Molecular dynamics simulation of the fragile glass former orthoterphenyl: a flexible molecule model. II. Collective dynamics

Molecular Insights into Dipole Relaxation Processes in Water-Lysine Mixtures

Dielectric spectroscopy is a robust method to investigate relaxations of molecular dipoles. It is particularly useful for studies of biological solutions because of the potential of this method to cover a broad range of dynamical time scales typical for such systems. However, this technique does not provide any information about the nature of the molecular motions, which leads to a certain underemployment of dielectric spectroscopy for gaining microscopic understanding of material properties. For such detailed understanding, computer simulations are valuable tools because they can provide information about the nature of molecular motions observed by, for example, dielectric spectroscopy and to further complement them with structural information. In this work, we acquire information about the nature of dipole relaxation, in n-lysine solutions by means of molecular dynamics simulations. Our results indicate that the experimentally observed main relaxation process of n-lysine has different origins for the single monomer and the polypeptide chains. The relaxation of 1-lysine is due to the motions of whole molecules, whereas the experimentally observed relaxation of 3-lysine and 4-lysine is due to the motions of the residues, which, in turn, are promoted by water relaxation. Furthermore, we propose a new structural model of the lysine amino acids, which can quantitatively account for the experimental dielectric relaxation data. Hydrogen bonding and the structure of water are also discussed in terms of their influence on relaxation processes.

Discontinuity in Fast Dynamics at the Glass Transition of ortho-Terphenyl

The dynamics of the molecular glass former ortho-terphenyl through the glass transition were observed with two-dimensional infrared vibrational spectroscopy measurements of spectral diffusion using the small probe molecule phenylselenocyanate. Although the slow diffusive motions were not visible on the experimental time scale, a picosecond-scale exponential relaxation was observed at temperatures from above to well below the glass transition temperature. The characteristic time scale has a smooth temperature dependence from the liquid into the glass phase, but the range of vibrational frequencies the probe samples displayed a discontinuity at the glass transition temperature. Complementary pump-probe experiments associate the observed motion with density fluctuations. The key features of the dynamics are reproduced with a simple corrugated well potential energy surface model. In addition, the temperature dependence of the homogeneous vibrational dephasing was found to have a T² functional form, where T is the absolute temperature.

Slow processes in supercooled o-terphenyl: relaxation and decoupling

We mapped the relaxation times of inter- and intramolecular correlations in o-terphenyl by a quasielastic scattering method using nuclear resonant scattering of synchrotron radiation. From the obtained map, we found that the slow β process is decoupled from the α process at 278 K, and this temperature is clearly below the previous decoupling temperature of 290 K, at which the α-relaxation dynamics changes. Then, it was also concluded that sufficient solidlike condition achieved by further cooling from 290 K is required to decouple the slow β process from the α process and, due to the difference of the length scales between the α and the slow β processes, these two averaged relaxation times <τ> are concluded not to cross as an extrapolation assumed so far. Furthermore, evidence of the restricted dynamics of the slow β process could be obtained as an anomalous momentum transfer (q) dependence of <τ>(<τ> ∝q(-2.9)) at 265 K, observed at q values of 18-48  nm(-1).

Cooperative dipolar relaxation of a glycerol molecular cluster in nanoscale confinement-a computer simulation study

We performed an all-atoms molecular dynamics simulation of a glycerol molecular cluster confined in single-walled carbon nanotubes of different diameters to study the confinement size effect on the dipolar relaxation of glycerol molecules. We show that the many-body approach proposed by Dissado and Hill can be directly applied to simulation data and provides quantitative information concerning the cooperative nature of the dipolar relaxation of molecules in nanoscale confinement.

Molecular Dynamics Study of Translational and Rotational Diffusion in Liquid Ortho-terphenyl

NVT molecular dynamics simulations were performed on liquid o-terphenyl as a function of temperature in the range 320-480 K. Computed translational diffusion coefficients displayed the non-Arrhenius behavior expected of a fragile glass-forming liquid and were in good, semiquantitative agreement with experimental results. Rotational correlation functions calculated for various vectors within the molecule exhibited a very short time (0-1 ps) initial decay, followed by a reversal, which corresponds to free reorientation within the "solvent" cage prior to collision with a wall. Rotational correlation times of three orthogonal vectors fixed on the central benzene were close to equal at all temperatures, indicating nearly isotropic overall molecular reorientation. The average correlation times exhibited a non-Arrhenius temperature dependence and were in very good agreement with experimental values derived from 2D and 1H NMR relaxation times. Correlation times of vectors located on the lateral phenyl rings were used to calculate the "spinning" internal rotation diffusion coefficients, which were approximately twice as great as the overall rotational diffusion constants, indicating rapid internal rotation of the phenyl side groups over wide ranges of angle in the liquid.

