AN INVESTIGATION OF SOME FACTORS CONTROLLING SOLUTE TRANSPORT ACROSS LIQUID MEMBRANES

. . . Acknowledgments. List of Tables . List of Figures I. Objectives I

( equation. Si ilar y, t he uptake of phenobarbital by a liquid memb rane system appeare t o obey the Arrhenius relationship until a 11 critical ' · temperature or temperature range was reached (about 43°C). Above the critical temperature, the transport rates of phenobarbital decreased as temperature was increased. Increasing the viscosity of the liqu·d membrane at the critical temperature did not help to improve transport at this temperature range.
Alterations in the physical properties of liquid membranes resulted in changes in the rate of solute uptake. Increasing viscosity and oil/ water ratio both resulted in reduced transport rate constants for the uptake of solutes. These effects can be readily interpreted using classical diffusion theory. Liquid membranes which were frozen and thawed were satisfactorily used for drug removal indicating the surprising robustness of these systems.
Studies reported in this thesis indicate that previous use of liquid membranes for solute transport does not materially affect their further use as sinks. These systems also demonstrated the ability to remove simultaneously two solutes from a multicomponent system at rates which were of the same magnitude as that measured in one solute systems.
Method of manufacture and appropriate surfactant blends were detennined to be key factors in the development of emulsions for use as liquid membranes formulated using only Generally Regarded As Safe components(GRAS).
As evidenced by the literature, extensive work has been done in the area of liquid membrane application. Although the .effect of some of the physical and chemical properties of liquid membrane systems on drug transport have been studied (15), only recently has work been initiated in the area of development of kinetic models to rationalize the transport of solutes across liquid membranes. Yang and Rhodes investigated the fonnulation variables of liquid membranes on transport (17), while Chilamkurti and Rhodes studied the effect of molecular structure on transport across liquid membranes (18). Some preliminary work was done by Yang and Rhodes on the effect of electrolytes on the transport of water through the liquid membranes (17).
In their work, Yang and Rhodes were able to detennine the macro rate ccnstants (a and e) for the systems studied, but they were unable to detennine the micro rate constants. For some systems Chilamkurti and Rhodes we~e able to establish micro rate constants (18), while in other systems their attempts were unsuccessful. A. Principles of Drug Transport 1. Transport Processes 4 "A membrane transport system is created where there is passage of a solute across a membrane" (23). This passage may be active, passive, or facilitated. Active transport is a process which requires energy to allow movement of a solute against a concentration or activity gradient. Facilitated transport makes use of additives(i.e. carrier) to improve the rate of solute transport over the rate of transport expected from passive diffusion (11,12,14). Most drugs seem to be transported by passive diffusion.
Passive transport, or free diffusion, may be described as the . random movement of liquid or solute molecules from a solution of high concentration into a pure solvent until equilibrium is reached. The driving force in this transport is the concentration gradient (23,24).
Considering passive transport thennodynamically, this process is driven entropically. The system undergoes an overall decrease in free energy due to an increase in entropy in accordance with the The partition coefficient, the ratio of distribution of solute between two inmiscible liquids, is expressed as follows : Equation (3) Where c 1 and c 2 are the equilibrium concentrations of solute in liquid 1 (nonaqueous) and liquid 2 (aqueous) and k is the partition coefficient.
The rate of solute transport is directly porportional to its partition coefficient for a particular system.  Since the unionized species has a greater partition coefficient than the ionized species, it would be expected that as the fraction 6 of unionized solute increases, the rate of solute transport increases.
Acidic drugs are transported most easily from solutions with a low pH since the drug does not ionize to any great extent. As the pH of the solution increases, the extent of ionization increases, thereby reducing the percent of unionized drug available to transfer across a membrane.
Conversely, basic drugs are transported best from solutions with a high pH. As the pH of the solution increases, the extent of ionization decreases and the percent of drug available for transport increases.
The pH-partition theory as it applies to the absorption of drugs in the stomach and small intestines . has been studied in both humans (26) and animals (27)(28)(29)(30).

