The mechanism of solute retention in reversed-phase liquid chromatography

Theory is developed to describe the retention of small molecules by the grafted chain phases used in reversed-phase liquid chromatography. The affinity of the solute for the grafted phase is determined by the entropy of mixing of the solute, the configurational entropy of the grafted chains, and the contact interactions among solute, solvent, and chains, treated here in the random mixing approximation for single or binary eluents. Retention can be predicted from simple physical quantities, such as the oil-water partition coefficient, but generally not from the solvent surface tension alone, as proposed in the "solvophobic theory". These grafted chains are semiordered at the interface; therefore, (i) solutes should partition less into the grafted phase than into amorphous oillike phases, and (ii) hydrophobic solutes should concentrate nearer the free than the grafted ends. It is shown that retention is principally due to solute partitioning into, rather than adsorption onto, the grafted chains.