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Complementary neutron spin-echo and x-ray experiments and molecular-dynamics simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2) glassy crystal. Static, single-molecule reorientational dynamics and collective dynamics properties are investigated. Our results confirm the strong analogy between molecular liquids and plastic crystals. The orientational disorder is characterized at different temperatures and a change in the nature of rotational dynamics is observed. A careful check of the rotational diffusion model is performed using self-angular correlation functions Cl with high l values and compared to results obtained on molecular liquids composed of A-B dumbbells. Below the crossover temperature at which slow dynamics emerge, we show that some scaling predictions of the mode coupling theory hold and that alpha-relaxation times and nonergodicity parameters are controlled by the nontrivial static correlations.

Analogy of the slow dynamics between the supercooled liquid and supercooled plastic crystal states of difluorotetrachloroethane

Slow dynamics of difluorotetrachloroethane in both supercooled plastic crystal and supercooled liquid states have been investigated with molecular dynamics simulations. The temperature and wave-vector dependence of collective dynamics in both states are probed using coherent dynamical scattering functions S (Q,t). Our results confirm the strong analogy between molecular liquids and plastic crystals for which alpha-relaxation times and nonergodicity parameters are controlled by the nontrivial static correlations S (Q), as predicted by the mode coupling theory.

Structural relaxation in supercooled orthoterphenyl

We report molecular-dynamics simulation results performed for a model of molecular liquid orthoterphenyl in supercooled states, which we then compare with both experimental data and mode-coupling-theory (MCT) predictions, aiming at a better understanding of structural relaxation in orthoterphenyl. We pay special attention to the wave number dependence of the collective dynamics. It is shown that the simulation results for the model share many features with experimental data for real system, and that MCT captures the simulation results at the semiquantitative level except for intermediate wave numbers connected to the overall size of the molecule. Theoretical results at the intermediate wave number region are found to be improved by taking into account the spatial correlation of the molecule's geometrical center. This supports the idea that unusual dynamical properties at the intermediate wave numbers, reported previously in simulation studies for the model and discernible in coherent neutron-scattering experimental data, are basically due to the coupling of the rotational motion to the geometrical-center dynamics. However, there still remain qualitative as well as quantitative discrepancies between theoretical prediction and corresponding simulation results at the intermediate wave numbers, which call for further theoretical investigation.

Dynamics and configurational entropy in the Lewis-Wahnström model for supercooled orthoterphenyl

We study thermodynamic and dynamic properties of a rigid model of the fragile glass-forming liquid orthoterphenyl. This model, introduced by Lewis and Wahnström in 1993, collapses each phenyl ring to a single interaction site; the intermolecular site-site interactions are described by the Lennard-Jones potential whose parameters have been selected to reproduce some bulk properties of the orthoterphenyl molecule. A system of N=343 molecules is considered in a wide range of densities and temperatures, reaching simulation times up to 1 micros. Such long trajectories allow us to equilibrate the system at temperatures below the mode coupling temperature T(c) at which the diffusion constant reaches values of order 10(-10) cm(2)/s and thereby to sample in a significant way the potential energy landscape in the entire temperature range. Working within the inherent structures thermodynamic formalism, we present results for the temperature and density dependence of the number, depth and shape of the basins of the potential energy surface. We evaluate the total entropy of the system by thermodynamic integration from the ideal-noninteracting-gas state and the vibrational entropy approximating the basin free energy with the free energy of 6N-3 harmonic oscillators. We evaluate the configurational part of the entropy as a difference between these two contributions. We study the connection between thermodynamical and dynamical properties of the system. We confirm that the temperature dependence of the configurational entropy and of the diffusion constant, as well as the inverse of the characteristic structural relaxation time, are strongly connected in supercooled states; we demonstrate that this connection is well represented by the Adam-Gibbs relation, stating a linear relation between logD and the quantity 1/TS(c). This relation is found to hold both above and below the critical temperature T(c)-as previously found in the case of silica-supporting the hypothesis that a connection exists between the number of basins and the connectivity properties of the potential energy surface.