Kinetics of Transport
The amount of solute transported from one compartment to another per unit of time, or the rate of solute transfer, may be expressed using the specific r.ate constant, k, for that process. A reaction may proceed by zero, first, or second order kinetics with k calculated accordingly. ( ( 7 Whereas for zero-order kinetics t e ra t e at wh ich the reaction proceeds is independent of solute concentration, the rate of a firstorder reaction is directly proportiona l to concentration, as expressed by the following equation: dx/dt = -k C Equation (5) Where: dx/dt = rate of transfer, k = first-order rate constant, C = transferable concentration.
By rearranging Equation 5 and integrating between time 0 and time t, the following equation is obtained: log C = log ct 0 (kt/2.303) Equation (6) ~~here: co = concentration at time 0, ct = concentration at time t, k = first-order rate constant, t = time.
By plotting log C against time, a straight line is obtained, the slope of which is k. Equation (6) permits the calculation of concentration at any time, t, when k is known.
The effect of temperature on the rate of a reaction may be described by the Arrhenius equation (25); k =A e-Ea/RT Equation (7) Where: k = specific rate constant, A = frequency factor (frequency of collisions between molecules), E = energy of activation (minimum kinetic energy a molecule a must possess in order to undergo a reaction}, ( ( R = gas constant (1.987 cal/mole°K or 8. 314 Joule/mole°K), T =abso lu te temperature (°K). 8 By plotting the l og of k against l/T, a straight line with a negativ_ slope is obta~n ed wh i ch indicates that with a rise in temperature, the rate of a reaction increases. As a general rule, the rate of a reaction may increase by a factor of 2-3 with every 10° rise in temperature. The slope of the line (Ea/2.303R) may be used to calculate the energy of activation (Ea) for the system. Ea may be calculated also using the following equation of ingredients which will enhance the degradation of a product (24). At low temperatures accelerated decomposition due to dehydration of the system by freezing or due to contamination by microorganisms may result in a change of slope (25). which is dispersed as globules in the other liquid phase" (25).
The two immiscible phases of an emulsion consist of a polar liquid such as water and a nonpolar liquid such as oil.
These systems may be classified according to which phase is dispersed or internal in the continuous or external phase.
If the aqueous phase is dispersed in the oil phase, the emulsion is classified as a water-in-oil (w/o) type. Alternatively, in cases where the oil phase is dispersed throughout a continuous aqueous phase, the emulsion is an oil-in-water (o/w) type. Complex or multiple emulsions may b.e formed by dispersing an emulsion in another liquid which becomes the final external phase. In this way, a w/o emulsion dispersed in an aqueous phase forms a water-inoil-in-water (w/o/w) multiple emulsion. If an o/w emulsion is dispersed in a oil phase, an oil-in-water-inoil (o/w/o) emulsion results. Generally, the particle size diameter of the dispersed phase of a two phase emulsion may range from 0.01 to 100 µm with the majority of them being 0.1 to 10 µm.  In pharmaceutical preparations, this ratio, kn own as the hydrophil ic-1 ipophil ic balance (HLB), is a number between 1 and 18.
Those materials with HLB values below 9 are considered lipophilic in character; whereas those with HLB values above 11 are hydrophilic. Intermediate substances fall into the range of 9 to 11. Those emulsifying agents in the range of 3 to 6 tend to produce w/o emulsions and those in the range of 8 to The HLB approach in emulsion formulation aids in the choice of emulsifiers for a particular system. By determining the required HLB of the components or combination of components in an emulsion and matching it to a single emulsifying agent or blend of emulsifying agents with the same HLB, a suitable formulation may be developed (31). When a blend of emulsifying agents is desired, the ratio of emulsifiers required to achieve a particular HLB may be determined by the following equation: % A = 100 (X-HLBB) I (HLBA -HLB 8 ) Equation (

Preparation of Emulsions
The details of emulsion preparation must be planned carefully in advance of ~he manufacturing process in order to achieve a stable product. The components of a phannaceutical emulsion must be selected with regard for patient safety and ·for efficiency (33,37). Addition order and temperature of the components as well as the manufacturing process will detennine the final characteristics of the system (33,(38)(39)(40)(41). The emulsification process may be accomplished spontaneously (42), by simple stirring~ homogenization, colloid milling, or -ultrasonic vibrations (33). In research and small scale production, simple stirring or shaking methods are employed frequently. Blenders, turbine mixers, mortar and pestles, and closed containers may be used.
For scale-up operations and in industrial manufacturing, homogenizers are used. Adjustments in a formulation are often required in the scale-up production of an emulsion. An emulsion made by stirring or shaking may be a noticeably different product when made by homogenization. Homogenized products usually have a smaller overall particle size diameter. The particle size and particle size distribution of an emulsion are important in predicting the stability of the system.
In general, an emulsion with a smaller mean particle size will exhibit greater physical stability than those with a larger mean particle size. Since the radius term in the equation is squared, particle size has the greatest effect on the rate of creaming. A smaller particle 15 size results in a slower rate of creaming. With reference to particle size distribution, Stokes' law assumes that the particles are spherical and uniform in shape. Deviations from this ideal condition affect the creaming of a system. Differences between the density of the internal and external phases will influence the rate and direction of particle settling. If the density of the internal phase is less than that of the external phase, a negative value for velocity is obtained. This indicates an upward settling or creaming of the particles. A positive velocity suggests a downward settling. If the emulsion viscosity is increased, the rate of settling will be slower. These aspects concerning Stokes' law are discussed in detail by a number of authors (25,(32)(33)(34)(35)(36).
Irreversible separation of an emulsion into two distinct phases is referred to as breaking or cracking. Particles of the dispersed phase coalesce as a result of destruction of the emulsifier film surrounding ( 16 these particles (37). Vigorous agitation will not restore the system to its fonner state. When an emulsion has cracked, it is no longer phannaceutically useful.
An unstable emulsion may be observed when phase inversion or the conversion of a system from one emulsion type to another occurs.
Factors affecting phase inversion include the phase volume ratio of the system, pH, temperature, the emulsifying agents, and the addition of other materials to the system. This inversion may render an undesirable product in tenns of palatability and drug release rate.

Evaluation of Emulsions
Emulsions may be evaluated in terms of flow (viscosity), appearance (colo_ r, phase separation), particle size, and emulsion type. Detennination of emulsion type may be made by either the phase dilution, conductivity, or dye solubility methods (33). The phase ( ( 17 dilution method is explained in Section III of this paper. The conductivity method is based upon the ability of the external phase to conduct an electric current. Whereas the external oil phase of an w/o emulsion will not conduct electricity, the external aqueous phase of an o/w emulsion will conduct electricity. Considering the dye solubility method, a water soluble dye will diffuse through the external aqueous phase of an o/w emulsion and color the system. A w/o emulsion will not be colored when a water soluble dye is added to the system.

