Structure and Transport Properties of a Charged Spherical Microemulsion System Alex Evilevitch,* ,† Vladimir Lobaskin, ‡ Ulf Olsson, † Per Linse, † and Peter Schurtenberger ‡ Division of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden, and Physics Department, Soft Condensed Matter Group, University of Fribourg, Perolles, CH-1700 Fribourg, Switzerland Received August 18, 2000. In Final Form: November 15, 2000 Structure and transport properties of an oil-in-water microemulsion of weakly charged spherical micelles were studied experimentally using viscosity, NMR self-diffusion, and static and dynamic light scattering as well as theoretically by Brownian dynamics and Monte Carlo simulations and the Poisson-Boltzmann equation. The micelles contain decane covered by the nonionic surfactant pentaethylene glycol dodecyl ether (C12E5) and the ionic surfactant sodium dodecyl sulfate. The system has a constant surfactant-to-oil ratio, and the total volume fraction of surfactant and oil, Φ, is varied between 0.01 e Φ e 0.46. The micelles were made weakly charged by replacing a small fraction (0.01, 0.04, and 0.06) of the nonionic surfactant with ionic surfactant, retaining the micellar size. Comparison between self-diffusion and viscosity coefficients measured as a function of concentration showed that the system obeys the generalized Stokes-Einstein relation at lower micellar concentrations. At higher micellar concentrations, a slightly modified equation can be used upon the addition of an extra frictional factor due to stronger interactions. The collective diffusion coefficient shows a maximum as a function of the volume fraction. This result is in good agreement with predictions based on a charged hard-sphere model with hydrodynamic interactions. Other static and dynamic properties such as osmotic pressure, osmotic compressibility, and self-diffusion coefficient were obtained theoretically from simulations based on a charged-sphere model. The static and dynamic properties of the charged hard-sphere model qualitatively describe the behavior of the charged microemulsion micelles. At high volume fractions, Φ > 0.1, the agreement is quantitative, but at Φ < 0.1 the effect of the charge is smaller than what is predicted from the model. 1. Introduction Microemulsions are thermodynamically stable isotropic fluid mixtures of water, oil, and surfactant. The surfactants assemble as dividing surfaces between oil and water domains. Previous studies on the structure of microemul- sions have shown that the structure can vary from discrete swollen micelles in solution to disordered bicontinuous networks as a function of either temperature or composi- tion. 1 In many respects, the properties of swollen micelles resemble those of small colloidal particles. In particular, by introducing a small amount of charged surfactants such microemulsions composed by micelles are electrostatically stabilized. The stabilization of colloidal particles in general by electrostatic particle-particle repulsion has a long his- tory. 2 The use of surface charges allows for a control of the phase behavior and the rheological properties by ma- nipulating, for example, the salt content or the pH of the solution (if titrable surface groups exist). An important factor in predicting electrostatic stabilization is the surface charge density of the particles, a property often difficult to uniquely measure experimentally. 3,4 The Derjaguin-Landau-Verwey-Overbeek (DLVO) 5 theory constitutes the classical theoretical foundation for describing charged stabilized colloidal suspensions. In this theory, the attractive van der Waals force promoting aggregation is counteracted by a repulsive force described on the basis of the Debye-Hu ¨ ckel solution of the linear Poisson-Boltzmann equation. During the last two de- cades, the DLVO theory has been challenged by experi- mental observations indicating that charged latex particles in aqueous solution attract each other and that the attraction has an electrostatic origin. 3,6,7 Such observations have renewed theoretical interest in charged colloidal systems. 6,8-10 In this work, we were particularly interested in the case in which the microemulsion consists of spherical oil- swollen micelles dissolved in water. To obtain the desired system, the temperature for our three-component mixture was kept at the phase boundary between the single microemulsion phase (L 1 ) and one in coexistence with excess oil (L 1 + O), termed “emulsification failure”. 11 At the emulsification failure phase boundary, the micro- emulsion spheres are of low polydispersity 12 (≈ 16%) and have a concentration invariant size. Therefore, they can † Lund University. ‡ University of Fribourg. (1) Evans, D. F.; Wennerstro ¨m, H. The Colloidal Domain, Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed.; Wiley- VCH: New York, 1999. (2) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1989. (3) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Langmuir 1999, 15, 4198. (4) Horn, M. F.; Richtering, W.; Bergenholtz, J.; Willenbacher, N.; Wagner, N. M. J. Colloid Interface Sci. 2000, 225, 166. (5) Verwey, E. J.; Overbeek, J. Th. G. Theory of the stability of lyophobic colloids; Elsevier: Amsterdam, 1948. (6) Arora, A. K.; Tata, B. V. R. Ordering and Phase Transitions in Charged Colloids; VCH: New York, 1996. (7) Larsen, A. E.; Grier, D. G. Nature 1997, 385, 230. (8) Schmitz, K. S. Langmuir 1999, 15, 4093. (9) Vlachy, V. Annu. Rev. Chem. 1999, 50, 145. (10) Hansen, J.-P.; Lo ¨wen, H. Submitted for publication. (11) Olsson, U.; Wennerstro ¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113. (12) Bagger-Jo ¨rgensen, H. Polymer Effects on Microemulsions and Lamellar Phases. Ph.D. Thesis, Lund University, Sweden, 1997. 1043 Langmuir 2001, 17, 1043-1053 10.1021/la0011883 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/19/2001