Molecular Insights of Refrigerants (R-32, R-125, R-134a) In Amino acid based Ionic Liquids.

CONTEXT: The separation of refrigerant mixtures is an important challenge in the development of sustainable cooling technologies, and ionic liquids (ILs) have emerged as promising alternative absorbents because of their tunable physicochemical properties. In this work, molecular-level interactions between three hydrofluorocarbon refrigerants (R-32, R-125, and R-134a) and amino acid (AA)-based ILs were investigated to understand their separation behavior. The ILs consisted of tetrabutylphosphonium ([TBP]) as the common cation paired with leucine ([LEU]) and isoleucine ([ILE]) anions. Structural, dynamic, and thermodynamic analyses revealed distinct differences in the behavior of the refrigerants within the IL environment. Among the three refrigerants, R-32 exhibited self-diffusivity approximately one order of magnitude higher than that of R-125 and R-134a in both ILs, indicating greater molecular mobility. RDF and interaction analyses showed that the [TBP] cation forms stronger intermolecular interactions with the refrigerants than the amino acid anions. Free-energy calculations further demonstrated that R-134a exhibits approximately 75% more favorable (i.e., more negative) solvation free energy than R-32 in [TBP][ILE], suggesting a stronger affinity for this IL. These findings provide molecular-level insights into refrigerant-IL interactions and support the rational selection of amino acid-based ILs for efficient refrigerant separation.

METHODS: MD simulations were performed to investigate the structural organization, diffusion behavior, and thermodynamic properties of refrigerants in amino acid-based ILs. Systems containing R-32, R-125, and R-134a dissolved in [TBP][LEU] and [TBP][ILE] were simulated under equilibrium conditions. Structural properties were characterized using RDF and coordination analyses, while molecular mobility was evaluated using MSD and self-diffusion coefficient calculations. Thermodynamic behavior was examined by calculating the solvation free energy of the refrigerants in each IL. All MD simulations and trajectory analyses were carried out using the force field and simulation software described in the manuscript.