MOBILE-PHASE EFFECTS ON SINGLE-COMPONENT AND COMPETITIVE ADSORPTION-ISOTHERMS IN REVERSED-PHASE SYSTEMS

The distribution of phenol and resorcinol as the model compounds betwe en octadecyl silica adsorbents and water or aqueous methanol as the mo bile phases can be described by the Langmuir adsorption isotherms. The dependence of the coefficients of the isotherms on the composition of the mobile phase can be described by three-parameter equations over t he range 0-40% methanol in water. Various approaches for processing th e data acquired were investigated to obtain an adequate description of the two-component experimental distribution data. Best fit was obtain ed using Langmuir isotherms with the coefficients determined at a cons tant concentration ratio of the two sample compounds. For the data acq uired at different concentration ratios, the competitive Langmuir isot herm with the coefficients b determined by multi-linear regression and with forced single-component Langmuir coefficient a fits the experime ntal distribution data of the sample solutes better than the competiti ve isotherm with single-component coefficients. The agreement between the experimental distribution of sample compounds and the data calcula ted from the quadratic isotherm for more strongly retained compounds w as better than with the data calculated from the Langmuir competitive isotherm, while the opposite behaviour was observed for the earlier el uted solutes. The Le Van-Vermeulen isotherm with single-component Lang muir coefficients did not fit the data.