Differential scanning calorimetry determination of phase diagrams and water activities of aqueous carboxylic acid solutions

We have investigated the solid-liquid equilibria phase diagrams and water activities of the several carboxylic acid/water binary systems using differential scanning calorimetry (DSC) method. These mixtures were formed by water and low molecular weight carboxylic acids (formic acid, acetic acid, propionic acid and butyric acid). Water activities of the investigated solutions were obtained from the freezing point depression data in the range of the ice melting envelope. The experimental results indicated that all the aqueous carboxylic acid systems present a eutectic point in their respective phase diagrams. The activity coefficients of two components in the liquid phase were used to adjust the parameters of the Wilson and NRTL models. For all of the investigated systems, the best correlations of the solid-liquid equilibrium data have been determined by using the NRTL model. Finally, the phase behaviors of the systems were compared to the ideal state and the influence of the chemical structure of the carboxylic acids on the magnitude of the deviation from the ideal state was discussed.