Heterogeneous dynamics of ionic liquids from molecular dynamics simulations

Dynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide studied by nuclear magnetic resonance dispersion and diffusion

NMR was used to study the dynamics of the ionic liquid Bmim-Tf2N in the supercooled regime. Rotation and translation molecular dynamics exhibit a transition at T_c∼1.26 T_g.

Time scale of dynamic heterogeneity in model ionic liquids and its relation to static length scale and charge distribution

We find a general power-law behavior: , where ζ_dh ≈ 1.2 for all the ionic liquid models, regardless of charges and the length scale of structural relaxation.

Molecular dynamics study of coagulation in silica-nanocolloid-water-NaCl systems based on the atomistic model

In the present work, large scale molecular dynamics (MD) simulations of nanocolloidal silica in aqueous NaCl solutions were performed using a fully atomistic model to study the microscopic structures and dynamics of the systems that lead to aggregation or gelation. Our attention is focused on the self-organizations that occur in the structures of the colloidal silica and water for various concentrations of NaCl. As the salt concentration increased, coagulation developed through the direct bonding of SiO4 units. The trend was explained by the systematic changes in the pair correlation functions related to the barrier height in the potential of mean force [J. G. Kirkwood, J. Chem. Phys., 1935, 3, 300]. Network structures of silica were visualised, and their fractal dimensions were examined by computing the running coordination numbers of Si-Si pairs and also by the analysis of two dimensional images. The calculated dimension by the former method was comparable to the experimental observations for the aggregation of silica colloids, and at longer length scales, super-aggregation was evident in the gelation process. The result from the 2D images is found to be insensitive to the differences in the structure. Clear changes in both the structure and mobility of the water were observed as the NaCl concentration increased, suggesting the importance of the solvent structures to these processes, although such a feature is lacking in the conventional models and most simulations of colloids.

The role of structural and fluidic aspects of room temperature ionic liquids in influencing the morphology of CdSe nano/microstructures grown in situ

RTILs as media to synthesize a variety of nanomaterials are gaining momentum owing to their unique physicochemical properties. However, the fundamental questions regarding the role of the inherent structure of the IL in directing the morphology and the growth mechanism of the nanoparticles are still unexplored. Therefore, an attempt was made in this respect wherein CdSe nanoparticles were synthesized in a neat room temperature ionic liquid (RTIL), 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), under ambient conditions. The IL was found to play three roles, as a solvent, as a stabilizing agent and as a shape directing template. The primary nanoparticles were of the sizes in the range of 2-5 nm, as determined by HR-TEM. These primary nanoparticles grow into nanoflake-like units which further self-assemble and transform into a mixture of anisotropic nanostructures (predominantly 2D sheets and flower-like 3D patterns) as revealed by the SEM studies. The co-existence as well as the stability of these nanomorphologies point towards the intrinsic microheterogeneity prevailing in the IL. Furthermore, the vibrational spectroscopic studies comprising of FT-IR and Raman spectroscopy clearly indicate a sort of accord involving the π-π stacked aromatic geometry and the hydrogen bonding network (between the cation and the anion) of the IL with the CdSe nanoparticles. Therefore, a suitable mechanism has been provided for the resulting anisotropic nanostructures on the basis of the structural and the fluidic aspects of the IL in conjunction with the surface properties of the transient morphologies involved in the process. To further supplement this, control experiments were facilitated by diluting the IL with different amounts of water and the morphology of the CdSe nanostructures was examined at respective mole fractions of water as well as at different time intervals.

Molecular dynamics study of one-component soft-core system: thermodynamic properties in the supercooled liquid and glassy states

Molecular dynamics simulations were performed to study the thermal properties of a supercooled liquid near the glass transition regime and of glasses in a one-component soft-core system with the pair potential φn(r) = ɛ(σ/r)(n), in which n = 12. The results are examined along a phase diagram, in which the compressibility factor defined by [Formula: see text] is plotted against the reduced density ρ* = ρ(ɛ/kBT)(3/n) (or the reduced temperature T* = ρ*(-n/3)). Similarly, a time-dependent dynamical compressibility factor can be plotted against the time-dependent reduced density [Formula: see text] (or the reduced time-dependent temperature). Analytical expressions of the specific heats CV and CP and of the entropy, S, were obtained as a function of [Formula: see text] or of the scaled potential U*. Even for a rapid cooling process, the CV values are found to be affected by non-equilibrium relaxations in the [Formula: see text] region, where [Formula: see text] is the given initial value of [Formula: see text]. The problem of the Kauzmann paradox is discussed using these expressions. The fluctuation of the time-dependent temperature, Tt*, which determines CV, is characterized by the spectra that are obtained by multitaper methods. The thermal fluctuation along the non-equilibrium relaxation under NVE conditions was also examined.

Cation dynamics in the pyridinium based ionic liquid 1-N-butylpyridinium bis((trifluoromethyl)sulfonyl) as seen by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been used to study the cation dynamics in the pyridinium based ionic liquid (IL) 1-N-butylpyridinium bis((trifluoromethyl)sulfonyl)imide (BuPy-Tf(2)N). This IL allows for a detailed investigation of the dynamics of the cations only, due to the huge incoherent scattering cross section of the cation (σ(inc)(cation) >> σ(inc)(anion)). The measured spectra can be decomposed into two Lorentzian lines, indicative of two distinct dynamic processes. The slower of these two processes is diffusive in nature, whereas the faster one can be attributed to localized motions. The temperature dependence of the diffusion coefficient of the slow process follows an Arrhenius law, with an activation energy of E(A) = 14.8 ± 0.3 kJ/mol. Furthermore, we present here results from experiments with polarized neutrons. These experiments clearly show that the slower of the two observed processes is coherent, while the faster one is incoherent in nature.

