Molecular Dynamics Simulations of Water Droplets On Hydrophilic Silica Surfaces

Wetting is essential and ubiquitous in a variety of natural and technological processes. Silicon dioxides-water systems are abundant in nature and play fundamental roles in a vast variety of novel science and engineering activities such as silicon based devices, nanoscale lab on a chip systems and DNA microarrays technologies.Although extensive experimental, theoretical and computational work has been devoted to study the nature of the interaction between silica and water, at the molecular level a complete understanding of silica-water systems has not been reached. Contact angle computations of water droplets on silica surfaces offers a useful fundamental and quantitative measurement in order to study chemical and physical properties of water-silica systems. For hydrophobic systems the static and dynamic properties of the fluid-solid interface are influenced by the presence of air. Hence, nanobubles have been observed and proposed as the origin of long range ``hydrophobic'' forces even for hydrophilic silica-water interfaces unusual phenomena related to nanobubbles have been observed. In this work we study the role of air on the wetting of amorphous silica-water systems. We conduct molecular dynamics (MD) simulations of a hydrophilic air-water-silica system using the MD package FASTTUBE. We employ quantum chemistry calculation to obtain air-silica interaction parameters for the simulations. Our simulations are based in the following force fields: i) The silica-silica interaction is based on the potential model developed by Guissani et al. ii) The water-water interaction is simulated based on the classical rigid SPC/E water model. iii) The silica-water interaction is based on the force field model developed by Hassanali et al. iv) The air-silica interaction is simulated using a Lennard-Jones (LJ) 12-6 potential, parameterised using two different approaches therefore from quantum chemistry computations, and from Lorentz-Berhelot (LB) mixing rules with values obtained from the universal force field (UFF) and from Guissani et al. v) The water-air interaction is simulated using a LJ 12-6 potential with parameters obtained using LB mixing rules and values obtained from Jiang et al.38 for nitrogen and air oxygen and from Werder et al. for water oxygen.