Experimental and Theoretical Exploration of Liquid–Liquid Equilibrium of Ternary Mixtures (Water + Lactic Acid + Esteric Solvents) at T = 298.2 K

In this research, liquid–liquid equilibria of two ternary systems (water + lactic acid + dimethyl succinate or dimethyl phthalate) were investigated at T = 298.2 K and 101.3 kPa. Acid–base titrations and high performance liquid chromatography measurements were employed to measure the tie-line data. NRTL and UNIQUAC thermodynamic models were applied for the correlation of the measured tie-line points. The root-mean-square deviation values (less than 0.4%) prove the quality and accuracy of modeling. Optimized and validated NRTL binary interaction parameters suggested that the strongest solute–solvent attraction energy is between lactic acid and water. The NRTL equation was employed to calculate the activity coefficients, revealing that the activity coefficients of lactic acid display a negative deviation in both the aqueous and organic phases. Conversely, the activity coefficients of the solvents in both phases and water in the organic phase exhibit positive deviations. The water activity coefficients in aqueous phases are found to be near unity with minor positive and negative deviations. We found that the investigation of the obtained activity coefficients is necessary due to the presence of numerous validated results displaying implausible values. Distribution coefficients and separation factors were calculated to evaluate the extraction capability of the solvents. Dimethyl phthalate had the larger separation factors and wider biphasic area, while dimethyl succinate showed the greater distribution coefficients at the cost of smaller separation factors.