CORRELATION OF MOLECULAR-ORGANIZATION AND SUBSTRATE WETTABILITY IN THE SELF-ASSEMBLY OF N-ALKYLSILOXANE MONOLAYERS

Monolayers of n-octadecylsiloxane (CH3(CH2)(17)SiOxHy;ODS) were self-assembled from n-octadecyltrichlorosilane solutions onto a series of OH- and CH3-containing surfaces prepared from the self-assembly of controlled composition mixtures of HO(CH2)(16)SH and H3C(CH2)(15)SH on gold (RS/Au). Using null ellipsometry, infrared spectroscopy, and hexadecane contact angles; the coverages, chain structures, and surface wetting of the formed ODS assemblies were determined as a function of the OH fraction, f(OH) = [OH]/[CH3 + OH], in the starting RS assembly. Three distinct ODS adsorption regimes were observed: (1) on pure CH3 surfaces no stable adsorbed layer forms; (2) for 0.1 less than or similar to f(OH) less than or similar to 0.8, the coverage is incomplete and monotonically increases with f(OH) and the ODS structures consist of a range of coexisting domains of nearly all-trans chains and disordered, liquid-like components with maximum disorder content, estimated as > 80%, arising near f(OH) similar to 0.5; and (3) for f(OH) > 0.8, a high coverage, close-packed monolayer is formed with predominantly all-trans chains tilted at 8-12 degrees from the surface normal, a distinctly different tilt than the known value of 26-30 degrees for the RS underlayer and an indication of strong structural decoupling (incommensurability) between the two highly organized layers. The f(OH)-dependence of the structures is explained on the basis of a previously proposed hypothesis that a continuous preadsorbed, substrate-bound water film is required for achieving maximum organization during n-alkylsiloxane self-assembly and that, in the present case of the OH/CH3 surfaces, the required water film structure at the preparation solution/substrate interface is not reached until high f(OH) values.