Methods of particle size determination have been investigated and
reviewed extensively by a number of authors, including (33,36,37,42,(44)(45)(46)(47). Although time consuming, microscopic determination of particle size using an ocular micrometer is a reliable method.
Photomicroscopy offers the advantage of obtaining a permanent record.
The use of accelerated aging studies and factors designed to impose a stress on an emulsion have been considered extensively with regard to stability (33,36,37,(49)(50)(51)(52)(53)(54). The use of heat, cyclic temperatures, freezing/thawing (37,46), centrifugation (46), and the addition of differing quantities of various electrolytes (36,54) have all been studied to determine their effects on the stability of emulsions. Heat and cyclic temperature treatments impose some of the greatest stress on an emulsion system.  In order to maintain the overall .stability of the liquid membrane system, surfactants, stabilizers, and . viscosity-inducing agents may be added· to either the primary em~lsion or the continuous phase of the system. The size· of the . dispersed. globules formed depends on the

Basic Separation Mechanisms Used with Liquid Membranes
Separation mechanisms as they apply to liquid membranes have been discussed by Frankenfeld, Cahn, and Li' (14) and are described below.

a. Selective Penneation
This simple mechanism, which was described by Li in reference to the separation of hydrocarbons (2), is dependent only upon the selectivity of the liquid membrane to allow specific materials to pass through it.

b. Chemical Reaction Inside Droplet
This mechanism is based upon the diffusion of an unionized species with significant oil solubility (2) from an area of high concentration (external aqueous phase) through the liquid membrane and into the internal aqueous phase which contains a trapping agent. This process has been described by Matulevicius and Li (11) as a type of facilitated transport through maximization of t he concentration gradient. The unionized species, through a chemical reaction with the contents of the internal phase, is convert ed to its ionized fonn, a form which · does not readi ly transport, and is tr~pped within the internal phase.
The concentration of transportable species is maintained at zero, thus creatjng a favorable concentration gradient. Acidic materials may be trapped by an encapsulated aqueous base. Conversely, a basic solute may be trapped by an encapsulated aqueous acid. Substances such as phenol (12), and phenobarbital and acetylsalicylic acid (15,16) have been trapped by this method.
The removal of a1TU11onia from waste water is a classic example used to illustrate this separation mechanism as depicted in Figure 1.
Awmonia (NH 3 ), in a pH 9 external aqueous phase, exists predominantly in its unionized, lipophilic fonn. This species easily transfers from the external aqueous phase, through the liquid membrane, and into the internal aqueous acid phase whe re upon contact it is ionized to an oil insoluble ammonium ion (NH 4 +) (13).
In addition to acidic or basic solutions, plasma proteins, activated charcoal, and specific antibodies have been proposed as possible trapping agents.
c. Carrier in Membrane Phase or Facilitated Transport In the previous example, the oil solubility of the solute was an important factor in its transport through the l i quid membrane. In order to assist in the transport of a solute through a liquid membrane by improving its solubility in the oil phase, a carrier may be used.
The carrier, which is incorporated into the external aqueous phase, reversi bly binds with the solute to fonn a complex capable of  the membrane surface as a mechanism .for achieving separation (14) .

Applications of Liquid Membranes a. General Applications
Since the initial development of liquid membranes, they have been tested for a number of potential uses. In the area of waste water purification, liquid membranes have been studied with regard to their ability to reduce and separate nitrates and nitrites by encapsulated enzymes (6,13), the extraction of heavy metals from waste water (11,13,14), and the separation of phenols from waste water (12).
Liquid membranes are capable of separating hydrocarbons (2,7,11) and have exhibited promise as a potential way of oxygenating blood by a less traumatic, more efficient means (3,4). Research has been directed toward developing a liquid membrane which will serve as an adjunct to kidney dialysis through the removal of uremic toxins (8,9).
Enzyme encapsulation by liquid membranes as a means of protecting enzymes from destructive materials or to immobilize them also appear feasibie (5,6,10). Liquid membranes have also been proposed for use in the pharmaceutical industry. The liquid membranes may be administered easily and in a single dose. Since they have the consistency and appearance of a milk shake and may be flavored, minimal patient resistance would be anticipated (iO). Using a stomach tube, the system may be administered even if the patient is unconscious (15). The results of the in vitro work to evaluate the potential usefulness of liquid membranes for the rapid removal of drug from the gastrointestinal tract showed that 95% of the phenobarbital present in the external aqueous phase was removed within 5 minutes and essentially complete removal occurred in 10 minutes. The rate of acetylsalicylic acid uptake was slightly faster. The rate of uptake followed first order kinetics (16).
The rate of drug uptake by liqu i d membranes was observed to be influenced by variables introduced into the system. The viscosity of the liquid membrane proved to be a very important variable. As the viscosi~y of the membrane decreased, the rate of drug removal increased at the expense of the membrane integrity. Membranes composed of lower viscosity oil phases ruptured and leaked. It was observed that at higher temperatures the rate of uptake increased (16), and increasing the pH of the external aqueous phase slowed the rate but did not affect the completeness of drug removal (15). The presence of bile salts in the external aqueous phase had an adverse effect on ( ' l 26 drug removal (:~) ,and t he ratio of liquid membra ne to external aqueO'!S phase was no+ea as an important variable infl uencing the rate of dru g transport (1 6) .
As previousl y entioned, the manufacture of a stable liquid membrane is a cnal 1enging task. This is especially true for those liquid membrane systems proposed for use in pharmaceutics. For example, buffer solution trapping agents add electrolytes to this sensitive system which may be a source of instability (20). In order to insure a stable emulsion which will function as a liquid membrane, an optimum ratio of oil to surfactant must be determined. For phannaceutical use, both of these components must be regarded as safe (GRAS).
More extensive studies on the effect of formulation variables on the transport of solutes across liquid membranes were conducted by Yang and Rhodes (17). Using acetylsalicylic acid and phenobarbital as model drugs, they evaluated the effect of liquid membrane oil to water ratio, the size the internal aqueous phase droplets, liquid membrane viscosity, and internal aqueous phase pH on solute transport.
Confinning the results of earlier work, they found that the lower the membrane viscosity, the faster the rate of drug removal. Faster drug removal was observed also with membranes having smaller internal phase droplets. Increasing the pH of the internal aqueous phase resulted in more efficientacetyl salicylic acid removal from the external aqueous phase.

