Orientational wetting and dynamical correlations toward glass transition on the surface of imidazolium-based ionic liquids

Surface induces many fascinating physical phenomena, such as dynamic acceleration, surface anchoring, and orientational wetting, and, thus, is of great interest to study. Here, we report classic molecular dynamics simulations on the free-standing surface of imidazolium-based ionic liquids (ILs) [C4mim][PF6] and [C10mim][PF6]. On [C10mim][PF6] surface, a significant orientational wetting is observed, with the wetting strength showing a diverging tendency. Depth of the wetting was captured from the density and orientational order profile by a static length, which remarkably increases below the temperature Tstat upon cooling down. The dynamical correlation length that measures the distance of surface-dynamics acceleration into the bulk was characterized via the spatial-dependent mobility. The translational correlation exhibits a similar drastic increment at Tstat, while the rotational correlation drastically increases at a lower temperature Trot. We connect these results to the dynamics in bulk liquids, by finding Tstat and Trot that correspond to the onset temperatures where the liquids become cooperative for translational and rotational relaxation, respectively. This signifies the importance of collective dynamics in the bulk on the orientational wetting and surface dynamics in the ILs.

Unified polarizable electrode models for open and closed circuits: Revisiting the effects of electrode polarization and different circuit conditions on electrode-electrolyte interfaces

A precise understanding of the interfacial structure and dynamics is essential for the optimal design of various electrochemical devices. Herein, we propose a method for classical molecular dynamics simulations to deal with electrochemical interfaces with polarizable electrodes under the open circuit condition. Less attention has been paid to electrochemical circuit conditions in computation despite being often essential for a proper assessment, especially comparison between different models. The present method is based on the chemical potential equalization principle, as is a method developed previously to deal with systems under the closed circuit condition. These two methods can be interconverted through the Legendre transformation, so that the difference in the circuit conditions can be compared on the same footing. Furthermore, the electrode polarization effect can be correctly studied by comparing the present method with the conventional simulations with the electrodes represented by fixed charges, since both of the methods describe systems under the open circuit condition. The method is applied to a parallel-plate capacitor composed of platinum electrodes and an aqueous electrolyte solution. The electrode polarization effects have an impact on the interfacial structure of the electrolyte solution. We found that the difference in the circuit conditions significantly affects the dynamics of the electrolyte solution. The electric field at the charged electrode surface is poorly screened by the nonequilibrium solution structure in the open circuit condition, which accelerates the motion of the electrolyte solution.

Pressing matter: why are ionic liquids so viscous?

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

Nonlinear measurements of kinetics and generalized dynamical modes. II. Application to a simulation of solvation dynamics in an ionic liquid

Solvation dynamics in ionic liquids show features that are often associated with supercooled liquids, including "stretched" nonexponential relaxation. To better understand the mechanism behind the stretching, the nonlinear mode-correlation methods proposed in Paper I [S. R. Hodge and M. A. Berg, J. Chem. Phys. 155, 024122 (2021)] are applied to a simulation of a prototypical ionic liquid. A full Green's function is recovered. In addition, specific tests for non-Gaussian dynamics are made. No deviations from Gaussian dynamics are found. This finding is incompatible with rate heterogeneity as a cause of the nonexponential relaxation and appears to be in conflict with an earlier multidimensional analysis of the same data. Although this conflict is not resolved here, this work does demonstrate the practicality of mode-correlation analysis in the face of finite datasets and calculations.

Coarse-graining strategy for modeling effective, highly diffusive fluids with reduced polydispersity: A dynamical study

We present a coarse-graining strategy for reducing the number of particle species in mixtures to achieve a simpler system with higher diffusion while preserving the total particle number and characteristic dynamic features. As a system of application, we chose the bidisperse Lennard-Jones-like mixture, discovered by Kob and Andersen [Phys. Rev. Lett. 73, 1376 (1994)], possessing a slow dynamics due to the fluid's multi-component character with its apparently unconventional choice for the pair potential of the type-A-type-B arrangement. We further established in a so-formed coarse-grained and temperature-independent monodisperse system an equilibrium structure with a radial distribution function resembling its mixture counterpart. This one-component system further possesses similar dynamic features such as glass transition temperature and critical exponents while subjected to Newtonian mechanics. This strategy may finally lead to the manufacturing of new nanoparticle/colloidal fluids by experimentally modeling only the outcoming effective pair potential(s) and no other macroscopic quantity.

