Proton Thermodynamics in a Protic Ionic Liquid, Ethylammonium Nitrate.

Area of Science:

• Physical chemistry
• Ionic liquid research
• Solvation thermodynamics

Background:

• Protic ionic liquids (PILs) are essential solvents with unique proton solvation properties.
• Understanding proton behavior in PILs is crucial for their application in various chemical processes.
• Water serves as a benchmark solvent for comparing thermodynamic properties.

Purpose of the Study:

• To investigate the proton solvation state in ethylammonium nitrate (EAN), a representative protic ionic liquid.
• To determine the acid dissociation enthalpies and entropies of various compounds in EAN.
• To compare proton solvation thermodynamics in EAN with those in water and explain observed differences.

Main Methods:

• Experimental determination of acid dissociation enthalpies and entropies for eight compounds in EAN.
• Comparative analysis of thermodynamic data between EAN and water.
• Theoretical explanation based on differences in proton solvation structures (HNO3 in EAN vs. H3O+ in water).

Main Results:

• Acid dissociation enthalpies and entropies in EAN were consistently higher than in water (24 kJ/mol enthalpy, 65 J/mol·K entropy).
• Protons in EAN exist as HNO3, exhibiting higher reaction energy and less-structured solvation compared to H3O+ in water.
• The proposed enthalpy and entropy windows effectively visualized similarities and differences between EAN and water.

Conclusions:

• The observed thermodynamic differences are attributed to distinct proton solvation structures in EAN and water.
• The entropic effect of proton solvation structure is likely applicable to all PILs.
• Enthalpy and entropy windows offer a novel perspective for solvent characterization and comparison.