The glass transition and the distribution of voids in room-temperature ionic liquids: a molecular dynamics study

The glass transition in prototypical room temperature ionic liquids has been investigated by molecular dynamics simulations based on an Amber-like empirical force field. Samples of C(4)mim]PF(6)], C(4)mim]Tf(2)N], and C(3)mim]Tf(2)N] have been quenched from the liquid phase at T = 500 to a glassy state at T ～ 0 K in discontinuous steps of 20 K every 1.2 ns. The glass temperature estimated by simulation (T(g) = 209 K for C(4)mim]PF(6)], T(g) = 204 K for C(4)mim]Tf(2)N], and T(g) = 196 K for C(3)mim]Tf(2)N]) agrees semi-quantitatively with the experimental values (T(g) = 193÷196 K for C(4)mim]PF(6)], T(g) = 186÷189 K for C(4)mim]Tf(2)N], and T(g) = 183 K for C(3)mim]Tf(2)N]). A model electron density is introduced to identify voids in the system. The temperature dependence of the size distribution of voids provided by simulation reproduce well the experimental results of positron annihilation lifetime spectroscopy reported in G. Dlubek, Y. Yu, R. Krause-Rehberg, W. Beichel, S. Bulut, N. Pogodina, I. Krossing, and Ch. Friedrich, J. Chem. Phys. 133, 124502 (2010), with only one free parameter needed to fit the experimental data.