Nonlinear isotherm of benzene and its derivatives by frontal analysis

Abstract

Adsorption behavior of a solute is one of the most important factors to consider when designing a batch and a continuous liquid chromatographic separation process. In liquid chromatography, this behavior is based on the adsorption equilibrium between the liquid mobile-phase and solid stationary-phase. However, most retention models have been developed under a linear adsorption isotherm: very few researchers have investigated the relationship between the adsorption parameters and the mobile phase composition, and some empirical models have been introduced. In this work, adsorption isotherms were obtained by a frontal analysis for three small molecular compounds (benzene, toluene, and chlorobenzene) on a commercial C₁₈ bonded silica column. The absorption based on the Langmuir, Freundlich, and Langmuir-Freundlich models were investigated according to changes of the composition of methanol highly enriched eluent. The calculations and analysis of the coefficients obtained for the three models confirm that the adsorption data for solutes are best modeled with the Langmuir-Freundlich isotherm. In spite of the acceptable accuracy, Langmuir and Freundlich isotherm models couldn’t satisfactorily describe the mechanism and provide objective information on the physical nature of the absorption.