Computer modeling and simulation of water adsorption on silica and silicate glass surfaces

Molecular dynamics (MD) has been used to simulate the atomic scale structure of silica and silicate glass surfaces, and several modeling techniques are applied to characterize the surface sites with respect to adsorption of water. A detailed 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 new techniques are applied to two types of silica glass surfaces: a "fracture surface" with high energy coordination defects and a low energy defect-free frozen "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. 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 semi-quantitative map of chemical reactivity across a surface.; The effect of glass composition on water adsorption was investigated by simulating sodium silicate, calcium silicate, sodium aluminosilicate, and calcium aluminosilicate glasses and their surfaces, and comparing them with silica. The trend of increasing reactivity from silica to sodium aluminosilicate to sodium silicate was also evident from silica to calcium aluminosilicate to calcium silicate, although the calcium containing glasses were less reactive than the sodium analogs. The effect of surface hydroxylation on the adsorption properties of glass surfaces has been examined using physisorption energy and chemical reactivity mapping techniques on the simulated glasses. Hydroxylation greatly reduced the reactivity of the most reactive surfaces, and once hydroxylated, the melt and fracture surfaces all had a similar low level of reactivity, regardless of composition. The physisorption energy distributions of bare and hydroxylated glass surfaces are compared with experimental inverse gas chromatography (IGC) energy distribution results for silica and calcium aluminosilicate glass fibers. Altogether, a new set of modeling techniques for examining water/glass interactions is presented as a means of relating the atomic scale of surface adsorption sites with macroscopic hydration and reactivity properties of glass surfaces.