Positional effect of solute functional group among positional isomers in hydroxyl group–solvent and carbonyl group–solvent specific interactions in methanol–water mixed solvents monitored by high-performance liquid chromatography

We have evaluated the hydroxyl group–solvent and carbonyl group–solvent specific interactions by using mostly an Alltima C18 stationary phase and subsidiarily squalane-adsorbed C18 phase, and by measuring the retention data of carefully selected solutes in 60:40, 70:30, 80:20 and 90:10 (%, v/v) methanol–water eluents at 25, 30, 35, 40, 45 and 50°C. The selected solutes are four positional isomers of phenylbutanol, 5-phenyl-1-pentanol, three positional isomers of alkylarylketone derived from butylbenzene, and 1-phenyl-2-hexanone. The magnitudes of carbonyl group–solvent specific interaction enthalpies are larger than those of hydroxyl group–solvent specific interaction enthalpies in general. We observed clear discrepancies in functional group–solvent specific interactions among positional isomers. The spatial accessibility of the functional group by the solvent molecules seems to govern the strength of interaction. The relationships between molecular structures and functional group accessibilities have been discussed. The specific functional group–mobile phase interactions obtained by the Alltima C18 stationary phase were systematically different from those obtained by the squalane-impregnated C18 stationary phase, which may be due to structural differences between the two phases.