Formulation variables have been considered by other investigators
in attempts to formulate systems suitable for drug delivery. In general, it is agreed that the concentration and nature of the surfactants (20,21,55), the incorporation of other additi ves into the formulation (19), and the composition of the internal aqueous phase greatly influence the successful manufacture of the finished product.
The effect of molecular structure on the transport of solutes across liquid membranes was investigated by Chilamkurti and Rhodes (18).
The authors were able to reiate differences in rates of solute transport to molecular structure, to show that some ionic species are capable of transporting across the membrane, and to demonstrate the effect of solute ionization on transport.  ..     Glassware and conman laboratory equipment as available in the College of Phannacy.   Where: A = absorbance, a = absorptivity, b = sample path lengt~ c = concentration.
Applying Beer's law, the slope of the straight line obtained by the absorbance versus concentration plot is equivalent to the term ab in Equation (14).
By rearranging Equation (14) to Equation (15) the concentration of an unknown sample may be calculated.

b. High Pressure Liquid Chromatographic Determination (HPLC)
A calibration curve was generated for each drug assayed by high pressure liquid chromatography (HPLC). The HPLC system included a Waters Intelligence Sample Processor (WISP) unit which was prograrrmed to inject automatically a range of sample volumes from one standard solution into the system for assay. For each drug, a series of dilutions from a standard solution was made also and the absorbance of each sample was determined. A Beer's plot was obtained for each drug by plotting the absorbance of each sample volume or sample dilution against its known concentration. Based on the principles described above in section 3 a, these plots were used to determine unknown concentrations of solutes.

Preparation of Emulsions a. Using the HLB Approach
Sixty milliliter quantities of 50% water-in-mineral oil emulsions were prepared using blends of emulsifying agents having a HLB of 6.0the HLB of mineral oil. The ratio of emulsifiers required to achieve this HLB was determined using Equations ( 9) and (10).
The emulsions were prepared by adding 33.3 ml of mineral oil to the emulsifier blend and heating to 72°C. The required quantity of aqueous phase, also heated to 72°C.was poured into a clear 120 ml ( ( bottle containing the heated oil/surfactant mixture. The total preparation was hand shaken for 2 mirutes and stored at room temperature.
In order to scale-up the quantity of emulsion prepared, the same procedure was followed using a homogenizer instead of the bottle for mixing. The tota1 preparation was homogenized for 1.5 minutes.
b. Using the Davis Approach 38 Sixty milliliter quantities of water-in-mineral oil emulsions (50%) were prepared based on a formula developed by Davis (19) (see Table I).
The required quantities of ingredients comprising the oil phase were heated to 72°C, as was the aqueous phase. The two phases were hand shaken in a clear 120 ml bottle for 2 minute s and stored at room temperature.
In order to scale-up the quantity of emulsion prepared, the same procedure was followed except a homogenizer was used for 1.5 minutes instead of the bottle for mixing.

Evaluation of Subsequent Solute Uptake
The experimental scheme depicted in Figure 4, was devised in order to investigate the ability of liquid membranes to be reused for the purpose of subsequent solute uptake. Using L.M. 573-119 A, an uptake experiment using either salicylic acid 1.0 g/l in O.lN HCl or phenobarbital 0.6 g/l in pH 2 buffer was conducted for 120 minutes with HPLC sample analysis. The liquid membrane system remaining at the end of the run (LMS I) was collected, measured, and mixe~ in a beaker until further use. The glassware used in the initial uptake run was washed and dried.
A pH 2 buffer solution containing either salicylic acid 1.0 g/l or phenobarbital 1.2 g/l was used as the external aqueous phase for the second part of the experiment. Samples collected from LMS II for 120 minutes were assayed for both drugs.  Two replicate runs were conducted to determine the effect of the presence of one solute on the rate of uptake of another solute when the initial concentrations of each solute were different. Table III lists the combinations of solutes and their initial concentrations in pH 2 buffer. L.M. 573-120 A was used in these experiments. Samples collected were assayed using HPLC.    Table V describes the emulsions prepared by the procedure described in Section III, C-4a. These emulsions were analyzed as described in Section III, C-5. Table VI lists the emulsions prepared as described in Section III, C-4b. These emulsions were analyzed as described in Section III C-5.  ( 50) Span 80 (66) Tween 80 (33) Span 80 (83) Tween 80 (17) Span 80 (83) Tween 80 (17) Span 60 (66) Tween 60 (33) Span 60 (71) Tween 60 (29) Span 60 (87) Tween 60 ( 13) Tables I and II of t heir work (17)), in many instances only two points were available for estimation of a ; however, there were some instances where as many as 11 points could be used for that purpose.

