Simulation studies of ionic liquids: Orientational correlations and static dielectric properties

Abstract

The ionic liquids BMIM+I-, BMIM+BF4-, and BMIM+PF6- were simulated by means of the molecular dynamics method over a time period of more than 100 ns. Besides the common structural analysis, e.g., radial distribution functions and three dimensional occupancy plots, a more sophisticated orientational analysis was performed. The angular correlation functions g(00)110(r) and g(00)101(r) are the first distance dependent coefficients of the pairwise orientational distribution function g(rij,Omega1,Omega2,Omega12). These functions help to interpret the three dimensional plot and reveal interesting insights into the local structure of the analyzed ionic liquids. Furthermore, the collective network of ionic liquids can be characterized by the Kirkwood factor Gkappa(r) [J. Chem. Phys. 7, 911 (1939)]. The short-range behavior (r<10 A) of this factor may be suitable to predict the water miscibility of the ionic liquid. The long-range limit of Gkinfinity is below 1 which demonstrates the strongly coupled nature of the ionic liquid networks. In addition, this factor relates the orientational structure and the dielectric properties of the ionic liquids. The static dielectric constant epsilon(omega=0) for the simulated system is 8.9-9.5. Since in ionic liquids the very same molecule contributes to the total dipole moment as well as carries a net charge, a small, but significant contribution of the cross term between the total dipole moment and the electric current to epsilon(omega=0) is observed.

Dynamics of RTILs: A comparative dielectric and OKE study

“…This points to a breathing mode of mesoscopic aggregates essentially normal to the average dipole orientation of the cluster. Aggregates of π-stacked cations and their surrounding anions are reasonable candidates compatible with MD simulations [40,41,42] and scattering experiments [43]. It should be noted that the lifetime of the caging effects observed in MD simulations of RTIL diffusion, see e.g.…”

Spotlight on ionic liquids

“…3͒ and demonstrates the sensitivity of IL interactions to alkyl group substitution. [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] …”

“…These effects have been addressed in a number of articles describing MD simulations of ILs. 31,33,[68][69][70][71][72][73][74][75][76][77] [69][70][71][72][73][74] reported several studies of both equilibrium and nonequilibrium solvation, orientational dynamics, vibrational relaxation, and electron-transfer reactions in ͓emim͔ + / ͓PF 6 ͔ − . Song 77 has presented a method to extend dielectric continuum models for solvation to include precise molecular shapes and an extension beyond the DebyeHückel model for high concentrations of charges, up to neat ILs.…”

Terahertz dynamics of ionic liquids from a combined dielectric relaxation, terahertz, and optical Kerr effect study: evidence for mesoscopic aggregation

“…The structure of a neat RTIL is largely determined by a combination of long-range Coulombic forces, hydrogen-bonding interactions, and packing factors. Computer simulations have long predicted the emergence of mesoscopic structure in RTILs [6][7][8][9][10][11][12][13] but this has been difficult to verify experimentally [14]. Recent small-angle x-ray scattering experiments and light scattering experiments have indicated the presence of structure in RTILs [15].…”

“…DR measures the <μμ(t)> correlation function of the (collective) dipole moment. Through our collaboration, we have been able to measure DR spectra over an extended frequency range: Regensburg & Murdoch (0.2 -89 GHz), Freiburg (0.1 -3 THz), and Strathclyde (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). This uses classic dielectric relaxation spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and Fourier-transform infrared (FTIR) spectroscopy [19,20].…”