Degree

Doctor of Philosophy (PhD)

Department

Chemistry

Document Type

Dissertation

Abstract

Deep eutectic solvents (DESs) are attractive systems for extraction and separation of compounds. One example of this is the DES formed by the combination of lauric acid (LA) and N-methylacetamide (NMA), system that has been shown to efficiently separate oil/water mixtures through gel formation between LA and oil molecules. Structural characterization of this system using small-angle X-ray scattering (SAXS) and vibrational spectroscopy revealed the existence of nanometer-scale LA domains embedded in a continuous polar NMA phase, where LA and NMA interact through hydrogen bonding. These LA domains are responsible for the separation efficiency of linear organic molecules. However, this DES exhibits a limited ability to solubilize bulky nonpolar compounds. To elucidate how microscopic structure governs solvation, we combined molecular dynamics (MD) simulations with vibrational spectroscopy and benchmarked the simulations against SAXS and spectroscopic data. We developed the molecular dynamics instantaneous frequencies of molecules (MD‑IFM) method for direct comparison with experiments. MD‑IFM is a transferable, parametrization-free approach that reproduces vibrational observables. Our results showed the formation of crystalline-like LA domains that prevent bulky nonpolar molecules, such as tungsten hexacarbonyl, from diffusing into the nonpolar domains, forcing them to remain at the LA-NMA interface.  The presence of LA domains confines molecules solvated in the NMA phase, slowing their solvation dynamics. This knowledge was used to introduce modifications to the LA-NMA DES. The first modification involved replacing LA with a weaker hydrogen-bond-accepting molecule, lauryl alcohol (LAlc), while the second involved disrupting amide-amide interactions by introducing N-isopropylacetamide (NIPAc). These modifications revealed that, similar to the original DES, the LAlc-based system also forms organized nonpolar domains, whereas disruption of amide-amide interactions with NIPAc leads to the loss of crystalline-like LA domains. Furthermore, we observed that while highly ordered domains hinder the insertion of bulky nonpolar solutes, the structured LA-NMA and LAlc-NMA interfaces enhance the solubility of nonpolar solutes by up to a factor of two with respect to pure NMA. In contrast, systems lacking highly organized structures and interfaces, such as the LA-NIPAc DES, show no enhancement in solubility with respect to pure NIPAc despite the presence of less ordered nonpolar domains.

Date

6-10-2026

Committee Chair

Daniel Kuroda

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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