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 measur ements of the retention of model compounds. The octadecylsilylated sil ica 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 part icles are not wetted when the methanol content in the mobile phase is lower than 10% (v/v). After equilibration with water, the octadecylsil ylated silica particles remain nonwetted until the methanol content re aches 65% (v/v). The nonwetted phase showed significantly reduced chro matographic retention. A much longer equilibration time is required wh en the octadecylsilylated silica particles are not well solvated or no nwetted. The retentive behavior of the column will depend on the histo ry of mobile-phase exposure.