Adsorption from solution occurs by solvent displacement in the interphase. When the adsorbate differs appreciably .in size from the previously adsorbed solvent, the relative size factor (x) of adsorbate and solvent determines (a) the form of the relation for free energy of adsorption and (b) the form of the configurational function in terms of fractional coverage f3 or site fraction X. Various types of isotherms that have been proposed for this situation are critically compared in relation to the statistics oft he solvent-lattice replacement process that occurs in adsorption, eg, of organic substances at electrodes. It is concluded that the correct configurational term to be used when x > 1 is I$.?-'(1 -8)=.
Using this term in an equilbrium adsorption isotherm then allows standard free energies of adsorption and derived quantities to be properly calculated from experimental coverage or surface excess measurements.
It is shown that it is strictly incorrect to refer to an isotherm containing the configurational term e/x(1 -8)las the " Flory-Huggins isotherm," since Flory-Huggins statistics leads to an isotherm having a different form for the configurational term, viz that containing eX-l, as above. For adsorption of molecules for which x > 1, failure to include the size factor term in the isotherm leads to errors in the evaluation of any interaction effects in the adsorption behaviour.