Interactions of N-alkyl-N-methylmorpholinium based ionic liquids with acetonitrile studied by density and velocity of sound measurements and molecular dynamics simulations

Abstract

Morpholinium-based ionic liquids (ILs) and their mixtures with polar co-solvents are an interesting class of emerging electrolytes in electrochemistry that is relatively poorly studied. In this work, densities and sound velocities of four ILs, N-ethyl-N-methylmorpholinium tetrafluoroborate, N-butyl-N-methylmorpholinium tetrafluoroborate, N-octyl-N-methylmorpho-linium tetrafluoroborate and N-decyl-N-methylmorpholinium tetrafluoroborate were measured in acetonitrile (AN) at T = (298.15–328.15) K and at atmospheric pressure. These data were used to calculate the apparent molar volumes and the apparent molar compressibilities as a function of molality. Then, an extrapolation procedure based on Masson and Redlich–Meyer equations were applied to evaluate the values of the limiting apparent molar volume and the limiting apparent molar compressibility. The limiting apparent molar expansibility coefficients were obtained from the slopes of the apparent molar volume at infinite dilution versus temperature. Using molecular dynamics simulations the degree of association of ILs in AN was calculated. Moreover, limiting apparent molar volumes were estimated and compared with the experimental values. The results indicate that the most significant factor controlling the volumetric and acoustic properties of the studied ILs in AN is the size of cation of the ionic liquid. Moreover, AN interacts more strongly with N-alkyl-N-methylmorpholinium tetrafluoroborates than dimethylsulfoxide and N, N-dimethylformamide, not only by strong ion-dipole interactions, but also by packing effect.