Molecular dynamics study of a one component soft-core system: thermodynamic properties in the crystalline state

In the present work, molecular dynamics (MD) simulations have been performed to study thermal properties of crystals in a one component soft-core system. The system was examined along the phase-diagram using the compressibility factor, P[combining tilde](T*) ≡PV/NkBT plotted against the reduced density ρ* (or reduced temperature, T*). The specific heat, CV, of the system is presented as the function of P[combining tilde](ρ*) and also as the function of the reduced potential energy, U*. These CV values are expressed using coefficients of the expansion around the harmonic term. We also derived the expression for entropy using these coefficients. The contribution of an anharmonic term in the specific heat and entropy was clarified by this. These results are compared with those obtained from the fluctuation of kinetic energy. Several quantities related to the distribution of time dependent reduced temperature and dynamical compressibility factor (PV/NkBT)t are also examined and three T* regions of thermal behaviors are characterized. In the diagram, the glass transition by cooling or compression can be treated on the same basis. Results of the present work will be helpful for a better understanding of the changes in the specific heat during the glass transition in the future works.

Nuclear magnetic resonance studies on the rotational and translational motions of ionic liquids composed of 1-ethyl-3-methylimidazolium cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts

Room temperature ionic liquids (ILs) are stable liquids composed of anions and cations. 1-ethyl-3-methyl-imidazolium (EMIm, EMI) is a popular and important cation that produces thermally stable ILs with various anions. In this study two amide-type anions, bis(trifluoro-methanesulfonyl)amide [N(SO(2)CF(3))(2), TFSA, TFSI, NTf(2), or Tf(2)N] and bis(fluorosulfonyl)amide [(N(SO(2)F)(2), FSA, or FSI] were investigated by multinuclear NMR spectroscopy. In addition to EMIm-TFSA and EMIm-FSA, lithium-salt-doped binary systems were prepared (EMIm-TFSA-Li and EMIm-FSA-Li). The spin-lattice relaxation times (T(1)) were measured by (1)H, (19)F, and (7)Li NMR spectroscopy and the correlation times of (1)H NMR, τ(c)(EMIm) (8 × 10(-10) to 3 × 10(-11) s) for the librational molecular motion of EMIm and those of (7)Li NMR, τ(c)(Li) (5 × 10(-9) to 2 × 10(-10) s) for a lithium jump were evaluated in the temperature range between 253 and 353 K. We found that the bulk viscosity (η) versus τ(c)(EMIm) and cation diffusion coefficient D(EMIm) versus the rate 1/τ(c)(EMIm) have good relationships. Similarly, linear relations were obtained for the η versus τ(c)(Li) and the lithium diffusion coefficient D(Li) versus the rate 1∕τ(c)(Li). The mean one-jump distances of Li were calculated from τ(c)(Li) and D(Li). The experimental values for the diffusion coefficients, ionic conductivity, viscosity, and density in our previous paper were analyzed by the Stokes-Einstein, Nernst-Einstein, and Stokes-Einstein-Debye equations for the neat and binary ILs to clarify the physicochemical properties and mobility of individual ions. The deviations from the classical equations are discussed.

Several routes to the glassy states in the one component soft core system: revisited by molecular dynamics

Molecular dynamics simulations have been performed to study the glass transition for the soft core system with a pair potential φ(n)(r) = ε(σ∕r)(n) of n = 12. Using the compressibility factor, PV/Nk(B)T=P̃(ρ*), its phase diagram can be represented as a function of a reduced density, ρ∗ = ρ(ε∕k(B)T)(3∕n), where ρ = Nσ(3)∕V. In the present work, NVE relaxations to the glassy or crystalline states starting from the unstable states in the phase diagram have been revisited in details and compared with other processes. Relaxation processes can be characterized by the time dependence of the dynamical compressibility factor (PV/Nk(B)T)(t) (≡g(ρ(t)*)) on the phase diagram. In some cases, g(ρ(t)*) reached a crystal branch in the phase diagram; however, metastable states are found in many cases. With connecting points for the metastable states in the phase diagram, we can define a glass branch where the dynamics of particles are almost frozen. The structures observed there have common properties characterized as glasses. Although overlaps of glass forming process and nanocrystallization process are observed in some cases, these behaviors are distinguishable to each other by the characteristics of structures. There are several routes to the glass branch and we suggest that all of them are the glass transition.

Many-ion Dynamics: The Common View of CM and MC

For solution of the problem of conductivity relaxation and diffusion of ions in ionic conductors with high density of ions, it is essential not to neglect treatment of the effects of many-ion dynamics. This view is shared by the Coupling Model (CM) and the MIGRATION CONCEPT (MC), although the treatment, emphasis and some predictions of the two models are different. Notwithstanding, a basic element is common to both models, namely the primitive relaxation, which performs two important functions. It terminates the caged ion dynamics at short times and initiates the many-ion dynamics at longer times. We demonstrate by experiments and molecular dynamics simulations the existence of the primitive relaxation, and the two functions it performs. The relation of the primitive relaxation time to the conductivity relaxation time predicted by the CM is shown to hold in all cases considered.

Correlations in the Structure and Dynamics of Ionic Liquids

A very great deal of the experimental work on room-temperature ionic liquids was done after high-performance computing became readily available for quantum-chemical or molecular-dynamic calculations. I explore the use of modern computational methods to guide or aid laboratory work, and the importance of ‘old-fashioned’ theory, from before the age of fast computers. Debye and Hückel published the first really important theoretical work on correlations between charged particles, and the Nernst–Einstein formula is still used to understand electrical conductivities. I assess the usefulness of all these theoretical methods and ideas, and discuss the particular difficulties presented by ionic liquids.