Influence of solute size and site-specific surface interactions on the prediction of retention in liquid chromatography using the solvation parameter model

Abstract

The solvation parameter model was used to characterize the retention properties of silica and a cyanopropylsiloxane-bonded silica sorbent in liquid–solid chromatography using hexane and various volume fractions of methyl tert-butyl ether as a mobile phase. The relative capacity of the solvated sorbent for dipole-type interactions and hydrogen-bond interactions, solute size and differences in the apparent phase ratio have to be considered to explain retention and selectivity differences for the two sorbents. Dipole-type and hydrogen-bond interactions favor retention whereas increasing solute size reduces retention for both sorbents, although the sorbent capacity and solvent dependence for these interactions are different. Solvent composition (range 10–50% v/v methyl tert-butyl ether) produces a similar trend for changes in cohesion and the solvated sorbent’s capacity for dipole-type interactions and capacity as a hydrogen-bond base, but different results for sorbent lone-pair electron and hydrogen-bond acid interactions. The quality of the model fits is excellent for the cyanopropylsiloxane-bonded sorbent but only approximate for silica (excluding nitrogen-containing bases in both cases). The poor fit for silica is probably due to the inadequacy of the characteristic volume to represent the projection of the cross-sectional area of the solute at the solvated sorbent surface and site-specific interactions of polar compounds with the heterogeneous surface sorption sites of the solvated sorbent. The anomalous behavior of the nitrogen-containing bases is possibly due to electrostatic interactions resulting from ion-exchange behavior that is not included in the model.