J . Phys. Chem. 1989, 93, 4198-4204
4198
The adsorption of the micelles induces only a small conformational change in the polymer. The confinement of the polymer chain near the micelles leads to a contraction of the polymer, whereas excluded volume effects expand the polymer coil. The net effect is a slight increase in the measured hydrodynamic radius from 21 nm in pure toluene to 24 nm for the polystyrene ( M , = 575000 g mol-') saturated with micelles (Wo = 5 and I O ) . The saturation value drat is determined by counteracting effects. First, the overall attraction between a micelle and the polymer, and second, the repulsion between adsorbed micelles. This may be expressed in the following form "sat
= Mmic/("MmonA)
(7)
Here, M,,, and M,,, are the molecular weights of a micelle and a monomeric unit of the polymer, respectively; N* is the adsorbance capacity of a micelle as given by eq 5 , and A is a factor taking into account the mutual repulsion between the adsorbed micelles. Substitution of N* in eq 7 yields "sat
= (Mmic/b2)(a2/Mmon) / ( 4 7 t A )
(8)
where b is the radius of a micelle and a the length of a monomeric unit of the polymer along the chain. The value of tA was found
to be 0.34 for Wo = 5 , and 0.22 for W, = 10. An independent determination o f t and A requires a separate measurement of N* or t . Finally, eq 8 is incorporated into eq 6
Equation 9 describes the adsorption of micelles on polymers within the following assumptions: The concentration of surfactant is higher than the cmc, c, > cmc. An excess of micelles is present, c, >> ( Y ' , ~ ~ c ~ . Shape and size of the spherical micelles do not change in the presence of the polymer. Free surfactant molecules do not adsorb on the polymer. The adsorbed micelles are in equilibrium with free surfactant molecules. The polymer is not branched, is in a good solvent, and is in the dilute regime. The degree of polymerization N is higher than the adsorbance capacity of a micelle, N > N*. Registry No. AOT, 577-1 1-7; polystyrene, 9003-53-6; toluene, 10888-3.
Reverse Micelle and Microemulsion Phases in Supercritical Xenon and Ethane: Light Scattering and Spectroscopic Probe Studies John L. Fulton, Jonathan P. Blitz, Joel M. Tingey, and Richard D. Smith* Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory,? Richland, Washington 99352 (Receioed: August 4 , 1988; In Final Form: November 23, 1988)
The properties of reverse micelle and water-in-oil type microemulsion phases in supercritical xenon and ethane have been investigated. Structure, size, and the solvent environment of sodium bis(2-ethylhexyl) sulfosuccinate aggregates in these fluids were studied by using dynamic light scattering and view cell determinations of phase behavior. In addition, a new spectroscopic probe technique is reported for study of the solvent environment of the micelle and, indirectly, micelle size. The formation of a xenon-water clathrate in equilibrium with a xenon microemulsion is observed, and the effect of clathrate formation upon the polar core of reverse micelles in supercritical xenon is discussed. Properties such as size and solvent environment of the aqueous region under isothermal conditions are found to be somewhat dependent on the density of the continuous phase. Evidence of strongly pressure dependent micelle-micelle attractive interactions is presented. The properties of microemulsions in near-critical and supercritical fluids are compared to those in conventional liquid systems.
Introduction The formation of reverse micelles and water-in-oil (w/o) microemulsions in liquid hydrocarbons using the surfactant sodium bis( 2-ethylhexyl) sulfosuccinate (AOT) has been widely studied.'-3 In nonpolar liquid solvents, these molecular aggregates generally consist of 3- to 20-nm-diameter spherical shells of surfactant molecules surrounding a polar core, which is typically an aqueous solution. This combination of hydrophilic, hydrophobic, and interfacial environments in one solvent has created potential applications in separation^,^,^ in chromatography,6 and for catalytic' reactions. It has been recently demonstrated that A O T forms micelles and microemulsion phases in supercritical fluids (dense A supercritical fluid is a substance above its critical temperature and pressure whose properties are highly dependent on pressure due to the proximity to the critical point.l0X1' In supercritical fluids, density. dielectric constant, and viscosity. as well as other properties, can be continuously varied between the gas- and liquid-phase limits by manipulating pressure. Fluids that are supercritical at moderate temperatures and pressures include ethane 'Operated by Battelle Memorial Institute.
