Substrate effects on the formation of alkylsiloxane monolayers

Submonolayers of octadecylsiloxane (ODS) were prepared by adsorption from d ilute solutions of octadecyltrichlorosilane (OTS) onto a series of differen t substrates: mica, native silicon (Si/SiO2), and mica coated with a define d number n(SiO) of SiO2 monolayers (n(SiO) = 1, 2, 4, 6). Atomic force micr oscopy (AFM) was used to investigate the adsorption rate and the submonolay er island morphology as a function of the substrate composition. Two types of substrate effects were observed-first, an abrupt change of the shape, si ze, and height distribution of the submonolayer islands between mica and Si O2-coated mica or silicon substrates, and second, an exponential decrease o f the adsorption rate with n(SiO) up to a thickness of about 6 SiO2 monolay ers. The first effect is independent of the SiO2 film thickness and the nat ure of the underlying substrate (mica or Si) and is therefore believed to a rise from the different surface concentrations of OH groups on mica and SiO 2 surfaces. The adsorption rate decrease with n(SiO), in contrast, appears to be a long-range, bulk effect of mica and might reflect an electrostatic interaction between the negatively charged mica surface and the polar head groups of the film molecules, which accelerates the adsorption in compariso n to that of an uncharged substrate such as silicon.