(291b) Microscopic Properties of the Electric Double Layer at Metal Oxide Surfaces and the Effect of Hydrogen Bonding

The interface between metal oxides and aqueous systems is of common occurrence in many natural and industrial environments. The complex nature of the interface, involving protolytic and complexation reactions, leads to its charging and formation of the electric double layer (EDL). A proper theoretical model of this heterogeneous region must take into account a number of interrelated local effects including hydrogen bonding, water dissociation, or ion adsorption, and also long range properties of both bulk phases. We present an analysis of the strucutre of water and ion adsorption at the (110) surface of cassiterite (α-SnO2) and rutile (α-TiO2) at ambient conditions. The systems are studied by means of MD simulations and the results are compared to X-ray crystal truncation rod experiments and interpreted with the help of the revised MUSIC model of surface protonation. The electrolyte solution is modeled by the SPC/E water and ions (Li+, Na+, Rb+, Ca2+, Sr2+, Zn2+, and Cl-) with interactions optimized for use with the SPC/E. The interactions of metal oxides are described by recently developed forcefields based on ab initio calculations [1,2]. The structure of the surface molecular layers, including density, hydrogen bonding, orientation of water dipoles, and ion complexation, is studied in dependence on the surface charge and the degree of hydroxylation. The analysis is complemented by the studu of water diffusivity in the interfacial region. Differences between cassiterite and rutile systems are discussed and related to the lattice parameters of bulk crystals and the thermodynamics of water adsorption [3]. To make a link to macroscopic pH titration experiments, we tested the predictions of the revised MUSIC surface protonation model[4]. Explicit use of the ab initio metal-oxygen bond lengths and H-bond configurations from MD simulations resulted in a predicted points of zero charge that agrees very well with those that have been determined experimentally.

References: [1] Bandura, A. V., and Kubicki, J. D., J. Phys. Chem. B, 2003, 107 (16), 11072. [2] Bandura, A. V., Sofo, J., and Kubicki, J. D., J. Phys. Chem. B, 2006, 110 (16), 8386. [3] Vlcek, L., Zhang, Z., Fenter, P., Machesky, M. L., Rosenquist, J., Wesolowski, D., and Cummings, P.T., Langmuir, submitted. [4] M. L. Hiemstra, T., Venema, P., and van Riemsdijk, W. H.,J. Colloid Interface Sci. 1996, 184, 680-692.