Dye interactions with surfactants in colloidal dispersions

Dye Interactions with Surfactants in Colloidal Dispersions ORNELLA ORTONA, VINCENZO VITAGLIANO, AND BRIAN H. ROBINSON ~ Dipartimento di Chimica, Universita di Napoli, 4 Via Mezzocannone, 80134 Naples, Italy Received June 24, 1987; accepted September 30, 1987 Stacking of the metachromatic dye acridine orange has been observed spectrophotometricaUy in surfactant-containing colloidal dispersions, including water-in-oil microemulsions, bilayer systems in water, and aqueous micellar solutions. A quantitative indication of dye-stacking has been obtained through measurement of the dimerization equilibrium constant. In all cases, the dye is located within the surfactant "pseudo-phase." Dye-stacking is much weaker than in aqueous solution, but is comparable to that in nonaqueous solvents. © 1988AcademicPress,Inc.

INTRODUCTION

The stacking of acridine dyes in aqueous solution has been much studied over recent years and the general features of the association process are now well-understood (1-7). In addition, the interaction of dyes with linear polymers, and DNA, has received much attention recently (1, 5, 8-12). Rather surprisingly, the aggregation of dyes on two-dimensional surfaces, such as are present in micellar and microemulsion systems, and self-association in the hydrophobic environments which are present in those systems, have not been systematically investigated so far. This paper reports a spectrophotometric investigation of the association of acridine orange hydrochloride in the following surfactantstabilized organized systems: (a) Aerosol-OT (AOT) or cetyltrimethylammonium bromide (CTAB)-stabilized water-in-oil microemulsion systems (which are known to contain discrete water droplets) (13-15), (b) sodium dodecyl sulfate micellar solution in water, and (c) AOT dispersion in water containing bilayer structures (16). Visiting Professor at the Facolt~ di Scienze of the University of Naples, Spring 1986. Permanent address: School of Chemical Sciences, University of East Anglia, Norwich, United Kingdom.

H3C\ N~N./CH3 CHa

CH a

SCHEME 1. Monoprotonated Acridine Orange Cation (0 < pH < 10).

For the AOT-stabilized microemulsions the oil used was n-heptane; for the CTAB-stabilized microemulsion, the oil medium was 50/ 50 v/v n-heptane/chloroform. The aims of the paper are to establish the preferred location of the dye in the dispersion and the tendency to self-associate in that environment through the evaluation of the dimerization equilibrium constant. Considering the possible locations of the dye in the microemulsion system, the dye may be found (i) in the water core, (ii) in the aqueous solutionsurfactant interface region, (iii) in the surfactant domain, i.e., the curved surfactant monolayer which compartmentalizes the water, or (iv) in the oil-continuous solvent. The determination of the preferred location of the dye is relatively easy using acridine orange since dye association is accompanied by a pronounced shift in the visible spectrum on planar (sandwich-type) stacking. In addition

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ORTONA, VITAGLIANO, AND ROBINSON

acridine orange stacks more efficiently than other dyes of the acridine family so that stacking in less favorable environments is more easily observed (17). Another advantage is that the visible dye spectrum is sensitive to the polarity of the dye environment. It is already known that the tendency to self-associate is primarily driven by hydrophobic interactions, the dimerization constant being decreased by a factor of 100 to 1000 on transfer from an aqueous to an organic medium such as dimethylformamide or methanol (17). The rate constant for association of acridine orange in water is close to but less than the diffusioncontrolled limit; the lifetime of the dimer is of the order of 20 #s in water, increasing with the ionic strength of the medium (2). There is also much current interest in the solubilization of macromolecular species, such as polymers or enzymes, in the compartmentalized water present in W/O microemulsions. For example, a-chymotrypsin may be readily dispersed in the aqueous core of microemulsions where it retains its native conformation and activity without denaturation. Recent reviews of this topic are available (15, 18). The use of dyes as probes in studies of macromolecular conformation and dynamics in such systems is then possible, but it is first necessary to establish, as is done in this paper, the preferred location of the dye in the absence of the macromolecular solubilizer.

moved by passage through a column of aluminum oxide.

