusion.-The apparent diffusion
constants obtained for the diffusion of PAPP from the solutions of
various pH values through 3 mil Silastic membrane have been reported
in Table VIII. The D pH profile constructed from the values
reported in Table VIII is shown in Fig. 12. The apparent diffusion
constant approaches an asymptotic value of 3.66 at higher pH values.
The pH corresponding to half the asymptotic value of 1.83 is 2.45
which is the pKa of the drug obtained from the diffusion experiments.
The pKa of the drug obtained spectrophotometrically is 2.42. (Table I)
Effect of ethanol on the rate of diffusion of 4'-aminopropio-
phenone.--The results obtained in the steady state diffusion of PAPP
through 3 mil Silastic membrane are recorded in Table IX. Set A in
Table IX lists the rates of diffusion when ethanol is present on both
sides of the membrane in equal concentration. A decline in the
specific rate of diffusion was observed with the increase in the
concentration of ethanol present in contact with PAPP. The plots of
the absorbance of the diffused PAPP versus time for different
concentrations of ethanol in PAPP solution are shown in Fig. 13. Set
D in the Table IX shows the same results obtained with saturated
solutions of the drug in aqueous ethanol and 0.1 N HCI containing
the same concentration of ethanol. In set B the diffusion constants
and the specific rates of diffusion are invariant for the different
concentrations of ethanol present in HCI solution. Set C shows that
when the concentrations of the drug are varied in solutions with a
constant concentration of ethanol, the specific rates of diffusion
with respect to the concentration, and the apparent diffusion constants
The data obtained from quasi-steady state experiments were
treated according to
V2/(Kp2VI + KplV2)ln[Kp2CO/Kp2CO CI(Kp2V1 + Kpl 2)/V2] = D'St/XVI
where subscripts 1 and 2 refer to the compartments containing solutions
without PAPP and with PAPP respectively at zero time. VI and V2 are
the volumes of the solutions with PAPP concentrations of C, and C2
respectively. Kpl and Kp2 are the apparent partition coefficients
between membrane material and aqueous solutions in the two compartments.
Equation 30 has been derived in Appendix C III.
When the numerator and the denominator of the logarithmic
term in Eq. 30 are multiplied by the intrinsic diffusion constant D'
for diffusion within the membrane alone, the resultant equation after
.rearrangement of terms becomes
In[D'Kp2COV2/D'Kp2CoV2 CI(D'Kp2V1 + D'KplV2)]
(D'Kp2V1 + D'KplV2)St/XVIV2 (Eq. 31)
If Kpl = Kp2 = Kp as is the case when the concentration of ethanol on
both sides of the membrane is the same, Eq. 30 reduces to
In[COV2/(COV2 C1(VI + V2)] = D S t(V1 + V2)/XVIV2 (Eq. 32)
where D is the apparent diffusion constant, D'Kp. The apparent
diffusion constants of PAPP for diffusion from 0, 10, 20 and 30%
ethanolic phosphate buffer into ethanolic phosphate buffer of the same
composition were calculated from the slopes of the plots of logarithmic
term in Eq. 32 versus time and their values are given in set A of Table X.
These values of D'Kp were used in places of D'Kpl and D'Kp2 in
Eq. 31 for comparable compositions of ethanolic phosphate buffers.
These values of D'Kpl and D'Kp2 were used to determine the slope
(D'Kp2VI + D'KplV2)S/XVIV2, from the plot of the calculated left hand
term of Eq. 31 versus time for various concentrations of ethanolic
phosphate buffer on the two sides of the Silastic membrane. The values
of D'KplV2 + D'Kp2VI derived from the measured slopes and calculated
from the known values of D'Kpl, D'Kp2, V, and V2 are given in Table X
for purposes of comparison.
The specific rates of diffusion of ethanol from phosphate
buffer containing 18% ethanol and different concentrations of PAPP
were found to be independent of the drug concentration (Table XI).
The specific rates of diffusion of ethanol from different concen-
trations of ethanol in phosphate buffer are also reported in Table XI.
The specific rates of diffusion of PAPP from solution in
phosphate buffer through Silastic membrane pretreated with ethanol
solutions of different concentrations was observed to be the same
in all cases indicating that the membrane is not altered physically
by ethanol; and that the alteration, if any, is reversible.
Effect of ethanol, propyl alcohol, isopropyl alcohol and
t-butyl alcohol on the diffusion of 4'-aminopropiophenone.-The con-
centrations of all the alcohols in the phosphate buffer were 2 M.
The specific rates of diffusion of PAPP from solutions containing
above listed alcohols were 5.05, 5.95, 6.41 and 5.12 x 10-3 hr.-1
respectively. It was therefore concluded that all four alcohols
modify the rate of diffusion of the drug to the same extent.
