The surface activity coefficients of ideal mixtures adsorbed on the heterogeneous surfaces were calculated systematically by using the 2 site heterogeneous surface model. The apparent surface activity coefficients are always <1; i.e., there are neg. deviations from Raoult's law and the results always found experimental On the other hand, the calculated surface activity coefficient curves are always nearly sym. if the total surface coverage is ~1 (>99% of the saturation value of unity). This situation is comparable with those of adsorption from solns. Therefore, the asym. curves for the experimental measured surface activity coefficients are probably not due to the heterogeneity of the surface, but rather to nonidealities in the mol. interactions between the unlike components of the adsorbed mixture1

Key words: CALCULATION, SURFACE CHEMISTRY, ACTIVITY, SURFACE PROPERTY, SURFACE TENSION, SOLID-LIQUID MIXTURE, ADSORPTIVITY