New approaches to prepare hydride silica.

Two synthetic schemes to produce a hydride-modified support that serves as an intermediate for the preparation of bonded phases for liquid chromatography (LC) and capillary electrophoresis (CE) are investigated. The strategies differ in the silane reagent utilized (trichlorosilane (TCS) or triethoxysilane (TES)) and the manner water is incorporated into the reaction. In the first approach, TCS in toluene reacts with a previously humidified silica substrate so that the reaction is confined to the silica surface. In the second approach, TES and a small amount of aqueous HCl are dissolved in THF, and this hydrolysate is diluted by a great factor in cyclohexane, prior to reaction with the silica substrate. Atomic force microscopy (AFM) images of the hydride film on wafers revealed that, unlike the traditional approach that produced a patchy coating, both new methods provided a homogeneous layer on the substrate's surface. IR and NMR spectra from porous silica particles clearly confirmed a successful surface modification. AFM and water contact angles (WCA) were used to examine the effect of dilution of the TES hydrolysate in cyclohexane on the trend of the film to polymerize on wafers and found that a dilution factor of at least 100 is required to attain a molecularly thin hydride layer. WCA and CE also revealed a strong susceptibility of the hydride silica intermediate to hydrolyze, even at low pH. Compared to TCS, the lower reactivity and volatility of TES resulted in a much more desirable experimental approach.