Solubilization of phenolic compounds in nonionic surface-active agents. I. binding pattern and parameters of phenol, cresols, and xylenols

Solubilization of Phenolic Compounds in Nonionic Surface-Active Agents I. Binding Pattern and Parametersof Phenol, Cresols, and Xylenols E M M A AZAZ AND M A X D O N B R O W

School of Pharmacy, Hebrew University, Jerusalem, Box 12065, Israel Received July 14, 1975; accepted January 27, 1976 The binding of phenol and nine of its methyl derivatives by an aqueous solution of a hexadecyl polyoxyethylene ether, cetomacrogol, was found by a modified potentiometric method to be concentration dependent and to follow Langmuir's adsorption equation. The product of the adsorption parameters was adopted as a parameter characterizing each system and represented the distribution coefficient (K0) of solute between micelles and water in an infinitely dilute solution of the solute. K0 values were inversely related to water solubility of the phenols and directly to their partition coeffÉcients between heptane or octanol and water. The validity of the K0 concept was verified by utilizing uv spectral shifts. The hypothetical absorption bands of two of the phenols in pure surfactant were calculated from their bands in very dilute solutions using K0 values. Identical hypothetical bands were obtained from systems containing different concentrations of surfactant. The significance of K0 as compared with saturation capacity and maximum additive concentration is discussed. INTRODUCTION

saturation in micellar systems. As systematic studies are lacking, the present investigation was undertaken for theoretical reasons and for establishing free and bound concentrations for antibacterial and pharmaceutical applications. The distribution was studied by the potentiometric method (2, 6, 8-11) which has not previously been used for phenols. Substantial modifications were introduced in place of the titration techniques used hitherto (11) to improve accuracy and rapidity.

A number of contradictions exist in the literature describing the distribution of aromatic compounds between the aqueous and the micellar phases of non-ionic surfactants; some authors found the distribution to be concentration-dependent, e.g. (1-4), whereas others (5, 6) assumed a single distribution constant. Mukerjee (7) considered that the concentration dependence in the case of methyl phydroxy benzoate conformed with regular solution theory. He obtained a linear relationship between the activity in the micelles and the free concentration, thus admitting nonlinear distribution when free and bound concentrations rather than activities are considered. Phenolic compounds hitherto studied were chosen largely for their microbiological interest. With few exceptions, studies were conducted at saturation or at a single concentration. I n practice, phenols are generally present below

EXPERIMENTAL

Materials Cetomacrogol 1000 B.P.C. (Texofor AlP, Glovers Chemicals, Ltd., Leeds, England) was characterized as previously (10). Average formula --CH~ (CH2) 15(OCH2CH2) 2~OH. Phenols. Their sources, method of purification and physical properties are summarized in Table I. 11

Copyright ~ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal oJ Colloid and Interface Science, Vol. 57, No. 1, October 1976

AZAZ AND DON-BROW

12

TABLE I Source and Characteristics of Phenols Phenol

Source, grade~

m.p. b (deg)

Xmax c (nm)

~ maxc

Phenol

A

a

268

1465

m-Cresol p-Cresol o-Cresol 2,3-Xylenol 2,4-Xylenol 2,5-Xylenol 2,6-Xylenol 3,4-Xylenol 3,5-Xylenol

B B C C C C C C C

a a a 74b a 74b 46.5b 68b 64b

270 278 269 271.5 278 274.5 270.5 277.5 272.5

1440 1710 1660 1150 1800 1570 1150 1725 1175

(A) Merck, Darmstadt, A.R. Grade (B) B.D.H., Lab. Reagent Grade, (C) Aldrich, Puriss Grade. bPurification method (a) vacuum distillation (b) recrystallisation from petroleum ether bp 60-80°C. "Solvent: water, measured using a Uvispek H 700 spectrophotometer. Values in close agreement with literature (12, 13).