Using the David Approach
In their work, Yang and Rhodes also reported t hat their data showed that the simple model yielding the equation wh i ch describes a two compartment model (Equation 13 of this thesis), is JI . . suitable for the definition of the kinetics of transport across a liquid membrane.JI They further state that in a previous paper by Chang, et~· (16), 11 it was reported that the uptake of drugs 58 by liquid membranes obeyed simple first-order kinetics. For those systems in which the a phase is very short, the description will essentially be valid for all practical purposes. However, for other systems, the s phase will be of greater magnitude than a and thus the apparent simple monoexponen~ial kinetics will be obeyed." (17).
Variation in the a values is greater than that observed for the s values. The method of sampling prohibits the acquisition of more data points in the early a stages of the experiment. As Yang and Rhodes have points out, 11 • errors in sampling times are relatively greater at the start of experiments than at later times 11 (17). Thus, those data points acquired for the a phase of a process are subject to more error than those points of the s phase. Although the s rate constants may be determined from actual experimental points, t he a rate constant is calculated indirectly by a "feathering" technique (62).
Although Autoan was used to indicate the best mathematical fit of the data (i.e. one or two compartment model), the simplest model that adequately described the data was chosen when di fferences between the two models were not sufficient to justify the use of the more complex two compartment model. In the two cases in which this procedure was adopted, the difference in the value assigned to the apparent B ·rate constant by use of the one or two compartment model was very small, as illustrated in Table VII.  The application of significance to the results has been viewed in three ways.
The first is data which appears to be different by inspection but when tested statistically, the differences were not significant.
Although the Student's T test does not show the means to be significantly different, the data may be reviewed for possible trends.
Increasing the sample size may help to clarify these cases. Obtaining additional data was hampered by the limited availability of the liquid membrane supplied by Exxon Research and Engineering Company.
Secondly, in some situations the acquisition of more data may prove it to be statistically significant; however, from a practical point of view, were these liquid membranes to be used for the purpose of drug delivery, it may appear that the differences \'/ould not be significantly large to be of practical importance.
In the final case, the data was found to be statistically significant and also of likely practical significance. An example of ( 60 this is the situation in which the ~atr. constants between different initial donor phase concentrations were determi~ed to be significant.
This finding also has an effect on the efficiency of the liquid membrane to function under varicus conditions. For example, one potential use of liquid membranes is their application as 11   HPLC can be used to identify and quantify a compound with a high degree of sensitivity. The sensitivity of the HPLC techniques used in this project was demonstrated in two ways. Firstly, the qua l itative sensitivity was determined. For example, it was noted by absorbance readings using the UV spectrophotometer that the complete uptake of salicylic acid had apparently occurred within a period of ten minutes.
Using HPLC the presence of drug was detected over the course of 120  An advantage of particular importance to this work is the ability · of HPLC to be a differential assay, thereby allowing analysis of two or more drugs without interference. This added another dimension to the study of drug transport across liquid membranes -investigation of co-uptake of drugs from one donor phase solution. Figure 5 shows the baseline separation obtained when acety l salicylic and salicylic acids .
are in solution together. Figure 6 shows a typical chromatogram obtained for phenobarbital.
The HPLC assays used in this study were si milar to those described in the literature (59,60). However, unlike the assay described in the Water's Product Literature (60), in this project detection of phenobarbital was ach ·ieved at 254 nm rather than 195 nm. The assay was adapted for use with a 254 nm lamp with good accuracy, precision, and sensitivity. Use of this wavelength was necessary since a variable wavelength detector was not available. The fact that the a~say could be run with satisfactory results at 254 nm, a generally available wavelength, is of considerable value.

Evaluation of Sampling Technique
The sampling technique was quite reproducible.

Acetylsalicylic Acid
The rate of uptake of acetylsalicylic acid as a function of initial donor phase concentration was investigated (refer to Figures 7 and 8).
The apparent s rate constants for each replicate run of acetylsalicylic acid 1.0 g/l and 0.5 g/l and the mean apparent s rate constants are listed in Table XIII       However, it will be appreciated that the above discussion is in the realm of reasonable speculation rather than firm conclusions.

Salicylic Acid
Four donor solutions, each with a different initial molar concentration of salicylic acid (2.5 mM; 4.1 mM; 5.6 mM; 7.2 mM) were studied for affect of concentration on solute uptake. Figure 9 shows the mean apparent ~ rate constants for each se~ of uptake runs as a function of molar concentration. These are listed in Table XV. The same trend of decreasing apparent 8 with increased donor concentration over a wider range was observed for the uptake of salicylic acid as was noted for acetysalicylic acid. Thus in summary, the results for these two solutes do indicate that as the initial concentration of the drug in the donor phase increases, the apparent 8 does have a tendency to decrease.   Extraction curves in Figure 10 show the effect of liquid membrane oil/water ratio on t he rate of uptake of salicylic acid 1.0 g/1. The ( mean apparent a rate constants for these uptake processes are tabulated in Table XVI. A plot of the mean apparen t a rate versus liquid membrane oil/water rat i o is presented in Figure 11. As the oil/water ratio increased, the drug was extracted le ss rapidly with the exception of the 3:1 liquid membrane. In this case a variation in the apparent a rate constant was observed and thisapparent a rate was greater than that of the 2:1 iiquid membrane. 78 It is hypothesized that the oil phase acts as "resistance" in the transport process. As the volume or the thickness of the liquid membrane increased, the apparent ~ rate constant consequently reduced, as would be expected from a consideration of Fick 1 s law (Equation (2)).   (2) 49.67 (2) 18.33 (2) 33.10 (2) E. Effect of Donor Phase pH on the Rate of Solute Uptake 1. Acetylsalicylic Acid 10-2 (min-1) The effect of donor phase pH on the rate of acetylsalicylic acid uptake was investigated using three different pH donor solut i ons (pH 1, pH 2, pH 3). Figure 12 shows a plot of mean apparent S as a function of pH. This mean apparent S rate constants are listed in Table XVII. For acetylsalicylic acid, since at low pH more drug exists in its unionized form than at high pH, it was expected that the apparent S would decrease with increased pH. In fact, however, the apparent S rate constant increased and then decreased with a change in pH from 1 to 2 differing from that which was expected.
A more extensive study of acetysalicyl i c acid uptake as a function of pH is necessary before any definite conclus i ons may be drawn.     (2) 5 . 52 (2) The effect of donor solution pH on the rate of salicylic acid 1.0 g/l uptake is shown in Figure 13. The mean apoarent B rate . .

