ADSORBED MIXTURES ON A HETEROGENEOUS SURFACE - THE LATTICE-GAS MODEL

A lattice gas model of mixed adsorption on a heterogeneous surface is considered. The assumption that the differences between the energies o f the components on each adsorption site are the same makes this model an ideal mixture. It is shown that mixing at constant spreading press ure in this case is equivalent to that at constant coverage. Interacti on between adsorbed molecules induces deviations from ideality, but th e excess chemical potentials of the components calculated in the rando m mixing (mean field) approximation depend linearily upon mole fractio ns, contrary to the regular bulk solutions which in the same approxima tion have a quadratic dependence on mole fractions. A new method of de scribing the experimental data is proposed in which two experimental i sotherms are fitted by two model equations with adjustable parameters under the additional constraint that some parameters in the two equati ons must be equal.