Methods Visible absorption spectra were recorded on a Perkin-Elmer Model 320 spectrophotometer using cells of different pathlength, ranging from 10 to 0.1 mm. They were located in a thermostated cell holder such that the temperature in that cells could be maintained constant to +0.2°C. For preparation of W/O microemulsion systems containing dye, acridine orange solutions were made up in water by dissolving acridine orange hydrochloride, assuming one molecule of hydration water in the salt. The dye exists entirely as the singly-charged cation (the pKa of the acridine orange cation being 10.7 (20)). This solution of dye in water was then added in the desired amount to a solution of AOT in heptane. An optically clear microemulsion formed spontaneously on shaking for a few seconds. A similar procedure was used for the CTAB-stabilized systems. RESULTS AND DISCUSSION

Water-in-Oil Microemulsions Stabilized by AOT

These systems have been well-characterized from a structural point of view using a variety of physical methods, including small-angle neutron and X-ray scattering (13, 14, 21), EXPERIMENTAL SECTION photon correlation spectroscopy (22-25), and ultracentrifugation (23). It has been shown that Materials the AOT-stabilized systems contain discrete Acridine orange hydrochloride (AO) was droplets on the millisecond, and shorter, time obtained from Merck and was purified as de- scale, comprising an aqueous core surrounded scribed previously (9, 17). Aerosol-OT (AOT) by a skin of surfactant (26). The radius of the was obtained from Sigma and was used as re- water core depends primarily on the compoceived. It contained negligible amounts of acid sition of the system, specifically the molar ratio and base impurities which are occasionally of water to surfactant, which will be denoted present in commercial samples (19). Sodium by R. The relationship between radius of the dodecyl sulfate (SDS) and cetyltrimethylam- water core (r) and composition is then found monium bromide (CTAB) were purchased experimentally to be given to a reasonable first from Sigma. Heptane (Baker) was used as re- approximation by Eq. [1] over the R range ceived. Chloroform was obtained from Merck, from 5 to 50 and for AOT concentrations in and ethanol (present as a stabilizer) was re- n-heptane from 0.1 to 0.01 mole/dm3: Journal of Colloid and Interface Science, Vol. 125,No. 1, September1988

273

DYE INTERACTIONS WITH SURFACTANTS

r, n m = 0.175R,

[1]

where R = [HzO]/[AOT]. Therefore, for an R = 20 system, the radius of the water core is 3.5 nm. It should be noted that the droplet size does not vary significantly on dilution with oil at fixed R or with changes in temperature, provided the temperature limits of instability of the single-phase microemulsions are not approached. In fact, Eq. [ 1] can be readily predicted from simple geometrical considerations, given the assumption that all the surfactant is located at the interface (19). Then r = (3 Vw/S)R where Vw is the volume of a water molecule and S is the head-group surface area of the surfactant at the interface, The skin of AOT surrounding the water droplet has a thickness of about 1.2 nm. Although the droplets are essentially discrete for the purposes of structural characterization, they do have a finite lifetime in the microemulsion medium. A small fraction of droplet encounters, which take place as a result of Brownian motion, leads to exchange of solubilizer between droplets with concomitant exchange of water and surfactant. Typically, the lifetime of a discrete AOT-stabilized droplet is of the order of t 0 -4 S at R = 20 (and an AOT concentration of 0.1 mole/dm 3) (27). Recently, the diffusion coefficient (D) of the surfactant around the aqueous core of the droplet has been determined using quasielastic neutron scattering techniques. D(AOT) = 6 × 10-1° m2/s for an R = 20 system (28). Using the same technique, the value of the diffusion coefficient of water was D(H20) = 1.3 X 10-9 m2/s (28). The implications of the "fluid" nature of the microemulsion system is that diffusion of dye within the droplets is relatively easy, regardless of whether the dye is located in the surfactant skin or in the aqueous core. Transport of dye between droplets is also relatively fast as indicated above. Initially, our aim was to determine the preferred location of AO in the microemulsion.