The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro- methylsulfonyl)imide Ionic Liquids

Extensive atomistic simulations demonstrated that a gradual substitution of hexyl chains with phenyl groups in tetraalkylphosphonium cations results in remarkable changes in hydrogen bonding interactions, liquid structures and scattering structural functions, and rotational dynamics of hexyl chains and phenyl groups in tetraalkylphosphonium bis(trifluoromethylsulfonyl)imide ionic liquids. Hydrogen donor sites in hexyl chains present competitive characteristics with those in phenyl groups in coordinating anions, as well as their continuous and intermittent hydrogen bonding dynamics. Cation-cation and anion-anion spatial correlations show concomitant shift to short distances with decreased peak intensities with variations of cation structures, whereas cation-anion correlations have a distinct shift to large radial distances due to decreased associations of anions with neighboring cations. These microstructural changes are qualitatively manifested in shifts of prominent peaks for prevalent charge alternations and adjacency correlations between ion species in scattering structural functions. Meanwhile, rotational dynamics of hexyl chains speed up, which, in turn, slow down rotations of phenyl groups, whereas anions exhibit imperceptible changes in their rotational dynamics. These computational results are intrinsically correlated with conformational flexibilities, molecular sizes, and steric hindrance effects of phenyl groups in comparison with hexyl chains, and constrained distributions of anions around cations in heterogeneous ionic environments.

Transport dynamics of complex fluids

Thermal motion in complex fluids is a complicated stochastic process but ubiquitously exhibits initial ballistic, intermediate subdiffusive, and long-time diffusive motion, unless interrupted. Despite its relevance to numerous dynamical processes of interest in modern science, a unified, quantitative understanding of thermal motion in complex fluids remains a challenging problem. Here, we present a transport equation and its solutions, which yield a unified quantitative explanation of the mean-square displacement (MSD), the non-Gaussian parameter (NGP), and the displacement distribution of complex fluids. In our approach, the environment-coupled diffusion kernel and its time correlation function (TCF) are the essential quantities that determine transport dynamics and characterize mobility fluctuation of complex fluids; their time profiles are directly extractable from a model-free analysis of the MSD and NGP or, with greater computational expense, from the two-point and four-point velocity autocorrelation functions. We construct a general, explicit model of the diffusion kernel, comprising one unbound-mode and multiple bound-mode components, which provides an excellent approximate description of transport dynamics of various complex fluidic systems such as supercooled water, colloidal beads diffusing on lipid tubes, and dense hard disk fluid. We also introduce the concepts of intrinsic disorder and extrinsic disorder that have distinct effects on transport dynamics and different dependencies on temperature and density. This work presents an unexplored direction for quantitative understanding of transport and transport-coupled processes in complex disordered media.

Understanding the charging dynamics of an ionic liquid electric double layer capacitor via molecular dynamics simulations

We investigate the charging phenomena of an electric double layer capacitor (EDLC) by conducting both equilibrium and non-equilibrium molecular dynamics (MD) simulations. A graphene electrode and 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]+[SCN]-) ionic liquid were used as a system for the EDLC. We clarify the ionic layer structure and show that an abrupt change of the ionic layers leads to a high differential capacitance of the EDLC. The charging simulations reveal that the charging dynamics of the EDLC is highly dependent on the rearrangement of the ionic layer structure. Particularly, the electrode charge during the charging process is consistent with the perpendicular displacement of ionic liquid molecules. From this property, we analyze the contribution of each molecular ion to the electrode charge stored during charging. Charging of the EDLC is largely dependent on the desorption of the co-ions from the electrode rather than the adsorption of the counter-ions. In addition, the contribution of bulk ions to the charge stored in the EDLC is as important as that of ions adjacent to the electrode surface contrary to the conventional viewpoint. From these results, we identify the charging mechanism of the EDLC and discuss the relevance to experimental results. Our findings in the present study are expected to play an important role in designing an efficient EDLC with a novel perspective on the charging of the EDLC.

Slow Dynamics of Ring Polymer Melts by Asymmetric Interaction of Threading Configuration: Monte Carlo Study of a Dynamically Constrained Lattice Model

Abnormally slower diffusional processes than its internal structure relaxation have been observed in ring polymeric melt systems recently. A key structural feature in ring polymer melts is topological constraints which allow rings to assume a threading configuration in the melt phase. In this work, we constructed a lattice model under the assumption of asymmetric diffusivity between two threading rings, and investigated a link between the structural correlation and its dynamic behavior via Monte Carlo simulations. We discovered that the hierarchical threading configurations render the whole system to exhibit abnormally slow dynamics. By analyzing statistical distributions of timescales of threading configurations, we found that the decoupling between internal structure relaxation and diffusion is crucial to understand the threading effects on the dynamics of a ring melt. In particular, in the limit of small but threaded rings, scaling exponents of the diffusion coefficient D and timescale τ diff with respect to the degree of polymerization N agree well with that of the annealed tree model as well as our mean-field analysis. As N increases, however, the ring diffusion abruptly slows down to the glassy behavior, which is supported by a breakdown of the Stokes⁻Einstein relation.

