Structure of Water at Charged Interfaces: A Molecular Dynamics Study Shalaka Dewan, † Vincenzo Carnevale, ‡ Arindam Bankura, ‡ Ali Eftekhari-Bafrooei, † Giacomo Fiorin, ‡ Michael L. Klein, †,‡ and Eric Borguet* ,† † Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States ‡ Institute for Computational Molecular Science, Temple University, Philadelphia, Pennsylvania 19122, United States ABSTRACT: The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To character- ize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a “compact layer” of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na + vs Cs + ) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na + vs Cs + , the depth (∼1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies. ■ INTRODUCTION The unique properties of interfacial water impact diverse fields such as biology 1-3 (e.g., protein folding and lipid membrane properties), geology 4,5 (e.g., mineral dissolution, the stability of colloids, and wastewater treatment), atmospheric chemistry 6 (e.g., effect of water aerosols on pollution), electrochemistry 7 (e.g., wetting and corrosion), materials science 8 (e.g., heterogeneous catalysis), and technological applications 9 (e.g., hydrogen fuel cells and biosensors). Despite the profound differences in the physicochemical processes involved in the above examples, they all depend on the interaction of water molecules with surfaces and the influence of the surface on the structure of the extended hydrogen bond network. 10 Water molecules at interfaces have different properties than their bulk counterparts 11,12 due to the abrupt break in the bulk hydrogen- bonding network caused by the presence of a phase boundary. It is this peculiar orientation and ordering of the first layer of water molecules at the interface that influence many macro- scopic interfacial processes. 11,13 At the well-studied air/water interface, experimental and theoretical results show that the interfacial depth, i.e., the region where the water structure is not bulklike, is rather shallow (∼3-7 Å), corresponding to one or two monolayers. 14,15 A different picture is expected at mineral/water or electrode/ electrolyte interfaces, where surface charge may be the main factor influencing the local hydrogen-bonding environment of water. In particular, the region that is inherently oriented due to the presence of the surface may extend well beyond the first layer. 16-18 The surface charge induces an electrostatic field that can align water molecules, inducing an orientational order that persists over a certain depth into the bulk, as described by simple electrostatic models 10 and confirmed experimen- tally. 19-21 In the presence of electrolytes, the surface charge is balanced by the distribution of counterions and oriented water dipoles near the interface, forming an electric double layer (EDL) region, our understanding of which has been ever-evolving since the early introduction of classical theoretical models. 22,23 Received: March 23, 2014 Revised: June 5, 2014 Published: June 30, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 8056 dx.doi.org/10.1021/la5011055 | Langmuir 2014, 30, 8056-8065