Preparations of Solutions Cetomacrogol solutions. 20% w / v stock solution was prepared from ground, mixed material, using gentle heat, and refrigerated under N2. For use in potentiometric studies, the stock solution was deionized by successive passage through columns of anion and cation-exchange resins (Amberlite I R 120 and IRA 401, Fisher Scientific Company). Eluate was collected in soda lime protected containers, the p H of batches ranging between 6.0 and 7.0. The cetomacrogol content (c% w/v) was determined from the refractive index n ~5 which was linearly related to concentration, observing the equation :

aqueous blanks in phenol and NaOH concentrations contained the requisite amount of deionized cetomacrogol and were also measured when fresh. Introduction of NaC1 was from a 5 N stock solution added just before the sample was diluted. Air-equilibrated water. Distilled water is supersaturated with carbon dioxide (16) and was found unsuitable for obtaining reproducible p H values of the accuracy needed. Air was washed with 10% H2SO4, then with distilled water and was passed through freshly distilled water for about 10 h. The p H of this water was 6.5 ± 0.2 and remained constant for at least 4 weeks.

pit Determinations

n 25 = 1.3325 + 0.00135c

Aqueous

blanks.

Suitable concentration ranges of phenols below saturation, (phenol to 0.3 M, cresols to 0.08 M, xylenols to 0.06 M) were prepared in air-equilibrated water, adding sufficient standard NaOH solution for halfneutralization before final dilution. The p H was measured immediately to avoid autoxidation of the surfactant (14, 15) and oxidation of phenates; discoloration did not however affect the p H values within 24 h. Micellar solutions. Solutions parallel to the

instrumentation. A Vibron Electrometer 33B-2 (Electronic Instruments Ltd., England) with pH measuring unit C33B was used (accuracy + 0.002 p H units). Instrument, electrodes and cell were enclosed in an aluminium cage and all were earthed to the same point. The instrument was offset to read over the range 5.00-11.00 by standardizing at 7.60 using 9.60 buffer (NaOH-NaHCO3). Glass electrode response was checked periodically using a series of NaOH-NaHCO3 buffers. The

Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976

SOLUBILIZATION I measuring cell 25°C 4- 0.1.

was

water-jacketed

at

13

10.3

pKc and pH determinations in micellar solutions. The pH of half-neutralized aqueous micellar solutions was measured. It was found necessary to determine the pKc by using a blank for each appropriate phenol-phenate concentration (11) because the relation between pK~ and pKa was atypical of regular activity coefficient effects in all the phenols and concentrations studied. Change in the unionized acid concentration, in the presence of micelles, should not affect the pKc (Fig. 1, see also (2, 6, 17, 18)). Values of pK~ were calculated, using Henderson's equation, from pH readings for ~, ½, and neutralized p-cresol in which the concentration of p-cresol was changed while the NaOH was kept constant. Concentrations of free and bound acid were found (2, 9-11) using Eq. [la-1 and [lb-]

[ttA]w

=

[A-][antilog (--ApH)]

[HA],, = [ A - I l l -

antilog (--ApU)]

10.2

2o_ 10.1 -

1

I

I

I

0.02

0.0t,

0.06

0.08

Concentration No+ M

FIG. 1. Effect of degree of neutralization on pK, of p-cresol: ( - - A - - A ) 2:1 p-cresol: p-cresolate; (--Q--O) 1:1 p-cresol: p-cresolate; ( - - O - - O ) 1:2 p-cresol: p-cresolate. turbid and last clear solution, the mean weight being used.

[-la]

Ultraviolet Spectra

[-lb]

Ultraviolet spectra were measured using a Unicam SP 800 recording spectrophotometer with a 1-cm cell, except for p-cresol in 20% cetornacrogol for which 0.1-crn cells were used to avoid light scattering effects at this concentration (20). Hypothetical uv spectra of phenol and pcresol in pure surfactant were calculated as in the following example : At 4.39.10-4 M phenol in 2% cetomacrogol, the unbound phenol per liter of solution, i.e., per 20 g of cetomacrogol, is calculated from the simultaneous equations:

where [-HA-]~= concentration of free (aqueous) unionized acid in rnicellar solution, [HA-Ira = concentration of micelle-bound unionized acid, I-A--1 = concentration of ionized species, and ApH = pH difference between half-neutralized rnicellar solution and parallel aqueous solution. For calculation of free acid, the aqueous volume was corrected for micellar partial volume using Florence's data (19). Evidence that the salt is not solubilized in the micelle was obtained by pNa+ measurements using a sodium responsive glass electrode in aqueous and micellar solutions.

