Thermodynamics of adsorption of binary aqueous organic liquid mixtures on a RPLC adsorbent

The surface excess adsorption isotherms of organic solvents commonly used in RPLC with water as co-eluent or organic modifiers (methanol, ethanol, 2-propanol, acetonitrile and tetrahydrofuran) were measured on a porous silica surface derivatized with chlorotrimethylsilane (C1-silica with 3.92 micromol C1 groups per m2 of SiO2), using the dynamic minor disturbance method. The 5 microm diameter particles were packed in a 150 mm x 4.6 mm column. The isotherm data were derived from signals resulting from small perturbations of the equilibrium between the aqueous-organic solutions and the adsorbent surface. The partial molar surface area of the adsorbed components were assumed to be the same as those of the pure components. The difference sigma-sigmai* between the surface tensions of the adsorbed mixtures and that of the pure liquids was measured as a function of the organic modifier molar fraction. A simple and unique convention for the position of the Gibbs dividing surface was proposed to delimit the Gibbs's adsorbed phase and the bulk liquid phase. The activity coefficients of the organic modifiers and of water and their thermodynamic equilibrium constants between the two phases were measured. The strong non-ideal behavior of the adsorbed phase is mostly accounted for by the surface heterogeneity. Some regions of the surface (bonded -Si(CH3)3 moieties) preferentially adsorb the organic compound while the regions close to unreacted silanols preferentially adsorb water.