Diffusion of 4'-Aminoacetophenone and 3'-Aminoacetophenone
The apparent diffusion constants for the steady state
diffusion of these two drugs through 3 mil Silastic membrane from
solutions in pH 6.8 phosphate buffer into 0.12 N HCI have been
reported for two temperatures of 25.00 and 37.50 in Table IV. (see
also Fig. 10) Table IV also lists the apparent diffusion constants
for 4'-aminopropiophenone, the partition coefficients of these three
drugs and their solubilities together with the activation energies of
Diffusion of Barbituric Acid Derivatives
The apparent diffusion constants and the specific rates of
steady state diffusion of amobarbital, barbital, butabarbital,
cyclobarbital, diallylbarbituric acid, mephobarbital, metharbital,
pentobarbital, phenobarbital, secobarbital, thiamylal, and thiopental
through 3 mil Silastic membrane from acetate buffer solutions are
given in Table XII for two temperatures 24.600 and 37.50. The plots
of the concentration of diffused drugs against time have been shown
in Figs. 14 and 15.
The apparent diffusion constants for the steady state diffusion
of barbital and pentobarbital from solutions of different pH values
through Silastic membrane are reported in Table XIII. The plots of
apparent diffusion constants versus pH for the two drugs are shown
in Fig. 16. From these plots the pKa values of barbital and pento-
barbital were calculated to be 7.50 and 7.72 respectively. The pKa
values of these two drugs obtained titrimetrically were 7.45 and 7.65
The plots of the logarithm of apparent diffusion constants
for some of these compounds against the reciprocal of the absolute
temperatures have been shown in Fig. 17. The activation energies of
diffusion aEa for barbituric acid derivatives have been reported in
Diffusion of Phenylalkylamines
The apparent diffusion constants and the specific rates of
diffusion from the steady state diffusion of a-methylphenethylamine,
3-amino-1-phenylbutane, 1-methyl-5-phenylpentylamine, a-ethyl-
phenethylamine and 2-amino-4-methyl-4-phenylpentane from their
solutions in borate buffer at pH values of 8.93, 9.40, 9.60, 9.28
and 9.45 through 3 mil Silastic membrane into 200 ml. of pH 6.8
phosphate buffer are reported in Table XIV. The plots of absorbance
of the drug diffused versus time are shown for three drugs in Fig.
18. The concentrations given in Table XIV are the total concentra-
tions and the concentrations of the uncharged species calculated
from the equation (97) where the pKa values are given in the table.
pH = pga + log [base] / [salt] (Eq. 33)
Diffusion of Dextromethorphan and Progesterone
The data obtained in the steady state diffusion study of
dextromethorphan were plotted as concentration of the drug diffusing
from solutions in peanut oil, mineral oil and pH 10.1 borate buffer
through 3 mil Silastic membrane into phosphate buffer versus time
as shown in Fig. 19. The apparent diffusion constants calculated
from these plots are given in Table XV.
The diffusion of progesterone was studied using a steady
state diffusion cell and the data obtained were plotted as shown in
Fig. 20. It is observed from Fig. 20 that the concentrations of
progesterone approach asymptotic values in the two plots. The pH
adjustment of the solution on the desorption side of the membrane
to force the steady state diffusion conditions was not possible
as progesterone does not have a pKa. The data obtained were
therefore treated by the quasi-steady state equation,
(Appendix C IV) using the equilibrium concentration values.
The apparent diffusion constants calculated for progesterone are
given in Table XVI.
Diffusion of Drugs from Silastic Capsules
The average area of Silastic membrane in the capsule was
found to be 5.58 cm.2. The average membrane thickness was 3.134 x
10-2 cm. The adhesive sealed the capsules well as tested for their
airtightness by pressing between fingers. However the portion of the
adhesive spread over a large part of the capsule making thickness at
those places different from average thickness of the capsule membrane.
The apparent diffusion constants and the specific rates of
diffusion obtained for the diffusion of barbital, phenobarbital,
pentobarbital from saturated solutions in acetate buffer, for 4'-
aminopropiophenone from saturated solution in phosphate buffer and
for dextromethorphan from saturated solutions in borate buffer,
peanut oil and mineral oil are given in Table XVII.
Permeability of Silastic Membrane
Silastic membrane is impermeable to chloride ions and phosphate
buffer salts. The apparent diffusivities of 4'-aminopropiophenone,
pentobarbital and barbital (Figs. 12 and 16) decrease with decreased
concentrations of uncharged species in the solution. Increased concen-
trations of charged species on the desorption side of a Silastic
membrane had no effect on the rate of transfer of the uncharged species
through the membrane. This has-been shown for all the compounds
studied e.g. the rate of transfer of dextromethorphan (pKa = 8.25)
from saturated solution in borate buffer or mineral oil was invariant
with increasing concentrations of protonated dextromethorphan in the
phosphate buffer on the other side of the membrane (Fig. 19). In fact,
the concentrations of dextromethorphan on the desorption side of the
membrane greatly exceeded the total concentration on the diffusing side.
