Physicochemical properties of choline chloride-based deep eutectic solvents and excess properties of their pseudo-binary mixtures with 1-butanol

Abstract

Ionic liquids and deep eutectic solvents have aroused interest as alternative solvents for separation processes due to their favourable properties. For instance, deep eutectic solvents are usually biodegradable, non-toxic, low cost and also share some properties with ionic liquids, such as their tuneability to be used in specific applications. Recently, deep eutectic solvents have been studied more extensively but their physicochemical characterization is still in the initial stages if the final objective is to select alternatives for industrial processes. Decomposition temperature, surface tension, density, and viscosity were measured for deep eutectic solvents based on choline chloride as the hydrogen bond acceptor and levulinic acid, ethylene glycol and phenol as hydrogen bond donors (in a 1:2 mol ratio). Density, viscosity and excess enthalpy were measured for pseudo-binary mixtures of the different deep eutectic solvents with 1-butanol. The excess properties were calculated to understand the behaviour of the deep eutectic solvents in the mixture. The results suggest that deep eutectic solvents are formed due to strong intermolecular forces between their constituents, with no reaction observed. This generates a higher thermal stability of the deep eutectic solvents compared to their constituent hydrogen bond donor. Also, with the addition of 1-butanol to the deep eutectic solvents, the mixture has a lower volume compared with the ideal case due to the creation of new networks of hydrogen bonds and interstitial site accommodations. Hence, the excess molar volumes and excess enthalpies have a negative deviation from the ideal behaviour. The viscosity deviation is related to the Arrhenius equation observing a negative deviation for the deep eutectic solvents containing levulinic acid and phenol, and a positive deviation for the one with ethylene glycol. Experimental data for the different properties allows a qualitative understanding of the magnitude of the interactions that occur when mixing deep eutectic solvents with 1-butanol.