SOLVATOCHROMIC STUDIES OF SOLVATED CHROMATOGRAPHIC STATIONARY PHASES

A series of solvatochromic dyes has been used to probe the solid/solution interface of C-18 bonded silica in mobile-phase mixtures consisting of methanol and water and acetonitrile and water. Chromatographic capacity factors (k') were measured in order to determine the fraction of dye sorbed at the surface. An adaptive Kalman filter has been used to resolve the spectrum of the surface-phase dye from that due td dye in the adjacent solution. Values for the Kamlet-Taft dipolarity/ polarizability pi* parameter for octadecylsilica in contact with the methanol/water mobile phase ranged from 0.72 to 0.75, indicating that the stationary phase is considerably more polar than bulk alkane solutions. The interfacial values for pi* and a (the Kamlet-Taft hydrogen bond acidity parameter) support the hypothesis that the chemistry and solvation of the interphase region is strongly influenced by the presence of residual surface silanol groups.