Progesterone does not have a pKa and cannot exist as a charged
species. The effect of decreased concentration gradient with increase
in the concentrations in the solution on the desorption side is
apparent from the approach to an asymptotic concentration (Fig. 20).
These facts demonstrate that Silastic membrane is impermeable
to charged molecules. This phenomenon was used to force steady state
conditions in the diffusion experiments throughout this investigation.
Fick's Law of Diffusion
The observed effects of the concentrations of 4'-aminopropio-
phenone (PAPP) solutions and the thickness of Silastic membrane on the
diffusion of PAPP were in accordance with Fick's law of diffusion
dA/dt = D S dC/dx (Eq. 4)
where A is the amount in moles diffused in time t through a membrane
with surface area of S cm.2. The differential dC/dx is the concentra-
tion gradient and D is the apparent diffusion constant. Under the
steady state conditions where the concentration of the diffusable
species on the desorption side of the membrane is essentially zero,
Eq. 4 becomes
D/X = dC/dt (V/SC2) (Eq. 34)
where dC/dt is the slope (Eq. 26) of the linear plot of the concen-
tration of PAPP appearing on desorption side of the membrane with time
(Fig. 7), V is the volume and C2 is the concentration of PAPP in
solution from which the diffusion is occurring. The linearity of such
plots indicate that the specific rate of diffusion, D/X, is inversely
proportional to the thickness of the membrane. The validity of this
relationship is shown by the linearity and zero intercept of the plot
in Fig. 8.
The effect of concentration of PAPP on the rate of diffusion
is shown in Fig. 9 and is in accordance with Eq. 25 rewritten as
dC/dt = DSC2/XV
which states that the rate of diffusion is directly proportional to the
concentration of diffusing species provided that the concentration is
zero on the desorption side of the membrane.
Effect of pH on Diffusion
The plot of the apparent diffusion constant D versus pH of
the diffusing solution is shown in Fig. 12 for PAPP and in Fig. 16 for
barbital and pentobarbital. The apparent diffusion constants were
calculated by Eq. 24 where C2 was the total concentration and included
both charged and uncharged species. These apparent diffusion constants
decrease with decreased concentrations of the uncharged species in the
solution since Silastic membrane is impermeable to charged species.
The pKa values of barbital and pentobarbital were the pH at the mid-
point of the sigmoid curve of apparent diffusion constants versus pH
(Fig. 16). The pKa of PAPP was the pH at half the asymptotic value
in sigmoidal curve of apparent diffusion constants versus pH (Fig. 12).
Apparent Diffusion Constants From Steady and Quasi-steady
State Diffusion Experiments
The value of the apparent diffusion constants (D in Eq. 24)
for the steady state diffusion of uncharged 4'-aminopropiophenone
(PAPP) through Silastic membrane at 25.00 is(3.76 0.19)x 10-10 cm.2/
sec. These results have been shown to be highly reproducible
(Appendix C VIII). The apparent diffusion constant (D in Eq. 22)
obtained for the same compound at the same temperature from quasi-
steady state diffusion experiment was 3.94 x 10-10cm.2/sec. This
value lies within the confidence interval calculated for the apparent
diffusion constants for the steady state diffusion data. The coinci-
dence of the apparent diffusion constants obtained from the two
different methods shows the validity of the results.
Non-dependence of Apparent Diffusion Constants of
Drugs on Molecular Weights
The diffusion constants for the diffusion of a substance in
an isotropic medium has been related to molecular weight in several
equations e.g. Eqs. 1 and 2. Barrer and Chio (98) have simplified
these relations, lumping together the constants, to
D = a Mb (Eq. 36)
where D is the diffusion constant, a and b are arbitrary constants
and M is the molecular weight. They have claimed that this expression
holds moderately well for the diffusion of inert gases through
The diffusion of barbituric acid derivatives through Silastic
membranes was studied to test this relationship. The apparent diffusion
constants (Table III) were plotted against the cube root and square
root of the molecular weights of the respective compounds according to
Eqs. 1 and 2 respectively. Similarly, the logarithm of the apparent
diffusion constants of barbituric acid derivatives (Table III) and
those of the other compounds (Tables IV and XIV) were plotted against
the logarithm of their molecular weights in accordance with the
logarithmic transformation of Eq. 36. The points were too scattered
to permit any assumption of correlation between molecular weightsand
the apparent diffusion constants.
