Ordinarily Redlich-Kister equation and SSF equation are used for correlating excess enthalpies.Both equations are multinomials in which the number of parameters is undetermined.The equation with three parameters proposed by Liu Guojie has a fixed form

but besides three undetermined parameters

the molar volume of the solution needs to be known.A model with three parameters that does not have this demand was proposed by the present author

but the composition of solution is shown by mol fraction

so it is difficult to reflect the special effect of difference in molecular sizes.In this paper

a modified model for correlating the excess enthalpies of binary systems is developed in which the macro-composition of solution is shown with volume fraction to emphasize the special mutual effect of molecules caused by different molecular sizes.Tests with experimental data of more than 100 non-symmetrical binary systems in which the components have different molecular sizes

shapes and structures

show that for non-symmetrical binary systems the modified model is of an apparent advantage over the former one in accuracy

and is still tenable in the limiting condition of φ→0 and φ→1.