Determination of Solubility of Phenols in Water Solids were equilibrated at 25 °, filtered using a filter stick and determined spectrophotometrically (see Table 1). For liquids, weighed samples were shaken with 50 ml portions of water at 25°C; in the vicinity of saturation, 3-5 rug increments distinguished between first

(1)

x = Koc (see Discussion)

(2)

20x + c = 4.39.10 -4

where x = moles bound phenol per gram micelles, c = moles unbound (aqueous) phenol per liter, and Ko = 0.042 1/g for phenol. Solution of these equations gives c = 2.39.10-4 M. Assuming the total experimental absorption to be additive, the absorption of the unbound phenol (2.39.10-4 M) is calculated point by point at 1-2 nm intervals from the extinction

Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976

14

AZAZ AND DON'BROW

1"51 o 2,5-XyLenot o 2,S..... zi 3.&.....

• PhenoL

0,5

~

1.0 .~

0,5 xE

CmH

Fro. 2. Solubilization isotherms of some phenols in 2% cetomacrogolat 25°C. coefficients in water and deducted from the experimental values of the 2°/v cetomacrogol solution to obtain the hypothetical absorption curve of the bound phenol. RESULTS AND DISCUSSION The solubilization isotherms of seven of the compounds studied are represented in Fig. 2. All ten compounds show nonlinear binding which can be described by Langmuir's Eq. 1-2] as was found for benzoic acid (2, 4, 10). x = K 1 K ~ d ( 1 -q- K~c)

for K1 and K , separately from a plot of either x-1 versus c-i or x / c versus x. While the first plot gives a very small intercept, making K2-I subject to great error, the second gives a very low slope value, the scatter of points making graphical determination of K1 inaccurate. Only the combined parameter K1K2 could be obtained with reasonable accuracy from either treatment. Reciprocal plots for the three compounds not included in Fig. 2 are shown in Fig. 3 and are typical of those of the other compounds. From the slopes, the products of the two parameters for all 10 compounds were obtained and are included in Table II. The combined parameter K1K2 is a physical property specific to each of the systems studied; it may be defined as the distribution coefficient K0 of the solubilizate at infinitely dilute solubilizate concentration, a concept which hitherto has not been used in micellar systems. Its value would characterize ideal behavior of the solute both in the aqueous and the micellar phase, hence it would be subject to an activity coefficient correction with increasing concentration on the basis of regular

E27

where x = solute bound (mmole/g micelle), c = concentration of free unionized solute (mM), K1 = binding constant (1/mmole), K2 = solute bound at hypothetical saturation (mmole/g micelle). Linearization of Eq. [2-] would be expected to enable derivation of the separate parameters g l and K2 using Eqs. 1-3-]or 1-4-]

1

1 -

x

E "~

~

3,5-Xyleno[

t

IP i

]

[

2

I

'

o- Cresot

1 I-3-]

4

K2

2 [[J ~ l~l

~/"

K1K2c

X

= K1K2 -- K l x .

-

[4-1

2t

£

A straight line was actually obtained with both over a wide range of concentrations for all the compounds studied, indicating that binding conformed essentially to the above equation. However, it was difficult to solve graphically

I p-Creso[iI 02

04

06

O8

1

FIG. 3. Langmuir reciprocalplots of some phenols in cetomacrogol at 25°C: (--11--O) in water, ( - - O - - O ) in 0.1N NaC1, (--E]--Fq) in N NaC1.

