Micellar versus hydro-organic reversed-phase liquid chromatography: A solvation parameter-based perspective

The performance of the solvation parameter model is examined for micellar liquid chromatography. The results are compared with those offered with hydro-organic eluents, intending to reveal the properties that influence the retention and distinguish the particular behaviour of micellar systems. The retention data of several series of non-ionisable and ionisable compounds (mainly steroids, polyaromatic hydrocarbons, phenols, sulfonamides, β-blockers, phenethylamines, antihistamines, and diuretics) were used as probe compounds. The micellar mobile phases contained an anionic (sodium dodecyl sulphate), non-ionic (Brij-35), or cationic (cetyltrimethylamonium bromide) surfactant, with or without the addition of an organic solvent (either propanol, butanol, pentanol or acetonitrile). In some instances (steroids, sulfonamides, β-blockers and diuretics), the processed data were obtained in both micellar and hydro-organic chromatographic modes using the same column. Accuracy in predictions is critically examined and a correction term that takes into account contributions not considered in the original Abraham model, such as electrostatic or steric ones, is suggested to improve the correlations. The proposed correction takes into account differences between the descriptors of ionic and neutral species. The case of compounds showing different degrees of ionisation is discussed. Three solvation parameter approaches that allow the prediction of retention at varying mobile phase composition are proposed, which also revealed differences between the micellar and hydro-organic modes.