The dye is quite soluble in water at neutral pH (to ~0.1 mole/dm 3) but extensive planar stacking takes place. Were the dye to be located in the aqueous core of the droplets, it would be expected that a similar pattern of stacking would be observed spectrophotometrically in the microemulsion system. The dye is insoluble in hydrocarbon media, so significant partitioning of the dye into the continuous oil domain can be discounted, it is well-known that cationic dyes can interact strongly on linear negatively charged surfaces such as poly(glutamate) or poly(styrene sulfonate), via cooperative interactions (1, 8), If the dye is not inside the water core, it may be located either at the aqueous solution-surfactant interface or in the surfactant skin "'pseudophase." In the latter location, association would be expected to be relatively weak, as only Van der Waals forces would be operative. The various possible locations of the dye in the droplet dispersion are indicated in Fig. 1. Concerning the association tendency of the dye, the various contributions to the free energy of stacking are shown in Eq. [2]. AGO =

- R T In Km

= AGOyd "1- A G O -4- AGel¢c, 0

[2]

where st, hyd, vw, and elec represent stacking, hydrophobic, Van der Waals, and electrostatic forces, respectively, and Km is the association .m

f

4

oil medium

i

~

FIG. 1. Model showing the possible location of dye molecules in the micrOemulsion system, Journal of Colloid and Interface Science,

V o l . 125, N o . t , S e p t e m b e r

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ORTONA, VITAGLIANO, AND ROBINSON

(dimerization) equilibrium constant. The most significant contributor to the equilibrium constant for dimerization is generally provided by hydrophobic interactions, but these will only be of importance when the monomer dye has some contact with the aqueous phase. The charge on the AO cation is well dispersed and it is generally accepted in this case that the unfavorable electrostatic interaction between the dye molecules is likely to be of least significance. In Fig. 2, visible absorption spectra are shown at different concentrations of AO for the system R = 20, AOT concentration = 0.1 mole/dm3 of total dispersion. As the dye concentration is increased, a new absorption peak appears at 470 nm, which is identified with associated dye species. However, high concentrations of dye are required to induce stacking. The absorption peak of the monomer dye is at 495 nm, which is as observed in methanol and formamide (17). In contrast, the absorption maximum of the dye in water is at

i

5

i

i

492 nm. This result immediately suggests that the dye is preferentially located in a nonaqueous environment, namely, the surfactant pseudo-phase. Confirmation of this assignment will now be sought from the determination of Kin. A good indication of the extent of association is provided by the ratio Er = Am/A d

[3]

where Am and Ad are the absorbances of the solution at the wavelengths of maximum absorption of the monomer and dimer dye (for AOT 495 and 470 nm), respectively. The variation of this ratio on dilution with n-heptane at two R values, R = 20 and R = 7.5, corresponding to different droplet sizes (Eq. [1 ]) is shown in Fig. 3. The near constancy of this ratio as the overall concentration of AOT, and, hence, the concentration of droplets, is changed indicates that the dye environment does not change appreciably with droplet concentration and that the droplets retain their integrity. The evaluation of the dimerization constant K m may be carried out in two ways, as described previously for dye-stacking in nonaqueous media (17): (a) Since the stacking is weak as compared with the situation in bulk water, association beyond the dimer stage can be generally neglected (except for the most concentrated dye solutions at low R, where there is spectrophotometric evidence for association beyond dimer stage). The fraction of monomer in the system, a, may be estimated using

