A simple thermodynamic model used to describe the excess Gibbs free energy of a liquid mixture. It uses activity coefficients that are a measure of the deviation from ideality of solubility of a compound in a liquid. See raoult’s law. In the case of a binary mixture, the excess Gibbs free energy is expressed as a power series of the mole fraction in which the constants are regressed with experimental data. The activity coefficients are found by differentiation of the equation. Unlike other (p. 230) models such as UNIQAC and NRTL, the model has the ability to predict a wide range of reliable thermodynamic values for the coefficients. It is named after Austrian scientist Max Margules (1856–1920) who developed it in 1895.