Structural and dynamical properties of ionic liquids: The influence of charge location

Abstract

The properties of ionic liquids depend on the chemical structure of the constituent ions. An important difference between molten inorganic salts and room temperature ionic liquids (RTILs) is that in RTILs the charge is frequently not located at the center of mass. This paper describes a molecular dynamics investigation of the influence of charge location on the structure and transport properties of ionic liquids. The model considered consists of univalent spherical ions with the cation charge moved away from its center of mass. It is shown that the charge location has an important influence on the liquid properties. As the charge is moved off center, the electrical conductivity initially increases, and the shear viscosity decreases. However, when the charge exceeds a certain displacement, this behavior is reversed. With further charge displacement, the conductivity decreases sharply and the viscosity increases rapidly. This behavior reversal can be traced to the formation of directional ion pairs that are present in sufficient numbers, and have lifetimes sufficiently long to strongly influence the liquid properties. We suggest that the influence of directional ion pairing can explain what appear to be anomalously low conductivities and high viscosities observed for some RTILs. The rotational and reorientational motions of the cations are examined, and shown to be strongly influenced by ion-pair formation when the charge is far off center. The temperature dependence of the transport properties is considered for selected systems, and deviations from Arrhenius behavior are found to be most important for the conductivity. Based on our results, this possibly indicates that directional ion pairs create an additional "barrier" to charge transport in some ionic liquids.

Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues

“…Counter ion association is recognized as a molecular feature that significantly influences the viscosity of ionic liquids. 33,39,104,114,115 Weak ion association decreases viscosity. 33,114 However, this trend is reversed if directional, long living ion pairs are formed.…”

“…33,39,104,114,115 Weak ion association decreases viscosity. 33,114 However, this trend is reversed if directional, long living ion pairs are formed. 33 Our gas phase calculations (TPSS-D3(RI)/6-31++G**) of one ion pair revealed an inter- .…”

“…33,114 However, this trend is reversed if directional, long living ion pairs are formed. 33 Our gas phase calculations (TPSS-D3(RI)/6-31++G**) of one ion pair revealed an inter- . It is decreased to 1.2 kJ/mol if thermodynamic corrections are considered.…”

Structural and dynamical properties of ionic liquids: Competing influences of molecular properties

“…8 The model employed consisted of spherical, univalent ions of the same size, with the cation charge displaced from the center of mass ͑CM͒ ͑see Fig. 8 The model employed consisted of spherical, univalent ions of the same size, with the cation charge displaced from the center of mass ͑CM͒ ͑see Fig.…”

Spotlight on ionic liquids

“…[33][34][35] Such intricate details of IL structure have been picked apart by careful analysis and examination of structure factors and higher-order correlation functions obtained from molecular dynamics simulations. 31,[33][34][35][36][37] …”