ROLE OF SOLVENT ON THE SILANIZATION OF GLASS WITH OCTADECYLTRICHLOROSILANE

The relative surface concentrations of octadecyltrichlorosilane (OTS) deposited on glass substrates in the presence of 11 different solvents were evaluated by X-ray photoelectron spectroscopy, with supporting e vidence from contact angle measurements. The density of the observed O TS films cannot be explained entirely on the geometrical similarities or differences between the solvent and the octadecyl moiety, as it was observed that n-pentane produces an OTS film much sparser than that o f hexadecane or toluene. A new mechanism involving extraction of surfa ce moisture into the bulk solvent, followed by OTS hydrolysis and subs equent deposition, is proposed. Aromatic solvents (benzene and toluene ) which are capable of extracting significant amounts of water from th e substrate surface yield the densest OTS films, indicating a preferen ce for OTS hydrolysis in the bulk solvent phase rather than at the sub strate surface. A novel protocol was developed to quantitate the surfa ce water extracting capacities of pentane and toluene. Dehydrated sili ca powder was rehydrated with D2O, followed by extraction with solvent , and the dissolved heavy water was trapped by treatment with phenylli thium to form deuterobenzene. The ratio of mass 79 (deuterobenzene) to mass 78 (benzene), determined by GC-MS, enables the quantitation of e xtracted D2O. The results show that toluene can extract significant am ounts of D2O from the silica surface, whereas n-pentane is much less e ffective. OTS dissolved in anhydrous toluene only reacts with the surf ace silanol groups, producing a low density film. However, the same to luene, when spiked with a small quantity of D2O, forms an OTS film nea rly twice as dense as that obtained under anhydrous conditions. The op timum quantity of moisture for the formation of a closely-packed monol ayer is about 0.15 mg/100 mL of solvent.