SOLVATOCHROMIC STUDIES OF SOLVATED CHROMATOGRAPHIC STATIONARY PHASES

A series of solvatochromic dyes has been used to probe the solid/solut ion interface of C-18 bonded silica in mobile-phase mixtures consistin g of methanol and water and acetonitrile and water. Chromatographic ca pacity factors (k') were measured in order to determine the fraction o f dye sorbed at the surface. An adaptive Kalman filter has been used t o resolve the spectrum of the surface-phase dye from that due td dye i n the adjacent solution. Values for the Kamlet-Taft dipolarity/ polari zability pi parameter for octadecylsilica in contact with the methano l/water mobile phase ranged from 0.72 to 0.75, indicating that the sta tionary phase is considerably more polar than bulk alkane solutions. T he interfacial values for pi and a (the Kamlet-Taft hydrogen bond aci dity parameter) support the hypothesis that the chemistry and solvatio n of the interphase region is strongly influenced by the presence of r esidual surface silanol groups.