Barrer et al. (99) have shown that the diffusion of hydro-
carbons in silicone rubber is "less sensitive to size and shape of the
penetrant molecule, and thus silicone rubber is less selective than
natural rubber as a separation medium." Their data presented for the
diffusion of hydrocarbons in silicone rubber did not permit correla-
tion with the molecular weights of the compounds.
Postulated Mechanism of Transport in Silastic Membrane
The transport of compounds from a solution into a membrane,
through the membrane and then into another solution has been shown
to obey Fick's law of diffusion. The transfer of compounds from the
solution into the membrane must be non-rate determining in the concen-
tration range studied. Fick's law with its dependence on membrane
thickness and concentration gradient fully describes the transport
process from very low to saturation concentrations of PAPP. The
diffusion constants obtained from the steady state and the quasi-
steady state experiments are equal.
The non-dependence of the apparent diffusion constants on
the molecular weights of the diffusing species may be explained on the
basis of a partition hypothesis. In the partition hypothesis, the non-
charged diffusing species partitions from the solution into the adjacent
membrane monolayer, is transported across the membrane and then
repartitioned into the solution on the other side of the membrane.
The partitionings on either side of the membrane are rapidly effected.
Apparent Diffusion Constants and Partition Coefficients
The partition hypothesis states that the apparent diffusion
constant D is related to the partition coefficient Kp
D = D'Kp (Eq. 37)
as may be readily seen from Eqs. 17 and 20 where D' is the intrinsic
diffusion constant for a compound in Silastic membrane.
This hypothesis was tested by plotting (Fig. 21) the apparent
diffusion constants of barbituric acid derivatives against their
coefficients of partition between chloroform and acetate buffer
solutions (Table III). A fairly good straight line could be drawn
through the points. This linear relationship implies that the parti-
tion coefficients of all the barbituric acid derivatives between
Silastic membrane and the acetate buffer are linearly related to the
partition coefficients between chloroform and acetate buffer. It
assumes that the intrinsic diffusion constant D' is the same for all
the barbituric acid derivatives in the Silastic membrane. These
assumptions may be reasonably true for compounds belonging to a
homologous series. However not all the barbituric acid derivatives
studied belong to such a series. Hence the deviations of the few
compounds (Fig. 21) from the linear plot are to be expected. The
diffusion constants of thiamylal and thiopental are extremely high,
commensurate with their high partition coefficients(Table III).
The partition hypothesis is further substantiated by the
good linear plot (Fig. 22) of diffusion constants against the
coefficients of partition of aminoalkylphenones between chloroform
and phosphate buffer solutions (Table IV).
Apparent Diffusion Constants and Solubilities
The partition coefficient may be defined as the ratio of the
activities and/or solubilities of a compound in two phases (100).
Kp = Cm/Ca = Sm/Sa (Eq. 38)
where Kp is the partition coefficient, Cm and Ca are the concentrations
of a compound in membrane and the aqueous buffer solution respectively,
and Sm and Sa are the solubilities of the compound in the membrane and
the aqueous buffer solution respectively. When Eq. 38 is substituted
into Eq. 37 the resultant equation obtained is
D = D' Sm/Sa (Eq. 39)
Barbituric acid derivatives do not significantly partition
into silicone liquid 200 from acetate buffer solutions. The estimates
of partition coefficients between the two solvents was in great error.
When the solubilities and the intrinsic diffusion constant D' of
barbituric acid derivatives in Silastic membrane are assumed to be
equal, the product D'Sm in Eq. 39 will be constant. The plot of the
apparent diffusion constants against the reciprocal of the solubilities
of barbituric acid derivatives in acetate buffer (Table III) show a
reasonably linear correlation (Fig. 23). The deviations of some
points from the straight line relationship are to be expected since
the simplifying assumptions of constancy of solubility and intrinsic
diffusion constant in Silastic membrane are not rigidly true in all cases.
The phenylalkylamine series of drugs were diffused through
Silastic membrane to confirm the relationship between apparent
diffusion constants (Table XIV) and partition coefficients, and
solubilities of the drugs. Unfortunately the ease of emulsification
of the liquid amines prevented the ready estimates of the solubilities
and the partition coefficients of this series of drugs.
Effect of Ethanol on Diffusion of 4'-Aminopropiophenone
Through Silastic Membrane
The results of the steady state diffusion of PAPP from
ethanolic phosphate buffers (Table IX) demonstrate that the rate of
diffusion of PAPP decreases with increased concentration of ethanol
in contact with the drug (Table IX, Set A). The rate of diffusion
is invariant with different concentrations of ethanol in the HCI
solution on the desorption side of the membrane (Table IX, Set B).
