Pressure-induced phase transition of MyTAB micelles in water

Pressure-Induced Phase Transition of MyTAB MiceUes in Water H. W. O F F E N AND W. D. T U R L E Y Department of Chemistry and The Marine Science Institute, University of California, Santa Barbara, California 93106 Received July 10, 1981; accepted October 5, 1981 The phase boundary between hydrated crystals and micelles in water has been examined for the surfactant myristyltrimethylammonium bromide (MyTAB) as a function of pressure (1-4000 bar) at several isotherms (15, 23, 32, and 41 °C). The phase separation is characterized by a large pressure hysteresis, a linear slope dT/dP = 0.015 K bar -~ and a transition temperature of 10.0°C at atmospheric pressure. The dT/dP magnitude is similar to that measured for phospholipid bilayers and n-alkanes.

been characterized. Several liquid crystalline phases as well as hydrated surfactant crystals are possible separation products at high pressures. Since the V,H characteristics of hydrated crystalline leaflets are not expected to differ appreciably from those of the lamellar liquid crystalline phase, it will be tentatively assumed that a crystalline suspension is formed at high pressures. This makes it possible to use AH and AV values in Eq. [ 1] based on measurements for crystals and micellar solutions. In this work, the transition pressures are measured optically along several isotherms for myristyltrimethylammonium bromide (MyTAB) micelles in water. The pressure shift of the transition temperature is of similar magnitude to that observed for the melting of paraffins and the gel-to-liquid crystalline transition of lipid bilayers. This suggests that the volume-enthalpy relation for surfactant solubility is closely related to other transitions of organized structures in water and, for the most part, determined by the hydrocarbon chain conformations.

I. I N T R O D U C T I O N

Amphiphilic salts form aggregates in aqueous solution, provided the amount dissolved exceeds the critical micelle concentration (CMC) (1, 2). These micelles are stable over an extended range of the P, T, c phase diagram, but increasing surfactant concentration eventually leads to the micellar solubility limit and the separation of lyotrophic liquid crystalline phases and hydrated crystals. The effect of pressure on the phase boundary between extended aggregates and micellar solution is measured in this work to determine the transition characteristics and to compare these with transitions in n-alkanes (3, 4) and phospholipids (5, 6) of comparable hydrocarbon chain length. Accordingly, the phase transition is described by the Gibbs free energy condition d & G = 0 and the Clapeyron-Clausius equation dT dP

TAV -

- -

AH

[l]

The volume change AV and the enthalpy change AH determine the P,T characteristics of the phase separation at the transition temperature T. The pressure-induced phase separating from micellar solutions has not

II. M E T H O D S A N D M A T E R I A L S

The high-pressure optical cell and P-T apparatus have been described (7). The 442

0021-9797/82/060442-05502.00/0 Copyright © 1982 by AcademicPress,Inc. All rightsof reproductionin any form reserved.

Journal of Colloidand InterfaceScience,Vol. 87, No. 2, June 1982

443

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aqueous micellar solution is placed within a cylindrical quartz capsule fitted with a stainless-steel piston and sealed by a Teflon O-ring. The loaded capsule is positioned in the high-pressure optical cell and surrounded by water so that dilution would be the only effect of minor leakage past the Teflon seal at elevated pressures. The latter precaution was taken to exclude the possibility that the observed transition characteristics are influenced by trace solubilization of hexane, the pressure transmitting fluid, into the micellar solution. The P-T apparatus is used for compressing and decompressing aqueous MyTAB solutions in the ranges 0-41°C and 1-4000 bar (10 bar = 1 MPa). Phase transitions are indicated by abrupt changes in turbidity measured either in the forward or 90 ° direction. Light intensities from both paths, along with the output of the pressure transducer, are indicated on a three-pen Soltec recorder as the pressure is continuously applied or removed. The marked change in sample turbidity accompanying the onset of phase separation is reported for 90 ° scattered light intensity/from a 0.5 mW He-Ne laser. Pressure-induced alterations in transmitted light essentially confirm the I observations and alert the operator to potential window leaks. The surfactant CI4H29N(CH3)3Br (MyTAB) is purchased from Aldrich Company (Lot No. 090777) and purified by repeated solvent pair (ethanol/ether) recrystalliza-

tions. Purity is verified by the absence of a minimum in a surface tension vs log [MyTAB] plot and CMC --- 3.4 X 10-3 m. Deionized, distilled, and filtered water (conductivity below 1 X 10-5 Sm -1) is used in all sample preparations. Four MyTAB solutions of concentration 0.012, 0.035, 0.062, and 0.304 m (0.4, 1.2, 2.1, and 9.3 wt%) are chosen to correspond to multiples of the CMC by factors of approximately 4, 10, 20, and 100, respectively. Since we were unable to locate a literature value for the heat of solution of MyTAB, a calorimetric determination (8) was made to give AH = 12.5 kcal mol -~ extrapolated to the CMC. III. RESULTS

