Measurement and PC-SAFT modeling of solid-liquid equilibrium of deep eutectic solvents of quaternary ammonium chlorides and carboxylic acids

In this study the solid-liquid equilibria (SLE) of 15 binary mixtures composed of one of three different symmetrical quaternary ammonium chlorides and one of five different fatty acids were measured. The experimental data obtained showed extreme negative deviations to ideality causing large melting-temperature depressions (up to 300 K) that are characteristic for deep eutectic systems. The experimental data revealed that cross-interactions between quaternary ammonium salt and fatty acid increase with increasing alkyl chain length of the quaternary ammonium chloride and with increasing chain length of the carboxylic acid. The pronounced decrease of melting temperatures in these deep eutectic systems is mainly caused by strong hydrogen-bonding interactions, and thermodynamic modeling required an approach that takes hydrogen bonding into account. Thus, the measured phase diagrams were modeled with perturbed-chain statistical associating fluid theory based on the classical molecular homonuclear approach. The model showed very good agreement with the experimental data using a semi-predictive modeling approach, in which binary interaction parameters between quaternary ammonium chloride and carboxylic acid correlated with chain length of the components. This supports the experimental findings on the phase behavior and interactions present in these systems and it allows estimating eutectic points of such highly non-ideal mixtures. This work was developed in the scope of the project CICECO e Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2013) and LSRE-LCM, POCI-01-0145- FEDER-006984jUID/EQU/50020/2013, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. M.A.R.M acknowledges FCT for her PhD grant (SFRH/BD/87084/2012). FCT is also acknowledged for funding the project DeepBiorefinery (PTDC/AGRTEC/ 1191/2014). P.V.A.P., G.J.M., M.D.H. and E.A.C.B thank the national funding agencies CNPq (National Council for Scientific and Technological Development) (305870/2014-9, 309780/2014, 406856/2013-3), FAPESP (Research Support Foundation of the State of S~ao Paulo) (2014/21252-0, 2016/08566-1), FAEPEX/UNICAMP (Fund for Research, Teaching, and Extension) (0125/16) and CAPES (Coordination of Improvement of Higher Level Personnel) for financial support and scholarships. E.A.C thanks Erasmusþ program of the European Union for co-funding. info:eu-repo/semantics/publishedVersion