MD simulations explain the excess molar enthalpies in pseudo-binary mixtures of a choline chloride-based deep eutectic solvent with water or methanol

Leon de Villiers Engelbrecht; Xiaoyan Ji; Carlo Maria Carbonaro; Aatto Laaksonen; Francesca Mocci

doi:10.3389/fchem.2022.983281

MD simulations explain the excess molar enthalpies in pseudo-binary mixtures of a choline chloride-based deep eutectic solvent with water or methanol

Front Chem. 2022 Nov 14:10:983281. doi: 10.3389/fchem.2022.983281. eCollection 2022.

Authors

Leon de Villiers Engelbrecht¹, Xiaoyan Ji², Carlo Maria Carbonaro³, Aatto Laaksonen^{1

2

4

5

6}, Francesca Mocci¹

Affiliations

¹ Department of Chemical and Geological Sciences, University of Cagliari, Cagliari, Italy.
² Division of Energy Science, Energy Engineering, Luleå University of Technology, Luleå, Sweden.
³ Department of Physics, University of Cagliari, Cagliari, Italy.
⁴ Division of Physical Chemistry, Arrhenius Laboratory, Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
⁵ Center of Advanced Research in Bionanoconjugates and Biopolymers, "Petru Poni" Institute of Macromolecular Chemistry, Iasi, Romania.
⁶ State Key Laboratory of Materials-Oriented and Chemical Engineering, Nanjing Tech University, Nanjing, China.

Abstract

The addition of molecular liquid cosolvents to choline chloride (ChCl)-based deep eutectic solvents (DESs) is increasingly investigated for reducing the inherently high bulk viscosities of the latter, which represent a major obstacle for potential industrial applications. The molar enthalpy of mixing, often referred to as excess molar enthalpy H ^E-a property reflecting changes in intermolecular interactions upon mixing-of the well-known ChCl/ethylene glycol (1:2 molar ratio) DES mixed with either water or methanol was recently found to be of opposite sign at 308.15 K: Mixing of the DES with water is strongly exothermic, while methanol mixtures are endothermic over the entire mixture composition range. Knowledge of molecular-level liquid structural changes in the DES following cosolvent addition is expected to be important when selecting such "pseudo-binary" mixtures for specific applications, e.g., solvents. With the aim of understanding the reason for the different behavior of selected DES/water or methanol mixtures, we performed classical MD computer simulations to study the changes in intermolecular interactions thought to be responsible for the observed H ^E sign difference. Excess molar enthalpies computed from our simulations reproduce, for the first time, the experimental sign difference and composition dependence of the property. We performed a structural analysis of simulation configurations, revealing an intriguing difference in the interaction modes of the two cosolvents with the DES chloride anion: water molecules insert between neighboring chloride anions, forming ionic hydrogen-bonded bridges that draw the anions closer, whereas dilution of the DES with methanol results in increased interionic separation. Moreover, the simulated DES/water mixtures were found to contain extended hydrogen-bonded structures containing water-bridged chloride pair arrangements, the presence of which may have important implications for solvent applications.

Keywords: MD simulations; choline chloride; cosolvents; deep eutectic solvent; excess properties; pseudo-binary solvent mixture.