However, the rate is slightly but significantly higher than that in
complete absence of ethanol.
The rates of diffusion of PAPP and ethanol from solutions
of different concentrations of PAPP in ethanolic phosphate buffer
show very little variation (Table IX, Set C and Table XI). This
demonstrates that the rates of diffusion of PAPP and ethanol are
independent of each other.
The data from the quasi-steady state diffusion were treated
specifically (Eqs. 31 and 32) to show the dependence of the apparent
diffusion constants on the partition coefficients for PAPP between
membrane and solutions on its two sides. The values of (D'Kp2V1 +
D'KplV2) (Table X) where D' is the intrinsic diffusion constant, Kpl
and Kp2 are the partition coefficients and V1 and V2 are the volumes
of the solutions on two sides of the membrane, were obtained from the
graphical analysis of the data and also calculated from the known
values of the apparent diffusion constants D'Kp for comparable
conditions obtained from separate experiments (Table X, set A). In
all the cases (Table X, sets B, C, D and E), the two sets of value
are quite close to each other. It is apparent from the results
(Table X) that partition coefficients on either side of the membrane
form an integral part of the apparent diffusion constants. This
observation together with the fact that the rates of diffusion of
ethanol and PAPP are independent of each other (Table XI) indicate
that the intrinsic diffusion constant of PAPP in Silastic membrane
is not significantly dependent on the ethanol concentration on
either side of the membrane.
It was noted previously that the partition coefficient is
a ratio of the solubilities (100) in two media. In the case of
diffusion of PAPP through Silastic membrane in the presence of
ethanol, the premise of constancy of solubility in the membrane is
warranted since only one compound is involved. Consequently the
plot of the apparent diffusion constants of PAPP from ethanolic phos-
phate buffer through Silastic membrane versus the reciprocal of the
solubilities of PAPP in these solutions (Table V) should be linear
according to Eq. 39. This was found to be true as seen from linearity
of the plot in Fig. 24. The regression analysis of the plot in Fig.
24 showed that at the 5% level of significance the intercept is not
significantly different from zero.
Temperature Dependence of Diffusion in Silastic Membrane
The temperature dependence of diffusion in polymeric films
may be expressed by
D = Do e -AEa/RT (Eq. 3)
where LEa is the apparent activation energy of diffusion. Barrer
and Chio (98) have reported that the activation energy of diffusion
of inert gases through silicone membrane is about 4 Kcal./mole.
The activation energy of diffusion in solution is theoreti-
cally about one-third of the energy of vaporization (101) and is in
the range of 3-5 Kcal./mole. in many liquids. These calculations are
based on the assumption that the diffusing molecules are large
compared to the molecules of the medium into which they are diffusing
and that the movement of the solvent molecules determines the rate
of diffusion of the solute (102). It has also been pointed out that
large diffusion constants have small temperature coefficients and
conversely "more slowly diffusing substances have to form relatively
large holes for.the activated state; the activation energy and hence
the temperature coefficient of diffusion are consequently large" (102).
In the diffusion of substances through polymers, the diffusing
molecules are small compared to the molecules of the polymer. The
polymer molecules are also rigidly fixed in position with respect
to one another necessitating the diffusing species to move on its own.
This should demand higher activation energies than the 3-5 Kcal./mole.
postulated for diffusion in' liquid media. However the results for
inert gases and hydrocarbons (98, 99) demonstrate that the silicone
rubbers may be considered to be liquidlike elastomers with great
flexibility of atoms in an individual chain.
The effect of temperature on the rate of diffusion of
4'-aminopropiophenone through Silastic membrane is shown in Fig. 9.
The apparent activation energies of diffusion Ea were calculated
(Eq. 29) from the plots of the logarithm of the apparent diffusion
constants versus the reciprocal of the absolute temperature for amino-
alkylphenones (Fig. 10) and for barbituric acid derivatives (Fig. 17).
The lEa values reported (Table IV and XI) vary within the small
range of 4.9 to 7.5 Kcal./mole. except for mephobarbital and thiamylal.
The values of the apparent diffusion constants for amino-
alkylphenones and barbituric acid derivatives are smaller than those
reported (98, 99) for inert gases and the hydrocarbons. Consequently
the apparent activation energies were higher as had been anticipated.
