Aqueous NaCl and CsCl Solutions Confined in Crystalline Slit-Shaped Silica Nanopores of Varying Degree of Protonation Tuan A. Ho, †,§ D. Argyris, †,§ D. R. Cole, ‡ and A. Striolo* ,† † School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States ‡ School of Earth Sciences, Ohio State University, Columbus, Ohio 43210, United States * S Supporting Information ABSTRACT: All-atom molecular dynamics simulations were conducted to study the dynamics of aqueous electrolyte solutions confined in slit-shaped silica nanopores of various degrees of protonation. Five degrees of protonation were prepared by randomly removing surface hydrogen atoms from fully protonated crystalline silica surfaces. Aqueous electrolyte solutions containing NaCl or CsCl salt were simulated at ambient conditions. In all cases, the ionic concentration was 1 M. The results were quantified in terms of atomic density distributions within the pores, and the self-diffusion coefficient along the direction parallel to the pore surface. We found evidence for ion-specific properties that depend on ion-surface, water-ion, and only in some cases ion-ion correlations. The degree of protonation strongly affects the structure, distribution, and the dynamic behavior of confined water and electrolytes. Cl - ions adsorb on the surface at large degrees of protonation, and their behavior does not depend significantly on the cation type (either Na + or Cs + ions are present in the systems considered). The cations show significant ion-specific behavior. Na + ions occupy different positions within the pore as the degree of protonation changes, while Cs + ions mainly remain near the pore center at all conditions considered. For a given degree of protonation, the planar self-diffusion coefficient of Cs + is always greater than that of Na + ions. The results are useful for better understanding transport under confinement, including brine behavior in the subsurface, with important applications such as environmental remediation. 1. INTRODUCTION Recent advances in nanofabrication 1-3 led to the development of many useful applications at the nanoscale such as nanofluidics and “lab-on-chip” processes, 4-7 better under- standing of biological membranes and ion-channels, 8,9 and the design of ion-exclusion desalination membranes. 10-12 Understanding solvent-electrolyte behavior under confine- ment is required to design nanoporous materials for other advanced applications, including sensors and supercapacitors. A detailed understanding of fluid-solid interactions will also provide needed insights for deploying and preserving subsur- face energy systems. 13 Although long simulation times are required to achieve properly equilibrated states, the growing interest in ion-exclusion processes, ion selectivity, and ion transport through pores and membranes justifies the utilization of atomistic simulations for realistic systems. 14-19 Computer simulation studies have been used extensively to describe the structural properties of water near solid surfaces, 20-28 and also its dynamic behavior, although to a lesser extent. 29-32 The results of these investigations suggest that interfacial water properties differ significantly from those observed in the bulk. A number of experiments support these observations. In a recent review, an extensive summary is provided for both simulation and experimental investigations on this matter. 33 Regarding fundamental studies on the structure and dynamics of aqueous electrolyte solutions at interfaces, Feng et al., 34-36 Yang et al., 37 Wander and Shuford, 38,39 and Shao et al. 40 investigated aqueous electrolytes in carbon slit pores for developing supercapacitors. Along the same research direction, Chialvo and Cummings 41 and Kalluri et al. 18 investigated the partition of aqueous NaCl between bulk and slit-shaped carbon pores. Kerisit et al. 42 studied the adsorption properties of monovalent ions on neutral (100) goethite surface. They pointed out that the structure of hydrated ions, the strength of ion-water interactions, and the effect of ions on water arrangement at the interface are key factors determining location and extent of ions adsorption. Argyris et al. 43 simulated NaCl and CsCl solutions in fully protonated silica-based slit pores. Shirono et al. 44 employed Monte Carlo and molecular dynamics simulations to study KCl distribution and transport in silica nanopores that contain both hydrophobic and hydrophilic surface patches. Their results suggest adsorption of Cl - at the silica walls and diffusion of K + through the center of the hydrophilic pore region. Marry et al. 45 suggested that the nature of counterions does not significantly alter the structure and Received: September 14, 2011 Revised: November 23, 2011 Published: December 13, 2011 Article pubs.acs.org/Langmuir © 2011 American Chemical Society 1256 dx.doi.org/10.1021/la2036086 | Langmuir 2012, 28, 1256-1266