Evaluation of the COSMO-SAC Model for the Prediction of Liquid–Liquid Equilibrium in the Ternary Chloroform–Methanol–Water System

Thermodynamic modeling of liquid–liquid equilibrium in multicomponent mixtures is essential for the design and optimization of separation processes, particularly when solvents with contrasting chemical characteristics are involved. The ternary chloroform–methanol–water system represents a challenging case due to the coexistence of polar, nonpolar, and associative interactions, making it an important benchmark for predictive models based on molecular fundamentals. In this work, the capability of the COSMO-SAC model to estimate the equilibrium behavior of this system at 20 °C was investigated.

The adopted methodology consisted of calculating activity coefficients using the COSMO-SAC model implemented via JCOSMO, employing molecular information obtained from quantum chemical calculations. Phase equilibrium conditions were determined by imposing the equality of chemical activities of the components in both phases, while the lever rule was applied to relate phase compositions to the overall system composition. This approach allowed the determination of the complete biphasic region, binodal curves, and a set of tie-lines, without the use of adjustable parameters.

The calculated results showed good qualitative agreement with available experimental data, successfully reproducing the overall shape of the immiscibility region and the orientation of the tie-lines. Quantitative deviations were observed in specific composition ranges, indicating limitations of the model in accurately describing certain interactions. Nevertheless, the results demonstrate that COSMO-SAC is a promising tool for the prediction of liquid–liquid equilibria in complex ternary systems, particularly for exploratory studies and solvent screening applications.