Ion-specific and concentration-dependent adsorption on mica surfaces: A molecular dynamics study

Abstract

Ion adsorption at solid-liquid interfaces and ion transport in confined spaces are central to many natural processes and practical applications in life and technology. For example, surface charge regulated membranes can enable high ion selectivity, and effective flow control for desalination and electrochemical energy storage systems [1]. Another physical process is clay swelling, a key phenomenon for oil and gas industry, is caused mainly due to the ion exchange of flowing fluid in rocks. Clay swelling significantly depends on the structure of the mineral and the composition of the aqueous environment [2]. For such applications, cation-specific effects and concentration-dependence is still little known. In this study, we carried out molecular dynamics (MD) simulations of aqueous electrolytes with various compositions confined between two negatively charged mica surfaces in order to explore processes such as ion adsorption, hydration and electric double layer (EDL) structuring, and ion transport at the solid-liquid interface. Muscovite mica is a widely used representative mineral to study the adsorption and hydration at solid-liquid interfaces owing to its hexagonal crystal structure with the perfect cleavage and inherent negative charges [3]. Our simulation models consist of mica surfaces containing 8x4 unit cells in which surfaces are separated with a 6 nm slab thickness. As electrolyte, we use chloride solutions of different metals, namely Cs+, Li+, and Ca+2, which represent monovalent and divalent metals. The number of salts is varied to obtain a wide range of ion concentrations. Additional cations are added in simulation system to balance the negative surface charge of mica surfaces [4]. A schematic configuration of our simulation model is shown in Figure 1. We examine the variations in ion concentration, interfacial water density, molecular orientations, and ion mobility. Our simulation results show that Cs+ ions have the most prominent concentration peaks at the surface, indicating a stronger ion adsorption compared to Li+ and Ca+2 ions at the same ionic concentration. Interestingly, the number of Cs+ ions adsorbed at the surface exceeds the amount of net surface charge of mica. This refers to a phenomenon called as “charge overscreening”. As a result, the surface becomes positively charged, and the diffuse layer of EDL becomes co-ion dominated. However, this is not the case for Li+ and Ca+2 which they less strongly attach to the surface. Adsorbed Li+ ions at the surface slightly undercharge the mica, while the net surface charge due to Ca+2 adsorption is the least among three different metals. Our simulations also show that almost all adsorbed Cs+ ions occupy the center of hexagonal cavity of mica lattice, creating a diamond-shape pattern on the surface (See Figure 1). On the contrary, Li+ and Ca+2 ions are located one of the binding oxygens of the mica next to the isomorphic substitution site. These structural arrangements are mainly attributed to the combined effects of the ion size, electron density and surface-ion interaction strength. We further observe that water densities of LiCl solution show more pronounced layering at the interface. Although there are less Li+ ions at the surface compared to Cs+, more water molecules come near to the surface from the center of the channel. Water molecules at the interface can pass through between adsorbed Li+ layer and mica surface, strongly hydrating Li+ ions whereas this is not possible for Cs+. There are almost three times less water molecules available per Cs+ ion at the surface. Accordingly, adsorbed Cs+ ions are found to be more stagnant at the surface due to the weaker hydration while the adsorbing Li+ ions are more mobile, allowing constant exchange of Li+ ions from the channel center. These results further indicate that hydration is the driving force in the LiCl solution whereas the surface-ion interactions are the dominant force in CsCl solution. By assessing the competitive behavior of charged species at the surface, the ion adsorption coverage is quantified as a function of the bulk ion concentration. Our results show that Cs+ coverage significantly increase with the increase of ion concentration while a linear but less prominent increase is obtained for Li+ adsorption. On the other hand, increased ion concentration shows a negligible influence on the Ca+2 coverage. MD simulation results highlighting the ion adsorption as a function of type and concentration is critical to understand the interfacial thermodynamics directly from atomic force microscopy (AFM) imaging. [1] U. Ramach, J. Lee, F. Altmann, M. Schussek, M. Olgiati, J. Dziadkowiec, L.L.E. Mears, A.T. Celebi, D. Lee, and M. Valtiner. Faraday Discuss. 246, 487-507 (2023). [2] L.S. de Lara, V.A. Rigo, and C.R. Miranda. J. Phys. Chem. C, 121(37), 20266-20271 (2017). [3] R. M. Pashley, J. Colloid Interface Sci. 83(2), 531-546 (1981). [4] I.C. Bourg, S.S. Lee, P. Fenter, and C. Tournassat J. Phys. Chem. C, 121, 9402-9412 (2017).