82
constants were calculated and are listed in Table ~VITI Table XVIIIl An attempt was made to predict the rate of salicylic acid 1.0 g/l uptake from donor solutions of various pH us i ng the Hendersor.-( ( The simple model used to describe drug uptake a~pears to be insufficient when used in conjunction with CSMP to predict drug uptake as a function of pH in a quantitative manner. One probable explanation for the failure of the simple model 83 and CSMP to quantitatively predict the rate of drug uptake is the fact that the surfactant used in the preparation of these emulsions contains an amino functional group. At low pH these groups will be largely ionized, whereas at higher pH they will be predominately unionized. Ionization of the amino functional groups will tend to cause mutua1 repulsion of the head groups as well as alter the effective HLB of the system. This head group repulsion is likely to reduce the packing density of the surfactant molecules in the con-  As was expected, a slower rate of salicylic acid 1.0 g/l uptake was observed as the viscosity of the liquid membrane increased. Mean apparent s rate constants were calculated and can be found in Table XIX. A plot of salicylic acid as a function of liquid membrane viscosity is shown in Figure 15.     (2) 31.17 (2) 10.00 (2) The effect of temperature on the rate of salicylic acid uptake was studied at 4, 16, 28, 37.5, and 50°C. A plot of percent drug uptake versus time is shown i n Figure 16. Table XX lists the mean apparent e rate constants. As expected, the results show that as the temperature increased, the apparent s rate constants for the process The Arrhenius relationship was apparently obeyed as indicated, but this does not necessarily mean that the Ea calculated was the true Ea.
Since, as previously discussed, alterations in the temperature of the liquid membrane system not only affect the solute per se but also affectsthe properties of the liquid membrane, it is unlikely that the transport process is occurring between the same standard states. 39.95 (2) 63.45 (2) ---** **Approximately 6% of drug remained in external aqueous phase after 1 minute.

Phenobarbital
The effect of temperature over the range of 4-45°C on the removal of phenobarbital from the external donor phase was investigated. A semilog plot of removal of drug as a function of time over the temperature range is shown in Figure 19. The a and s rate constants were calculated and are listed in Table XXI. The effect of temperature on these systems was not as expected. As the temperature was increased, the a rate constant increased over the range of 4-41°C.
After reaching a maximum at 41°C, the a rate constant was then reduced.
Essentially the same phenomena occurred with the B rate constant.
~~is rate constant increased with increasing temperature up to 41°C, but decreased with any rise in temperature thereafter.  .µ There are several possible explanations as to why this behavior was observed. As indicated by the uptake of salicylic acid at 50°C, 94 it appears as though increasing the temperature alters the physical properties of the emulsion in such a way as to change the characteristics of the interfaces. Although, as has been predicted, emulsion properties wi 11 change with increasing temperatur· e, the data shows that there is a small temperature change at which there is a radical change in the ability of the liquid membrane to trap drug. It is suspected that this critical temperature (43°C) reflects a substantial disruption of one or both interfaces. At all temperatures a steady uptake of drug into the liquid membrane was noted. This process occurred at a faster rate than the t~ansport of drug from the liquid membrane into the internal aqueous phase since a was always greater than a. Therefore, the ability of the liquid membrane to remove solute at any temperature was maintained.
Another consideration may be the formation of a specific complex between phenobarbital and the surfactant depending on the temperature sensitivity of the complexation and membrane transport process. This could possibly be a factor involved in the type of data reported here.  ,. ...
.. Since increasing the viscosity of the liquid membrane would likely have a stabilizing effect, it seems possible that at the critical temperature it would be advantageous to use a liquid membrane with a greater viscosity. However, the data presented in Table XXII and Figure 22 shows that even at the critical temperature a lower viscosity system had greater a and 8 rate constants than a higher viscosity system. There was no obvious relationship between viscosity and the rate constants. This is not 5urprising since viscosity may itself be dependent upon a critical temperature.

