Molecular dynamics study of water and ion behaviors of mixed salts solutions on extended quartz surface

The adoption and evolution of water molecules and ions in mixed electrolytes at the surface play vital roles in the physical properties and chemical reactions of SiO2-like corrosion. The effect of salt type and concentration on the structure and dynamics of water molecules and ions at silica surfaces are studied using all-atom molecular dynamics simulations taking the case of the NaCl, MgCl2, and NaCl–MgCl2 aqueous solutions. The ability of ion hydration is in the order of Mg2+ > Na+ > Cl−, being opposite to their hydration Gibbs free energies, which directly influence the weak interaction in the solution and the diffusion rate of the particles. Mg shows stronger destruction to weak interactions than Na does, and ionic hydration of Mg2+ decelerates the self-diffusion coefficient of water molecules significantly due to the enhanced Coulomb effect and the interruption of solution continuity. Meanwhile, the self-diffusion coefficient of particles decreases with the concentration improvement in the single salt solution as increased ionic hydration. In the mixed salt solution, the order of diffusion rate is Cl− > Na+ > Mg2+ as a result of the different confinement effects of the protonated pore. Interestingly, a small amount of Na+ addition can promote the self-diffusion of Mg2+, but a great many of Na+ addition slows the diffusion of Mg2+. This work provides comprehensive insight into the behavior of mixed salt solutions at silica surfaces, shedding light on the practical applications of geological sciences, cultural relics protection, and colloidal sciences.