The relaxation dynamics of a confined glassy simple liquid

We use molecular-dynamics computer simulations to study the relaxation dynamics of a confined simple liquid. Two types of confining walls are considered: A rough wall and a smooth wall. The simulation is set up in such a way that the static properties of the confined system are identical to the ones of the bulk. Nevertheless, we find that upon cooling the relaxation dynamics of the confined systems differ strongly from the one of the bulk. In particular, we find that close to the rough/smooth wall this dynamics is slowed down/accelerated by orders of magnitude. Using these results we are able to extract a dynamical length scale of the system and we show that this length shows an Arrhenius dependence.Received: 1 January 2003, Published online: 14 October 2003PACS: 64.70.Pf Glass transitions - 68.15. + e Liquid thin films - 02.70.Ns Molecular dynamics and particle methods     

Studies of diffusion coefficients in disordered porous matrices confined in a slit-pore

We present a series of molecular dynamics simulations to study the structure of porous matrices confined in a slit-pore. The matrices were prepared by two different methods. In the first method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. In both methods the matrices were confined by two parallel walls (slit-pore) modeled by continuous solid surfaces. The results show that the matrix structure and porosity were affected by the method of preparation of the porous matrices. Moreover, we found smaller void cavities in these matrices than in matrices prepared without walls. Finally, diffusion of a fluid inside the matrices was investigated and it was found that the diffusion coefficient did not decrease with the fluid density, and presented a maximum at certain values of the fluid density.     

Segmental dynamics of polymers in nanoscopic confinements,as probed by simulations of polymer/layered-silicate nanocomposites

In this paper we review molecular modeling investigations of polymer/layered-silicate intercalates, as model systems to explore polymers in nanoscopically confined spaces. The atomic-scale picture, as revealed by computer simulations, is presented in the context of salient results from a wide range of experimental techniques. This approach provides insights into how polymeric segmental dynamics are affected by severe geometric constraints. Focusing on intercalated systems, i.e. polystyrene (PS) in 2 nm wide slit-pores and polyethylene-oxide (PEO) in 1 nm wide slit-pores, a very rich picture for the segmental dynamics is unveiled, despite the topological constraints imposed by the confining solid surfaces. On a local scale, intercalated polymers exhibit a very wide distribution of segmental relaxation times (ranging from ultra-fast to ultra-slow, over a wide range of temperatures). In both cases (PS and PEO), the segmental relaxations originate from the confinement-induced local density variations. Additionally, where there exist special interactions between the polymer and the confining surfaces (e.g., PEO) more molecular mechanisms are identified.Received: 1 January 2003, Published online: 14 October 2003PACS: 83.10.Rs Computer simulation of molecular and particle dynamics - 81.07.Nb Molecular nanostructures - 81.07.Pr Organic-inorganic hybrid nanostructures     

Slow dynamics of embedded fluid in mesoscopic confining systems as probed by NMR relaxometry

Mesoscopic media such as porous materials or colloidal dispersions strongly influence the dynamics of the embedded fluid. In the strong-adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long-time pathology, enlarging the time domain of the embedded-fluid dynamics towards the low-frequency regime. An interesting way to probe such a slow interfacial process is to use the field-cycling NMR relaxometry. This technique is used here to probe the fluid dynamics in two types of interfacial systems: i) a colloidal glass made of thin and flat particles; ii) a fully saturated porous media, the Vycor glass. Experimental results are critically compared to either a simple theoretical model of NMR dispersion involving elementary steps of the fluid dynamics near an interface (loops, trains, tails) or Brownian-dynamics simulations performed inside 3D reconstructions of these confined systems.Received: 1 January 2003, Published online: 14 October 2003PACS: 76.60.Es Relaxation effects - 61.43.Gt Powders, porous materials - 82.70.Dd Colloids     

Dynamics of wing cracks and nanoscale damage in glass

We investigate initiation, growth, and healing of wing cracks in confined silica glass by molecular dynamics simulations. Under dynamic compression, frictional sliding of precrack surfaces nucleates nanovoids which evolve into nanocrack columns at the precrack tip. Nanocrack columns merge to form a wing crack, which grows via coalescence with nanovoids in the direction of maximum compression. Lateral confinement arrests the growth and partially heals the wing crack. Growth and arrest of the wing crack occur repeatedly, as observed in dynamic compression experiments on brittle solids under lateral confinement.     

Radial-fluctuation-induced stabilization of the ordered state in two-dimensional classical clusters

Melting of two-dimensional (2D) clusters of classical particles is studied using Brownian dynamics and Langevin molecular dynamics simulations. The particles are confined either by a circular hard wall or by a parabolic external potential and interact through a dipole or a screened Coulomb potential. We found that, with decreasing strength of the interparticle interaction, clusters with a short-range interparticle interaction and confined by a hard wall exhibit a reentrant behavior in its orientational order.     