3.04
For most of the experiments in this project the uptake of drug from an external aqueous phase was monitored. In this section the back   of drug in the external aqueous phase following exposure to the liquid membrane for 120 minutes. According to the pH-partition theory, the unionized species transports easily compared to the ionized species.
At low pH, salicylic and acetylsalicylic acids are predominately unionized and therefore available for transport across the liquid membrane into the internal aqueous phase. Once inside this internal phase the drug is trapped by conversion to its ionized species. If, as might well be expected, the ionized species of the solute is unable to cross the hydrophobic area of the liquid membrane, then transport will not be possible. The data shown in Table XXIII that with acetylsalicylic and salicylic acids this was the case when the ( ( ( ionized form. Therefore, there is small but finite quantity of unionized species which could be available for back transport. This seems to be unlikely since the unionized species is present in a very low concentration. It is possible that ionized phenobarbital had undergone back transport. It is noteworthy that Chilamkurti and Rhodes (18)    In the first system the uptake of phenobarbital following the removal of salicylic acid at 50°C was slow but steady for one run ( Figure 23). When this was repeated, the uptake was slow and variable as a function of time ( Figure 23). Variability in this data is somewhat disappointing but not entirely unexpected. As reported in this thesis, the extraction curves for one component system are very reproducible and very much better than might be expected for a complex dispersed system. However, whenever one tries to study the extraction of a second solute when the system has already been perturbed by the extraction of a compound, nonreproducible results may be obtained.
It is very easy to postulate that such factors as mean liquid membrane thickness or interfacial area may be substantially modified.    (1) A simiiar trend was observed for the second and third systems studied. In these systems the removal of salicylic acid from the external aqueous phase subsequent to phenobarbital remova~ was investigated at 37.5 and 4s:c. Under both conditions the removal of salicylic acid was steady and essentially complete (Figures 24 and 25).
The mean apparent B rate constant calculated for these systems are listed in Table XXIV   It is interesting to note that essentially the same phenomena 112 occurred with the co-uptake of acetylsalicylic acid and phenobarbital.
The mean apparent 8 rate constant for acetylsalicylic acid was less when it was in a multicomponent donor phase with salicylic acid than with phenobarbital (6.74 x 10-2 (min-1 ) verus 11.41 x 10-2 (min-1 )).
Moreover the mean apparent s rate constant for acetylsalicylic acid in System I was approximately· the same as that calculated for this drug in a non-equimolar ~ulticomponent system with salicylic acid (6.95 x 10-2 (min-1 ) versus 6.74 x 10-2 (min-1 )).
The total molar concentration of solute in the external donor phase with these two drugs in equimclar concentrations was less than the total molar concentration of solute in the previous co-uptake system of these two drugs (11.2 mM versus 12.8 mM of solute). As previously observed for these two solutes as single component systems, a decrease in concentration resulted in increased mean apparent s rate constants. The mean apparent s rate constants obtained for the present system seem to support the proposed speculations of membrane surface property alterations and changes in the size of the internal aqueous phase droplets. At this lower concentration alterations in the properties of the membrane are not as great as the more concentrated system and the internal aqueous phase droplets were not exposed to as much solute. This resulted in increased apparent s rate constants.
For System II the rate of . salicylic acid uptake was too rapid to calculate a rate constant for the process. After two minutes less than one percent of the drug remained in the external donor phase. This rapid removal of salicylic acid was similar to both the removal of this drug from a single component 2.5 mM system and from the previous salicylic acid/phenobarbital co-uptake system. The percent of phenobarbital remaining in the external aqueous phase after 120 minutes decreased as compared with the percent remaining in the previous salicylic acid/phenobarbital co-uptake system (9.48 versus 33.78).
In a co-uptake system, the rate constant and percent of phenobarbital remaining in the external aqueous phase appears to be affected by the total solute concentration in the donor phase.
Speculation regarding the mechanisms involved as discussed previously with phenobarbital/salicylate uptake may be applied to this system.
The effect of phenobarbital appears to be insignificant to the uptake of salicylic acid at these lower solute concentrations of the external aqueous phase. It is also possible that the lower concentration of solute in the external aqueous phase enhanced the phenobarbital uptake in the same manner.
Because of a short supply of liquid membrane, the combinations of solutes for co-uptake was limited. For this reason the interpretation of the data described above must of necessity be somewhat speculative.
Although differences were noted through the comparison of some apparent e rate constants, additional data would aid in determining if these differences were significant. From a practical point of view these changes in the apparent e rate constants may not be of such a magnitude as to be significant.

Solutes
In general, the effect of liquid membrane oil/water ratio on the co-uptake of phenobarbital (2.5 mM) and salicylic acid (7.2 mM) was as ( l 117 expected. As the ratio of 1il/water increased, the uptake of drug by the system was slower. For this situation two parameters must be considered--the oil/water ratio of the membrane and the presence of two drugs in the external aqueous phase . Figure 28 shows that the mean apparent s rate constant decreased as a function of increasing oil/water ratio for the uptake of phenobarbital as expected.     As can be seen from Table XXVIII, there appears t o be an optimum oil/water ratio (1.0) for the uptake of salicylic acid from the multicomponent system. As a general trend, the higher oil/water ratios gave a lower s rate constant. However, the range of apparent s rate constants varied widely at the lower oil/water ratios. Therefore, it was difficult to designate a specific order of decreasing apparent s rate constants for these three liquid membranes (O.S; 0.67; 1.0).
Additional studies at the lower oil/water ratios may help to clarify the order of rate constants in this situation.
Based on the previous results of co-uptake of phenobarbital and salicylic acid at one oil/water ratio (1.0), it was expected salicylic acid would behave similarly when the oil/water ratio was varied. In general this was the case.

