Experimental and Computational Studies of Choline Chloride-Based Deep Eutectic Solvents

Choline-chloride based deep eutectic solvents (DES) have been used for several different applications (e.g., solubility, electrochemistry, and purifications) due to their relative inexpensive and readily available nature. In this work, three choline chloride-based DESs are simulated using molecular dynamics to study the hydrogen bonding interactions of the system. Three hydrogen bond donors (HBD) are studied in order to determine the changes in the hydrogen bonding interactions when the HBD is different in the DES. One dicarboxylic acid and two polyols (with different number of OH groups) were chosen as the HBDs of interest. First, the simulations are validated by comparing simulated and experimental thermodynamic and transport properties, when possible. Then, for maline (choline chloride/malonic acid), the more anomalous system studied here, molecular simulations complement results obtained from an FTIR spectroscopic study in order to further understand this unique system. Good agreement with experimental values was obtained for simulated density, heat capacity, and transport properties. A high relative percent of hydrogen bonding is observed for interactions between the anion and the HBD for the three main systems studied here, consistent with the nature of how these moieties interact in DESs. Comparison is also done with a previous DES studied in our group. From the infrared spectroscopic study conducted on maline films, band assignments were discussed highlighting a "free" carbonyl group of the carboxylic acid group in the eutectic mixture when the OH group is hydrogen bonded to something else. Additionally, a band is assigned to a hydrogen bonded carbonyl group. These band assignments are consistent with findings in the molecular simulations and highlight the predominant interactions of the system.