Aqueous Electrolytes near Hydrophobic Surfaces: Dynamic Effects of Ion Specificity and Hydrodynamic Slip ² David M. Huang, Ce ´cile Cottin-Bizonne, Christophe Ybert, and Lyde ´ric Bocquet* UniVersite ´ de Lyon, UniVersite ´ Lyon 1, LPMCN, and CNRS, UMR 5586, F-69622 Villeurbanne Cedex, France ReceiVed July 19, 2007. In Final Form: September 20, 2007 We demonstrate, using molecular-dynamics computer simulations, the strong influence of surface wettability on the equilibrium structure of the electrical double layer at solid interfaces and on electrokinetic transport in aqueous electrolytes due to the effects of interfacial ion specificity and hydrodynamic slip. In particular, we show that anomalous electrokinetic effects such as nonzero zeta potentials for uncharged surfaces are general features of electro-osmotic flow in hydrophobic channels for electrolytes with substantial cation/anion size asymmetry, as a result of the stronger attraction of the larger ion to the “vapor-liquid-like” interface induced by a hydrophobic surface. We establish that the simulated velocity profiles obey continuum hydrodynamics on the nanoscopic length scales studied and show that the anomalous flow profiles can be accurately predicted by using a modified Poisson-Boltzmann description for the ion density distributions that incorporates an ion-size-dependent hydrophobic solvation energy as a crucial component. We also demonstrate that, counterintuitively, the flow for a charge-neutral fluid is independent of the solid-fluid friction coefficient. 1. Introduction The rapid development of microfluidics 1,2 has spurred great interest in recent years in the fundamental physics of fluid flow on micrometer and smaller length scales. The violation of the no-slip boundary condition (BC) of macroscopic hydrodynamics that has been observed in a variety of systems at these length scales 3-5 has particularly important implications for microfluidics in that it offers the possibility of substantial flow amplification. Hydrodynamic slip at the solid-liquid interface has been shown to be controlled largely by the wetting properties of the liquid on the solid surface: although the no-slip BC is satisfied by solvophilic (hydrophilic for the case of water as the solvent) surfaces, solvophobic (hydrophobic) surfaces can exhibit finite slip. 6,7 The small length scale structure of the fluid interface, besides controlling hydrodynamic slip, also plays an important role in electrokinetic transport, a popular means of inducing fluid flow in microfluidic devices, 2 because the magnitude and direction of fluid flow depend sensitively on the interfacial distribution of dissolved ions. In fact, the interplay between hydrodynamic slip and the interfacial ion distribution can provide a powerful means of microfluidic flow control. 8 The interfacial structure of electrolyte interfaces also plays an important role in many other phenomena, both static and dynamic, mediated by ions at aqueous interfaces, in areas as diverse as biology, 9 atmospheric chemistry, 10 and colloid science. 11 The study of interactions between hydrophobic surfaces in aqueous solutions (generally containing dissolved ions) is an area of particular interest 12 because of their importance in processes such as protein folding and micelle formation. Many such interfacial phenomena are strongly affected by the identity of the cation or anion in solution, 9,13,14 contrary to traditional theories of electrolyte interfaces such as the Gouy- Chapman model of the electrical double layer 11 in which only differences in ion valency are taken into account. Such trends in physical phenomena as a function of cation or anion type are generally referred to as Hofmeister series and are ubiquitous in biology and physical chemistry. 13 Although in some cases the observed ion-specific Hofmeister series are likely due to particular ion-surface interactions, 15 in others the effects arise purely as a result of the interfacial structure of water. For example, the significant dependence of the air-water surface tension on anion type for halide salts 15 has been explained in terms of the differing propensities of the ions for the vapor- liquid interface. Recent spectroscopic experiments 16,17 and computer simulations 18,19 have indeed shown that the larger bromide and iodide ions exhibit enhanced interfacial concentra- tions. It has been suggested that the depleted water density near ² Part of the Molecular and Surface Forces special issue. * To whom correspondence should be addressed. E-mail: lyderic. bocquet@univ-lyon1.fr. (1) Stone, H. A.; Stroock, A. D.; Ajdari, A. Annu. ReV. Fluid Mech. 2004, 36, 381-411. (2) Squires, T. M.; Quake, S. R. ReV. Mod. Phys. 2005, 77, 977-1026. (3) Granick, S.; Zhu, Y. X.; Lee, H. Nat. Mater. 2003, 2, 221-227. (4) Lauga, E.; Brenner, M. P.; Stone, H. A. In Handbook of Experimental Fluid Dynamics; Foss, J., Tropea, C., Yarin, A., Eds.; Springer, New York, 2005; Chapter 15. (5) Bocquet, L.; Barrat, J.-L. Soft Matter 2007, 3, 685-693. (6) Cottin-Bizonne, C.; Cross, B.; Steinberger, A.; Charlaix, E Ä . Phys. ReV. Lett. 2005, 94, 056102. (7) Barrat, J.-L.; Bocquet, L. Phys. ReV. Lett. 1999, 82, 4671-4674. (8) Ajdari, A.; Bocquet, L. Phys. ReV. Lett. 2006, 96, 186102. (9) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. Q. ReV. Biophys. 1997, 30, 241-277. (10) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Science 2000, 288, 301-306. (11) Hunter, R. J. Foundations of Colloid Science, 2nd ed.; Oxford University Press: Oxford, U.K., 2001. (12) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15739-15746. (13) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9,1-18. (14) Jungwirth, P.; Tobias, D. J. Chem. ReV. 2006, 106, 1259-1281. (15) Bostro ¨ m, M.; Kunz, W.; Ninham, B. W. Langmuir 2005, 21, 2619-2623. (16) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M. Science 2005, 307, 563-566. (17) Petersen, P. B.; Johnson, J. C.; Knutsen, K. P.; Saykally, R. J. Chem. Phys. Lett. 2004, 397, 46-50. (18) Vrbka, L.; Mucha, M.; Minofar, B.; Jungwirth, P.; Brown, E. C.; Tobias, D. J. Curr. Opin. Colloid Interface Sci. 2004, 9, 67-73. (19) Archontis, G.; Leontidis, E.; Andreou, G. J. Phys. Chem. B 2005, 109, 17957-17966. 1442 Langmuir 2008, 24, 1442-1450 10.1021/la7021787 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/06/2007