2 10

4

3

2 a = (E -

450

500

550

FIG. 2. Extinction coefficients of AO hydrochloride absorbed on AOT-water microemulsions in n-heptane at 25°C. E in units of dm 3 (of total solution)/(mole, cm); AOT = 0.1 mole/dm3; R = 20; AO molality (mole o f AO/ kg of AOT): (1) 7.85 × 10 -5, (2) 7.85 × 10-3; (3) 1.57 x 10-2, (4) 3.14 × 10-2. Journal of Colloid and Interface Science, Vol. 125,No. 1, September1988

E2)/E1 -

E2),

[4]

where E is the extinction coefficient of the solution and E1 and E2 are the molar extinction coefficients of the monomer and dimer, respectively, expressed in units of dm 3 (of total solution)/(mole • cm). All analyses were made at the wavelength corresponding to maximum absorption of monomer dye. Then Km(app) is given by Km(app) = (1 -

a)/(2c~2rn),

[5]

275

DYE INTERACTIONS WITH SURFACTANTS I

I

I

I

I

I

I

I

I

I

description of the approach used to obtain Eq. [6]). The Km values computed using the a values obtained from Eq. [6] are also shown in Fig. 4. It can be seen that, as previously found (17), the extrapolated Km value agrees with that obtained by procedure a.

1.2

1

Er -O

0.8

0.6

C A O T • 10 2 I

I

2

I

I

4

I

I

6

[

I

I

8

I

10

FIG. 3. E,, ratio of the absorbance of AO at 495 n m to that at 470 n m (Eq. [3]), as a function of microemulsion dilution with n-heptane. C(AOT) = concentration of AOT in mole/dm 3. (1) R = 20, AO molality = 1.66 × 10-2 (mole/kg of AOT), (2) R = 7.5, AO molality = 1.14 × 10-2 (mole/kg of AOT).

where rn is the molality of the dye, expressed as moles of dye per kilogram of the surfactant phase (the assumption was made that all the surfactant was present at the interface). Actually, E2 cannot be obtained experimentally; however, by plotting Km(app) versus m for various arbitrarily chosen values o f E2 (as shown in Fig. 4 for the R = 20, 0.1 mole/ dm 3 AOT system), it can be seen that the Km value extrapolated to infinite dye dilution is almost independent of the value taken for E2. (b) An analysis of the AO data in aqueous and nonaqueous solutions (17) has previously shown that the a value (for a > 0.45) is a linear function of the ratio Er (as defined in Eq. [3]). The empirical relation proposed in Ref. (17) can be fitted to the A O T - A O system by

Procedure b was preferred for computing the Km as it is simpler, independent of optical path, and it minimizes the effect of small concentration errors. A value of Km of 38 kg/mole is obtained from the intercept on the y axis (Fig. 4), where Km(app) = Kin. At R = 7.5, a significantly higher value o f Km of ~ 152 kg/mole was obtained at 25°C. The dependence o f Kin on temperature was also studied by making measurements from 5 to 50°C. Measurements of droplet size, by photon correlation spectroscopy, indicate a negligible size change in the aqueous core over this temperature range (22). Values of the enthalpy of dimerization were obtained from plots ofln Km (app) versus the reciprocal of the absolute temperature. A value of AH ° for the R = 20 system o f - 18 M/mole was obtained, which is considerably less than that measured in bulk aqueous solution ( - 3 5 kJ/mole) (7).

I

I

I

150 1

K~ 100

a = 1.095Er/(E ° - 0.070) 5C

- 0.0766/(E ° - 0.070) - 0.095

3

[6] 4

where E ° is the Er ratio for the monomeric dye E ° = 1.728 - 0.0033T.

