Selectivity of Brij-35 in Micellar Liquid Chromatographic Separation of Positional Isomers

Implementation of Brij-35, a nonionic surfactant, as a mobile phase for separation of positional isomers is investigated. Chromolith C-18 SpeedROD is used as a stationary phase. The effect of surfactant and organic modifier (propanol) concentration on the separation of some selected isomers is studied and evaluated in terms of linear solvation energy relationship (LSER). Shape selectivity is assessed by α value of sorbic and benzoic acid, which is found to be 1.339 by using mobile phase composed of 0.5% aqueous solutions of Brij-35 and propanol in 9？:？1. Isomers of parabens, nitroanilines, nitrophenols, and quinolinols are successfully separated using mobile phases composed of various percentages of surfactant and propanol. System constants for nonionic MLC using LSER analysis show that hydrogen bond basicity and dipolarity may be major contributors to selectivity, while excess molar refraction helps fine-tuning the separation which also imparts unique selectivity to nonionic surfactants as compared to ionic ones. 1. Introduction Use of surfactants as mobile phases in HPLC above critical micellar concentration (cmc) gave birth to a new branch of chromatography, now known as micellar liquid chromatography (MLC) [1]. Since its first use for separation of PAHs in 1980 [2], a number of publications, books, and monographs appeared in the literature covering the？applications and characterization of MLC systems [3–5]. The most commonly used surfactant in liquid chromatography and in overall analytical chemistry is sodium dodecylsulfate (SDS) [6], an anionic surfactant as compared to other cationic and nonionic surfactants [7]. In nonionic surfactants, Brij-35 (polyoxyethylene dodecyl ether) has found applications in liquid chromatography as it is non-UV absorbing, which is a drawback of other nonionic surfactants having aromatic ring into their structure. Brij-35 is reported as mobile phase for the separation of a number of compounds [7]. In a previous report [8] by our group, we reported that isomers of propyl-parabens could be better separated by Brij-35 as compared to SDS. Takayanagi et al. [9, 10] have also reported the separation of various positional isomers using Brij-35 as a surfactant in capillary electrophoresis. Vlasenko et al. [11] have studied the dissociation constants of hydroxybenzoic acids and parabens and found that addition of Brij-35 alters the dissociation of these compounds. Inspite of the number of publications on MLC using non-ionic surfactants as mobile phases, none of them is specifically focused on separation of isomers. Separation