The apparent diffusion constant D, is a product of intrinsic
diffusion constant D' in the membrane and the partition coefficient,
Kp (Eq. 37). Since Kp is the ratio (Eq. 38) of the solubilities of
the compound in the membrane Sm and the solvent for the drug solution
Sa, Eq. 3 may be rewritten as
log D = log D'Kp = log D'Sm/Sa = log DO 6Ea/2.303 RT (Eq. 40)
If log Sm = log SmO 6Hm/2.303 RT (Eq. 41)
log Sa = log SaO AHa/2.303 RT (Eq. 42)
and log 0' = log D'0 LED/2.303 RT (Eq. 43)
then log D'Sm/Sa = log SmoD'o/Sao-[E D + (tHm Ha)]/2.303 RT
where EDI, is the true activation energy of diffusion inside the
membrane and --Im and -Ha are the heats of solution for the compound
in the membrane and the solvent phase. Smo, SaO and D'0 are the pre-
exponential factors in the Arrhenius equation, Eq. 3. Thus the
apparent activation energy of diffusion is the sum of the true activa-
tion energy of diffusion inside the membrane and the difference between
the heats of solutions of the diffusing species in the membrane and
the solvent phase.
The diffusion of drugs through Silastic membrane in steady
state conditions is of zero order (Figs. 7, 9, 14, 15 and 18) when
the concentration of the drug in the solution on one side of the
membrane is kept constantly high by use of large volume of the
solution of high concentration or by using a saturated solution in
presence of large quantity of undissolved drug. The zero order
release of a drug from a dosage form at a predetermined rate to
exactly compensate the loss of drug from the body is ideal for a
sustained release formulation (103). The diffusion of drugs from
saturated solutions in Silastic capsules was studied with this view
in mind. The concentrations of the drugs inside the capsules were
thus kept constant for a long period of time.
The apparent diffusion constants for the compounds from
the capsules are given in Table XVII, together with those obtained
from the steady state diffusion studies in the diffusion cells, for the
purpose of comparison. The two sets of values are comparable to
each other. But the apparent diffusion constants calculated for
diffusion from Silastic capsules are significantly lower than those
from the diffusion cells. This is due to the fact that the area
available for diffusion in Silastic capsules was calculated from
geometrical considerations and the air entrapped in the capsules
reduced this available area. Also the thickness of the part of the
membrane changed to an unknown value because of the spread of the
glue used for sealing the holes in the capsules. The two sets of
values of apparent diffusion constants were, therefore, normalized
with respect to the apparent diffusion constant of barbital in each
set. The close agreement between these two sets of normalized values
demonstrate that the apparent diffusion constants from Silastic
capsules differed from those obtained with diffusion cells, by a
The apparent diffusion constants of dextromethorphan and
progesterone (Table XV, XVI and XVII) for diffusion from saturated
solutions in non-aqueous solvents are much lower than those from
saturated solutions in aqueous solvents. However because of the
higher solubilities of the drugs in the non-aqueous solvents, higher
concentrations are permissible and thus from these solvents the rates
of diffusion of these drugs are greater than those from aqueous
solvents (Figs. 19 and 20).
The specific rates of diffusion and apparent diffusion
constants of dextromethorphan from mineral oil were higher than those
from peanut oil (Table XV and XVII). This was expected as dextro-
methorphan is more soluble in peanut oil than in mineral oil and
consequently the coefficient of partition from peanut oil to the
membrane would be lower than that from mineral oil to the membrane.
However the rate of diffusion is higher from saturated solution in
peanut oil than that from saturated solution in mineral oil due to
the higher concentration of drug attainable in the peanut oil. Thus
the choice of a solvent to obtain a particular rate of diffusion from
saturated solutions may be made on the basis of the solubility of the
drug in a series of solvents.
Control and Prediction of Diffusion of Drugs
through Silastic Membrane
It has been shown conclusively that the diffusion of drugs
through Silastic membranes conform to Fick's law of diffusion. The
apparent activation energy for diffusion has been shown to vary within
a narrow range of 4.9 to 7.5 Kcal./mole. for most drugs studied.
If the diffusivity of a drug through Silastic membrane is
obtained experimentally at one temperature, the apparent diffusion
constants and the rates of diffusion at any other temperature can be
predicted from Eqs. 4 and 29
dA/dt = DS dC/dx (Eq. 4)
log D = log DO AEa/2.303 RT (Eq. 29)
using a mean value of 5.7 Kcal./mole. for the term LEa, the apparent
activation energy of diffusion. The optimum area S, and the thickness
X of the membrane to obtain a pre-determined rate of diffusion may be
calculated from Fick's law (Eq. 4) which reduces to
dA/dt = D S C/X (Eq. 45)
for a concentration C on the diffusing side of the membrane and when
the concentration of the diffusing species is negligible on the
desorbing side of the membrane.
The apparent diffusion constant has been shown to be a
linear function of the partition coefficient or the ratio of the
D = D'Kp = D'Sm/Sa (Eq. 39)
where K is the partition coefficient and Sm and Sa are the solubilities
of a compound in the membrane material and aqueous solvents respectively.
