A Study of Alkyl Chain Conformational Changes in Self-Assembled n-Octadecyltrichlorosilane Monolayers on Fused Silica Surfaces

The adsorption and conformational changes of n-octadecyltrichlorosilane (OTS) self-assembled monolayers on fused silica surfaces are monitored by a nonlinear optical technique, sum-frequency generation (SFG) spectroscopy, and lateral force microscopy (LFM). The effect of a small amount of water in the OTS deposition solution on the alkyl chain conformation within the resulting OTS monolayers is also investigated. Results show that the alkyl chain conformational changes of OTS films on the surface of fused silica occur in three stages. The initial stage involves OTS adsorption from solution and the beginning of island formation. During this stage, the alkyl chains within the film are almost completely disordered, though a significant increase in surface coverage occurs. The second stage shows dramatic changes in alkyl chain conformation within the film from disordered gauche conformation to an ordered mainly all-trans conformation, while only a small increase in surface coverage occurs. The final stage is a much slower adsorption process. Although the surface coverage increases from 90% to a complete monolayer, only a slight increase in the SFG band intensities is observed during this stage. The early stages of OTS adsorption and the kinetic effect of water in the solvent on the resulting OTS monolayers are also studied by LFM. Island structures are observed, and the correlation of SFG, LFM, and contact angle measurements suggests that both uniform growth (monomer deposition) and island growth (aggregate deposition) occur during the OTS adsorption. The effect that water in the deposition solution has on each of these mechanisms is discussed.