Wetting of octadecylsilylated silica in methanol-water eluents

The wetting of an octadecylsilylated silica in methanol-water mixtures was studied by optical transmittance, visual observations, and measurements of the retention of model compounds. The octadecylsilylated silica particles remain wetted as the methanol content is decreased from 100% (v/v) to 20% (v/v). With the methanol content in the range of 20% (v/v) to 10% (v/v), the octadecylsilylated silica particles are still wetted, but the degree of solvation of the C-18 chains decreases with decreasing methanol concentration. The octadecylsilylated silica particles are not wetted when the methanol content in the mobile phase is lower than 10% (v/v). After equilibration with water, the octadecylsilylated silica particles remain nonwetted until the methanol content reaches 65% (v/v). The nonwetted phase showed significantly reduced chromatographic retention. A much longer equilibration time is required when the octadecylsilylated silica particles are not well solvated or nonwetted. The retentive behavior of the column will depend on the history of mobile-phase exposure.