(470c) Effect of Polar Environments on the Aluminum Oxide Shell Surrounding Aluminum Particles:

Density functional theory (DFT) calculations were performed to understand molecular alterations of a polar environment on an alumina surface. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. Recent studies have shown a link between the solvation media used for Al powder intermixing and the reactivity of Al with fluorine containing reactive mixtures. Flame speeds show a threefold increase when polar liquids are used as the carrier fluid when intermixing aluminum and fluoropolymer powder mixtures. It was hypothesized that the alumina lattice structure could be transformed due to the hydrogen bonding forces exerted by these solvents that induce modified bond distances and charges that influence reactivity. In this study, the alumina surface was analyzed by using DFT calculations and model clusters as isolated systems embedded in polar environments (acetone and water). Five defect models for specific active â??OH sites were investigated in terms of structures and vibrational â??OH stretching frequencies. The observed changes of the surface OH sites invoked by the polar environment were compared to the bare surface. The calculations revealed a strong connection between the impact of carrier fluid polarity on the hydrogen bonding forces between the surface OH sites and surrounding species. Changes were observed in the OH characteristic properties such as OH distances (increase), atomic charges (increase), OH stretching frequencies (decrease); and these consequently improve OH surface reactivity. The difference between medium (acetone) and strong (water) polar environments was minimal.