Retention mechanisms in reversed-phase chromatography. Stationary phase bonding density and solute selectivity.

Chromatographic selectivity for small, non-polar solutes has been determined as a function of monomeric octadecyl stationary phase bonding density over the range 1.74-4.07 mumol/m2. Phenyl or shape selectivity increases with increasing bonding density, whereas methylene selectivity remains approximately constant. These findings are in agreement with the mean field statistical thermodynamic theory of Dill, which predicts that increased stationary phase chain density should lead to increased anisotropic chain ordering and increased solute-shape selectivity. These studies provide further evidence that partitioning, not adsorption, is the dominant mode of retention for small, non-polar molecules in reversed-phase liquid chromatography.