Effect of Pressurized Solvent Environments on the Alkyl Chain Order of Octadecylsilane Stationary Phases by Raman Spectroscopy

The effect of pressure on solute retention in HPLC has been controversial, since direct evidence for structural changes in the stationary phase due to pressure is not readily available. In this report, the effect of pressurization in four solvent environments on the conformational order of one monomeric and one polymeric octadecylsilane stationary phase is investigated using Raman spectroscopy. Normalized changes in a Raman spectral indicator of conformational order after exposure to these pressurized solvent environments are compared to those observed by exposure to the same solvent at atmospheric pressure. Although solvation has a greater impact on conformational order than pressurization, measurable changes induced by pressure are observed for both stationary phases. Small effects (<2–3% change in order) due to pressure are noted for the monomeric stationary phase in all solvents; the magnitudes of these effects generally correlate with isothermal compressibility of the solvent. Slightly larger pressure effects are observed for the polymeric stationary phase (4–7% change in order), with pressurization in polar solvents inducing greater changes in order than in nonpolar solvents. Collectively, these results are interpreted in terms of differences in the local surface bonding density and interchain spacing of the alkylsilanes of these two stationary phases.

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