( T , = 32.2 OC,P, = 48.8 bar), xenon ( T , = 16.6 OC,P, = 58.4 bar), and carbon dioxide ( T, = 3 1.1 O C , P, = 7 3 . 8 bar). Microemulsions formed in supercritical fluids have been previously characterized by light scattering,I2 conductivity, density, and phase (1) Huang, J. S.; Kotlarchyk, M.; Chen, S. H. J. Phys. Chem. 1985,89, 4382-4386. (2) Clarke, J. H. R.; Brown, D. J . Phys. Chem. 1988, 92, 2881-2888. (3) Wong, M.:Thomas, J. K.; Nowak, T . J . Am. Chem. Soc. 1977, 99, 4730-4736. (4) Fletcher, P. D. I.; Parrott, D. J . Chem. Soc., Faraday Trans. I 1988, 84, 1131-1 144. ( 5 ) Hatton, T. A.; Goklen, K. E. Sep. Sci. Technol. 1987, 22, 831-824. (6) Dorsey, J . G.; Hernandez-Torres, M. A,; Landy, J. S. Anal. Chem. 1986, 58, 744-747. ( 7 ) Fendler, J. H. In Reverse Micelles; Luisi, P. L., Straab, B. E., Eds.; Plenum: New York, 1984; pp 305-322. (8) Smith, R. D.; Fulton, J. L.; Gale, R. S. J . Am. Chem. Soc. 1987, 109, 920-92 1. (9) Smith, R . D.; Fulton, J. L. J . Phys. Chem. 1988, 92, 2903-2907. ( IO) Chemical Engineering at Supercritical Fluid Conditions; Paulaitis, M. E., Penninger. J. M . L.. Gray, R. D., Davidson. P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983. ( I I ) Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M . A., Krukonis. V . J . , Eds.; Elsevier: Amsterdam, 1985. (12) Blitz, J. P.: Fulton. J. L.: Smith. R . D. J . Phys. Chem. 1988, 92, 2707-27 I O .
0022-3654/89/2093-4l98$0~.50/0 C 1989 American Chemical Society
Reverse Micelle and Microemulsion Phases in Xe and C2H6
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4199
b e h a ~ i o r . ~These surfactant/supercritical fluid systems have potential applications in enhanced oil recovery13 and in reaction14 and separation processes where the high diffusivities of solutes in the fluid continuous phase may greatly increase reaction or extraction rates. I n conventional liquid microemulsions, the properties of the surfactant interfacial region are of primary importance in determining the size and shape of surfactant aggregates. The ionic interactions between surfactant head groups and interactions with their counterions, as well as the hydrogen bonding within the aqueous core, are important factors that lead to aggregation. The equilibrium structure of the interfacial region is also determined by a delicate balance of several additional factors. Small changes in the surfactant’s hydrocarbon tail ~ t r u c t u r e as ’ ~ well as small changes in the composition of the nonpolar s o l ~ e n t cause l~~~~ profound changes in the aggregate structure or size as evidenced by the large changes in the amount of water or surfactant that can be solubilized by the microemulsion. The aggregation or clustering of two or more micelles can also occur when the micelles are of sufficient size or at high enough concentrations.17 Indeed, the phase behavior of a micelle species in solution is known to resemble that of a simple molecular fluid that has