Chromatographic interfaces monolayers

Model chromatographic interfaces were examined using sum-frequency generation spectroscopy (SFG) in order to study the effect of various solvents on the structure and conformation of the stationary phase. Monolayers formed from mixed alkyltrichlorosilanes on fused silica were examined in contact with air, acetonitrile, isooctane, and water. Results show that monolayers of 100% Cig composition show little disorder in the alkyl chains for all the solvents examined, though a very distinct solvent shift is apparent in the methyl symmetric stretch. When mixed composition (Cis and Ci) monolayers were examined, significant disorder was induced in the alkyl chains for all the solvents. However, the largest change occurred when the monolayer was in contact with water. These results support a collapsed structure model for the mixed monolayer in contact with water. 

To further explore the nature of chromatographic interfaces in situ, we examine model chromatographic systems composed of 100% Cig and mixed (Cig and Ci) monolayers in several solvent environments using sum-frequency generation spectroscopy. Sum-frequency generation (SFG) spectroscopy is a powerful surface analysis tool that has proven to be invaluable in the elucidation of molecular structure at many interfaces obscured by adjacent bulk phases. The forbidden nature of the process in isotropic media within the electric dipole approximation permits this technique to be used to examine the structure and properties of the minute interfacial region between two isotropic materials. Interfaces that have been successfully explored include those at gas-liquid, liquid-liquid, liquid-solid, and gas-solid interfaces (8). 

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