Neural Networks Reveal the Impact of the Vibrational Dynamics in the Prediction of the Long-Time Mobility of Molecular Glassformers.

Two neural networks (NN) are designed to predict the particle mobility of a molecular glassformer in a wide time window ranging from vibrational dynamics to structural relaxation. Both NNs are trained by information concerning the local structure of the environment surrounding a given particle. The only difference in the learning procedure is the inclusion (NN A) or not (NN B) of the information provided by the fast, vibrational dynamics and quantified by the local Debye-Waller factor. It is found that, for a given temperature, the prediction provided by the NN A is more accurate, a finding which is tentatively ascribed to better account of the bond reorientation. Both NNs are found to exhibit impressive and rather comparable performance to predict the four-point susceptibility χ4(t) at τα, a measure of the dynamic heterogeneity of the system.

Mutual Information in Molecular and Macromolecular Systems.

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari-Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation.

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Vibrational scaling of the heterogeneous dynamics detected by mutual information.

The correlations detected by the mutual information in the propensities of a molecular viscous liquid are studied by molecular-dynamics simulations. Dynamic heterogeneity is evidenced and two particle fractions with different mobility and relaxation identified. The two fractions exhibit the scaling of their relaxation in terms of the rattling amplitude of the particle trapped in the cage of the first neighbours 〈u2〉 . The scaling master curve does not differ from the one found for bulk systems, thus confirming identical results previously reported in other systems with strong dynamic heterogeneity as thin molecular films. The excitation of planar and globular structures at short and long times with respect to structural relaxation, respectively, is revealed. Some of the globular structures are different from the ones evidenced in atomic mixtures. States with equal 〈u2〉 are found to have identical time dependence of several quantities, referring to both bulk and the two fractions with heterogeneous dynamics, at least up to the structural relaxation time [Formula: see text].

Mutual information does not detect growing correlations in the propensity of a model molecular liquid.

The dynamical spatial correlations detected by the mutual information (MI) in the isoconfigurational particle displacements of a monodisperse molecular viscous liquid are studied via molecular-dynamics simulations by changing considerably both the molecular mobility and the degree of dynamical heterogeneity. Different from atomic liquids, the MI correlation length does not grow on approaching the glass transition by considering the liquid both in full detail as a collection of monomers and as a coarse-grained ensemble of molecular centers of mass. In the detailed picture, it is found that: (i) the MI correlations between monomers are largely due to inter-molecular correlations, (ii) the MI length scale is numerically identical, within the errors, to the correlation length scale of the displacement direction, as drawn by conventional correlation functions. The time evolution of the MI spatial correlations complies with the scaling between the fast vibrational dynamics and the long-time relaxation. Our findings suggest that the characteristics of the MI length scale are markedly system-dependent and not obviously related to dynamical heterogeneity.

Highly concentrated "solutions" of metal cations in ionic liquids: current status and future challenges.

In this article we summarize a series of conditions that allow highly concentrated "solutions" of metal cations in ionic liquids to be obtained, evidencing structural features and some of the most important physico-chemical properties of these systems. We try to emphasize aspects and problems that are not conventionally discussed in detail in the literature. In particular, we provide a full length discussion on the topics of: (i) solvation of metal salts in ionic liquids, (ii) anion coordinating ability and homologous and heterogeneous complexes. A brief outlook into future perspectives of metal containing ILs is also provided.

Excess entropy scaling of diffusion in room-temperature ionic liquids.

Excess entropy scaling relationships for diffusivity of ions in room-temperature ionic liquids are tested using molecular dynamics simulations for a model ionic liquid, dimethyl imidazolium chloride. The thermodynamic excess entropy of the single ions (estimated from the ion-ion pair correlation functions) is shown to be very strongly correlated with the diffusivity. An essential feature of these systems, the fact that the heavier and larger cation has a greater diffusivity with respect to the anion, is correctly captured by the excess entropy calculations, which estimates the diffusivity ratio between the two ions with noticeable precision.

Ab Initio Study of the Diels-Alder Reaction of Cyclopentadiene with Acrolein in a Ionic Liquid by KS-DFT/3D-RISM-KH Theory.

We study the Diels-Alder reaction between cyclopentadiene and acrolein in a model room-temperature ionic liquid ([mmim][PF6]) as a solvent. The calculations have been performed with the KS-DFT/3D-RISM-SCF theory, where the reactants and transition state (TS) have been represented at a QM level, while the solvent is represented by a 3D distribution of classical (charge + LJ) sites obtained by solving the 3D-RISM integral equation. We show that this method, being computationally efficient, is able to reproduce the main experimental features displayed by the experiments, concerning the reaction rate enhancement and augmentation of the endo/exo ratio in ionic liquids (ILs). We find that the IL distorts noticeably the transition state geometry, inverting the order of the frontier orbitals and leading to an enhancement of the asynchronicity of the reaction. Finally, we find, in agreement with recent work, that formation of the hydrogen bond between the unique C2 hydrogen of the imidazolium ring is not essential to explain the peculiar features of these reactions in ILs.

Ab initio study of ionic liquids by KS-DFT/3D-RISM-KH theory.

