Synthesis and Physicochemical Characterization of a Choline-Tryptophan Based Ionic Liquid in Aqueous Medium at Different Temperatures

In the present work, synthesis, and physicochemical investigations of choline-tryptophan [Ch][Trp] based IL have been reported. The synthesized [Ch][Trp] IL was fully characterized by Fourier transform infrared (FTIR), ultraviolet (UV), fluorescence, nuclear magnetic resonance (1H and 13C NMR) spectroscopy. The thermophysical properties namely density and ultrasonic velocity of the synthesized IL in aqueous medium have been monitored in the temperature range of 293.15–313.15 K at 5 K interval. Derived properties like apparent molar volume ( $${{V}_{\phi }}$$ ), partial molar expansibility ( $$E_{\phi }^{0}$$ ), isentropic compressibility ( $${{K}_{{\text{S}}}}$$ ), isothermal compressibility ( $${{K}_{T}}$$ ), apparent molar isentropic compressibility ( $${{K}_{{{\text{s}},\phi }}}$$ ) and hydration number ( $${{n}_{{\text{H}}}}$$ ) have been calculated.The increase in density values with rise in concentration of IL reveals that there is association between the constituents due to closeness. But as the temperature rises, the ionic mobility intensifies leading to expansion in volume and lowering of density. A decreasing trend of $${{V}_{\phi }}$$ has been noticed at higher concentration of IL due to large engagement of water molecules in the solvation process leading to strong electrostriction and overlapping of hydration co-spheres. It has also been observed that the water molecules located in the primary hydration shells are incompressible due to electrostriction but the water molecules out of the electrostatic field are main contributors to the isentropic compressibility of the aqueous IL mixture.