Electronic structure calculations of physisorption and chemisorption on oxide glass surfaces

A description of the physisorption and subsequent chemisorption of water on silica glass surfaces is presented that combines electronic structure calculations with classical molecular dynamics simulations. The method associates the strength of the physisorption sites with the gradient of the electrostatic potential, and the chemical reactivity with the chemical hardness of the surface. The physisorption results are compared with those of more classical physisorption energy mappings, and to ab initio calculations of water molecules physisorbing at specific sites. The chemisorption reactivity index is compared with calculated chemisorption energy barriers. The techniques are applied to two types of silica glass surfaces: a fracture surface with high energy coordination defects and a low energy defect-free ``melt surface.'' The mappings show that the strongest sites for physisorption are network coordination defects, but that a high physisorption energy is not necessarily an indicator of a reactive site. The physisorption and chemisorption mappings were converted to energy distributions and reactivity distributions for a direct comparison between the melt and fracture surfaces. Altogether, this approach combines the efficiency of classical molecular dynamics for structural determinations, with the chemical degrees of freedom provided by electronic structure calculations, to yield a semiquantitative map of chemical reactivity across a surface.