Rigidity and soft percolation in the glass transition of an atomistic model of ionic liquid, 1-ethyl-3-methyl imidazolium nitrate, from molecular dynamics simulations-Existence of infinite overlapping networks in a fragile ionic liquid

摘要

The typical ionic liquid, 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO3), was examined by molecular dynamics simulations of an all-atomistic model to show the characteristics of networks of cages and/or bonds in the course of vitrification of this fragile glass-former. The system shows changes of dynamics at two characteristic temperatures, T-B (or T-c) and the glass transition temperature T-g, found in other fragile glass forming liquids [K. L. Ngai and J. Habasaki, J. Chem. Phys. 141, 114502 (2014)]. On decreasing temperature, the number of neighboring cation-anion pairs, N-B, within the first minimum of the pair correlation function, g(r)(min), increases. On crossing T-B (>T-g), the system volume and diffusion coefficient both show changes in temperature dependence, and as usual at T-g. The glass transition temperature, T-g, is characterized by the saturation of the total number of "bonds," N-B and the corresponding decrease in degree of freedom, F = [(3N - 6) - N-B], of the system consisting of N particles. Similar behavior holds for the other ion-ion pairs. Therefore, as an alternative, the dynamics of glass transition can be interpreted conceptually by rigidity percolation. Before saturation occurring at T-g, the number of bonds shows a remarkable change at around T-B. This temperature is associated with the disappearance of the loosely packed coordination polyhedra of anions around cation (or vice versa), related to the loss of geometrical freedom of the polyhedra, f(g), of each coordination polyhedron, which can be defined by f(g) = [(3N(V) - 6) - N-b]. Here, 3N(v) is the degree of freedom of N-V vertices of the polyhedron, and N-b is number of fictive bonds. The packing of polyhedra is characterized by the soft percolation of cages, which allows further changes with decreasing temperature. The power spectrum of displacement of the central ion in the cage is found to be correlated with the fluctuation of N-b of cation-cation (or anion-anion) pairs in the polyhedron, although the effect from the coordination shells beyond the neighboring ions is not negligible. (C) 2015 AIP Publishing LLC.