The result of a typical experiment is indicated in Fig. 1 where the time progression of the scattered light intensity I and pressure P are compared. Phase separation from the micellar solution is accompanied by a sudden, large increase in L The high turbidity of the crystalline suspension remains until the dissolution process sets in with decreasing pressures. The pressure is applied continuously by adjusting high pressure valves manually; decompression is achieved by successive valve opening. Ramp pressures are generally ~ 100 bar min -I on the upstroke and average ~ 1 0 bar rain -I near the transition on the downstroke. The salient feature of Fig. 1 is the large pressure hysteresis relative to the narrow transition width: the onset of precipitation and dissolution of MyTAB solutions are ~2000 bar apart. Once precipitation commences, it cannot be reversed by a small reduction in applied pressure. Conversely, the dissolution process can be arrested by a small pressure increase. This behavior is expected for a supersaturated solution, i.e., dissolution of the high-pressure phase to form micelles is an equilibrium process. The onset of the transition in these aqueous, electrolytic solutions is identified with the abrupt change in L The scattered Journal of Colloid and Interface Science, Vol. 87, No. 2, June 1982

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OFFEN AND TURLEY

light intensity varies 50-1000-fold between the homogeneous solution and the crystalline suspension. The precise change in I not only depends on beam geometry and surfactant concentration but also on pressure history. Yet the transition pressures corresponding to the onset of the dissolution process are reproducible within +50 bar. The transition pressures for the onset of precipitation and dissolution at several temperatures are illustrated in Fig. 2. The T-P plot is linear with positive slope and the width of the metastable region appears pressure invariant in the 10-40°C temperature interval. The intercept at atmospheric pressure (T1 arm) is obtained by compressing the supercooled solution, releasing the applied pressure completely, and then slowly warming the sample. A least-squares analysis of 0.035 m MyTAB yields dT/dP = 0.015 + 0.001 K bar -1 and T] atm 10.0° CI This solubility temperature is nearly identical to the Krafft Point (defined as the intercept of the CMC and solubility curves) of aqueous MyTAB solutions. The transition pressures Pd for the equilibrium dissolution of samples containing different amounts of surfactant are illustrated in Fig. 3. The 0.012 rn micellar solution could not be induced to precipitate at 41°C within the accessible pressure range, so the measurement was made at 35°C instead. The Pd values of the different solutions =

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PRESSURE

(bar)

FIG. 2. The dissolution(--) and precipitation (---) curves for 0.035 m MyTAB. Journal of Colloid and Interface Science,

V o l . 8 7 , N o . 2, J u n e 1982

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FIG. 3. Isothermaldissolutionpressures (Pd) of four aqueous MyTABsolutions. appear to spread somewhat at higher temperatures. The significance of this concentration effect will be further investigated analogous to the deoxycholate sol-gel transition study by Sugihara et al. (9). However, the quantity of chief interest, dT/dP, is linear and varies in slope only by 20% between the highest and lowest concentrations used in these experiments. This is close to the experimental uncertainty in each slope; hence dT/dP = 0.015 K bar -1 is the transition slope quoted for the MyTAB surfactant crystalmicellar solution process. IV. DISCUSSION The measured dT/dP can be combined with the known AV to compute the transition enthalpy. The difference between the partial molal volume of the micellar solution (331.2 cm 3 mole-1) (10) and the molar volume of the crystal (298.8 cm 3 mole -1) (11) equals AV = 32.4 cm 3 mole-1 for the MyTAB transition. Combined with the T = 283°K, the Clapeyron-Clausius equation (Eq. [ 1 ]) gives AH = 14.6 kcal mo1-1 which differs by 15% from our measurement of the micellar heat of solution. This agreement is considered adequate and supports the hypothesis that the dissolution of surfactant crystals in water can be treated within the thermodynamic

