In situ voltammetric determinations of solute distribution coefficients in emulsions

In Situ Voltammetric Determinations of Solute Distribution Coefficients in Emulsions t TADASHI MATSUBARA AND JOHN TEXTER z Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received July 19, 1985; accepted October 18, 1985 A method to measure distribution coefficients (P) of electroactive species in situ in turbid oil-in-water emulsions has been developed and is illustrated for the partitioning of 4-methyl-3-amino-N,N-diethylaniline (2) between oil and aqueous phases in an aqueous gelatin/l-undecanol emulsion at 40°C (P = 55_+ 2). Voltammetry at a rotating platinum disk electrode is used as a probe of 2 concentration in the aqueous gelatin phase. Rheological effects due to the microheterogeneous nature of the emulsion on the limiting currents are quantitatively characterized by the use of 4-amino-3-methyl-N-ethyl-N-(fl-sulfoethyl)aniline (1), which does not partition measurably into the oil phase. © 1986AcademicPress,Inc.

INTRODUCTION

The in situ determination of solute distribution coefficients in turbid oil-in-water emulsions has received practically no attention in the literature, although associated heterogeneous transport phenomena are of great importance in a variety of technologies (1-6). This paper describes the application of the rotating platinum disk electrode (RPDE) to the first such determinations for electroactive solutes. Only recently have electrochemical techniques been applied successfully to the study of electroactive species solubilized in micelles and similar microheterogeneous media. Yeh and Kuwana (7), Hoshino et al. (8), Georges and Desmettre (9), McIntire et al. (10), and Eddowes and Gratzel (11) have illustrated the use of a variety of electrochemical techniques in the characterization of the equilibria and kinetics of solute partitioning in aqueous micellar systems. These applications have been

Presented in part at the Annual Meeting of the Society of Photographic Science and Technology o f Japan, May 1984, Tokyo, Japan. z To whom correspondence should be addressed.

successful partly because the partitioning kinetics of the solutes between aqueous and micellar phases and micelle diffusion itself are rapid enough to cause measurable changes in voltammetric half-wave potentials, which can be related to equilibrium constants for solutemicelle binding in terms analogous to those developed for CrEr (preceding reaction) mechanisms (12, 13). The emulsion systems we are interested in are very different from aqueous micellar solutions in many respects, such as particle size, turbidity, total volume concentration of oil, particle diffusion, and metastability rather than thermodynamic stability. The application of the RPDE method we describe is restricted to systems in which the solute exchange kinetics are slow compared to the time scale (5-10 ms, Ref. (12)) of measurement. This restriction, however, enables one to measure aqueous-phase solute concentrations directly and without interference from solute in the oil phase. The solutes of primary interest to us in this work are p-phenylenediamines (PPDs), which serve a dual role in the formation of indoaniline image dyes in a wide variety of color films and papers based on silver halides (14). PPDs 421

Journalof Colloidand InterfaceScience. Vol. 112,No. 2, August1986

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422

MATSUBARAAND

reduce silver halide to elemental silver in the development process (pH 10-11) and undergo a reversible two-electron oxidation to the corresponding quinonediimines (QDIs). The resulting QDI reacts with various types of incorporated couplers to form the final indoaniline dye image (15-17). These couplers are incorporated in films in the (nominal) form of oil-in-water emulsions. It is recognized that the kinetics of this dye formation are influenced by the equilibria and kinetics of PPD and QDI partitioning into these emulsion particles (17). The half-wave potentials for the reversible two-electron oxidations of PPDs to QDIs are about 0-200 mV with respect to the SCE potential (18). Aqueous solution PPD concentrations can be monitored by proportionality to the limiting oxidation currents at the RPDE. In an emulsion, such limiting currents are also proportional to the aqueous-phase concentration of PPD, subject to the condition that the PPD in the oil phase is shielded from the working electrode. The proportionalities in aqueous and emulsion media are different, however, owing to the differences between solution and emulsion rheologies. We examine here the partitioning of PPD 2 (see Fig. 1) in an aqueous gelatin/1-undecanol emulsion at 40°C, and we illustrate how limiting currents can be used to estimate the associated distribution coefficient (P). We make the simplifying assumption that the oil droplets and the aqueous gelatin phase are separated by an interface of infinitesimal thickness. We also assume that the oil phase

