KINETIC CONTROL IN THE FORMATION OF SELF-ASSEMBLED MIXED MONOLAYERS ON PLANAR SILICA SUBSTRATES

Self-assembled mixed monolayers (mixed SAMs) were formed by competitive adsorption of an n-alkyltrichlorosilane and either n-butyl- or tert-butyltrichlorosilane on planar silica substrates. These studies were undertaken to provide information about the structures of silica-supported mixed SAMs and the processes which control their formation. Three sets of mixed monolayer series were prepared. One set consisted of four series of mixed SAMs formed from combinations of n-butyltrichlorosilane with n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilane using total adsorbate concentrations of 1 mM. Two additional sets consisted of five series each of mixed SAMs formed from tert-butyltrichlorosilane in combination with n-butyl-, n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilane. These two sets differed with regard to the total solution concentration of alkyltrichlorosilane used (1 mM versus 10 mM). Within each series, the solution concentration ratio of the competing adsorbates was varied from zero to infinity in order to produce surfaces with a range of compositions. Monolayers were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). It was found that monolayer composition, as determined primarily by thickness measurements, varied with the solution concentration ratio of the shorter- and longer-chain adsorbate in a manner predicted by both simple kinetic and thermodynamic models for monolayer formation. Surface free energies for many of the mixed SAMs were found to be higher than those of single-component SAMs, which indicates that shorter- and longer-chain adsorbates are not macroscopically phase-segregated in these systems. n-Alkyltrichlorosilanes were found to adsorb with similar efficiency regardless of chain length and with considerably greater efficiency than tert-butyltrichlorosilane. In all, the results are consistent with a model in which monolayer composition is controlled by kinetic factors associated with the relative rates of surface adsorption of alkyltrichlorosilanes.