Spectroscopic and computational investigations of molecular interactions in gas-expanded liquids
Gohres, John Linton, III
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Gas-expanded liquids (GXLs) are a unique class of tunable solvents with unlimited potential. A wide range of solvent properties and solvent interactions and complexes are possible by adjusting the amount of the gas component (in situ) or changing the organic liquid. Aside from solvent tunability, there are environmental and processing benefits to using GXLs. Organic solvent use is decreased, the gas component can be vented off facile separations, and the gas can act as an antisolvent for selective solute precipitation. As a result there are numerous reaction and extraction schemes and materials processing applications that could benefit from GXL use. Unfortunately, important molecular-level details that can drive a chemical process are largely unknown and limit GXL use in industrial and specialty applications. The work presented in this uses a synergistic study of experiments and computer simulations to explore solvation processes and molecular interactions in GXLs and the effects on macroscopic observables like spectroscopy, transport, and reactions. Steady-state solvation of a laser dye is studied with spectroscopy (UV/vis and fluorescence) and molecular dynamics simulations (MD). Both experiment and theory show that organic enrichment occurs in the vicinity of the solute called the cybotactic region. Subsequently, the solvent dynamics arising by electronically perturbing the solute are studied with MD simulation. Unexpected dynamics are observed and are dependent on the organic component and gas composition. The diffusion of heterocyclic compounds is studied with MD simulations and compared to the Taylor-Aris diffusion study of former group members. The experiments and simulations do not agree, but solvent structures obtained by simulation are shown to provide valuable insight into solvent-dependent absorption spectroscopy, or solvatochromism. Finally, dissociation constants of alkylcarbonic acids that form in situ in CO2/alcohol mixtures are presented from spectroscopic measurements. Spectroscopic techniques to measure dissociation constants are well known; however, the high-pressure and multiple equilibria associated with alkylcarbonic acids hinder straight-forward measurement and analysis.