Liquid Chromatography of Macromolecules Under Limiting Conditions of Solubility (LC LCS): A Mechanistic Study

Abstract

A coupled liquid chromatographic technique combining an enthalpic contribution, precipitation, and the entropic size exclusion effect is reported. Liquid chromatography under limiting conditions of solubility (LC LCS) results in the elution of the polymer solute on the front shoulder of the injection zone. In the experiments carried out herein 70/30–65/35 wt.% THF/n-hexane mixtures were found to provide a retention independent of molar masses for poly(methyl methacrylate)s up to molar mass of approximately 1000 K daltons. This far exceeds the feasible range of liquid chromatography at the critical adsorption point. The effects of the mobile phase, composition and further flow rate, injected concentration, and temperature, were investigated. The polymer retention volumes were found to be dependent on the eluent composition and temperature and to a lesser extent also on injected polymer concentration and eluent flow rate. At high molar masses and high injected concentrations, the system can be overloaded, shifting the balance in favor of precipitation, while at low molar masses and flow rates the macromolecules have time to equilibrate and the balance shifts in favor of solubility. In LC LCS the enthalpic effects leading to adsorption and partition may be sufficiently strong to result in large retention of high molar mass compounds on the stationary phase, implying that the choice of the column packing is critical.