Incorporation of Phenoxy Groups in Self-Assembled Monolayers of Trichlorosilane Derivatives: Effects on Film Thickness, Wettability, and Molecular Orientation

To evaluate the possibilities for preparing monolayer films from trichlorosilane derivatives containing aromatic functional groups, model studies of monolayer-forming trichlorosilanes which incorporate a phenoxy moiety in various positions of a long alkyl chain were undertaken. Self-assembled monolayers of several such compounds could be successfully formed. The properties of these monolayers have been evaluated by utilizing eilipsometry, wettability measurements, and FTIR and compared with analogous alkyltrichlorosilanes. Monolayers prepared from 1-(trichlorosilyl)-1 1-(p-n-nonylphenoxy)undecane (1) show that the phenoxy group can be introduced into such monolayers with no loss of oleophobicity, as compared with octadecyltrichlorosilane (OTS), and with spectral features which indicate a high level of monolayer order and close packing. Linear dichroism measurements using FTIR-ATR are used to estimate the orientation of the alkyl chains and the phenoxy group in 1, and a model is proposed to explain the results. Shortening of the alkyl chain lengths in several examples led to more disordered, liquidlike monolayers. The orientation of the phenoxy group did not vary significantly with position in the alkyl chain.