Influence of the deposition process on the structure of grafted alkylsilane layers

Monofunctional alkylsilanes, CH3(CH2)(n-1)Si(CH3)(2)R with n varying from 4 to 30 and the hydrolyzable group R = Cl or N(CH3)(2), were deposited on quartz slides and on nanometric silica. Due to the presence of only one hydrolyzable group for these alkylsilanes, silane monolayers result from the grafting process. Two methods for silanization were considered: deposition from a solvent (derived from Sagiv's process) and vapor phase deposition (derived from Kovats's process). In vapor phase deposition, the physisorbed water is mainly removed from the surface during the conditioning of the specimens, whereas this is not the case for the solvent method. In addition, the two deposition processes do not take place at the same temperature: the silane needs to be heated under vacuum for the vapor deposition method, whereas the reaction takes place at room temperature for the solvent method. Characterizations of the grafting efficiency and of the structure of the grafted layers were performed with different techniques including contact angle measurements, cross polarization magic angle spinning Si-29 NMR, and gas chromatography. The comparison between the two methods of deposition showed that the grafting process has a great influence on the resulting monolayers. By considering the wetting behavior, we can evidence that silane monolayers deposited from a toluene solution are less well-ordered than those resulting from the vapor phase method. In fact, the choice of the solvent for deposition is crucial. This effect is due to the competition between the solvent and the silane molecules to reach the silica surface. This competition, which infuences the packing density of the grafted layer, is dependent on the polarity and the shape of the solvent molecules and on the deposition process.