Properties investigation of protic morpholinium-based ionic liquids by molecular dynamics simulation and quantum chemical calculations

Abstract

In this paper structure, thermodynamic, and dynamic properties were investigated for series of selected protic ionic liquids, methylmorpholinium ([MeMorph]+) and ethylmorpholinium ([EtMorph]+) with carboxylate formate ([For]−) and acetate ([Ace]−) anions, using quantum chemical calculations and molecular dynamics simulations. Relative geometrical structure, H atom transfer between anion-cation, charge transfer and the polarizability was calculated and studied. As a result, anion type (characterized by the alkyl chain length) has the major influence on the charge and H atom transfer with subsequent influences on physiochemical properties. Irregularity in the trend of thermodynamic properties of these ILs (observed experimentally) was described by the extent of charge transfer of atom we estimated by scanning the anion-cation distance. Molecular dynamics simulations indicated a substantially lower density for [MeMorph][Ace] than [MeMorph][For], consistent with reported experiment, and demonstrate the high impact of anion type on thermodynamic properties. Radial distribution function g(r), demonstrates cations and anions tightly bond in the first coordinate shell but bond loosely in second and third shells as the anion alkyl chain increases. Adhering to this, special distribution functions demonstrate the more compactness and higher anion distributing around cation for [MeMorph][For] than the other two ILs. As regards the accuracy of simulation of transport property (the viscosity) of ILs, the scaling down of partial-charges was found necessary to account for electrostatic interaction with the nonpolarizable force field used. Required scaling depends on the diversity of anion-cation charge transfer. These simulations, consistent highly with experiment, exclusively provide insight into the details of less impact ILs that would be obtained by larger size alkyl substituent on carboxylate anion.