Analysis of immobilized artificial membrane retention factors for both neutral and ionic species

Retention data on an immobilised artificial membrane have been taken from the work of Li et al. and from Liu et al., and have been correlated with a set of descriptors that includes descriptors for ionized species, that is anions from deprotonated acids and cations from protonated bases. Log k(IAM) values can be predicted for acids or bases that are partially ionized at the experimental pH and log k(IAM) values for acids and bases can be predicted as a function of the fraction present as the ionized species, equivalent to prediction as a function of pH. It is shown that anions reduce the value of log k(IAM) by about 1.1 log units but that cations have almost no effect by comparison to the neutral species. By comparison to non-polar solvents, carboxylate anions and protonated base cations are considerably stabilized by both water and the IAM phase to about 6-8 log units and so the rather small influence of anions (-1.1 log unit) and cations (-0.1 log unit) on log k(IAM) is due to substantial cancellation of these stabilization effects. Indeed, the effect of change of phase from water to IAM on the neutral species is at least as large as the effect of change of phase on the ionic species.