C2Hs.,~2H4 S03H

C2H5.~N/C2H5

~"~CH~ ~CH3 NH2 1

NHz 2

FIG. l. p-Phenylenediamine (PPD) structures: (1) 4amino-3-methyl-N-ethyl-N-03-sulfoethyl)aniline; (2) 4amino-3-methyl-N,N-diethylaniline. Journal of Colloid and Interface Science, Vo|. 112, No. 2, August 1986

TEXTER

is homogeneous, with a density of the bulk oil. These assumptions, necessary because of our lack of knowledge of the details of the oil phase and interfacial structures, make it possible to calculate the total volume of the oil phase. The possible importance of a diffuse interface containing oil, surfactant, and tightly bound gelatin can be appreciated by considering the volume fraction of the interface with respect to the total particle volume. For particles 1000 in diameter, an interfacial thickness of 25 corresponds to a particle volume fraction of 14%. MATERIALS

AND

METHODS

p-Phenylenediamines 1 and 2 were obtained from Mr. W. F. Coffey of the Kodak Research Laboratories. PPD 2 was obtained as the hydrochloride salt. Aqueous gelatin/1-undecanol emulsions were prepared by passing a premix of oil and aqueous phases through a Gaulin colloid mill five times. These emulsions were chill set and stored in the cold until used. The premix was obtained by mixing oil and aqueous phases at 50°C. The oil phase consisted of 120 g of 1-undecanol (Kodak Laboratow Chemicals). The aqueous phase was composed of 480 g of 12.5% aqueous gelatin, 60 g of 10% aqueous Alkanol-XC surfactant (Du Pont), and 880 g of distilled water. Gelatin was used because of its steric stabilization of the oil droplets. The largest droplets in the emulsions were 0.3-0.4 #m in diameter, as measured in optical micrographs. Rotating disk voltammetry was done with a Pine Instrument RDE3 dual potentiostat/ current amplifier coupled with an ASR2 analytical rotator. Traces of current vs potential were recorded on a Hewlett-Packard model 7004B X-Y recorder. The electrochemical cell was a conventional H-type cell with a water jacket for temperature control. The reference electrode compartment, filled with saturated aqueous KNO3, was connected to the main compartment through a fine-porosity glass frit (2 mm in diameter) at the bottom-center of

423

SOLUTE DISTRIBUTION IN EMULSIONS

the::main compartment. A Pine Instrument AFDD20PT RPDE with an electrode diameter of 7 m m was the working electrode. The auxiliary electrode was a spiral of platinum wire, placed at the bottom of the main compartment, and the reference electrode was a saturated calomel electrode (SCE). The electrochemical measurements were made at 40°C and pH 10. For the experiments with emulsion samples, the cell solutions contained 2% gelatin (w/v) and 4% 1-undecanol (w/v). All solutions were degassed with water-saturated argon or nitrogen. The cell solutions were kept under an argon atmosphere during the measurements. In a typical experiment, 50 ml of buffered emulsion or buffer (25 ml of emulsion or water plus 25 ml of 0.79 M K2CO3 and 0.63 M KHCO3) was introduced into the cell and deaerated. The residual current-potential curve was recorded at an electrode rotation speed of 1000 rpm and over a potential range o f - 2 0 0 to +400 mV. Then a 5-ml aliquot of stock 5 m M PPD solution was added, and the cell solution was deaerated for 5 min more before the current-potential curve was measured. This aliquot addition, followed by current-potential scanning, was generally repeated twice in a given experiment.

o . ~ ~

T~

i

o/ 0,4

o/

<

o/

o/

vE 0.2

/

o

0/ / A3 ~ j 5 . . . . ~ " ~ 1

°o

13~[]? 5

11o 03~ ( r o d I/2 s - I/2)

FIG. 3. Root angular velocity dependence of the limiting current at the RPDE of 0.833 m M 2 at 40°C and pH 10 (ionic strength 1.5 M) in (A) carbonate buffer and (B) buffered aqueous gelatin/1-undecanol emulsion.

