Fitting competitive adsorption isotherms to the experimental distribution data in reversed-phase systems

Distribution isotherms of phenol and resorcinol and o-cresol were determined between aqueous solutions with different concentrations of methanol and an octadecyl silica adsorbent. Single-component distribution data can be well described by either the Langmuir or the Jovanovic adsorption isotherms. The competitive Langmuir isotherm with single-component isotherm coefficients can describe moderately successfully the distribution in mixtures of phenol and o-cresol in 20-40% methanol, but it does not fit the experimental distribution data of the mixtures of phenol and resorcinol, which are better described by the competitive Jovanovic and the quadratic isotherms with single-component coefficients. Generally, the quality of the fit of the isotherms improves as the concentration of methanol in the solutions increases. The dependencies of the logarithms of the Langmuir isotherm coefficients and of the coefficients of quadratic and Jovanovic isotherms on the composition of the mobile phase can be described by second-degree polynomial equations over the range 0-40% methanol in water.