PC-SAFT Modeling of Solid-Liquid Equilibrium of Deep Eutectic Solvents

The concept of deep eutectic solvent (DES) was first described by Abbott and co-workers in 2003 as eutectic mixtures of two (or more) compounds with an eutectic temperature far below the melting temperatures of the pure... [ view full abstract ]

The concept of deep eutectic solvent (DES) was first described by Abbott and co-workers in 2003 as eutectic mixtures of two (or more) compounds with an eutectic temperature far below the melting temperatures of the pure compounds. Over the last decade, DESs have attracted considerable attention from researchers as green solvent alternatives to conventional solvents, capable of overcoming some of the limitations of ionic liquids (ILs). Based on a broader database and on a deeper understanding of the common novel properties, many DESs have been prepared and applied to various areas of chemistry and chemical engineering. However, despite the important information it provides on the operation window (temperature and composition ranges) as well as the donor-acceptor interactions, the phase equilibrium behind the formation of a DES is still a poorly studied topic. 

In this work, the solid-liquid equilibria (SLE) of different binary mixtures (most of them composed of quaternary ammonium salts and fatty acids) were measured. The experimental data obtained showed strong negative deviations to ideal-mixture behavior causing large melting-temperature depressions up to 300 K, which is characteristic for DESs. The data shows that cross-interactions between the quaternary ammonium salts and the fatty acid increase with increasing alkyl chain length of the quaternary ammonium salts and with increasing chain length of the fatty acid. The perturbed-chain statistical associating theory (PC-SAFT), which is able to explicitly account for the association phenomenon, was then used to model the measured phase diagrams. The PC-SAFT results showed a very good agreement with the experimental data using a semi-predictive modeling approach that correlates binary interaction parameters with the chain length of the DES constituents. The modelling results supported the experimental findings on the phase behavior and on interactions present in these systems and allowed for the estimation of the eutectic points (temperature and composition) of these highly non-ideal mixtures.