Exploration of Diverse Interactions of l-Methionine in Aqueous Ionic Liquid Solutions: Insights from Experimental and...

Here, we have investigated some physicochemical parameters to understand the molecular interactions by means of density (ρ) measurement, measurement of viscosity (η), refractive index(n _D) measurement, and conductance and surface tension measurements between two significant aqueous ionic liquid solutions: benzyl trimethyl ammonium chloride (BTMAC) and benzyl triethyl ammonium chloride (BTEAC) in an aqueous l-methionine (amino acid) solution. The apparent molar volume (Φ_v), coefficient of viscosity (B), and molar refraction (R _M) have been used to analyze the molecular interaction behavior associated in the solution at various concentrations and various temperatures. With the help of some important equations such as the Masson equation, the Jones-Doles equation, and the Lorentz-Lorenz equation, very significant parameters, namely, limiting apparent molar volumes (Φ_v ⁰ ), coefficient of viscosity (B), and limiting molar refraction (R _M ⁰), respectively, are obtained. These parameters along with specific conductance (κ) and surface tension (σ) are very much helpful to reveal the solute-solvent interactions by varying the concentration of solute molecules and temperature in the solution. Analyses of Δμ₁ ^0#, Δμ₂ ^0#, TΔS ₂ ^0#, ΔH ₂ ^0#, and thermodynamic data provide us valuable information about the interactions. We note that l-Met in 0.005 molality BTEAC ionic liquid at 308.15 K shows maximum solute-solvent interaction, while l-Met in 0.001 molality BTMAC aqueous solution of ionic liquid at 298.15 K shows the minimum one. Spectroscopic techniques such as Fourier transform infrared (FTIR), ¹H-NMR, and UV-vis also provide supportive information about the interactions between the ionic liquid and l-methionine in aqueous medium. Furthermore, adsorption energy, reduced density gradient (RDG), and molecular electrostatic potential (MESP) maps obtained by the application of density functional theory (DFT) have been used to determine the type of interactions, which are concordant with the experimental observations.