Journal oS Colloid and Interface Science, Vol. 57, No. 1, October 1976

15

SOLUBILIZATION I TABLE II Aqueous Solubility and Distribution Coefficient at Infinite Dilution between Cetomacrogol and Water of Phenols at 25°C Compound

Solubility in water (M X l 0 s)

Ko in water a

Solubility in NaCI (0.1 N) ( M ) < l0 s)

K0 in NaCI" (0.1 N)

Phenol o-Cresol p-Cresol m-Cresol 2,4-Xylenol 2,6-Xylenol 3,5-Xylenol 3,4-Xylenol 2,3-Xylenol 2,5-Xylenol

1000 240 199 142 51.0 49.5 40.0 39.0 37.4 29.0

42.0 79.5 76.4 85.1 125 114 132 151 169 197

233 188 133

117 80.6

190

Units: (l/g) X 10~ = 1/1000 g or dimensionless units assuming density of cetomacrogol is unity at 25°C. Measured in 2% cetomacrogol. solution theory. This would correspond to Mukerjee's postulated treatment of activity in the micelles as an alternative to the " a d sorption" model (7). This definition of K0 is justified b y Eq. [ 2 ] which at very low concentrations reduces to the linear relation: x = K1K:

= Koc.

[-5]

the phenol partition coefficients between organic solvents and water (21-23) in Fig. 4.II. The plots are substantially linear, both for n-octanol and n-heptane, indicating that linear free energy relationships are observed, but the deviant systems are significant. I n partition between n-heptane and water, o-cresol and 2,6-xylenol are much more hydrophobic

On this basis, Eq. [-2-] would be replaced by an equation of the form

- tog S 0

3"x = K o c

[6]

0.4

0.8

1.2

1.6

2.3 24 341~a"~ ' "

where 3, = activity coefficient of the solubilizate in the micelle for the standard state at infinite dilution, (3'--* 1 as x--* 0). Theoretically, K0 would be the slope of a straight line extrapolated from the initial portion of the adsorption isotherms in Fig. 2. Some experimental support for the K0 concept is given in the next section. Taking the values of K0 as a measure of the relative solubilizing power of cetomacrogol for different compounds, it can be seen from Table I I that the binding capacity is inversely related to the water solubility of the compound. I n fact, a log-log plot of these functions yields a straight line (Fig. 4.I). The micellar distribution coefficients, converted to dimensionless units (see Table I [ notes), are plotted against literature values of

2 1.7' /

,n Kp

log

-0.8 2,3

o

z0

26

3 2 4 .t~"

t

,

,

{n-heptone

-0.4 ,

,

i

i

0---o )

0 i

~

t

0.4

t

t

J

i

r

0.8 i

i.v

I

i

z3t~so' ' '

'°

~

1.7/~P.-/"" 1.2

-u'~ "~/.o

.p~.-~4 ~ • 1.6 log

2.0 Kp

2.4

2.8

(n-octano[ o ~ o )

FIG. 4. Relation between Ko (dimensionless) and (I) solubility in water (S) of phenols (II) distribution coefficients (Kp) of phenols between n-heptane and water ( - - [ ] - - [ ] ) and n-octanol and water ( - - O - - O ) (P = phenol; numbers indicate positions of methyl substituents in phenol).

Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976

16

AZAZ AND DONBROW

(Hansch ¶oH3 values 0.86; 1.16) than their isomers (¶c~3 values 0.55; 0.45-0.76) (21, 24) and these compounds deviate from the upper regression line in Fig. 4.II in a direction indicating that they do not show similarly enhanced binding in the micellar phase relative to the other isomers. 2,4-xylenol also appears to be less strongly micelle-bound than the 2,3 and 2,5 isomers, these three other isomers having closely similar heptane Kp values. However, the n-octanol-water partition coefficients of the three cresol isomers scarcely differ (¶CH3 values 0.48 4-0.05) (21) and significantly the lower regression line in Fig. 4.II shows a high degree of correlation, assuming the 2,6-xylenol point to deviate. The higher Kp values of ortho-substituted phenols towards the hydrocarbon-type solvent is considered by Golumbic, Orchin, and Weller to be due to lowered water solubility resulting from steric hindrance to hydrogen-bonding (24). Both octanol and the micellar ether region contain potential H-bond receptors for the phenol OH groups and steric hindrance in both these media would tend to cancel out the deviation in the graph. Nevertheless, the greater steric limitations of micellar organelles would impose greater restrictions on such interaction in the micelles and this would be reflected in a low K0 value as observed in the highly-hindered 2,6-xylenol. Furthermore, the K0 values are of the same order as the octanol values and much higher than the heptane values. It seems therefore that micellar-uptake of the phenols involves the polyoxethylene chain region predominantly. Further evidence supporting this will be presented in Parts II and III of this series. The results obtained for solubilization of phenols support previous work demonstrating that in many unsaturated systems the binding constant of solubilizates to surfactants is concentration dependent (1-4, 10, 25-27) and not a fixed value as used by many authors (5-7, 28-34). For example, Mulley and Winfield (33) who on the basis of saturation solubility studies supported linear distribution,

