Molecular simulation study of the glass transition in a soft primitive model for ionic liquids

Consistent and reproducible computation of the glass transition temperature from molecular dynamics simulations

In many fields, from semiconductors for opto-electronic applications to ionic liquids (ILs) for separations, the glass transition temperature (Tg) of a material is a useful gauge for its potential use in practical settings. As a result, there is a great deal of interest in predicting Tg using molecular simulations. However, the uncertainty and variation in the trend shift method, a common approach in simulations to predict Tg, can be high. This is due to the need for human intervention in defining a fitting range for linear fits of density with temperature assumed for the liquid and glass phases across the simulated cooling. The definition of such fitting ranges then defines the estimate for the Tg as the intersection of linear fits. We eliminate this need for human intervention by leveraging the Shapiro-Wilk normality test and proposing an algorithm to define the fitting ranges and, consequently, Tg. Through this integration, we incorporate into our automated methodology that residuals must be normally distributed around zero for any fit, a requirement that must be met for any regression problem. Consequently, fitting ranges for realizing linear fits for each phase are statistically defined rather than visually inferred, obtaining an estimate for Tg without any human intervention. The method is also capable of finding multiple linear regimes across density vs temperature curves. We compare the predictions of our proposed method across multiple IL and semiconductor molecular dynamics simulation results from the literature and compare other proposed methods for automatically detecting Tg from density-temperature data. We believe that our proposed method would allow for more consistent predictions of Tg. We make this methodology available and open source through GitHub.

Molecular Dynamics Simulation of the Influence of External Electric Fields on the Glass Transition Temperature of the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

We present the results of molecular dynamics simulations of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁im][NTf₂] in the presence of external electric fields (EEFs) of varying strengths to understand the effects of EEFs on the glass transition temperature T_g. We compute T_g with an automated and objective method and observe a depression in T_g when cooling the IL within an EEF above a critical strength. The effect is reversible, and glasses prepared with EEFs recover their original zero-field T_g when heated. By examining the dynamics and structure of the liquid phase, we find that the EEF lowers the activation energy for diffusion, reducing the energetic barrier for movement and consequently T_g. We show that the effect can be leveraged to drive an electrified nonvapor compression refrigeration cycle.

Diffusion and Velocity Correlations of the Phase Transitions in a System of Macroscopic Rolling Spheres

We study an air-fluidized granular monolayer composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never lose contact with the grid and so that the dynamics may be regarded as pseudo two dimensional (or two dimensional, if the effects of the sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all the cases, we found the coexisting phases display a strong energy non-equipartition. In the first transition, at a weak fluidization, a glass phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at a very low packing fraction and for a wide range of granular temperatures. Both phases are also characterized by strong anticorrelated velocities upon a collision. Thus, the dynamics observed for this quasi two-dimensional system unveil phase transitions with peculiar properties, very different from the predicted behavior in well-know theories for their equilibrium counterparts.

Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular dynamics simulations are used for predictions of the glass transition temperatures for a test set of five aprotic ionic liquids. Glass transitions are localized with the trend-shift method, analyzing volumetric and transport properties of bulk amorphous phases. A classical nonpolarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as a starting level of theory for all calculations. Alternative approaches of charge scaling and the Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former nonpolarizable model overestimates the glass transition temperature by tens of Kelvins (37 K on average). The charge-scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most subcooled liquids but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the subcooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.

Pseudo-two-dimensional dynamics in a system of macroscopic rolling spheres

We study in this work the dynamics of a collection of identical hollow spheres (ping-pong balls) that rest on a horizontal metallic grid. Fluidization is achieved by means of a turbulent air current coming from below. The upflow is adjusted so that the particles do not levitate over the grid, resulting in quasi-two-dimensional dynamics. We show that the behavior of diffusion and correlations in this system is particularly rich. Noticeably as well (and related to the complex dynamical behavior), a variety of phases appear, with important peculiarities with respect to analogous setups. We observe gas, liquid, glass, and hexagonal crystal phases. Most notably, we show that the melting of the hexagonal crystal occurs in coexistence with a liquid phase. This strikingly differs from the corresponding transition in a purely two-dimensional systems of air-fluidized disks, for which no phase coexistence has been reported in the literature.

Computational assessment of the crystallization tendency of 1-ethyl-3-methylimidazolium ionic liquids

A test set of 20 1-ethyl-3-methylimidazolium ionic liquids, differing in their anions, is subjected to a computational study with an aim to interpret the experimental difficulties related to the preparation of crystalline phases of the selected species. Molecular dynamics simulations of the liquid phases, quantum-chemical symmetry-adapted perturbation theory calculations of the interaction energies within the ion pair, and density functional theory calculations of the cohesive energies of the crystal phases are used in this work to obtain the structural, energetic, and diffusion parameters of the materials. Correlations of fusion temperatures and enthalpies and temperatures of the glass transitions with 15 calculated parameters are investigated in order to interpret the trends of the phase behavior of the selected ionic liquids. Correlations of a fair significance are found between the glass transition temperatures and selected energetic, cohesive, and diffusion-related characteristics of the liquids; however, the correlations of calculated transport and some enthalpic properties are blurred by the limited accuracy of the non-polarizable CL&P force field for predicting these properties. 1-Ethyl-3-methylimidazolium acetate is found to have an exclusive position among those in the test set due to several outlying characteristics, such as the short contact distance of its counterions in the liquid, high pair interaction energies, and importance of the dispersion interactions for the collective cohesion, impeding its crystallization significantly.