[7]

Equation [7] accounts for the slight absorption change of the monomer dye spectrum that has been observed on changing the temperature, T (in °C) (Ref. (17) should be consulted for a

I

I

I

1

2 mxto 2

3

4

FIG. 4. Apparent dimerization constant of AO bound to AOT in microemulsions, Km (in kg/mole), Eq. [5], as a function of dye molality (mole of AO/kg of AOT): ( 1)(3) a computed through Eq. [4], E2 = 20000, 15000, 10000, respectively, R = 20. (4) a computed through Eq. [6], R = 20. (5) a computed through Eq. [6], R = 7.5. Journal of Colloid and Interface Science, Vol. 125, No. 1, September 1988

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ORTONA, ViTAGLIANO, AND ROBINSON I

'

I

''

I

I

150 Krn (app.] c) IO0

50 o

1

2

I

i

4 r (nrn) 6

~

I

8

FIG. 5. Apparent dimerization constant of AO in AOT mieroemulsions as a function of droplet size; r, average droplet radius in nm. Dye molality = 1.70 × 10-2 (mole/ kg o f AOT), AOT concentration 0.1 mole/dm 3.

It is, however, comparable to that measured in several nonaqueous solvents (29). The effect of variation of droplet size on Km has also been systematically investigated. A plot of Km(app) versus r is shown in Fig. 5. It can be seen that the value of Km is essentially independent of R in the region from 20 to 50 (r from 3 to 9 nm), where the droplet size is changing considerably. This result implies that the environment of the dye in the surfactant skin does not change over this size range. However, as R is decreased below 20, there is a pronounced increase in Kin, suggesting that the dye is becoming more exposed to water. It is felt that the increased negative curvature at the surfactant-water interface as R is decreased allows some water penetration into the surfactant layer where the dye is located, hence, the increase in Kin.

Water-in-Oil Microemulsions Stabilized by CTAB Water droplets of similar size were formed in the CTAB system as for the AOT system (30). The spectral evidence of the monomeric dye (maximum absorption shifted from 492 to 497 nm) again supports the assertion that Journal of Colloid and Interface Science, VoL 125,No. 1, September1988

the dye is not in an aqueous environment. However, in this case, the surfactant is positively charged, so it might reasonably be predicted that the dye would locate preferentially in the surfactant skin pseudo-phase rather than at the aqueous solution-surfactant interface. Evidence from small-angle neutron scattering data lends some support to the notion that the interface between the surfactant and the water is sharper in CTAB as compared with AOT (30). The dye in the CTAB mieroemulsion may then be expected to be present in a more hydrophobic environment. Some primary data from which Kin, expressed in (kg of CTAB/mole of AO), was evaluated (Eq. [6] was used, with E ° = 1.874 at 25°C, for computing a). Significantly lower values for Km are obtained as compared with the AOT system. For R = 15, CTAB = 0.1 mole/dm a, a value of 2.6 kg/mole was obtained at 25°C. For statistical encounter of spheres in a homogeneous medium, a value of the dimerization constant Kd (dm3/mole) (the ratio Kin~ Kd, at infinite dilution, is the density of the solvent medium) is given by (3 l)

Kd(stat)/dma/mole = (~)TrNav(r). 4 ,3

[8]

For a contact distance r' in the dimer of 0.5 nm, a value of Kd(stat) of 0.3 dm3/mole is obtained. This value will differ slightly for encounters between squares or sheets (the latter being the most realistic model for the structure of acridine orange). However, a precise calculation is not feasible for acridine orange diffusing in the surfactant skin pseudo-phase, since there will be a preferred mutual trajectory for favorable formation ofdimers. However, taking all these factors into consideration, we conelude that the value of Km determined in the CTAB system indicates an association process which is stronger than that calculated on the basis of random diffusional encounter and separation in the surfactant skin by up to a factor of 5.