RESULTS AND DISCUSSION

The rotating-disk voltammogram of pphenylenediamine 2 was well defined in carbonate buffer at pH 10 (Fig. 2A). The limiting current, monitored at +0.4 V, was proportional to the concentration (C) of 2, as required by the Levich (19) equation for a diffusioncontrolled electrode process: il = 0 . 6 2 n F A D 2 / 3 C v - I / 6 w

0,4

A

Bockgroun~

o -0.2

o

oi,4 E(V)

no. 2. Rotating platinum disk voltammo~ams of 0.833

mM 2 at 1000 rpm, 40°C, and pH 10 (ionic strength 1.5 M) in (A) carbonatebufferand (B) bufferedaqueous gelatin/1-undecanol emulsion (scan rate, 20 mV/s).

i

1/2

[1]

where n is the number of electrons per molecule transferred at the electrode surface, F is the Faraday constant, A is the electrode area, D is the diffusion coefficient, C is the concentration, u is the kinematic viscosity, and w is the rotation speed. The limiting current was also proportional to the square root of the angular velocity (w 1/2) of the electrode (Fig. 3A), which also indicated a diffusion-controlled reversible electrode process. The voltammogram of 2 in the buffered emulsion medium was also well defined (Fig. 2B), but the limiting current was substantially smaller than that measured in the aqueous buffer. The half-wave potential of 2 in the emulsion medium matched (_+5 Journal of Colloid and Interface Science, VoL 112, No. 2, August 1986

424

MATSUBARAAND TEXTER

mY) that in the buffer medium (Fig. 2). The limiting current was proportional to 601/2, which shows that diffusion control also occurs in the emulsion medium (Fig. 3B). The large medium effect illustrated for the electrode oxidation limiting currents cannot be explained simply by the kinematic viscosity @) difference in these two media (0.74 and 1.41 cP-cm3/g, respectively, in buffer and emulsion). The measured viscosity difference is far too small (by a factor of 1500) to cause such a large difference in limiting currents. Instead, this observation clearly shows that a substantial portion of the PPD in the emulsion medium is shielded from the electrode oxidation process. The smaller limiting current observed in the emulsion medium is therefore attributed to the oxidation of aqueous gelatinphase PPD, with m i n i m u m interference from PPD in the oil droplets. It is necessary to express quantitatively how much of the current decrease (Fig. 2, from curve A to curve B) is due to partitioning and how much is due to viscosity effects of the emulsion medium. In our approach to corn-

pensate for these viscosity effects, we used 1, which is negatively charged at pH 10 and has a negligible distribution coefficient (<0.06) in separate, macroscopic two-phase aqueous buffer/1-undecanol systems (manuscript in preparation). This technique, illustrated in Fig. 4, involves four separate current measurements. Two measurements, one with 1 and one with 2, were first made in aqueous buffer. These measurements were then repeated in the buffered emulsion medium. The ratio of limiting currents obtained with the nonpartitioning PPD 1 gives the relative current decrease due to rheological and excluded volume differences between the two media. We denote this current ratio as 3'. The corresponding ratio obtained with the partitioning P P D 2 is denoted as ~ (fl < -g).

tl,buffJl - -C - 3' C*-y ll,buff]2

PPD 2

C

PPD 1

~

,-.----3

f

3TT/~A

3

2

I00

~Z=~7~"=0.265 .... •

. ...................

::iii:iiiii::)i

!,:S:

3

................................ ' ................................

___.~:'_2

288 _

1

............ 0.2

E(V)

0.4 -Or.2

I 0

I 0,2

I 0,4

E(V)

FIG. 4. Voltammograms used to determine the distribution coetficient of 2 in an aqueous gelatin/1undecanol emulsion at 40°C. The reduced data are listed in Table I. The dashed lines are background voltammogramsobtained beforethe addition of PPD (50 ml of buffer or bufferedemulsion). The solidlines are voltammograms obtained in buffer, and the dotted curves are voltammograms obtained in buffered emulsion. The numbers to the right of the curves indicate how many 5-ml aliquots of 5 mM PPD stock solution were added. The measurement and calculation of/3 and 7 after the second aliquot addition are illustrated. Limiting currents were measured 150 mV positive of the half-wavepotentials (rotation speed, 1000 rpm; scan rate, 20 mV/s). Journal of Colloid and Interface Science, Vol. 112, N o . 2, A u g u s t 1986

[2]

SOLUTE DISTRIBUTIONIN EMULSIONS

425

TABLE I PPD Voltammetric and Distribution Data at 40°C Limiting currents, uA PPD 1

PPD 2

PPD stock added (ml)

Buff.

Emul.

Buff.

Emul.

13

3"

3,113

voa

P

5 10 15

181 338 477

152 288 406

204 377 531

49 100 153

0.240 0.265 0.288

0.840 0.852 0.851

3.50 3.22 2.95

22.9 25.1 27.3

57 56 53

v~_~

These current ratios illustrate the respective proportionalities between current and aqueous-phase PPD concentrations. The concentration C* is necessarily less than C if 2 partitions into the oil phase of the emulsion. Equation [3] shows that the mole fraction of 2 in the aqueous phase of the emulsion can be expressed as the quotient of 3 and % If we designate n,q as this mole fraction and (1 - 3/ 3') as the mole fraction in the oil phase, we can write

no~,_l-3/y_(v) ,,.G 1

.

