Reactivity of free radicals on hydroxylated quartz surface and its implications for pathogenicity of silicas: Experimental and quantum mechanical study

We studied the adsorption of hydroxyl radicals and superoxide anion radicals on a hydroxylated α-quartz surface using cluster and periodic slab models by means of density functional calculations. Models of two hydroxylated α - quartz surfaces (0001) and (0111) - have been used in the simulations. The hydroxyl radical adsorbs readily on both surfaces. The subsurface Si-O bonds are weakened during the adsorption resulting in surface layer destabilization. This destabilization leads directly to surface disintegration in the case of•OH/(0111) adsorption. The product of the surface disintegration and reconstruction is a surface terminated by silanol groups (Si-OH) and siloxyl radicals (Si-O•). The model calculations suggest that adsorption of •OH on a hydroxylated quartz surface transforms a chemically inert, aged, silanol terminated surface to a very reactive, silicon-based radical terminated surface. The activated surface may then cause oxidative damage to the adsorbed biomaterial. The superoxide anion radical adsorbs on both surfaces, but the adsorption products are only weakly bonded to the surface. The calculated energy barrier for the O₂^•- activated subsurface Si-O bond dissociation is 10 kcal/mol, which is higher than for the •OH activated process (4 kcal/mol). The calculated weaker bonding to the surface and higher activation energy barrier suggest that the superoxide anion radical will be less efficient in reactivation of an aged, hydroxylated quartz surface than the hydroxyl radical. The importance of the specific geometry of the surface silicon atoms on the surface reactivity and adsorption properties is also discussed. The theoretical predictions are supported experimentally using chemiluminescence to monitor reactivation of the aged silica surface by superoxide anion radicals.