Retention mechanisms in reversed-phase liquid chromatography. Stationary-phase bonding density and partitioning.

The partitioning model of retention for reversed-phase liquid chromatography, described by mean-field statistical thermodynamic theory, asserts that one principal driving force for solute retention is the creation of a solute-sized cavity in the stationary phase. Beyond a critical stationary phase bonding density, increased grafted chain density should result in enhanced chain ordering, which will increase the energy necessary for solute cavity formation and result in decreased chromatographic partition coefficients. We have evaluated chromatographic partition coefficients over an octadecyl bonding density range of 1.6-4.1 mumol/m2 and have found a maximum in partition coefficient at approximately 3.1 mumol/m2. Retention, however, approximately plateaus due to compensating changes in the partition coefficient and stationary phase volume. This provides unequivocal evidence that partitioning is the dominant form of retention for small nonpolar solutes.