The diffusivity of compounds belonging to a homologous series
can be predicted reasonably from the experimental value for diffu-
sivity of a compound belonging to the series, and the partition
coefficients and/or solubilities of these compounds. The linear
relationship between the solubility and the diffusivity may be used
to choose the best solvent to obtain a pre-determined rate of diffusion
of a drug by studying the solubility of the drug in several solvents
as in the case of 4'-aminopropiophenone in ethanolic phosphate buffers
or dextromethorphan in peanut oil and mineral oil.
Thus the area and the thickness of the membrane and the
solvent for the drug may be used to control the rate of diffusion.
The Fick's law of diffusion, the apparent activation energy of
diffusion and the knowledge of partition coefficients and/or solubility
of drugs may be used for predicting the rates of diffusion of homologous
series of drugs from the experimental value obtained for one of the
members of the series.
SUMMARY AND CONCLUSIONS
1. Nine polymeric films were screened for their permeability
to aminoalkylphenones, sulfonamides, steroids, barbituric acid
derivatives, and other drugs. Silastic membrane was found to be
permeable to many drugs in measurable quantities.
2. Silastic membrane was shown to be impermeable to HCI
and phosphate buffer salts.
3. The rate of diffusion of 4'-aminopropi.ophenone through
Silastic membrane was independent of ionic strength, and minor changes
in the hydrostatic pressure.
4. The effect of concentration of drug solutions and
thickness of Silastic membrane on the rate of diffusion of 4'-amino-
propiophenone was studied by steady state diffusion technique. The
zero order transport of the drug through the membrane conformed to
Fick's law of diffusion
dA/dt = D S dC/dx (Eq. 4)
where D is the apparent diffusion constant, A is the amount trans-
ported in time t across a membrane of surface area S and with a
concentration gradient of dC/dx.
5. The apparent diffusion constants D and the specific
rates of diffusion [(dA/dt)/(concentration of diffusing drug)] through
Silastic membranes were calculated for aminoalkylphenones, barbituric
acid derivatives, phenylalkylamines, dextromethorphan and progesterone.
6. The effect of temperature on the rate of diffusion of
barbituric acid derivatives and aminoalkylphenones through Silastic
membrane were studied and the activation energy of diffusion was observed
to vary between 4.9 and 7.5 Kcal./mole. for these compounds except
thiamylal (21.78) and mephobarbital.(15.22).
7. The effect of pH on the rate of diffusion of barbital,
pentobarbital and 4'-aminopropiophenone through Silastic membrane was
studied. The rate of diffusion was shown to be directly proportional
to the concentration of the uncharged species. The pKa values of these
compounds calculated from the plot of apparent diffusion constants
versus the pH of the solutions were observed to be the same as those
obtained by other methods.
8. The apparent diffusion constants in Silastic membranes
were found to be independent of the molecular weights of the diffusing
9. The transport of a drug across Silastic membrane was
postulated to be due to partitioning of the drug from its solution
into the membrane, its transport across the membrane and then
repartitioning into the solution on other side of the membrane. This
postulated mechanism was substantiated by quasi-steady state diffusion
studies of 4'-aminopropiophenone from phosphate buffer solutions
containing varying concentrations of ethanol into ethanolic phosphate
buffer solutions. The intrinsic diffusion constant of 4'-aminopropio-
phenone in the membrane was independent of the composition of the
solutions, since the data could be fitted to the equations derived on
the assumption of constancy of intrinsic diffusion constant.
10. The apparent diffusion constants, D, of barbituric acid
derivatives and aminoalkylphenones in Silastic membranes were shown
to be lineraly related to their partition coefficients between
aqueous solutions and chloroform. This was shown to be in accordance
with the equation
D = D'K (Eq. 37)
where D' is the intrinsic diffusion constant of a drug inside the
Silastic membrane and Kp is the partition coefficient of the drug
between the membrane and the drug solution.
11. The apparent diffusion constants in Silastic membranes
were shown to be linearly related to the reciprocal of the solubilities
of barbituric acid derivatives in acetate buffer and 4'-aminopropio-
phenone in phosphate buffer containing varying concentrations of
ethanol. This was shown to be in accordance with equation
D = D'Sm/Sa (Eq. 39)
where Sm and Sa are the solubiliiies of a drug in the membrane and
the solvent for the drug solution.
12. The diffusion of dextromethorphan, pentobarbital,
phenobarbital, barbital, and 4'-aminopropiophenone from saturated
solutions in Silastic capsules was studied to investigate the
feasibility of using Silastic membranes as encapsulating material
for drugs. The rate of diffusion of dextromethorphan from peanut
oil was greater than that from mineral oil because of the high
concentration of the drug attainable in peanut oil. It was shown that
the rates of diffusion of a drug through Silastic membrane could be
controlled by choice of a solvent for the drug solution. If a
method of sealing the Silastic capsules, better than using a glue,
is available,Silastic membranes can be used to encapsulate a drug
solution to obtain its zero order release.
