In Situ Examination of the Structure of Model Reversed-Phase Chromatographic Interfaces by Sum-Frequency Generation Spectroscopy

Model reversed-phase chromatographic interfacial systems were examined using sum-frequency generation (SFG) spectroscopy to study the effect of solvent on the structure and conformation of the stationary phase. Monolayers formed from mixed C18 and C1 alkysilanes (both polymeric octadecyltrichlorosilane-methyl trichlorosilane mixed monolayers and monomeric octadecyldimethylchlorosilane-trimethylchlorosilane mixed monolayers) on fused silica were examined in contact with air, deuterated acetonitrile (CD3CN), D2O, and mixtures of CD3CN and D2O. When mixed composition (C18 and C1) monolayers were examined, significant disorder was observed in the alkyl chains for all solvents examined at intermediate alkyl chain densities for the polymeric materials, and at all chain densities for the monomeric materials. Maximum order for intermediate polymeric chain densities was found to occur in contact with 40−50 vol % D2O solutions. These results indicate that solvent microheterogeneity has a large effect on alkyl chain order.