liquid-gas-phase equilibria and a well-defined critical p ~ i n t . ’ ~ , ~ ~ The properties of a supercritical fluid continuous phase provide a useful tool with which to study surfactant aggregation. Haydon et aL20have shown that a low molecular weight liquid alkane, e.g., butane, penetrates and solvates the hydrocarbon tail region of the surfactant interface to a much greater extent than higher molecular weight liquids, e.g., hexadecane. Smaller molecules, such as xenon or ethane, possess the capability of even greater solvation of this hydrocarbon tail region. It is expected that the degree of solvation of the hydrocarbon tails by the fluid will be strongly density dependent; it is well-established that the solvation of simple nonpolar, higher molecular weight organic substances in supercritical fluids is highly density dependent.21 On a macroscale, the degree of solvation of the micelle species will also be dependent on pressure. When the pressure is changed at constant temperature, relatively large changes in the solvating power of the continuous phase occur, and this is expected to change both the molecular and macromolecular aggregation. In this paper we examine micelle size and clustering in supercritical xenon and ethane and compare their properties to micelles in liquid isooctane. Reverse micelle or microemulsion phases formed in a continuous phase of monatomic molecules, such as xenon, are particularly significant from a fundamental viewpoint. Modeling and theoretical studies of such systems should be more readily tractable. In addition, a range of spectroscopic studies (e.g., N M R and infrared) would be greatly facilitated by a simple monatomic continuous phase. We show that such studies can be complicated by the equilibrium with the xenon/water clathrate. We report measurements of microemulsion hydrodynamic radii in supercritical xenon and ethane using dynamic light scattering and compare these results with measurements of the aqueous core solvent environment using a spectroscopic probe technique.
Experimental Section The surfactant AOT (“purum” grade, Fluka) was purified as ( I 3) Smith, R. D.; Fulton, J. L. In Surfactant-Based Mobility Confro[; Smith, D. H., Ed.; American Chemical Society: Washington, DC, 1988; ACS Symp. Ser. No. 373. (14) Prausnitz, J . M.; Randolph, T. W.; Clark, D. S.; Blanch, H. W. Science 1988, 238, 387-390. ( 1 5 ) Evans, D. F.; Sen, R.; Warr, G. G. J . Phys. Chem. 1988, 92,774-783. (16) Eicke, H. F.; Hilfiker, R.; Kim, V . J . Colloid InterfaceSci. 1988, 121, 579-584. (17) Eicke. H. F.: Hilfiker, R. J . Chem. Soc., Faraday Trans. I 1987, 83, 162 1 -I 629. (18) Huang, J. S. J . Chem. Phys. 1985, 82, 480-484. (19) Roux, D.; Bellocq, A . M. In Surfactanfs in Solufion, Mittal, K. L., Lindman. B..Eds.; Plenum: New York. 1984; pp 1247-1261. (20) Gruen, D. W . R.; Haydon, D. A. Pure Appl. Chem. 1980, 52, 1229-1 240. (21) Schmitt, W . J.; Reid, R. C. J . Chem. Eng. Data 1986, 31, 204-212.
