Structure and Dynamics of Molecules at Water/Silica and Water/Carbon Dioxide Interfaces

by Hui Zhang

Institution: The Ohio State University
Department: Chemistry
Degree: PhD
Year: 2010
Keywords: Physical Chemistry; Stern layer; molecular dynamics; silica; interface; carbon dioxide
Record ID: 1871449
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=osu1290543413


The interface between silica and water is one of the most technologically relevant surfaces. An especially important aspect of this system is its inherent negative charges at most pH values, and the resulting electrokineticphenomena that take place in the fluid region. We have constructed a realistic model for the charged silica/water interface where many of these standard modelscan be tested. The model allows for undissociated and dissociated silanol groups. We have also conducted ab initio MD simulations of a smaller system consisting of a hydrated silica slab. The comparison of the radial distribution functions from the ab initio MD simulations and thoseobtained from the empirical model are favorable. The hydrophobic and hydrophilic nature of silanol-poor and silanol-rich regions of the amorphous silica surfaceobserved in our empirical model is reproduced in the ab initio MD simulations of the smaller slab. In the initial stages of our ab initio MD simulations, we observe various chemical processes that represent different hydroxylation mechanisms of the surface.To explain why dynamical properties of an aqueous electrolyte near a charged surface seem to be governed by a surface charge less than the actual one, the canonical Stern model supposes an interfacial layer of ions and immobile fluid. However, large ion mobilities within the Stern layer are needed to reconcile the Stern model with surface conduction measurements. Modeling the aqueouselectrolyte/amorphous silica interface at typical charge densities, a prototypical double layer system, the flow velocity does not vanish until right at the surface. The Stern model is a good effective model away from thesurface, but cannot be taken literally near the surface. Indeed, simulations show no ion mobility where water is immobile, nor is such mobility necessary since the surface conductivity in the simulations is comparable to experimentalvalues. Our studies suggest a richer, microscopic picture that allows for much greater mobility near the surface without a sharp boundary between mobile fluid and immobile ion layer, but still accounts for observed phenomena. The effect of salt concentration, surface charge density (which would be controlled experimentally by varying the pH) and local water viscosity on electrokinetic phenomena is explored.The structural properties of the interface between water and carbon dioxide are very important in many areas of chemistry and physics, such as supercritical extraction, electrochemistry and ion transport across membranes. In my study, the structural properties of the interface of water and CO2 are investigated by means of molecular dynamics (MD) simulations. The capillary wave theory is used to find the interface positions and the shape of the interface isdetermined by this theory. The density profiles of CO2 and water are extracted based on capillary wave theory. The density profiles are very helpful to calculate the surface excess and check whether there is a wetting transitionwhen the pressure is increased. Molecular…