doubted the validity of the concentration dependence on the grounds that change in ionic strength might have interfered. The effect of ionic strength was checked in this work using excess electrolyte in the potentiometric measurements on some of the phenols. From Fig. 3 and Table II it is seen that K0 increases on addition of 0.1 N NaC1, as has been found in other systems (35, 36). Since a swamping concentration of electrolyte was present in p-cresol and 3,5-xylenol, the ionic strength could be regarded as constant; nevertheless a concentration dependent isotherm was obtained. Mitchell and Brown (32) published several values for the distribution coefficient of benzoic acid in aqueous solutions of cetomacrogol. The variation between them was believed to be due to the effect of salt on micelle structure and micellar uptake. Their results have been recalculated by the present authors and the variations in distribution coefficient were found to be dependent on aqueous free concentration but independent of cetomacrogol or total salt concentration. The recalculated results observe the Langmuir isotherm (10). Thus work done at different degrees of saturation in water would result in different values of the distribution coefficient. A single K value obtained by the saturation solubility method, i.e., from M.A.C. (maximal additive concentration) (37), would give a misleading picture not only of the distribution in unsaturated solutions but also of the saturation capacity of the pure surfactant for solutes with limited water solubility, the micellar uptake being limited by the maximum aqueous free solute concentration, thus : M.A.C.

=

Cbound--]- Cfree

Cbound = f(x) = f(Ko, cf~) see Eqs. [2] and E6-]. Cbo,nd--*maximum as cfr~--~ saturation value in aqueous solution. The paradoxical implication of this relationship is that a compound with high affinity for cetomacrogol might give a lower M.A.C. due

Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976

SOLUBILIZATION I to the limitation in its free aqueous concentration (c in Eq. [-2]). In fact none of the compounds studied showed the plateau region expected from Langmuir's isotherm (Fig. 2) which should have been reached with compounds of unlimited solubility in water. The lower the water solubility of the compound, the narrower the range of concentrations measurable, as seen by comparing the xylenols with the cresols (Fig. 2). It is interesting to cite in this context data published by Mulley and Winfield (33). These authors studied solubilization of benzoic acid by the saturation method in a series of nonionic surfactants. Their data show that, invariably, the maximum micellar uptake in aqueous micellar solution was lower than in pure surfactant; the differences amounted to as much as 13.5 to 30%. Similarly a difference of 17 to 40% between the theoretical uptake and Mulley's observed maximum uptake was found when hypothetical maximal uptake was calculated using the Donbrow et al. equation, based on surface available for saturation adsorption (4). In both the above publications the theoretical saturation capacity of micel]es was not achieved, presumably due to the water solubility limitation, which was not considered by the authors. 10

5

,,1o ~ o

-'-

'q

I 250

,

"~,(nm )

~r

I

I 300

I 250

,

I 300

"J', ( n m )

Fro. 5. Ultraviolet absorption bands of phenol at different degrees of binding to cetomacrogol. I, Experimental bands. II, Hypothetical bands in pure micellar phase, calculated using K0 of 0.042 1/g ( - - O - - O ) in 2% cetomacrogol, ( - - O - - Q ) in 5% cetomacrogoI. Curves superimposed on semilog scale.

17

10

5

1 I 260

i

I 300

~, (nrn)

I

I

260

1_ 300

),(nm)

FIG. 6. Ultraviolet absorption bands of p-cresol at different degrees of binding to cetomacrogol. I, Experimental bands. II, Hypothetical bands in pure micellar phase, calculated using K0 of 0.076 1/g. ( - - O - - © ) in 2% cetomacrogol, ( - - e - - l ) in 20% cetomacrogol. Curves superimposed on semilog scale.