described by Kotlarchyk.22 The AOT solution was filtered through a 0.2-1m Millipore filter prior to drying in vacuo for 8 h. The AOT was stored in a desiccator over anhydrous calcium sulfate. The molar water-to-AOT ratio (W)was assumed to be 1 in the purified, dried solid.22 Water was distilled and filtered through a Millipore Milli-Q system. Ethane (“CP” grade, Linde) and xenon (Research grade, Linde) were used as received. Details of the high-pressure light scattering cell are given in ref 12. Briefly, the cylindrical light scattering window was a high-precision sapphire tube with an inside diameter of 1.9 cm and an outside diameter of 3.2 cm and an approximate volume of 1.5 mL. The axis of this tube was located within 0.002 cm of the axis of rotation of the goniometer. The cell was placed in an 8.25-cm-diameter, thermostated quartz vat filled with toluene. The alignment of the instrument was checked with 58-nm polystyrene latex spheres dispersed in H 2 0 , and the measured size of this standard was found to agree within 5% of the reported value. Experiments with supercritical xenon and ethane were done by adding 0.10 g of AOT (150 mM) into the scattering cell with either 16 p L of water ( W = 5) or no added water ( W = 1 ) . A miniature magnetic stir bar was also placed directly into the scattering cell. A Varian 8500 syringe pump filled with pure xenon or ethane was connected directly to the scattering cell. Upon pressurizing with pure continuous phase, the solution was mixed for 15 min and allowed to equilibrate for 1 h prior to data acquisition. The outlet of the scattering cell was connected to a pressure transducer (Setra Systems, No. 300C). A Malvern PCS-100 spectrometer (Malvern Instruments, Malvern, England) equipped with a 5-W Ar’ laser (488 nm) was used. The spectrometer and laser were mounted on a vibration-free optical table (Technical Manufacturing Corp.). All measurements were taken at a constant scattering angle of 5 4 O . The signal from the photomultiplier was processed on a 128-channe1, real time digital correlator (K7032-OS) using either a 50- or 100-ns sample time. Temperatures were maintained with a Malvern temperature controller at 25 f 0.1 “C for experiments conducted in xenon and 37 f 0.1 O C for experiments in ethane. The photon autocorrelation function was analyzed by the method of second-order c ~ m u l a n t s ~ ~ in which the logarithm of the normalized autocorrelation function, G(’)(q,t),was fitted to a polynomial equation by using a nonlinear least-squares fitting routine 1 - In [ G ( 2 ) ( t ) = ]
2
1 t2 ro- r,t + -r22 2
where rois ideally zero. F,, the first cumulant, is equal to the diffusion coefficient, DT,by
r, = &q2
(2)
where q is the scattering vector. The magnitude of the scattering vector, q, is given by
4?r sin %0 I L
4 =
X/n
(3)
where 0 is the scattering angle, X is the wavelength of the incident beam in a vacuum, and n is the index of refraction of the scattering medium. In the absence of interparticle interactions the second-order cumulant, r2,is related to the variance of the particle size distribution. The mean hydrodynamic diameter, dH,was calculated from DT via the Stokes-Einstein relation for spherical particles (4)
where k is Boltzmann’s constant, T i s the absolute temperature, and 7 is the viscosity of the solvent. The mean, *I standard deviation of five replicate measurements, is reported in all cases. (22) Kotlarchyk, M . ; Chen, S.: Huang, J. S . ; Kim, M . W. Phys. Rec. A 1984. 29, 2054-2069. (23) Koppel, D. E. J . Chem. Phys. 1972, 57, 4814-4815.
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The Journal of Physical Chemistry, Vol. 93, No. IO, 1989
Fulton et al.
TABLE I: Critical Parameters, Polarizability, van der Waals Volume, Dielectric Constant, and Wo"of Xenon, Ethane, and Isooctane T, Tc, pc3 polarizability, van der Waals press., "C O C bar cm3 x loz4 vo1,b cm3/mol bar WO
supercritical xenon
25
16.6
58.4
4.2
36.1
liquid ethane
25
32.3
48.8
4.5
39.7
180 200 250 400 250 400 1 100
supercritical ethane
37
32.3
48.8
4.5
39.7
liquid isooctane
25
270.8
25.6
15.9
147.0
200 270 3 20
200 300
[AOT] = 75 m M in all cases. bFrom ref 30. CDielectricof pure fluid "OT
I t must be remembered that the reported hydrodynamic diameters may reflect differences from the actual micelle diameters due to micelle-micelle interactions. To calculate micelle size and diffusion coefficient, the viscosity and refractive index of the continuous phase must be known (eq 2-4). It was assumed that the fluid viscosity and refractive index were equal to those of the pure fluid (xenon or ethane) a t the same temperature and pressure. W e believe this approximation is valid since most of the dissolved A O T is associated with the micelles; thus, the monomeric A O T concentration in the continuous phase is very small. The density of supercritical ethane a t various pressures was obtained from interpolated values.z4 Refractive indexes were calculated from density values for ethane by using a semiempirical Lorentz-Lorenz type r e l a t i o n ~ h i p . ~Viscosfties ~ of supercritical ethane were