Molecular Dynamics Study of Thermodynamic Scaling of the Glass-Transition Dynamics in Ionic Liquids over Wide Temperature and Pressure Ranges

Abstract

Experimentally, superpositioning of dynamic properties such as viscosity, relaxation times, or diffusion coefficients under different conditions of temperature T, pressure P, and volume V by the scaling variable TV γ (where γ is a material constant) has been reported as a general feature of many kinds of glass-forming materials. In the present work, molecular dynamics (MD) simulations have been performed to study the scaling of dynamics near the glass-transition regime of ionic liquids. Scaling in the simulated 1-ethyl-3-methylimidazolium nitrate (EMIM-NO 3 ) system has been tested over wide ranges of temperatures and pressures. TV γ scaling of the dynamics is well described by master curves with γ ) 4.0 ( 0.2 and 3.8 ( 0.2 for cation and anion, respectively. Structures and Coulombic terms of the corresponding states are found to be quite similar. The temperature and pressure dependence of the pair correlation function show similar trends and therefore can be superpositioned onto the master curve. Although the behaviors with γ ) 4 might be expected from the relation, γ ) n/3, for the dynamics with the soft-core-type potential U ) ε(σ/r) n , with n ) 12, pair potentials used in the MD simulation have a more complex form, and not all the repulsive terms can play their roles in the heterogeneous structures determined by ion-ion interactions. Scaling is related to the common part of effective potentials related to the pair correlation functions, including the many-body effect in real space.

Multifractal nature of heterogeneous dynamics and structures in glass forming ionic liquids

“…From the plot of the densities against temperature from the data in NPT ensembles, a clear change of the slope is found, and from which the glass transition temperature is determined to be 260 K [9,10]. Radial distribution function (distance dependence of the pair correlation functions) g(r) of each ion pairs is obtained at several temperatures between 100 and 1000 K. The results for g(r) are comparable to those previously obtained [9][10][11]17]. The g(r) for each pair is determined for the center of the mass position of each ion and we assume that the charge is simply on the center of the mass position in the following treatment, although the partial charges are used for each atom in the MD simulation.…”

“…Molecular dynamics simulations of 1-ethyl-3-methylimidazolium nitrate (EMIM-NO 3 ) have been performed in similar manner as described in our previous works or in related works [9][10][11][12][13][14][15][16][17] using an all atoms model with the generalized Amber field. The smaller system consisted of 64 EMIM + and 64 NO 3 − ions with a total 1472 atoms and the larger system had 256 EMIM + and 256 NO 3 − with a total of 5888 atoms.…”

“…(1) is representative for the charge density wave (CDW-or-oscillatory behavior of layers formed by cations and anions), while the G(r) is representative for the density wave (DW or oscillatory behavior of layers of density regardless the charge). The structure of the system is formed under the influence of both the repulsive terms and the Coulombic term, and g(r) has a priori a many body character [17,[23][24][25]. The function Q(r) and G(r) are concerned with the resultant structure, and therefore the forces acting by the charge-charge interaction and that by the repulsive one is not separated completely.…”

Scaling of the dynamics of flexible Lennard-Jones chains

“…We refer to this scaling as power-law * a.a.veldhorst@gmail.com † tbs@ruc.dk density scaling. To date, many more molecular liquids have been shown to obey power-law density scaling to a good approximation, including polymers, but also ionic liquids [16][17][18][19][20][21][22] and liquid crystals [23][24][25][26][27][28].…”

Thermodynamic scaling of dynamics in polymer melts: Predictions from the generalized entropy theory

“…[3][4][5][6]The exponent γ of the thermodynamic scaling also provides a measure of the relative importance of the density and temperature in controlling glassy dynamics. Hence, not surprisingly, a large body of experiments [7][8][9][10][11][12][13] and simulations [14][15][16][17][18][19][20] have probed the nature of thermodynamic scaling of glass-forming liquids since the first observation by Tölle et al for orthoterphenyl. [21] The existence of thermodynamic scaling is well established for as diverse materials as van der Waals liquids, polymers, ionic liquids, weakly hydrogen-bonded systems, etc.…”