Effect of stationary-phase sorption of organic modifier from a water-rich mobile phase on solute retention by an ODS bonded phase

Sorption isotherms for l-butanol (BuOH) and eucalyptol (Eu) on Partisil 10 ODS-3 are plotted as concentration sorbed versus activity in aqueous soluti on. The former follows the Langmuir equation and the latter follows the equ ation for an "associative bilayer isotherm. with limited solubility in the aqueous phase". Two series of simultaneous sorption experiments of BuOH and Eu were also performed. One series involved measuring the influence of BuO H, as an organic modifier solvent in water, on the sorption of Eu as solute . In the second series, the roles of BuOH and Eu were reversed. These studi es show that when sorption is treated as partitioning between two bulk liqu id phases, the stationary phase behaves as an ideal solution which is compo sed of C-18 chains, sorbed organic modifier, and sorbed solute. Combined wi th the Langmuir isotherm behavior of BuOH, this ideal solution behavior sup ports the view that BuOH is sorbed at the stationary-phase/mobile-phase int erface, with its hydrocarbon tail in the stationary phase.