UNDERSTANDING AQUEOUS SOLUTIONS AT ALPHA-ALUMINA SURFACES USING MOLECULAR DYNAMICS SIMULATIONS

Abstract

Water/oxide interfaces are ubiquitous on earth and show significant influence on many chemical processes. However, many questions persist such as how do solid surfaces perturb interfacial water structure? How do interfacial water molecules and adsorbed solutes affect solid surfaces and how do interfacial environments affect solvent and solute behavior. Although it is difficult for experimental techniques to provide detailed microscopic structural information, molecular dynamics (MD) simulations are able to answer these questions.In this work, I investigated the structure and dynamics of water/alumina interfaces because alumina is one of the most abundant minerals and has a wide range of industrial applications, including catalysis, water purification and desiccation. Understanding the microscopic structure of aqueous solution at alumina surfaces can inspire the design of better materials. First, I studied the simplest process, namely surface wetting, and found that the alumina (0001) and (112 ̅0) surfaces are superhydrophilic, a property that could not be determined by measuring the contact angle experimetally. Interactions between surfaces and interfacial waters promote a templating effect whereby the latter are aligned in a pattern that follows the underlying lattice of the mineral surfaces. Simulation results explain why alumina could be used as desiccants and why there are strong interactions between water and the surfaces. Next, in order to study how ions perturb aqueous interfaces, a more complicated system, the water/α-alumina(0001) interface was investigated. Simulations show that ions accumulate near the alumina(0001) surface, consistent with experimental measurements of F- adsorption. The results provide physical insights why activated alumina is used to remove harmful ions from water. The excess adsorbed Na+ reverses the surface dipole, beyond what traditional EDL theory predicts and difficult to be observed in experiments. I found that adsorbed ions affect local structures of the alumina surface, e.g., reorienting surface OH groups. The phenomenon is worth investigating because how ion adsorption affects solid structures is often neglected in previous research. This work provides detailed structural information of the water/alumina(0001) interface in the presence of ions. The method developed can be used to analyze more complicated process, such as biomolecule adsorption. To test the hypothesis that water/oxide interfaces played a crucial catalytic role in peptide formation, the self-assembly of a simple amino acid glycine at water/α-alumina(0001) interfaces is investigated. The pattern of adsorbed glycine zwitterions is similar to that of Na+ or Cl-. I observed the formation of long glycine chains that ready for condensation due to a templating effect of the solid. The probability to form a chain longer than 10 is at least 1010 times higher than that in the bulk. Our work provides preliminary insight on how mineral surfaces may induce configuration-specific assembly of amino acids and thus promote condensation reactions. One reason for abnormal water behavior is hydrogen bonds. Most of previous published papers focus on individual hydrogen bonds but not the hydrogen bond network topology. To quantify it, I develop algorithms to calculate the connectivity and dimensionality of Hbond networks. Simulations of several different interfaces show an increased number of Hbonds parallel to surfaces due to the high local water density and the missing of Hbonds counterpart on surfaces. Those parallel Hbonds increase the connectivity of Hbond networks, which are connected at lower thickness or number of waters than bulk water, indicating that Hbond networks at interfaces prefer to spread in two dimensions instead of three in bulk water. At the end of the work, I will present a short summary of all my work and the future of this field.