Langmuir 1991, 7,896-897
896
Salt Effects on Liquid-Liquid Phase Separation in Aqueous Micellar Solutions of the Zwitterionic Surfactant Cs-Lecithin Daniel Blankschtein,'*t Yao-Xiong Huang,t George M. Thurston,s and George B. Benedeks Department of Chemical Engineering and Department of Physics and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 14, 1990
It has recently proven possible to develop a thermodynamic theory of the equation of state and the phase separation of binary micellar solutions that exhibit micellar growth and high polydispersity in micellar size.' This theory incorporates into a Gibbs free energy model the physically important features of (i) interactions between micellar aggregates, (ii) entropy of mixing of the micellar solution, and (iii) micellar multiple chemical equilibrium. The theory is capable of providing a selfconsistent analytical representation of a broad spectrum of equilibrium properties of the micellar solution, including (i) the shape and location of the coexistence curve delineating the boundary of liquid-liquid phase separation, (ii) the osmotic compressibility of the micellar solution, and (iii) the concentration and temperature dependence of the micellar size distribution. Indeed, this representation has proven to be an accurate description of the experimental data in aqueous micellar solutions of the zwitterionicsurfactant dioctanoylphosphatidylcholine(c8lecithin), which exhibit liquid-liquid phase separation upon lowering the temperature and, therefore, display an upper consolute (critical) temperature, Tc. The theory has also proven to give an accurate description of experimental data in aqueous micellar solutions of the nonionic alkyl poly(oxyethy1ene) surfactant C1&, which exhibit a lower consolute temperature. The theory has also been utilized recently to characterize the effects of added urea on the location of the coexistence curve in aqueous C8lecithin micellar solutions.2 The theory in its present form contains two phenomenological parameters, C and Ap, which can be obtained experimentally. The parameter C describes the magnitude of the effective attractive intermicellar free energy. The parameter Ap describes the free energy advantage associated with micellar growth, that is, the larger its m a g nitude, the larger the micelles and associated micellar size polydispersity. It has long been recognized that the addition of salts can dramatically affect the phase behavior of colloidal systems and can therefore also serve as a practical means of controlling this phase behaviors3 Indeed, the effects of Department of Chemical Engineering. Present address: Department of Physics, Zhongshau Medical University, Guangzhou, People's Republic of China. 8 Department of Physics and Center for Materials Science and Ennineerinn. (1) Blankkhtein, D.; Thurston, G. M.; Benedek, G. B. J . Chem. Phys. t
t
1986,85,7268. (2) Carvalho, B.; Briganti, G.; Chen, S. H. J . Phys. Chem. 1989, 93,
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(3) Edsall, J. T.; Wyman, J. Biophysical Chemistry; Academic Press: New York, 1958.
added salts on liquid-liquid phase separation of surfactant solutions have been studied since the early part of this ~ e n t u r y .In ~ particular, in the past few decades, the effects of added salts on the liquid-liquid phase separation of aqueous micellar solutions of nonionic surfactants have received considerable attention.5 Much less work has been reported on the effects of added salts on the phase separation of aqueous solutions of zwitterionic surfactants. Tausk, Oudshoorn, and Overbeeks studied the effects of added salts on the phase separation of aqueous solutions of the zwitterionic surfactant Cglecithin. They found that the two salts NaCl and LiI had dramatically different effects on the location of the coexistencecurve. The relative effects of each of the added cations and anions, however, could not be determined from their work. Accordingly, we have continued on the path set by Tausk et al. In this paper, we briefly summarize the observations of the effects of the salts KI, NaI, LiI, KCl, NaCl, and LiCl on the coexistence curves of aqueous C8-lecithin micellar solutions. A detailed exposition of our results may be found in ref 5. Our data clearly show that salts containing iodide and chloride ions have very different effects on the phase separation of aqueous Ce-lecithin micellar solutions. Potassium iodide, sodium iodide, and lithium iodide all lower the critical temperature for phase separation, Tc, dramatically, while potassium chloride, sodium chloride, and lithium chloride first lower Tcand then raise Tcwith increasing salt concentration. The identity of the monovalent cations used has a relatively minor effect on Tc. In the context of a Gibbs free energy model for micellar solutions' composed of water and a single type of surfactant, we have deduced from our data the dependence of the interaction parameter C and the micellar growth parameter Ap on ionic identity and concentration. With use of these dependences, the calculated theoretical coexistence curves agree well with our experimentally determined cloud pointa and coexisting concentration determinations.5 The analysis above suggests that in the present context the dependence of Tcon solution conditions is principally determined by changes in the magnitude of the effective intermicellar interactions, as summarized by the parameter C. Naturally, it is now of considerable interest to understand at a molecular level the magnitudes of both C and Ap which have emerged from our experimental (4) McBain, J. W.; Pitter, A. V. J. Chem. SOC.1926, 129, 893. (5) Huang, Y. X.;Thurston, G. M.; Blankschtein, D.; Benedek, G. B. J. Chem. Phys. 1990, 92, 1956, and references therein.
(6) Tausk, R. J. M.; Oudshoorn, C.; Overbeek, J. Th. G. J. Biophys. Chem. 1974,2, 53.
0743-746319112407-0896%02.5010 , 0 1991 American Chemical Society I
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Salt Effects on Phase Separation
findings, and the origin of their dependenceon salt identity and concentration. We have begun the first step in this direction by developing a molecular model for Ap, or more generally, for the free energy of micellization associated with any type of micellar morphology, including spheres, rods, disks, and bilayers.' Acknowledgment. We thank Ilhan Olmez and Mamoru Kondo for performingthe neutron activation analyses, which were supported in part by Massachusetts Institute of Technology Sponsored Research. We also thank Peter Schurtenberger and Simone Angehrn for their assistance with these investigations. This work was supported in (7) Puwada, S.;Blankechtain, D.J. Chem. Phys. 1990,92,3710.
Langmuir, Vol. 7, No. 5, 1991 897 part by the National Science Foundation (NSF), under Grant No. DMR 84-18718, and by a NSF-Presidential
Young Investigator (PYI)Award to Daniel Blankschtein. Daniel Blankschtein also acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, and is grateful for the support by the Texaco-Mangelsdorf Career Development Professorship at MIT. He is also grateful to the following Companies and Foundations for providing PYI matching funds: BASF, British Petroleum America, Colgate-Palmolive, Exxon, Hazardous Substances Management Program at MIT, Kodak, Rohm & Haas, Texaco, Unilever, Waters, and the Whitaker Foundation.
