The Margules equation is one of the oldest empirical expressions for liquid activity coefficients. It was derived from a polynomial expansion, with the form proposed by Redlich and Kister (1948) being the most popular form. When the expansion is truncated after the quadratic term, the resulting three parameter correlation is known as the four-suffix Margules equation.
Thus, for each binary pair in a mixture there are three parameters which are combined to determine the mixture parameters for the expansion equation. The Margules expression cannot model temperature dependence, therefore, this equation should not be used at temperatures substantially different from the range in which the parameters were fitted.
Refer "Margules Equation" under "Liquid Activity Coefficient Methods" in the PRO/II Reference Manual, Volume I, Chapter 2, for more information on this method