DYE INTERACTIONS WITH SURFACTANTS

Dye-Surfactant Interaction in Aqueous Solution The sodium dodecyl sulfate (SDS)-AO and the A O T - A O systems in aqueous solution have been studied by titrating an AO aqueous solution with a concentrated solution of surfactant. SDS-AO. The titration curves of S D S - A O indicate the existence of three characteristic regions as found previously (4, 32) (see Fig. 6). At low surfactant concentrations, the presence of SDS promotes association of the dye; an endpoint is observed by monitoring the extinction coefficient at 492 nm, corresponding to the formation of structures with a 1:1 S D S AO stoichiometry. It has been suggested that dye-surfactant aggregates are present in this concentration range (4, 31). When the vicinity of the critical micelle concentration (CMC) of SDS is reached, a sudden extinction increase is observed at 492 nm. The dye absorption spectrum then corresponds to that of a m o n o m e r i c dye with a

i

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~

i

i

E

~ee

6 ' E,9 .i0-4 4

2

~

/

1

"

A

o

o

Csos" 10a

FIG. 6. Titration of aqueous solutions of AO hydrochloride with SDS: E in units of dma (of total solution)/ (mole. cm). (A)--(1) AO concentration = 0.988 × 10-5, (2) dilution of a solution at constant suffactant to dye concentrations ratio, SDS/AO = 7.47. For C(SDS) < 2 × 10-3 M, the solution becomes turbid. (B) Evidence for dye association inside the SDS micelles: ratio of the dye absorbances at 495 and 475 nm as a function of the dye molality (mole of AO/kg of SDS). SDS concentration: 0.0135 mole/dm3.

277

m a x i m u m absorption shifted to 495 nm, which shows that the dye is solubilized inside the micelles. A set of runs at different AO over all concentrations has shown the following: (a) The dye associates in the micelle pseudo-phase, although the dimerization constant is very low, Km = 1.3 (in units o f k g micellar SDS/mole of AO) at 25°C (a was computed through Eq. [6] using E ° = 1.628 at 25°C). This is close to the value predicted for statistical encounters. (b) An increase in the a m o u n t of dye absorbed by the micelles increases the C M C but not to any great extent (compare curves 1 and 2 of Fig. 6A). (C) The m a x i m u m average a m o u n t of dye absorbed by micelles (solubilization capacity) is about 1 AO molecule per 5.7 micellar SDS molecules (see the most concentrated points in Fig. 6B). As amounts of dye in excess of this are added to the system, it becomes turbid.

AOT-AO. The A O T system in water is thought to form bilayers, rather than micelles, at concentrations higher than the CMC. The titration of AO with A O T shows a spectrophotometric trend similar to that of the S D S AO system; however, the m a x i m u m a m o u n t of dye that can be solubilized by the A O T bilayer is quite low, namely, 1 molecule of AO per 60-70 molecules of AOT. At this A O T AO ratio the dye is almost totally present as m o n o m e r so that it has been not possible to compute a dimerization constant for this system. It is, however, significant that there appears to be no binding of AO on the outside of the A O T aggregate structure, where stacking should be promoted by cooperative effects, as was observed previously for linear polymers. This result further supports the assignment of the preferential location for AO in the microemulsion systems as being within the surfactant layer. Journal of Colloid and Interface Science, V o l .