Electrostatic interactions in soft particle systems: mesoscale simulations of ionic liquids

Computer simulations provide a unique insight into the microscopic details, molecular interactions and dynamic behavior responsible for many distinct physicochemical properties of ionic liquids. Due to the sluggish and heterogeneous dynamics and the long-ranged nanostructured nature of ionic liquids, coarse-grained meso-scale simulations provide an indispensable complement to detailed first-principles calculations and atomistic simulations allowing studies over extended length and time scales with a modest computational cost. Here, we present extensive coarse-grained simulations on a series of ionic liquids of the 1-alkyl-3-methylimidazolium (alkyl = butyl, heptyl-, and decyl-) family with Cl, [BF4], and [PF6] counterions. Liquid densities, microstructures, translational diffusion coefficients, and re-orientational motion of these model ionic liquid systems have been systematically studied over a wide temperature range. The addition of neutral beads in cationic models leads to a transition of liquid morphologies from dispersed apolar beads in a polar framework to that characterized by bi-continuous sponge-like interpenetrating networks in liquid matrices. Translational diffusion coefficients of both cations and anions decrease upon lengthening of the neutral chains in the cationic models and by enlarging molecular sizes of the anionic groups. Similar features are observed in re-orientational motion and time scales of different cationic models within the studied temperature range. The comparison of the liquid properties of the ionic systems with their neutral counterparts indicates that the distinctive microstructures and dynamical quantities of the model ionic liquid systems are intrinsically related to Coulombic interactions. Finally, we compared the computational efficiencies of three linearly scaling O(N log N) Ewald summation methods, the particle-particle particle-mesh method, the particle-mesh Ewald summation method, and the Ewald summation method based on a non-uniform fast Fourier transform technique, to calculate electrostatic interactions. Coarse-grained simulations were performed using the GALAMOST and the GROMACS packages and hardware efficiently utilizing graphics processing units on a set of extended [1-decyl-3-methylimidazolium][BF4] ionic liquid systems of up to 131 072 ion pairs.

Molecular dynamics study of mesophase transitions upon annealing of imidazolium-based ionic liquids with long-alkyl chains

Molecular dynamics simulations are performed on a 1-dodecyl-3-methylimidazolium hexafluorophosphate ([C12mim][PF6]) ionic liquid using a united-atom model. The ionic liquid exhibits second step relaxation at temperatures below a crossover point, where the diffusion coefficient shows an Arrhenius to non-Arrhenius transition. Annealing below this crossover temperature makes an isotropic to mesophase transition, where the smectic A (SmA) phase or crystal-like smectic B (SmB) phase forms. Hundreds of nanoseconds are required for completing these transitions. A normal diffusion process is found for anions along the layer-normal and -lateral directions in the SmA phase, but only in the lateral directions in the SmB phase. We find a preserved orientational order for the imidazolium-ring rotational and the alkyl-chain reorientational dynamics in both of the smectic phases.

Cation Dynamics in Supercooled and Solid Alkyl Methylimidazolium Bromide Ionic Liquids

The molecular dynamics of alkyl methylimidazolium bromide ionic liquids with different side groups of the cation are studied over a wide range of temperatures, covering the supercooled and crystalline states. Nuclear magnetic resonance relaxation dispersion (NMRD) at different magnetic field strengths was combined with NMR pulsed field gradient (PFG) diffusion measurements in order to obtain a description of the temperature dependence of the cationic mobility. While an Arrhenius dependence of the correlation times was found at high temperatures, a deviation is observed below a critical temperature of T_dyn ∼ 275 K which corresponds to about 1.25 T_g for two of the substances. The macroscopic diffusion coefficient, on the other hand, is best described by a VFT dependence down to a similar temperature, and a much weaker temperature dependence below. Measurements carried out in the crystalline state of 1-butyl-3-methylimidazolium bromide (Bmim Br) exhibit a dramatically increased self-diffusion coefficient in agreement with earlier reports of strong dynamic heterogeneity in the presence of minute amounts of water.

Excitation-energy dependence of solvation dynamics in room-temperature ionic liquids

Influence of the excitation energy of a probe solute molecule on its solvation dynamics and emission spectrum in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF6 (-)) is studied via molecular dynamics simulations using a coarse-grained model description. By exciting the probe at different energies, each with an extremely narrow distribution, ensuing solvent relaxation and its dynamic variance are monitored using the isoconfigurational ensemble method. Resulting Stokes shift function, S(t), indicates that long-time solvent relaxation becomes slower with the decreasing excitation energy and approaches the equilibrium correlation function, C(t), of solvent fluctuations. This suggests that the system excited at the red-edge of the spectrum observes linear response better than that at the blue-edge. A detailed analysis of nonequilibrium trajectories shows that the effect of initial configurations on variance of relaxation dynamics is mainly confined to short times; it reaches a maximum around 0.1 ≲ t ≲ 1 ps and diminishes as time further increases. The influence of the initial velocity distribution, on the other hand, tends to grow with time and dominates the long-time variations of dynamics. The emission spectrum shows the red-edge effect in accord with previous studies.

Heterogeneous dynamics and its length scale in simple ionic liquid models: a computational study

We numerically investigate the dynamic heterogeneity and its length scale found in coarse-grained ionic liquid model systems.

Unusual linear dependency of viscosity with temperature in ionic liquid/water mixtures

The unusual linear scaling of the self-diffusion coefficient and viscosity leads to the violation of the SE and FSE equations and shows dynamic heterogeneity.