Density, sound velocity, viscosity, and refractive index of new morpholinium ionic liquids with amino acid-based anions: Effect of temperature, alkyl chain length, and anion

In this work, room temperature synthesis of twenty new ionic liquids (ILs) based on the N-alkyl-N-methylmorpholinium ([Mor (1,R)], R = 2, 3, 4, 6, 8) cation and N-acetyl-L-amino acid anions (L-alaninate, L-valinate, L-leucinate, L-isoleucinate) was described. The synthesized ILs were characterized by various spectroscopic techniques and high-resolution mass spectrometry. Furthermore, density (rho), sound velocity (v), viscosity (eta) and refractive index (n(D)) have been measured within the temperature range of T = (298.15 to 318.15) K and at 0.1 MPa pressure. The density, sound velocity and viscosity values were fitted to linear and Vogel-Tammann-Fulcher (VTF) equation. Moreover, on the basis of the refractive index values, molar refraction (R-m) and free volume (V-f) were calculated using the Lorentz-Lorenz equation. The thermodynamic properties such as thermal expansion coefficient (alpha) and isentropic compressibility (kappa(s)) were calculated using the experimental values of density and sound velocity. Finally, the obtained data were evaluated in order to determine the effect of such parameters as temperature, the nature of the anion and the length of the cation alkyl chain. (C) 2019 Elsevier B.V. All rights reserved.