Properties of molecules solvated in ionic liquids (ILs) are strongly affected by solvent environment. For this reason, to give reliable results, ab initio calculations on solutes in ILs, including ions constituting ionic liquid itself, have to self-consistently account for the change of both electronic and classical solvation structure in ILs. Here, we study the electronic structure of the methyl-methylimidazolium ion in the bulk liquid of [mmim][Cl] by using the self-consistent field coupling of Kohn-Sham density functional theory and three-dimensional molecular theory of solvation (aka 3D-RISM) with the closure approximation of Kovalenko and Hirata. The KS-DFT/3D-RISM-KH method yields the 3D distribution of the IL solvent species around the [mmim] solute, underlying the most important peculiarities of this kind of systems such as the stacking interaction between neighboring cations, and reproduces the enhancement of the dipole moment resulting from the polarization of the cation by the solvent in a very good agreement with the results of an ab initio MD calculation. The KS-DFT/3D-RISM-KH method offers an accurate and computationally efficient procedure to perform ab initio calculations on species solvated in ionic liquids.

Solvation thermodynamics of alkali and halide ions in ionic liquids through integral equations.

In this work, we study the solvation thermodynamics and other solvation properties of small ions in two room-temperature ionic liquids, dimethyl imidazolium hexafluorophosphate [mmim] [pf6] and dimethyl imidazolium chloride [mmim][cl] with the reference interaction site model (RISM). The nature of the charge affects several aspects of solvation, from electrostriction to the mutual disposition of cations around the solute; nevertheless, the long-range screening behavior of the liquid appears to be insensitive to both charge and dimensions of the solute. The ion solvation is energy driven, as expected for the nature of the solvent, and displays a marked asymmetry between cation and anion solvation chemical potential. Such asymmetry is dependent, even qualitatively, on the ionic liquid chosen as solvent. Partial molar volumes of ions in solution are found to follow the nature of ion-solvent interaction.

A RISM approach to the liquid structure and solvation properties of ionic liquids.

We test for the first time the performance of the reference interaction site model (RISM) to predict the liquid structure and solvation of room-temperature ionic liquids (RTILs) represented with different degrees of accuracy. The model gives satisfactory results, proposing itself as a possible method to explore and to describe at a chemically realistic level the solvation shell in ionic liquids, which is believed to play a fundamental role in the static electronic and vibrational properties of these systems.

Equilibrium dynamics of an associating polymer melt in narrow slits by computer simulation.

Molecular dynamics simulations have been used to study the dynamics of a coarse-grained model of a melt of polymer chains with associating terminal groups, confined in a narrow slit by two layers of Lennard-Jones sites. Simulations were carried out as a function of wall separation and attracting strength. We found that confinement has an important effect on the overall dynamics of the system. Strongly attracting walls can significantly modify the dynamics of the melt, giving an aggregation structure with extremely long relaxation times. A noticeable degree of anisotropy was found for the dynamics of both the individual chains and the aggregates formed by the associating terminal groups.

Dynamics of relaxation of entangled polymers in shear flow.

The application of shear flow to entangled polymer melts can strongly modify its rheological and physicochemical behaviors, giving rise to an acceleration of several chemical processes such as diffusion-controlled reactions. In the present work, we investigate the modification of conformational and diffusive properties of an entangled polymer in shear flow by numerical methods. The flow affects both the conformational and diffusive properties of the system, giving rise to a quasinematic ordering of the macromolecules which take prolate spheroid shape with the main axis aligned to the shear direction. The shear flow is found to accelerate the overall diffusion of the chains in all directions at times longer than the polymer relaxation time. The polymer chains display a quite peculiar displacement behavior in direction parallel to the flow. At the same conditions, the linear relation between the diffusion constant in direction perpendicular to the flow and the inverse of the relaxation time, usually adopted in equilibrium regimes, is shown to hold even in the presence of flow.

Confinement effect in diffusion-controlled stepwise polymerization by Monte Carlo simulation.

Diffusion-controlled stepwise polymerization of a linear polymer confined in nanoscopic slits is simulated through a Monte Carlo approach. A noticeable influence of the confinement on the kinetics is found. The confinement modifies both the spatial pair distribution function and the diffusive properties of the polymers. As a consequence, the confined system can show either faster or slower reaction kinetics with respect to the bulk system, depending on the strength of intermolecular interactions. The predicted polydispersity of the polymer is in agreement with recent theories of diffusion-controlled stepwise polymerization, and can be slightly affected by the confinement.

Theoretical study of electromagnetic scattering by metal nanoparticles.

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role of particle-particle and tip-sample interactions. In this perspective, we investigate theoretically, at a very moderate level of sophistication, the optical behavior of simple silver nanoparticle aggregates, in terms of a formalism involving a multipolar expansion of the fields involved, along with a simplified model for the optical behavior of nanostructures previously developed. In particular, the tip-sample interaction is taken into account roughly, treating the tip as an additional, single particle, characterized by proper dielectric behavior.

Structure of an associating polymer melt in a narrow slit by molecular dynamics simulation.

Molecular dynamics simulation has been used to study the equilibrium properties of a generic coarse-grained polymer melt with associating terminal groups, confined in a narrow slit by two atomically smooth walls. Simulations were carried out as a function of wall separation and attracting strength as well as polymer end-end interaction strength. We find that confinement has an important effect on the melt properties. In particular, strongly attracting walls can produce radical changes in chain conformation, the nature of the transient network, and the structure of the aggregates formed by the associating terminals.