Mobile phase effects on single-component and competitive adsorption isotherms in reversed-phase systems

The distribution of phenol and resorcinol as the model compounds between octadecyl silica adsorbents and water or aqueous methanol as the mobile 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 the range 0-40% methanol in water. Various approaches for processing the data acquired were investigated to obtain an adequate description of the two-component experimental distribution data. Best fit was obtained using Langmuir isotherms with the coefficients determined at a constant concentration ratio of the two sample compounds. For the data acquired at different concentration ratios, the competitive Langmuir isotherm with the coefficients b determined by multi-linear regression and with forced single-component Langmuir coefficient a fits the experimental distribution data of the sample solutes better than the competitive isotherm with single-component coefficients. The agreement between the experimental distribution of sample compounds and the data calculated from the quadratic isotherm for more strongly retained compounds was better than with the data calculated from the Langmuir competitive isotherm, while the opposite behaviour was observed for the earlier eluted solutes. The Le Van-Vermeulen isotherm with single-component Langmuir coefficients did not fit the data.