Acetic acid (HOAc) is a typical weak Brønsted acid that has been broadly used as a reactant in esterification reactions and can be used as such in ionic liquid (IL) solutions. The nature of the HOAc molecule as a solute in the IL 1-ethyl-3-methylimidazolium acetate (EMIMOAc) solutions was investigated with thermodynamic, vibrational spectroscopic, conductivity, and viscosity measurements. Calorimetry and vapor pressure-based thermodynamic measurements of the binary HOAc-EMIMOAc mixture were used to quantify the exothermic enthalpy of solution, H sol , as ~3.2-4.9 kJ/mol. FTIR and Raman spectroscopy showed that the structures of the HOAc molecule and IL molecular ions are unchanged in the solutions relative to the pure liquids, indicating that the HOAc is a negligibly-dissociated weak acid in the IL solutions. Temperaturedependent conductivity measurements quantify the effects of the HOAc on the IL solution conductivity and confirm that the HOAc remains a mostly neutral molecular solute that interacts with the EMIMOAc IL via ion-dipole and hydrogen bond interactions.
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