Stationary-phase contribution of 1-propanol organic modifier to changes in sorption of 1-hexanol on an ODS-bonded phase

Using the reversed-phase bonded-phase HPLC packing Partisil-10 ODS-3, sorption isotherms have been measured for the alcohols 1-propanol (PrOH) and 1-hexanol (HexOH), and as well, a simultaneous sorption curve for the two alcohols has been measured from solutions containing a low and constant concentration of HexOH as sample with increasing concentrations of PrOH as organic modifier. The mobile-phase effect of PrOH is quantified by solution-phase activity coefficients obtained either from vapor/solution equilibrium measurements or from cloud point measurements. Since sorbed alcohols are located at the ODS/solution interface, the stationary-phase effect of PrOH is modeled in terms of three processes: (i) competition for space; (ii) decrease of space required per mole with increasing concentration of sorbed PrOH; and (iii) change of free energy of sorption with increasing concentration of sorbed PrOH. The model yields excellent fits to the isotherms and to the simultaneous sorption curve. Comparison of the model-fitting parameters for the simultaneous sorption curve with those for the PrOH isotherm confirms that the stationary-phase effect of PrOH on HexOH is due exclusively to processes i and ii. Sorbed PrOH causes rearrangement of the C 18 chains of the ODS phase. For volume percent PrOH less than 15% in the mobile phase, the effect of PrOH on sample sorption is nearly exclusively in the stationary phase. Between 15 and 30%, both mobile- and stationary-phase effects are important.