Thermal stability of octadecylsilane monolayers on silica: Curvature and free volume effects

The thermal stability and dynamic behavior at elevated temperature (120 degreesC) of octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on fumed silicas [Aerosil 380(A380), 7 nm primary particle size, and Aerosil OX50 (OX50), 40 nm primary particle size] and nonporous silica beads (106-nm) were investigated using vibrational spectroscopy and calorimetry. In the case of well-ordered SAMs. formed by OTS adsorption on "superhydrated" silica, thermal disordering of the alkyl chains increased with increased surface curvature and thus increased free volume. A melt endotherm was observed only for the high curvature A380 fumed silica. In-situ Raman data at 120 degreesC supported these results and showed increased conformational disorder and mobility for A380 compared with OX50. The conformational structure of OTS SAMs was found to be largely reversible up to 200 degreesC, except for "superhydrated" A380. This suggests that, for close-packed SAMs, silanol condensation only occurs when the resulting Si-O-Si bond does not interfere with the hydrophobic interactions of the alkyl chains; the Si-O-Si bond distance is smaller than the van der Waals distance between alkyl chains. In the case of "superhydrated" A380, the increased free volume available due to the high surface curvature, as well as the void space left in the interfacial region after evaporation of the adsorbed water, resulted in some unfavorable cross-linking at high temperature and irreversible disordering of the chains. When triethylamine (TEA) was added to already-close-packed SAMs, in the presence of excess OTS, no further OTS adsorption occurred, and the conformational order remained high. For lower coverage fumed silica, the addition of TEA resulted in increased OTS coverage at the expense of chain order.