The effect of stationary-phase pore size on retention behavior in micellarliquid chromatography

One of the limitations that has restricted the applicability of micellar Li quid chromatography (MLC) is the weak eluting power of micellar mobile phas es compared to conventional hydro-organic mobile phases used in reversed-ph ase liquid chromatography. This may be the result of Donnan or steric exclu sion of the micelles from the pores of the stationary phase, within which n early all (greater than or equal to 99%) of the stationary phase resides an d the analytes spend most of their time. To determine whether wide-pore sta tionary phases would overcome this limitation in MLC, several C8 and C18 st ationary phases ranging from 100 to 4000 Angstrom were investigated using a diverse set of test solutes and micellar solutions of anionic, neutral, an d cationic surfactants as mobile phases. With the larger pore size stationa ry phases, the eluting power of the MLC mobile phases was enhanced with all surfactant types, the greatest effect being with the neutral surfactant. D ifferences in retention behavior were observed between various solute types and between the C8 and C18 stationary phases. These differences appear to be related to the relative hydrophobicity of the solutes and to differences in the surfactant-modified stationary phases. Partitioning behavior of rep resentative solutes on the large-pore C8 and C18 columns was shown to follo w the three-phase partitioning model for MLC. Methylene group selectivity d ata showed only minor differences in the stationary-phase characteristics b etween the small- and large-pore size C18 columns. The true eluting power o f micellar mobile phases was revealed with wide-pore stationary phases and was demonstrated by the separation and elution of an extended series of alk ylphenones on C18 columns.