13. The area and the thickness of Silastic membranes and
solvents for the drug solutions can be chosen to control the rate of
diffusion of the drugs through Silastic membranes.
14. The apparent diffusion constants for diffusion of drugs
through Silastic membranes can be estimated from the knowledge of
partition coefficients and/or solubility of drugs belonging to a
homologous series and the experimental apparent diffusion constant
of one of the drugs from the series.
WAVELENGTHS OF MEASURED ABSORBANCES, MOLAR ABSORPTIVITIES AND pKa
VALUES OF COMPOUNDS.
Compounds pKa Wavelength Molar
Measured Literature mp Absorptivity
Amobarbital 7.40 238 5850
Barbital 7.45 7.91 238 8300
Butabarbital 238 10410
Cyclobarbital 7.27 7.50 238 8700
turic acid 7.30 7.79 238 5990
Mephobarbital 7.45 238 6050
Metharbital 7.90 238 7700
Pentobarbital 7.65 8.11 238 8900
Phenobarbital 7.40 7.41 238 6800
Secobarbital 7.45 8.08 238 7700
Thiamylal 6.80 304 21000
Thiopental 7.12 304 13240
Dextromethorphan 8.25 277 3002
phenone 2.42 307 16016
phenone 312 13700
phenone 327 1840
ethylamine 9.07 520
ethylamine 9.30 520
4-phenylpentane 9.42 520
butane 9.30 520
pentylamine 9.50 520
Progesterone 248 16470
Cortisone 238 16000
Hydrocortisone 242 16200
Prednisolone 242 15200
Sulfadiazine 255 5370
Sulfabenzamide 255 4480
TABLE I I
DIFFUSION OF 4'-AMINOPROPIOPHENONE FROM SATURATED SOLUTION (2.36 x 10-3M)
IN pH 6.5 PHOSPHATE BUFFER THROUGH 3 MIL SILASTIC MEMBRANE INTO 200 ML.
OF 0.12 N HCI AT 37.30.
Date Time Time Since Tempera- Absorbance at 307 mpa
Zero Hrs. ture C 1 2 3 4 5
7/16 10.00 0 37.30 0.007 0.004 0.008 0.014 0.006
11.00 1.00 37.30 0.134 0.141 0.116 0.121 0.112
12.00 2.00 37.30 0.234 0.224 0.232 0.207 0.209
13.00 3.00 37.30 0.357 0.319 0.340 0.287 0.319
14.00 4.00 37.50 0.478 0.428 0.460 0.395 0.433
15.00 5.00 37.50 0.560 0.515 0.532 0.470 0.505
16.00 6.00 37.30 0.663 0.602 0.653 0.559 0.612
18.00 8.00 37.30 0.886 0.815 0.875 0.722 0.825
Volume in the cell in ml. 21.0 22.0 23.0 23.0 22.0
pH of cell solution 6.55 6.55 6.50 6.50 6.55
Volume of solution in
beaker in ml. 180 184 184 184 184
pH of solution in the
beaker 1.00 1.00 1.00 1.00 1.00
a The sample solutions were diluted 1:5
before measurement of absorbances.
with pH 6.5 phosphate buffer
( 4- (o (D (0 -
- (u 4- 4- 4- 4- (
(u 4- *- (u *- +- -
4- -O L -0 4- -- ru
- Q L (DU CO L L L 0 (C 4-
.-0 L u ( .0 (O -O u C(D L -
L 4- CO >- .- L 3 -0 (0 > (
0 -- -0 O O C O O E o0
-o n Co -C -C 4- c 0 (C 0
0 L +- U Cu 0- 4- C 0) U
E Cu >. : -- () (a) ) ) C r _-
< C 0 OC) 0 2 2 a- a- O F-
CO N 0 CO
C M N Ot 0
n o C -
Ot O O N
0 0 0
cO 0) c0 CO
U) d U) -
N CO 0 m
N 0co 00 r
O Cr- r- r-:
ko s in t Q n r\ in O
0 0 O 0 m Nm 01
S N 0 00
o 1 ^- ^- N
CO 4- 0
o I 0 0 00 -t I I
0) in CN L)n a)
> > >
r- 'o 0 rn NQ r
O 0 U n r-: O 0
(a C CD
4- S- C E
n o C
Cu L C
4- -0 0 (D
) 4- 4-
S 0 U1-
I C -
C 1) 4-
0 aS I V)
3 U (D (0
0 0 CL-
(U -0 U