Alkyl chain conformation of octadecylsilane stationary phases by Raman spectroscopy. 1. Temperature dependence

Raman spectroscopy is used for the first time to probe the effect of temperature on the conformational order of polymeric and monomeric octadecylsilane stationary phases. Spectral data in the v(C-C) and v(C-H) regions are interpreted in terms of alkyl chain conformational state and its dependence on temperature. In contrast to the liquid-like disordered state characteristic of these stationary phases at room temperature, at liquid N₂ temperatures, the alkyl chains exist in a more ordered state with a residual level of gauche conformational defects. Systematic studies between -15 and 95 °C reveal more subtle changes in conformational order as ascertained from empirical spectral indicators including the intensity ratios I[v(a)(CH₂)]/I[v(s)(CH₂)] and I[v(C-C)(T)]/I[v(C-C)(G)]. Plots of these ratios as a function of temperature reveal two distinct regimes of behavior. By extrapolating the linear regions of these plots, a surface 'phase transition' temperature of ~20 °C for both surface-confined octadecylsilane stationary phases is estimated that represents subtle changes in alkyl chain conformational order from a more ordered phase to a slightly more disordered phase. The similarity in behavior between the polymeric and monomeric octadecylsilane stationary phases is interpreted as evidence for similar interchain spacing of the alkylsilanes on these silica surfaces.