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Quartz-water interface – complementary experimental and molecular dynamics study

Properties of interfacial water differ significantly from those present in the homogeneous bulk water, whereby the presence of surface charges and the accumulation of ions at the solid-water interface can induce long-range ordering of water molecules and the formation of electrical interfacial layer. This, in turn, gives rise to a variety of different physical and chemical processes, such as heterogeneous catalysis, mineral dissolution, membrane chemistry etc. Using a model system, consisting of (0001) quartz surface and alkali metal (Na+, K+) halide (Cl–, I–) solutions at three different pH conditions (pH = 3, 6, and 9), we have characterized the behavior of water at quartz- water interfaces using complementary combination of molecular dynamics (MD) simulations and two experimental techniques, namely inner surface potential measurements (obtained by Single Crystal Electrode [1]), from which zeta-potential is obtained. Our strategy enabled us to compare experimental and theoretically calculated results in a direct manner, i.e. theoretically obtained zeta- potentials (obtained via non-equilibrium molecular dynamics (NEMD) simulations [2]) were compared with the measured zeta-potential data, thus enabling us to both critically assess the applicability of a chosen silica-water force field [3] and to describe the nature of the model quartz–water interface in appreciable detail. More specifically, we find that interactions between the surface silanol (≡SiOH) and siloxide groups (≡SiO–) with water and solvated ions have a dominant role in inducing a multidimensional layering profile of water. In this respect, we find that the interface region comprises of a “compact layer” of solvent next to the quartz surface that is not properly described in classical electric double layer theories, whereby the depth of the diffuse solvent layer is sensitive to the ionic strength, pH conditions and the identity of solvated ions.

quartz, interfacial water, molecular dynamic, adsorption of ions

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