Solvent Strength Parameters and Retention Factors in Pure Water Using UNIFAC-Predicted Activity Coefficients

It is well-known in reversed-phase liquid chromatography that the solute retention factor (k) in binary eluents is well modeled as a quasi-linear function of the eluent composition (Φ) by the equation log k = log k _s/w - SΦ, where the solute parameters k_s/w and S are the retention factor in pure water and the solvent strength parameter, respectively, and Φ is the volume fraction of the organic component of the eluent. S is related to the free energy of the solute transfer from water to bulk organic liquid, which in turn is related to the infinite-dilution activity coefficients (γ^∞) of the solute in water and bulk organic liquid. We report that the γ^∞ values obtained from a γ^∞ prediction model, UNIFAC, can be used to predict S directly and also to aid in the determination of best-fit k _s/w values for acetonitrile-water mobile-phase systems. More specifically, UNIFAC-based S values combined with experimentally determined k_s/w values for a limited set of solutes can be used to predict the variations in retention as a function of mobile-phase composition in acetonitrile-water systems within the calibration range for solutes that were not in the original data set.