An Experimental and Computational Study of Choline Chloride-based Deep Eutectic Solvents

Open Access
Author:
Perkins, Sasha Lianna
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 21, 2013
Committee Members:
  • Coray M Colina, Thesis Advisor
  • Paul C Painter, Thesis Advisor
Keywords:
  • Deep eutectic solvents
  • ionic liquids
  • choline chloride
  • fourier transform infrared spectroscopy
  • hydrogen bonding
Abstract:
Deep eutectic solvents (DES), considered readily available and relatively inexpensive ionic liquid (IL) analogues, show potential for many material science applications over conventional ILs. Molecular dynamics (MD) simulations have been performed on four different DES to provide atomistic insight into the hydrogen bonding interactions of these systems. The simulations showed excellent agreement with the available experimental values for the density, volume expansion coefficients, and heat capacities showing proper validation for the modified force field used. Atom-atom and center of mass (COM) radial distribution functions (RDF) are discussed showing contact distances for possible hydrogen bonding interactions. A thorough hydrogen bond analysis was performed on each system to determine the relative “mobility” of the moieties in the system and relative fraction of hydrogen bonding types in the system. The four systems showed either dominant anion-HBD and HBD-HBD interactions or anion-HBD and cation-anion interactions. Experimental infrared (IR) spectra are also reported for choline chloride-urea and choline chloride-malonic acid mixtures with a discussion on band assignments. The spectra of the mixtures are compared to the spectra of the pure constituents in order to see changes in the hydrogen bonding interactions. For the OH stretching region of the IR spectra of both systems (between 3500 cm-1 and 3000 cm-1), possible hydrogen bond interactions are compared to molecular simulation results in order to determine the most probable interactions. IR bands in the carbonyl stretching (and NH2 bending for the case of choline chloride-urea) region are curve-resolved and compared to interactions observed in the molecular simulations. For choline chloride-urea systems, it is suggested that there is a strong interaction between the NH2 of urea and the chlorine anion where the system wants to maximize the number of hydrogen bonds to the anion. Additionally, the disappearance of “free” carbonyl groups upon increasing concentrations of urea suggests that as this concentration increases, the anion-urea complex remains but with additional interactions that remove the “free” carbonyl band from the spectra. For choline chloride-malonic acid interactions, two main peaks assigned in the carbonyl stretching region in the pure malonic acid spectrum as a transition dipole coupling, now become two peaks at slightly different frequencies. In the spectra of the mixture, the higher frequency band in this region is assigned to the “free” carbonyl stretching mode where the OH of this carboxylic group interacts with a different moiety. The second band at a lower frequency is a hydrogen bonded carbonyl group. These two types of carbonyl interactions are consistent with interactions observed in the molecular simulations. Now that these systems have been simulated and analyzed through a detailed molecular simulations and vibrational study, the conclusions and knowledge of the interactions can be implemented towards finding new DES from the vast number of possible combinations and/or towards the design of DESs for different applications, e.g., the phase separation of oil and sand.