Excess Gibbs free energy in methanol + acetic acid and ethanol + acetic acid

Total vapour pressures have been measured by the isoteniscope method for methanol + and ethanol + acetic acid at 308.15 and 318.15 K. Activity coefficients for the liquid phase and compositions of the vapour phase were evaluated by numerical integration of the Gibbs-Duhem equation taking into consideration the dimerization of acetic acid in the vapour phase. The mixtures are both characterized by negative deviations from ideality. The excess Gibbs free energy G^E is almost symmetric about mole fraction x = 0.5, with minimum values of -235 and -215 J mol^-1 for methanol + acetic acid and -184 and -132 J mol^-1 for ethanol + acetic acid at 308.15 and 318.15 K respectively. The negative values of G^E indicate the formation of hetero-complexes-due to intermolecular hydrogen bonds-and this effect is stronger in methanol + acetic acid than in ethanol + acetic acid.