Quartz/Aqueous Electrolyte Solution Interface: Molecular Dynamic Simulation and Interfacial Potential Measurements Zlatko Brkljač a, † Danijel Namjesnik, † Johannes Lü tzenkirchen, ‡ Milan Pr ̌ edota, § and Tajana Preoč anin* ,† † Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR-10000 Zagreb, Croatia ‡ Institut fü r Nukleare Entsorgung, Karlsruher Institut fü r Technologie, P.O. Box 3640, 76021 Karlsruhe, Germany § Institute of Physics, Faculty of Science, University of South Bohemia, Braniš ovská 1760, 37005 C ̌ eské Budě jovice, Czech Republic *S Supporting Information ABSTRACT: In this complementary experimental and theoretical study, we employ surface and electrokinetic potential measurements and equilibrium molecular dynamics (MD) techniques to study the electrical interfacial layer between aqueous solutions of electrolytes and an oxide solid surface. More specifically, we investigate the behavior of a prototypical model system consisting of the (0001) quartz surface in contact with aqueous solutions of alkali metal salts under different conditions. The inner surface potential and electrokinetic ζ-potential were measured by means of single crystal electrodes and via streaming current measurements, respectively. Calculated ζ-potentials allowed us to benchmark MD simulations against experiments, thereby, on the one hand, verifying the validity of our strategy and, on the other hand, enabling a detailed molecular picture of the investigated phenomena and elucidating the role of both water and ions in the formation of the multilayered quartz/aqueous electrolyte interface. ■ INTRODUCTION Quartz is one of the most common minerals that occur in the environment. The quartz (0001) crystal face is the most stable plane with the lowest surface energy and is often considered as a “model surface”, convenient for modeling SiO 2 materials and hydrophilic surfaces in general. 1 In aqueous electrolyte solution, surface silica atoms react with water and form amphoteric SiOH silanol surface sites. The extent of the surface protonation and deprotonation of these silanol groups depends on pH and the composition of the aqueous electrolyte solution. Surface concentrations of positively and negatively charged surface groups determine the overall surface charge and ion distributions as well as the orientation and diffusion of water molecules within the interfacial layer. Surface charging and formation of the electrical interfacial layer (EIL) are complex and mutually related processes. The electrostatic surface potential is determined by the charge distribution at the quartz/electrolyte solution interface, resulting from an interplay of electrostatic and van der Waals interactions with key roles of surface charge and interfacial structure of the solvent. The inner surface potential, Ψ 0 , is the electrostatic potential at the solid plane exposed to the liquid medium. Because this potential markedly affects the state of charged species bound to the surface, it plays a dominant role in surface equilibration. The expressions for the inner surface potential depend on the assumed surface complexation model. 2 However, irrespective of the model, the inner surface potential depends on the bulk concentration of the potential determining ions (H + /OH − in the case of quartz), the thermodynamic equilibrium constants of surface complexation, and the ratio of surface concentrations of the charged groups. 3 The measurement of the inner surface potential, enabled by construction of single crystal electrodes (SCrE’s), 4 provides important information on the equilibrium at the interfacial layer and enables a critical examination of the theoretical models describing the interfacial equilibrium. 5−7 A SCrE consists of a single crystal mounted to a poly(methyl methacrylate) holder. Ideally, one specific crystal plane is exposed to the aqueous electrolyte solution and measurements of the electrode potential with respect to a reference electrode provides information about surface complexation and dis- tribution of ions within the EIL. A few limitations of this method make its application slightly difficult. This includes the required calculation of an absolute inner surface potential from the measured relative electrode potential, 5 the high resistance of the single crystal, and titration hysteresis. 8 The electro- kinetic potential, often called ζ-potential, is assumed to occur at the hypothetical slip (or shear) plane that divides the stagnant from the mobile part of the EIL. The position of the slip plane distance has often been estimated, by fitting experimental data, 9 to be about 1 nm from the metal oxide surface. Molecular dynamic studies have attempted to explain the molecular origins of the electrokinetic potential and the location of the slip plane. 10−12 Received: April 29, 2018 Revised: October 2, 2018 Published: October 2, 2018 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C 2018, 122, 24025-24036 © 2018 American Chemical Society 24025 DOI: 10.1021/acs.jpcc.8b04035 J. Phys. Chem. C 2018, 122, 24025−24036 Downloaded via ACADEMY OF SCIENCES CZECH REPUBLIC on October 25, 2018 at 06:50:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.