THERMODYNAMIC STUDY OF THE MARKED DIFFERENCES BETWEEN ACETONITRILE WATER AND METHANOL WATER MOBILE-PHASE SYSTEMS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY

A thermodynamic approach which considers partitioning of solute molecules between the mobile phase and stationary phase and relates the enthalpy, DELTA-H, and entropy,DELTA-S, of solute transfer to the solute partial molar excess quantities is described for the evaluation of the relative contributions of the two phases to solute retention in reversed-phase liquid chromatography (RPLC). The experimental results for DELTA-H and DELTA-S of several alkylbenzenes in methanol/water and acetonitrile/water mobile-phase systems are discussed in light of this approach. The changes in DELTA-H and DELTA-S as a function of the volume fraction of water, theta-1, are found to be significantly different in the two system. In the acetonitrile/water system, DELTA-H is fairly constant in the acetonitrile-rich region of theta-1 less-than-or-equal-to 0.45, and at theta-1 greater-than-or-equal-to 0.50, delta-H decreases with theta-1. In this system, initially, DELTA-S increases with theta-1 reaching a maximum around theta-1 = 0.50,then decreases with theta-1. In the methanol/water system, both DELTA-H and DELTA-S are found to decrease steadily which 01. The differences between tho two mobile-phase system are also revealed in the composition dependence of the total methylene group selectivity and the mobile-phase contribution to methylene group selectivity obtained from the RPLC retention data and activity coefficient data of alkylbenzenes, respectively. It appears that, while the methanol solution chemistry is governed by solvent-solvent hydrogen bonding, entropic effects arising from the solvation of solute molecules by clusters of acetonitrile govern solute retention in the acetonitrile/water system. The methylene group selectivity results support the fact that selectivity of nonpolar solutes in RPLC is controlled primarily by mobile-phase effects. l