Estimation of the reversed-phase liquid chromatographic lipophilicity parameter log k'w using ET-30 solvatochromism

The estimation of the reversed-phase liquid chromatographic lipophilicity parameter log k'w has been studied using the solvatochromic ET(30) polarity scale. Over 200 sets of chromatographic retention data were used to compare the log k'w estimates made from the volume percent of organic modifier (%) and ET(30) models of mobile phase strength. It was found that linear extrapolations of log k' versus ET(30) plots to the polarity of unmodified aqueous mobile phase gave a more reliable value of log k'w than linear regressions of log k' versus volume percent. This evaluation was based upon the relative value of the 95% confidence interval about log k'w, the point of intersection of log k' versus solvent strength plots for different modifiers, the scatter of estimations with different modifiers and the goodness-of-fit of the data to the linear model. Estimations of log k'w using the solvatochromic approach are found to give a more sensitive lipophilicity description than the volume percent approach. Recommended procedures for carrying out lipophilicity estimations with reversed-phase liquid chromatography are also discussed.