MyTAB MICELLES

framework of one-component, first-order phase transitions. This view of surfactant solubilities is strengthened by the similar dT/dP magnitude measured for surfactants and phospholipids. Among the several reports on the gelto-liquid crystalline phase transition in lecithin bilayers dispersed in water, we report the results of only two studies. The experimental approach of Liu and Kay (12) resembles that used in our work: pressure-induced isothermal phase transitions reported for decreasing pressures. Their result is a pressure-invariant (up to 270 bar) dT/dP = 0.023 K bar -1 for 5 wt% dispersions of dipalmitoylphosphatidyl choline in water. DeSmedt et aL (5) report dT/dP = 0.020 K bar -1 to be independent of pressure up to 1500 bar for the C14 homolog (dimyristoyllecithin). Another comparison can be made with the melting curves of aliphatic hydrocarbons. Corkill et aL (10) conclude from their density studies that two to four methylene groups attached to the charged headgroup are hydrated. Thus, the comparison is made with dodecane for which the liquid-, olid phase transition is characterized by d ) / d P = 0.020 K bar -1 (2iV = 25.7 cm 3 mc~e-l; AH = 8.86 kcal mo1-1) at 15°C and 1175 bar (3). In contrast to micelles and lipids, the melting curve of n-alkanes is perceptibly nonlinear over a 30 ° temperature interval. The comparable P, T dependence of micelle solubility and lecithin phase transitions in water suggests that the theoretical framework developed for cooperative phenomena is equally applicable to both organized structures. Accordingly, the order-disorder model (13) (hydrocarbon chain "melting") could be extended to encompass micelles: hydrocarbon chain conformation primarily determines the pressure shift of the crystal-micelle phase transition, i.e., the solubility curve. This hypothesis should be tested for other micellar systems before detailed comparisons are made and the subtle differences among lipids and micelles are explored. Sur-

445

factants are frequently suggested as model systems for biomembranes; the present results on aqueous solubility of surfactants suggest another avenue for such comparisons. The large pressure hysteresis observed for cationic surfactant solubilities above CMC is another result that deserves further attention (14). A previous, detailed pressure study of surfactant solubility by Tanaka et aL (15) also reports supersaturation, detected by solution conductance as a function of increasing and decreasing pressures. The superpressed state is also observed for liquid alkanes, reported by Nelson et aL (3) to be as large as 1200 bar for nonane at 60°C. Our diffusion studies of micelles under pressure (16, 17) show that discontinuous changes in transport properties are absent in the large metastable region. As in supercooled solutions (18) microcrystals are not present because nucleation of the supersaturated solution is the rate-determining step in phase separation. Additional work is planned to describe factors responsible for the large pressure hysteresis. ACKNOWLEDGMENT Partial support of the National Science Foundation under Grant NSF CHE 79-05965 is gratefully acknowledged. REFERENCES 1. Wennerstri3m, H., and Lindman, B., Phys. Rep. 52, 1 (1979). 2. Lindman, B., and Wennerstr6m, H., Topics Curt. Chem. 87, 1 (1980). 3. Nelson, R. R., Webb, W., and Dixon, J. A., J. Chem. Phys. 33, 1756 (1960). 4. W~irflinger, A., and Schneider, G. M., Ber. Bunsenges. 77, 121 (1973). 5. De Smedt, H., Olbrechts, H., and Heremans, K., Europhys. Conf. Abstr. 1A, 98 (1975). 6. Heremans, K., Rev. Phys. Chem. Japan 50, 259 (1980). 7. Dawson, D. R., and Often, H. W., Rev. ScL Instrum. 51, 1349 (1980). 8. Shoemaker, D. P., Garland, C. W., and Steinfeld, J. I., "Experiments In Physical Chemistry," 3rd. ed., p. 161. McGraw-Hill, New York, 1974. Journal of Colloid and Interface Science, Vol. 87, No. 2, June 1982

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9~ Sugihara, G., Ueda, T., Kaneshina, S., and Tanaka, M., Bull. Chem. Soc. Japan 50, 604 (1977). 10. Corkill, J. M., Goodman, J. F., and Walker, T., Trans. Faraday Soc. 63, 768 (1967). 11. Norbert, A., and Brun, B., Bull Soc. Ft. Mineral Cristallogr. 98, 111 (1975). 12. Liu, N., and Kay, R. L., Biochemistry 16, 3484 (1977). 13. Nagle, J. F., and Wilkinson, D. A., Biophys. J. 23, 159 (1978).

Journalof Colloidand InterfaceScience,Vol.87, No. 2. June 1982

14. Often, H. W., Rev. Phys. Chem. Japan 50, 97 (1980). 15. Tanaka, M., Kaneshina, S., Tomida, T., Noda, K., and Aoki, K., J. Colloid Interface Sci. 44, 525 (1973). 16. Nicoli, D. F., Dawson, D. R., and Often, H. W., Chem. Phys. Lett. 66, 291 (1979). 17. Dawson, D. R., Often, H. W., and Nicoli, D. F., J. Colloid Interface Sci. 81, 396 (1981). 18. Franses, E. I., Davis, H. T,, Miller, W. B., and Scriven, L. E., J. Phys. Chem. 84, 2413 (1980).