[41

The distribution coefficient can therefore be written as

P-

n°il Vaq - ( ~ - 1 ) ' V a q /2aq

l)oil

[5]

Voil

The data illustrated in Fig. 4 are reduced to tabular form in Table I, where an average distribution coefficient of 55 ___2 is obtained for 2 in this buffered emulsion of 1-undecanol. In separate experiments, this coefficient was measured as a function of the rotation speed. The adjacent hydrodynamic layer (20 times thicker than the diffusion layer) changes in thickness in the same proportion as the diffusion layer (19). Current was measured at eight rotation speeds over the 100-1500 rpm interval, and an average distribution coefficient of 55.5 ___2.0 (+3.6%) was obtained. The

idealized diffusion layer thickness varies by a factor of 4 over this interval. Measurements of distribution coefficients of four different p-phenylenediamines in emulsions of six different oils will be reported later. The results of that study show that distribution coefficients measured in emulsion systems correlate linearly with distribution coefficients measured in macroscopic two-phase buffer/oil systems. This observation and the invariance of the distribution coefficient reported here to marked changes in o~ are evidence that reequilibration in the diffusion or hydrodynamic layers is not a significant source of error under the conditions of our measurements. If such reequilibration were significant, the distribution coefficients obtained by this technique would represent lower bounds. We are not aware of any other techniques that are at present applicable to the determination of distribution coefficients in highly turbid emulsions. ACKNOWLEDGMENTS We thank Dr. Greg Mclntire for discussions and criticisms. We also thank a referee for commentsand suggestions. REFERENCES 1. Griffin, W.,J, Soc. Cosmet. Chem. 1, 311 (1949). 2. Bean, H. S., Konning, G. H., and Thomas, J., Amer. Perfum. Cosmet. 85, 61 (1970). Journal of Colloid and Interface Science, Vol. 112, No. 2, August 1986

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MATSUBARA AND TEXTER

3. Leo, A., Hansch, C., and Elkins, D., Chem. Rev. 71, 525 (1971). 4. Hermann, J., and Abd E1 Salam, L., Lebensm.- Wiss. Technol. 13, 123 (1980). 5. Minkov, E., and Titeva, S., Farmatsiya (Sofia) 33, 26 (1983). 6. "Interfacial Kinetics in Solution," Faraday Discuss. Chem. Soc. 77 (1984). 7. Yeh, P., and Kuwana, T., J. Electrochem. Soc. 123, 1334 (1976). ~8. Hoshino, K., Saji, T., Ohsawa, Y., and Aoyagui, S., Bull. Chem. Soc. Japan 57, 1685 (1984). 9. Georges, J., and Desmettre, S., Electrochim. Acta 29, 521 (1984). 10. Mclntire, G. L., Chiappardi, D. M., Casselberry, R. L., and Blount, H. N., J. Phys. Chem. 86, 2632 (1982). 11. Eddowes, M. J., and Gratzel, M., J. Electroanal. Chem. 163, 31 (1984).

Journal of Colloid and Interface Science. VoL 112, No. 2, August 1986

12. Bard, A. J., and Faulkner, L. R., "Electrochemical Methods: Fundamentals and Applications," pp. 163-164, 435. Wiley, New York, 1980. 13. Ohsawa, Y., and Aoyagui, S., J. Electroanal. Chem. 136, 353 (1982). 14. James, T. H., Ed., "The Theory of the Photographic Process." Macmillan Co., New York, 1977. 15. Tong, L. K. J., and Glesmann, M. C., J. Amer. Chem. Soc. 79, 583 (1957). 16. Tong, L. K. J., Liang, K , and Ruby, W. R., J. Electroana[. Chem. 13, 245 (1967). 17. Tong, L. K. J., "The Theory of the Photographic Process," pp. 339-352. Macmillan Co., New York, 1977. 18. Lee, W. E., and Brown, E. R., "The Theory of the Photographic Process," pp. 291-334. Macmillan Co., New York, 1977. 19. Levich, V. G., "Physicochemical Hydrodynamics." Prentice-Hall, Englewood Cliffs, N.J., 1962.