calculated from the fluid density via an empirical relationship.z6 Supercritical xenon densities were interpolated from tabulated values.z7 The Lorentz-Lorenz functionz8 was used to calculate the xenon refractive indexes. Viscosities of supercritical xenon were obtained from previously determined values.z9 A Varian Model 2200 spectrophotometer was used to measure solvatochromic shifts in the visible absorbance spectra of the selected probe molecule, thymol blue (Sigma), as a function of fluid density. The high-pressure cell used in these measurements was made of stainless steel with sapphire windows of 1.23-cm thickness. A gas-tight seal was made between the windows and the cell by compression of a silver-plated metal V-ring (Parker No. 8812-2001-0050). The resulting optical path length was 8.25 cm w i t h a sample volume of 8.3 mL. Pure xenon or ethane was introduced into the cell from a high-pressure syringe pump (Varian 8500). Fluid pressure was monitored to f 1 bar with an electronic transducer (Precise Sensors, Inc., No. C451). Reverse micelles were formed in liquid and supercritical ethane and supercritical xenon by first filling the high-pressure cell with the desired amounts of AOT, water, and thymol blue. The continuous-phase solvent was then added to the desired pressure. The fluid mixtures were stirred with a 1 /2-in.-long Teflon-coated stir bar driven by a magnetic stirrer. The pressure of the supercritical and near-critical reverse micelle systems was increased by adding pure fluid directly to the constant-volume high-pressure cell. Therefore, the surfactant and water molar concentrations remained constant over the entire range of pressures. Changes in the water-to-surfactant molar ratio upon the addition of the pure fluids are negligible due to the low water impurity levels in xenon ( 3), but to a much lesser extent than the reduction of size observed by light scattering. A major factor responsible for the observed changes in hydrodynamic size with pressure in the light scattering measurements appears to be micelle-micelle attractive interactions due to the decreased shielding of the interactions by the supercritical continuous phase at lower pressure, where a supercritical fluid is a relatively poor solvent. Solvation of the surfactant hydrocarbon tails by the supercritical fluid continuous phase may change the structure or size of a micelle. From simple geometric constraint^^^ on the surface area and volume of a micelle, any pressure- or solvent-induced micelle size change in a single-phase solution must be associated with a change in the surface area occupied by a surfactant molecule. The area occupied by a surfactant molecule is dictated by a balance between head group forces and hydrocarbon tail interactions. The ability of a solute to penetrate the hydrocarbon tail region should be partly based on steric considerations. Table I shows the van der Waals volumes of xenon, ethane, and isooctane. W e expect the degree of penetration and solvation by xenon and ethane to be much higher than for liquid alkanes such as isooctane. This is supported by observations of Haydon et a1.,20who have shown that n-butane solvates the hydrocarbon tail region to a much greater extent than high molecular weight alkanes such as decane or hexadecane. Clearly, one of the dominant forces in micellar aggregation is the ionic interactions of the head groups within the aqueous core. The repulsive interactions of the surfactant groups in the double layer of the exterior shell of the aqueous core strongly affect the micelle size. This is clearly demonstrated by increasing the ionic strength of the aqueous core. Such additions screen the head group charge, reducing their repulsive interactions leading to a reduction in the area occupied by a surfactant head W e expect that solvation of the hydrocarbon tails by the fluid may lead to a net reduction in the local dielectric constant around the surfactant anions. This may increase the repulsive forces, leading to an increase in an area occupied by a surfactant molecule and hence a reduction in micelle size. An equivalent explanation is that solvation of the hydrocarbon tail region imposes new steric forces on the A O T molecule, which results in an effective size reduction. In this light the experimental observations are consistent. The measured sizes of micelles (at high pressure) in xenon are smaller than in isooctane, where the extent of solvent penetration of isooctane into the hydrocarbon region is lower. In addition, supercritical fluids a t higher densities may be expected to better solvate the hydrocarbon tail region, leading to a reduction in micelle size. On a macroscale, micelle-micelle attractive interactions of the London-van der Waals type are of much greater importance in near-critical and supercritical fluids than in liquid solvents. A t (41) Rentzepis, P. M.; Douglas, D. C. Nurure 1981, 293, 165-166. (42) Smith, R.D.; Frye, S . L.; Yonker, C. R.; Gale, R. W. J . Phys. Chem. 1987, 91, 3059-3062. (43) Evans. D. F.; Mitchell, D.J.; Ninham, B. W. J . Phys. Chem. 1986, 90. 28 17-2825.