VERIFICATION OF THE Ko CONCEPT BY ULTRAVIOLET ABSORPTION Figure 5.I shows the spectra of 4.39 10-4 M phenol in 2 and 5% cetornacrogol solutions superimposed in log absorbance versus wavelength units. The shift observed was thought to be due to the different relation between the unbound and bound portions of phenol in each system. Therefore, an attempt was made to calculate the aqueous concentration in each system; for the low concentration of phenol used in uv spectra determinations, the definition of K0 as a linear distribution coefficient should apply. The value of K0 for phenol from Table II was used to calculate the aqueous (unbound) portion in each system (see Experimental). The contribution to absorption of this fraction of the phenol was then calculated and subtracted from the total absorbance at close intervals across the main absorption band. Thus h3qoothetical absorbances in pure cetomacrogol were obtained. Fig. 5.II shows the hypothetical spectra obtained from 2 and 5% cetomacrogol solutions superimposed on a semilog scale, which shows that they are identical and that the value of K0 is significant. An analogous picture is shown for p-cresol in 2 and 20% cetomacrogol systems in Figs. 6.I and 6.1I.

Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976

18

AZAZ AND DONBROW

I t is noticeable that the calculated curve for phenol has a shoulder which is not visible in the aqueous micellar solutions, due to summation of the overlapping water and cetomacrogol spectra. This occurs to a lesser degree in the cresol. This treatment was possible only for compounds with relatively low distribution coefficients, for which the contribution of the aqueous portion to the total spectrum was significant. SUMMARY 1. The solubilization of phenol, the cresols and xylenols in a nonionic surfactant, cetomacrogol, is concentration-dependent in the unsaturated rnicellar solution. The micellar uptake follows the Langmuir isotherm in all cases. 2. The distribution coefficient of a solubilizate at infinitely dilute solubilizate concentration, K0, is defined, and evaluated for each of the phenols from reciprocal Langmuir plots. K0 values are a measure of the binding capacity of the micelles for a solubilizate. The values are raised by salt addition. 3. K0 values are related inversely to the water solubility of the phenols and directly to their partition coefficients (Kp) between organic solvents and water. There is better correlation of the K0 values with the K~ values between n-octanol and water than between n-heptane and water, particularly for o-cresol and 2-6-xylenol, which show enhanced hydrophobicity in the latter solvent. This, with other evidence, indicates that phenols are solubilized in the polyoxyethylene chain region of the micelles. 4. The importance of the degree of saturation of the micelles in relation to experimental K values is discussed and the significance of the water-solubility of the solute in limiting the m a x i m u m additive concentration is explained. 5. Ko values have been tested for phenol and p-cresol by measuring the shifts in the uv spectra at different surfactant concentrations

and estimating the spectra in the micellar phase. ACKNOWLEDGMENTS The authors are grateful to Mrs. P. Fischer and Mr. Y. Peres for their technical assistance. Some of this work formed part of a thesis submitted by E. Azaz to the Hebrew University of Jerusalem for the Ph.D. degree. REFERENCES 1. PATEL, N. K. AND KOSTEZ'mAUDER,H. B., J. Pharm. Sd. 47, 289-293 (1958). 2. DO,BROW, M. Am) RaODES, C. T., J. Chem. Soc. Suppl. 2, 6166-6171 (1964). 3. PAXEL, N. K., Canad. J. Pharm. Sci. 2, 97-101 (1967). 4. DOrCBROW,M., MOLYN~UX,P., ANDRaODES, C. T., J. Chem. Soc. (A) 561-565 (1967). 5. THOMA,K., ULLMANZ~,H., AND FICYd~L,O., Arch. Pharm. Weinheim Get. 303, 297-304 (1970). 6. EVANS, W. P., J. Pharm. Pharmacol. 16, 323-331 (1964). 7. MUKERJEE, P., J. Pharm. Sci. 60, 1531-1534 (1971). 8. DONBROW, M. AND RHODES, C. T., J. Pharm. Pharmacol. 15, 233-238 (1963). 9. DONBROW, M. AND RHODES, C. T., 23rd F.I.P. Conference, Munster. Govi-Verlag GMBH, Frankfurt/Main, 1964, pp. 397-404, 1963. 10. DONBROW,M., AzAz, E., ANDHAMBURGER,R., J. Pharm. Sci. 59, 1427-1430 (1970). 11. DGNBRGW, M. AND RHODES, C. T., J. Pharm. Pharmacol. 17, 258-260 (1965). 12. HERINGTON,E. F. G. AND KYNASTON,W., Trans. Faraday Soc. 53, 138-142 (1957). 13. PLIEV,T. N., Dokl. Akad. Nauk. SSSR 184, 11131116 (1969). 14. HAMBURGER, R., AZAZ,E., AND DONBROW, M., Pharm. Acta Helvet. 50, 10-17 (1975). 15. DONBROW,M., HAMBURGER,R., AND AZAZ,E., J. Pharm. Pharmacol. 27, 160-166 (1975).

