Theoretical and experimental studies of electrostatic effects in reversed-phase liquid chromatography.
Electrostatic effects are important in many reversed-phase liquid chromatographic separations involving surfactants and ion-pair reagents. To give better understanding of such systems the Poisson-Boltzmann equation has been solved for two cases. The first is that of an infinitely long cylinder with charge density on the inner wall, and two phases in the interior, one in the center and the other around it. Salts with ions of various solubilities in each phase may be present. The solution is obtained for the low-potential case (Deltapsie / kT 1). The second case is that of a planar sandwich, wall/phase 1/phase 2/phase 1/wall, with the same chemistry as case 1 but without the low-potential restriction. In case 2, there is also a Langmuir adsorption isotherm for each ion. The theory shows that the information gained from the Stern-Gouy-Chapman theory, which is mathematically semi-infinite, only applies when the diffuse layer thickness is much smaller than the pore radius, an unlikely circumstance in chromatography. The volume-averaged electrostatic potential difference can be measured experimentally with the solutes ø(4)Si, ø(4)As(+), ø(4)B(-) (ø = phenyl). The results are in agreement with theory. They show that the electrostatics of the system is dominated by fixed charges on the silica when the salt present is insoluble in the stationary phase, but when ions with some solubility in the stationary phase are used, mixed effects are obtained.
Weber
S G
SG
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A.