Confocal Raman Microscopy Investigation of the Wetting of Reversed-Phase Liquid Chromatographic Stationary Phase Particles

Wetting phenomena in reversed-phase liquid chromatographic (RPLC) stationary phase particles strongly influence the retention of solutes with high water-content mobile phases. To better understand these phenomena, this work reports the spectroscopic observation of the wetting of the interior of individual C18-silica stationary phase particles with acetonitrile−water mobile phase solutions by confocal Raman microscopy. It was found that the pores of dry C18 silica do not wet when the concentration of acetonitrile is below 0.12 mol fraction (28% by volume). It was also found that there is a wetting hysteresis, where particles that had been previously exposed to acetonitrile solutions above the wetting transition remain filled with solution even at much lower concentrations of acetonitrile in the surrounding solution. Contact angles of acetonitrile−water solutions were measured at a planar C18-modified silica surface and used to predict the capillary wetting of the particles based on the Young−Laplace equation. The solution composition at the wetting transition detected by Raman microscopy is higher in acetonitrile concentration than predicted by the Young−Laplace equation, which may be due to the presence of a vapor or air gap at the interface between the hydrophobic pores and aqueous solution. Further evidence of this behavior is found in water porosimetry results, which show wetting pressures ∼5 times greater than predicted by the Young−Laplace equation and are consistent with only 50% of a water interface being in contact with the C18 surface. This fraction increases to 80% at an acetonitrile concentration of 0.12 mol fraction, leading to spontaneous and irreversible wetting of the hydrophobic pores.