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

Using the reversed-phase bonded-phase HPLC packing Partisil-10 ODS-3, sorpt ion isotherms have been measured for the alcohols I-propanol (PrOH) and 1-h exanol (HexOH), and as well, a simultaneous sorption curve for the two alco hols has been measured from solutions containing a low and constant concent ration 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 measu rements or from cloud point measurements. Since sorbed alcohols are located at the ODS/solution interface, the stationary-phase effect of PrOH is mode led in terms of three processes: (i) competition for space; (ii) decrease o f space required per mole with increasing concentration of sorbed PrOH; and (iii) change of free energy of sorption with increasing concentration of s orbed PrOH. The model yields excellent fits to the isotherms and to the sim ultaneous sorption curve. Comparison of the model-fitting parameters for th e simultaneous sorption curve with those for the PrOH isotherm confirms tha t the stationary-phase effect of PrOH on HexOH is due exclusively to proces ses i and ii. Sorbed PrOH causes rearrangement of the Cls chains of the ODS phase. For volume percent PrOH less than 15% in the mobile phase, the effe ct of PrOH on sample sorption is nearly exclusively in the stationary phase . Between 15 and 30%, both mobile- and stationary-phase effects are importa nt.