Fulton et al. low continuous-phase densities, solvent-micelle attractive forces are lower and hence micelle-micelle attractive forces are more important. These attractive forces can lead to clustering or eventual coalescence of micelles to form either a second reverse micelle phase or a second aqueous phase. Rouz and co-workers& have recently described what may be a significant type of attractive interaction due to overlapping and intersolvation of the hydrocarbon tails on the exterior of two or more micelles. Such interactions would also be strongly dependent on the solvent strength of the continuous-phase fluid when the micelle concentration and length and type of hydrocarbon tail are constant. Both the phase behavior and light scattering data show evidence of strong attractive interactions. The phase behavior is strongly dependent on pressure. As solvent strength of the continuous-phase fluid is reduced through a reduction in pressure, the droplets coalesce to form a second phase. From light scattering data, we see that as this two-phase boundary is approached, an apparent increase in micelle size occurs which is attributable to strong micelle-micelle attractive interactions.
Conclusions Micelle size and the aqueous core solvent environment of A O T reverse micelles in supercritical xenon, supercritical ethane, and liquid ethane are similar to those in higher molecular weight alkane liquids a t equivalent Wvalues. In contrast to liquids, micelle size appears to be somewhat affected by the pressure of the supercritical or near-critical fluid continuous phase. As the density (and pressure) of the fluid increases, a small reduction in the hydrophilic aqueous environment around the spectroscopic molecular probe is observed. This is interpreted as a small reduction in droplet size. Dynamic light scattering and spectrophotometric probe studies suggest that the micelle-micelle attractive interactions are much larger in the supercritical fluid phase than in subcritical liquids, and that the resulting effects due to the proximity of the two-phase boundary are significant. Stronger micelle-micelle attractive interactions are expected since the dielectric constant of the fluid phase is much less than in liquids. Xenon microemulsions are readily formed and have properties that may add to the fundamental understanding of surfactant aggregation. Xenon is a spherical monatomic molecule with well-characterized properties, allowing studies of solvation of micelle phases and the interfacial layer to be greatly simplified. Variation of supercritical xenon density, a t constant temperature, allows manipulation of the solvent strength of the continuous phase in the course of spectroscopic studies, which may provide a better understanding of the type and magnitude of micelle-micelle attractive interactions. In addition, theoretical modeling studies of these properties should be more readily tractable in a monatomic solvent. It also appears that the equilibrium with the xenon clathrate and the effective competition for water by these two structures has the potential for providing new insights into the thermochemistry of the microemulsion phases. Microemulsion phases in supercritical fluids provide a unique class of solvents that combine hydrophilic, hydrophobic, and amphiphilic solvating capabilities with the low viscosity and high diffusivities of the supercritical continuous phase. Acknowledgment. We thank the U S . Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC06-76RLO 1830, for support of this research. Registry No. AOT, 577-1 1-7; Xe, 7440-63-3; ethane, 74-84-0; thymol blue, 76-61-9. (44) Lemaire, B.; Bothorel, P.; Roux, D. J . Phys. Chem. 1983, 87, 1023-1 028.
