Noura Dawass

5 92 P ROPERTIES OF U REA -C HOLINE C HLORIDE M IXTURES 5.1. I NTRODUCTION DESs are produced by mixing quaternary ammonium salts and a hydrogen bond donor (HBD) in a ratio that results in a homogenous solvent with a melting tem- perature much lower than the melting temperatures of the constituting compo- nents [132] . DESs have many similarities with ILs such as the thermophysical behaviour, and tunability [133] . One of the most popular type of DESs is based on the ammonium salt choline chloride (ChCl), which is biodegradable, nontoxic and readily available [133] . To form a DES, ChCl can be easily mixed with vari- ous HBDs such as urea, ethylene glycol, or carboxylic acids [133] . ChCl–based DESs are increasingly considered to be a cheaper alternative to organic solvents and ILs [132, 133] . To use DESs in industrial applications, knowledge of their thermodynamic and transport properties is required. While there is an abun- dant number of publications on ILs [5, 9, 134– 138] , attention is moving towards studying DESs [132, 133, 139– 141] . In recent years, many experimental and com- putational studies focused on understanding the chemistry of DESs, providing thermodynamic and transport data [6, 11, 132, 133, 142– 146] . Molecular simulation is a powerful tool for predicting the properties of com- plex fluids such as ILs and DESs, while also studying the microscopic structure and interactions [5, 144, 147– 149] . As shown in previous chapters, the KB the- ory provides a theoretically sound framework to predict macroscopic properties from microscopic structure. When studying realistic and complex liquids, KBIs from molecular simulations can be used to: (1) predict a number of thermody- namic properties such as partial molar volumes, and derivatives of the chemical potential with respect to composition, (2) connect properties computed from molecular simulation to experimental measurements as in the case of Fick and MS diffusion coefficients (see section 1.4.3) , and (3) compute thermodynamic factors, which indicate the non-ideality of a mixture and the the affinity between the different components. KBIs of mixtures involving ILs and DESs [10, 150, 151] have beenmainly com- puted from experimental data using the inversion of the KB theory. In the lim- ited number of studies that use molecular simulation [44] , KBIs were computed by truncating KBIs of infinite systems. Truncating KBIs to a cutoff distance re- sults in poor estimates and is not physical (see chapter 2) . In general, com- puting KBIs of salt solutions, such as ILs and DESs, is not straightforward since the electroneutrality of the system has to be maintained, while KBIs are defined in an open ensemble. This theoretical disparity was discussed in section 1.4.2 of this thesis. The approach of Krüger and co–workers [74] offers a method to compute KBIs from closed systems while using open subvolumes that mimic the grand–canonical ensemble. In this approach, ions and cations can be treated as indistinguishable. In this way, KBIs of pseudo-binary mixture can be computed.