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 temper ature on the conformational order of polymeric and monomeric octadecylsilan e stationary phases. Spectral data in the v(C-C) and v(C-H) regions are int erpreted in terms of alkyl chain conformational state and its dependence on temperature. In contrast to the liquidlike disordered state characteristic of these stationary phases at room temperature, at liquid N-2 temperatures , the alkyl chains exist in a more ordered state with a residual level of g auche conformational defects. Systematic studies between -15 and 95 degrees C reveal more subtle changes in conformational order as ascertained from e mpirical spectral indicators including the intensity ratios I[v(a)(CH2)]/I[ v(s)(CH2)] and I[v(C-C)(Tau)]/I[v(C-C)(G)]. Plots of these ratios as a func tion of temperature reveal two distinct regimes of behavior. By extrapolati ng the linear regions of these plots, a surface "phase transition" temperat ure of similar to 20 degrees C for both surface-confined octadecylsilane st ationary phases is estimated that represents subtle changes in alkyl chain conformational order from a more ordered phase to a slightly more disordere d phase. The similarity in behavior between the polymeric and monomeric oct adecylsilane stationary phases is interpreted as evidence for similar inter chain spacing of the alkylsilanes on these silica surfaces.