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ORTONA, VITAGLIANO, AND ROBINSON ACKNOWLEDGMENTS

This research has been carried on with the financial support of the Italian Ministero della Pubblica Istruzione and of the Italian CNR. REFERENCES 1. Vitagliano, V., in "Aggregation Processes in Solution" (E. Wyn-Jones and J. Gormally, Eds.), pp. 271308. Elsevier, Amsterdam, 1983. 2. Robinson, B. H., Loftier, A., and Schwarz, G., J. Chem. Soc. Faraday Trans. 1 69, 56 (1973). 3. Robinson, B. H., Seeling-Lofller, A., and Schwarz, G., J. Chem. Soc. Faraday Trans. 1 71,815 (1975). 4. James, A. D., and Robinson, B. H., Adv. Mol. Relax. Proc. 8, 287 (1975). 5. Schwarz, G., Eur. J. Biochem. 12, 442 (1970). 6. Costantino, L., Ortona, O., Sartorio, R., Silvestri, L., and Vitagliano, V., Adv. MoL Relax. Proc. 20, 191 (1981). 7. Costantino, L., Guarino, G., Ortona, O., and Vitagliano, V., J. Chem. Eng. Data 29, 62 (1984). 8. Vitagliano, V., in "Chemical and Biological Applications of Relaxation Spectrometry" (E. WynJones, Ed.), pp. 437-466. Reidel, Dordrecht, 1975. 9. Vitagliano, V., Costantino, L., and Zagari, A., J. Phys. Chem. 77, 204 (1973). 10. Vitagliano, V., Costantino, L., and Sartorio, R., J. Phys. Chem. 80, 959 (1976). 11. Ortona, O., Vitagliano, V., Sartorio, R., and Costantino, L., J. Phys. Chem. 88, 3244 (1984). 12. Crescenzi, V., and Quadrifoglio, F., in "Polyelectrolytes and Their Applications" (A. Rembaum and E. S61~gny,Eds.), pp. 217-30. Reidel, Dordrecht, 1975. 13. Toprakcioglu, C., Dore, J. C., Robinson, B. H., and Chieux, P., J. Chem. Soc. Faraday Trans. 1 80, 13 (1984). 14. Kodarchyk, M., Chert, S. H., Huang, J. S., and Kim, M. W., Phys. Rev. A 29, 2054 (1984).

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15. Luisi, P. L., and Magid, L., Crit. Rev. Biochem. 20, 409 (1986). 16. Ekwall, P., Mandell, L., and Fontell, K., J. Colloid Interface Sci. 33, 215 (1970). 17. Vitagliano, V., Ortona, O., Sartorio, R., and Costantino, L., J. Chem. Eng. Data 30, 7 (1985). 18. Luisi, P. L., Angew. Chem. 24, 439 (1985). 19. Robinson, B. H., Fletcher, P. D. I., Perrins, N. M., and Toprakcioglu, C., in "Reverse Micelles" (P. L. Luisi and B. Straub, Eds.), pp. 69-73. Plenum, New York, 1983. 20. Zanker, V. Z., Z. Phys. Chem. 199, 225 (1952). 21. Cabos, C., and DeLord, F., J. Phys. Lett. 41, 455 (1980). 22. Zulauf, M., and Eicke, H. F., J. Phys. Chem. 83, 480 (1979). 23. Clarke, J. H. R., Doherty, J. V., Day, R. A., and Robinson, B. H., J. Chem. Soc., Faraday Trans. 1 75, 132 (1979). 24. Clarke, J. H. R., Nicholson, J. D., and Regan, K. N., J. Chem. Soc. Faraday Trans. 1 81, 1173 (1985). 25. Huang, J. S., and Kim, M. W., in "Proceedings SPEDOE Symposium on Enhanced Oil Recovering, SPE-ASME 1982," p. 901. 26. Fletcher, P. D. I., and Robinson, B. H., Ber. Bunsenges Phys. Chem. 85, 863 (1981). 27. (a) Fletcher, P. D. I., Howe, A. M., and Robinson, B. H., J. Chem. Soc. Faraday Trans. 1, in press; (b) Atik, S. S., Thomas, J. K., Amer. Chem. Soc. 103, 3543 (1981); 103, 7403 (1981); (c) Pileni, M. P., Chem. Phys. Lett. 81, 603 (1981). 28. Fletcher, P. D. I., Robinson, B. H., and Tabony, J., Z Chem. Soc. Faraday Trans. 1 82, 2311 (1986). 29. Ortona, O., Elia, V., Vitagtiano, V., Barone, G., and Costantino, L., in "IUPAC Conference on Chemical Thermodynamics, Univ. College, London 610 Sept. 1982," Abstract. 30. Mead, J., Ph.D. thesis, University of Kent at Canterbury, 1984. 31. Robinson, B. H., Steytler, D. C., and Tack, R. D., J. Chem. Soc. Faraday Trans. 1 75, 481 (1979). 32. Robinson, B. H., White, N. C., and Mateo, C., Adv. Mol. Relax. Proc. 7, 321 (1975).