16. KOLTHOFF,J. M. Am) SA~OELL,E. B., "Textbook of Quantitative Inorganic Analysis," 3rd. ed., Macmillan, New York, 1967. 17. DYER, D. L., J. Colloid Sci. 14, 640 (1959). 18. COLLET, J. H. Am) WITalNGrO~, R., J. Pharm. Pharmacol. 34, 241 S (1971). 19. FLORENCE,A. T., J. Pharm. Pharmacol. 18, 384-389 (1966). 20. DONBROW,M., "Instrumental Methods in Analytical Chemistry," Vol. 2. Pitman, London, 1967. 21. LEO, A., HANSCH,C., ANDELKINS, D., Chem. Rev. 71, 525-516 (1971). 22. FUJITA,T., ISAWA,J., ANDHANSCH, C., J. Amer. Chem. Soc. 86, 5175 (1964).

Journal of Colloid and Interface Science, Vol.57, No. 1, October 1976

SOLUBILIZATION I 23. DELIGNY,C. L., KREUTZER,J. H., ANDVISSERMAN, G. F., Rec. Tray. Chim. Pays Bas 85, 5 (1966). 24. GOLUMBIC,C., ORCHIN, M., AND WELLER, S., J. Amer. Chem. Soc. 71, 2624 (1949). 25. PATEL, N. K. AND Foss, N. E., Y. Pharm. Sci. 54, 1495-1499 (1965). 26. RHODES, C. T. AND DONBROW,M., J. Pharm. Sci. 54, 1130-1132 (1965). 27. CROOKS, M. J. ArCO BROWN, K. F., J. Pharm. Pharmacol. 26, 235-242 (1974). 28. CHAKRAVARTY,D., LACH,Y. L., ANDBLAUG,S. M., Drug Standards 25, 137-140 (1957). 29. EVANS, W. P. AND DUNBAR, S. F., in "Surface Activity and the Microbial Cell," S.C.I. Monograph No. 19. Society of Chemical Industry, London, 1965. 30. GOODHART,F. W. AND MARTIN, A. N., J. Pharm. Sci. 51, 50-54 (1962).

19

31. HUMPHREYS,K. J. AND RHODES, C. T., J. Pharm. Sci. 57, 79-83 (1968). 32. MITCHELL, A. G. AND BROWN, K. F., J. Pharm. Pharmacol. 18, 115-125 (1966). 33. MULLEY,B. A. ANDWINFIELD,A. J., J. Chem. Soc. (A) 1459-1464 (1970). 34. TnOMA, K., ULL~ANN, E., AND FICKEL, O., Arch. Pharm. Weinheim Get. 303, 289-296 (1970). 35. MANKOWlCH,A. M., Ind. Eng. Chem. 47, 2175-2181 (1955). 36. SAITO,H. ANDSmNODA,K., J. Colloid InterfaceSci. 24, 10-15 (1967). 37, ELWORTHe, P. H., F~ORENCE, A. T., Am) MACFARLA~, C. B., "Solubilization by SurfaceActive Agents and its Application in Chemistry and Biological Sciences," Chapman and Hall Ltd., London, 1968.

Journal of Colloid and Interface Science. Vol. 57, No. 1, October 1976