K. Effect of Freeze/Thaw on the Use of Liquid Membranes
The integrity of liquid membranes and their capacity to function as sinks was investigated by freezing and thawing three liquid membranes and then exposing them to drug donor solutions. Gross changes were observed after thawing. Nearly total phase separation was noted but the emulsions could be redispersed with vigorous agitation into a homogenous system. Figure 29 shows a plot of percent drug in the external aqueous phase versus time for each of the frozen/thawed systems. The apparent s rate constants are listed in Table XXIX.
Surprisingly, these rate constants were almost identical to the mean apparent s rate constants for salicylic acid using these membranes under ideal conditions (Table· XXIX). It is realized that these rate      color indicating a small mean particle size. Those emulsions which were the whitest tended to be the most stable as would be expected.
The major problems associated with the use of these emulsions as liquid membranes were their high viscosity (i.e. far too thick) and, in some instances, their incomplete dispersion as discrete glcbules in         The pH of the external donor phase was observed to affect the rate constants describing salicylic acid uptake in the expected manner. At low pH the apparent 8 rate constants were greater than at higher pH.
This was due to more n9ni oni zed drug being ava i 1ab1 e for transport at low pH values. An attempt was made to predict quantitat·ively salicylic acid uptake as a function of pH over 60 minutes us i ng a CSMP computer  The study of the effect of temperature on the uptake of solutes by liquid membranes revealed some interesting phenomena. Whereas the apparent s rate constant controlling salicylic acid uptake increased with increases in temperature, as expected, and apparently followed the Arrhenius relationship, this was found to be true for phenobarbital until a critical temperature was reached. It was discovered that a critical temperature range existed above which a sharp decrease in the a ands rate constants resulted. It is believed that the liquid membrane was beginning or nearly about to collapse. It is speculated that changes in the physical properties at this high and critical temperature were responsible for this or that secondary reactions between the surfactant and phenobarbital resulted in a temperature sensitive formation of a complex. Through this complex, it was thought that 'changes in the nature of the interfaces resulted in alterations in solute permeability. By increasing the viscosity of the liquid membrane used at this critical temperature, it was thought that solute transport might be improved. This was not found to be the case, in fact, there was no obvious relationship between the rate constants and viscosity at the critical temperature. Viscosity of the membrane itself may be dependent upon a critical temperature aGd further studies in this area may be useful in detennining if this is true.
Altering the physical properties of the liquid membranes was found to have the predicted affects on solute uptake. By increasing the oil/water ratio, the rate of solute uptake decreased. This was observed for salicylic acid in both single component and multicomponent systems and for phenobarbital in a multicomponent system. The increased thickness of the oil phase and any possible changes in the amount of surface area from the internal aqueous phase droplets are proposed as possible explanations for this occurrence.
As the viscosity of the liquid membrane increased, the rate of salicylic acid decreased, as would be expected by consideration of Fick's law. It is probable that as the oil was changed to achieve a more viscous membrane, the diffusion coefficient of the drug changed, hence slower transport.
Through evaluation of the reuse of liquid membrane, it was seen that whereas some drugs (acetylsalicylic and salicylic acids) were not capable of back transport (i.e. transport from the liquid membrane to the external aqueous phase), phenobarbital was able to transport from Freeze/thaw stressing of the liquid membranes proved that they are very stable systems. The apparent e rate constants obtained from experimental runs using these membranes was surprisingly similar to -those obtained for uptake under ideal conditions.
Although the formulation of liquid membranes with GRAS components was difficult, some success was achieved with the production of two systems which used liquid membranes in drug removal runs. Method of manufacture and appropriate surfactant blends were determined to be key factors in the development of these systems for use as liquid membranes.
The HLB approach to emulsion formulation was found to be a useful tool in the development of these systems.
In suwmary, the major impact of the data reported here on the theoretical aspects of liquid membrane transport will be on the deve1opmentoramodel which will describe this complex process.
Although in many instances the data obtained could readily be eAplained by the simple model proposed by Yang and Rhodes and Chilamkurti and Rhodes, it was insufficient to rationalize all aspects of liquid membrane transport when additional parameters were defined and included the model. Realizing that this is a complex system, it would be inappropriate at this time to develop a model which would quantitatively predict all aspects of liquid membrane transport. Clearly, further study of the kinetics of more complex liquid membrane systems is a fascinating and challenging area for research.
The results reported in this thesis have relevance for a number of practical pharmaceutical applications of liquid membranes.
Although the attempt to produce liquid membrane formulations using GRAS components was only partially successful and it is fully realized that none of the formulations developed in this study are such that they could be marketed in their present fonn, it is believed that the data reported overall tends to strongly support the potential development  Reuse of liquid membranes in terms of subsequent drug removal may have application in the multiabsorption of drugs in overdose situations.
Another area which has direct application is the use of liquid membranes overdose situations is the co-uptake of solutes. The liquid ( 135 membranes were found to behave well in the presence of multicomponent external aqueous phases. Although changes in the rate of uptake were observed in some cases, these changes are thought to have little effect on the overa 11 ability of . these membranes to remove two solutes simultaneously.
In conclusion, it is believed that the data reported in this thesis are likely to prove valuable in the development of a comprehensive, rational theory defining the properties of liquid membrane systems.
Also it is appreciated that the data reported in this thesis clearly defines a number of aspects pertinent to the practical application of liquid membranes as drug delivery systems or drug sinks in the emergency treatment of . drug overdose. 2. Viscosity and oil/water ratio of liquid membranes will influence the rate of solute uptake. 136 3. It appears that optimum environmental conditions are necessary for the efficient removal of solutes by liquid membranes. Whereas increasing the temperature will increase the rate of uptake of a solute, there appears to be a critical temperature for phenobarbital uptake.
Increasing the pH and solute concentration of the external aqueous phase decreases the rate of solute uptake.
4. Computer modeling techniques to describe the uptake of solutes with liquid membranes were of limited value. This was probably due to the complex nature of the system.

5.
Liquid membranes show potential for use in co-uptake and multidruguptake and they appear to be reusable.