9803884 Pantano The goal of this project is to gain a better understanding of the molecular structure of multicomponent silicate glass surfaces, and especially, the effect of modifier-ions upon the distribution and activity of surface adsorption sites. A coordinated study of multicomponent silicate glass surfaces is proposed in which computer simulation and experimental studies will be directed toward characterization and prediction of adsorption isotherms in terms of the atomic/molecular scale surface structure. The computer simulation of the atomic structure of the silicate-based glasses is based on a novel approach to the simulation of the vitreous silica surfaces. It is assumed that the surface of pure vitreous silica should be hydrophobic. To achieve this in a computer simulation, it is necessary to bring the structural relaxation time within the time limits of computer simulation. Whereas previous computer simulations of glass have focused on the ideal surface created by abrupt removal of periodic boundary conditions with some annealing, this study will focus more specifically on the annealing of surfaces in an effort to simulate lower energy melt surfaces. A novel experimental approach will be used for determining the isotherms and energies of adsorption on glass powders, glass fibers and gels. This approach is an extension of the comparison method in which the gas-adsorption isotherm of an adsorbent (with unknown adsorption characteristics) is plotted against the standard isotherm (of a reference material). The isotherms will be compared with computer simulated isotherms to obtain (i) detailed descriptions of the atomic structure of multicomponent glass surfaces, (ii) the local electric fields associated with modifiers, non-bridging oxygens or surface silanols, (iii) the spatial distributions of modifying cations on the surface and (iv) the mechanism of water (and other reactive specie) adsorption on these heterogeneous surfaces. Static-SIM S, FTIR and related spectroscopic techniques will be employed to further validate surface composition effects and irreversible chemisorption reactions. %%% The ability to accurately simulate the glass surface and physisorption isotherms using computer methods provides a powerful tool to predict the surface thermodynamic properties for new glass compositions, and/or to interpret the adsorption behavior of existing glasses and glass products. The surface and surface chemistry of silica and silicate glasses are fundamental to many of their properties including strength, corrosion, polymer adhesion, bioreactivity, and optical response. The project is being carried-out through the collaboration of scientists simulating and studying the theory of simple gas adsorption on model amorphous surfaces with those whose focus is characterization and processing of more complex glass surfaces. ***
