Dynamical Properties of Water and Ions at the Quartz (101)-Water Interface at a Range of Solution Conditions: A Classical Molecular Dynamics Study

The solution conditions such as the ion concentration and pH have a profound effect on the behavior of the silica/water interface, which dictates many of the surface properties of mesoporous silica nanoparticles (MSNs) and their utility in a range of nanotechnology applications. The interaction of water molecules with a model silica surface, alpha-quartz (101), at different surface charge densities, is investigated to evaluate the influence of pH on structure (density profile, radial distribution function) and dynamics (diffusion coefficient D) of the interfacial water in the presence of biologically relevant ions, Na+, K+, and Rb+ ions. Classical molecular dynamics were performed using a recently developed force field (Krona et al. J. Phys. Chem C 2015, 119, 9274-9286). Our results show the interfacial water is more structured and the diffusion of interfacial water molecules becomes slower as the surface charge becomes more negatively charged, as we increase the pH from acidic to neutral to basic pH. The self-coefficient diffusion (D) of water molecules and ions decrease with an increase in pH and is affected by concentration and type of ions in the system. The diffusion of water molecules around deprotonated oxygen atoms is slower than the diffusion of water molecules around oxygen protonated. The presence of the ions near to deprotonated oxygen atoms further decreases the diffusion of water molecules.