Solute Retention in Column Liquid Chromatography. IV. Extension of the Langmuir Sorption-Isotherm Function to Include Stepped Variation of Retentions with Composition of Multicomponent Mobile Phases

Abstract

Retentions are reported for substituted phenol solutes with a reverse-phase silica sorbent and acidified aqueous mobile phases containing 0-100% v/v tetrahydrofuran over the temperature range 10[ddot]-35[ddot]C. Curvature in plots of 1/k'_(M) against ⊘ is substantial, although monotone, while those of 1/V'_R(M) against ⊘ exhibit points of inflexion. All are shown to be described to well within experimental error by expansion of the Langmuir monolayer sorption isotherm relation presented and discussed previously in Parts I-III of this Series. Account is thereby taken of multilayer formation of sorbed mobile-phase constituents; the relation is then shown to describe all six BET isotherm Types of isocratic retention data, as well as retentions under conditions of gradient elution. Fully parabolic curve shapes for retentions derived from the literature are identified in addition that are not forecast or described by any other model heretofore put forth in column liquid chromatography, yet which are shown to be fitted nearly exactly also by the extended isotherm relation.