A generalized analytical theory for adsorption of fluids in nanoporous materials

An analytical theory is presented for the adsorption of fluids confined in zeolites, molecular sieves, and other nanoporous materials. The theory takes advantage of the localized adsorption sites within a zeolite and develops a statistical mechanical lattice model of adsorption. The theory is completely generalized and can be used to model the lattice of adsorption sites within any arbitrary zeolite. The theory also has the advantage of requiring very few parameters: it requires only 4 parameters to describe the adsorbent, which can be obtained from a potential energy map of the adsorbent. No molecular dynamics simulations are required for parametrization. The theory incorporates both the atomistic structure of the adsorbent and the fundamental physical mechanisms, which dictate the behaviour of fluids confined in nanoporous materials. Finally the theory has the practical advantage of computational efficiency. The theory can generate a complete isotherm in approximately 1 minute on a desktop PC (300 MHz), compared with tens of CPU hours of supercomputer or parallel cluster time necessary to perform the molecule dynamics simulations required to generate a few points of the same isotherm.     

Dynamics of capillary drying in water

We use atomistic simulations to address the question when capillary evaporation of water confined in a hydrocarbonlike slit is kinetically viable. Activation barriers and absolute rates of evaporation are estimated using open ensemble Monte Carlo-umbrella sampling and molecular dynamics simulations. At ambient conditions, the evaporation rate in a water film four molecular diameters thick is found to be of the order 10(5)(nm(2) s)(-1), meaning that water readily evaporates. Films more than a few nanometers thick will persist in a metastable liquid state. Dissolved atmospheric gas molecules do not significantly decrease the activation barrier.     

骨架柔性对短链烷烃分子在金属-有机骨架材料中扩散的影响

采用分子动力学方法,对短链烷烃在柔性和刚性具有相似拓扑结构的金属有机骨架材料(isoreticular metal-organic frameworks, IRMOFs)中的分子扩散进行了研究. 结果表明,分子在柔性骨架中的自扩散系数大于在刚性骨架中的自扩散系数,在柔性骨架中的活化能小于在刚性骨架中的活化能. 骨架的柔性对自扩散系数的影响随着温度的升高而增加,随着扩散分子数目以及扩散分子链长的增长而降低. 因此,在利用分子模拟方法研究烷烃在金属-有机骨架材料中的扩散行为时,尤其对于较高温度以及较短的烷烃分 关键词： 分子模拟 金属-有机骨架材料 柔性骨架 扩散     

Fluidity of hydration layers nanoconfined between mica surfaces

We perform molecular dynamics simulations to investigate the shear dynamics of hydration water nanoconfined between two mica surfaces at 1 bar pressure and 298 K. Newtonian plateaus of shear viscosity comparable to the bulk value for different hydration layers D=0.92-2.44 nm are obtained. The origin of this persistent fluidity of the confined aqueous system is found to be closely associated with the rotational dynamics of water molecules, accompanied by fast translational diffusion under this confinement.     

Molecular simulation of loading dependent slow diffusion in confined systems

An extension to transition state theory is presented that is capable of computing quantitatively the diffusivity of adsorbed molecules in confined systems at nonzero loading. This extension to traditional transition state theory yields a diffusivity in excellent agreement with that obtained by conventional molecular dynamics simulations. While molecular dynamics calculations are limited to relatively fast diffusing molecules or small rigid molecules, our approach extends the range of accessible time scales significantly beyond currently available methods. It is applicable in any system containing free energy barriers and for any type of guest molecule.     

Dynamic properties of propylene glycol confined in ZSM-5 zeolite matrix—A computer simulation study

We performed all atom computer simulations of molecular dynamics of propylene glycol confined in ZSM-5 zeolite host matrix in order to study the characteristic of dipolar relaxation process in this system and compare it with recently published results for similar molecular systems confined in single walled carbon nanotubes. We focused on the influence of the geometric confinement inside ZSM-5 1D channels and interaction with the host system on the observed change in the character of deviation from exponential relaxation, as well as on thermal activation characteristic of the process.     

Low-temperature behavior of water confined by biological macromolecules and its relation to protein dynamics

Confined water is an essential component of biological entities and processes and its properties differ from the ones of bulk water. Since protein and water dynamics are thought to be strongly coupled, and since macromolecular dynamics is crucial for biological function, the study of water confined by biological macromolecules is not only interesting on its own right but often provides useful information for understanding biological activity at the molecular level. Studies are reviewed that focus on the low-temperature behavior of water confined in protein crystals and in stacks of native biological membranes. Diffraction methods allowed the determination of characteristic changes that relate to the glass transition and crystallization of water. Protein crystallography and energy-resolved neutron scattering are employed to gain further insight into the coupling of solvent and protein dynamics.Received: 1 January 2003, Published online: 14 October 2003PACS: 87.15.He Dynamics and conformational changes - 87.50.Gi Ionizing radiations (ultraviolet, X-rays, gamma-rays, ions, electrons, positrons, neutrons, and mesons, etc.) - 64.70.Pf Glass transitions     

Influence of flow on the structure and dynamics of clusters in complex plasma systems

The influence of flow with different strengths, positions, and widths on the structure and dynamics of clusters is studied by two-dimensional (2D) Langevin molecular dynamics simulations. The particles are confined by a quadratic confining potential. The horizontal position of the system centre, average inter-particle distance, and coupling parameter are calculated to characterize the effect of changing the strength, position, and width of the flow on the cluster structure. The trajectories, the velocity autocorrelation function, and the mean square displacement are obtained to further uncover the dynamic properties of the 2D dusty plasma system.     

Excited states dynamics in time-dependent density functional theory

We present numerical simulations of femtosecond laser induced dynamics of some selected simple molecules -- hydrogen, singly ionized sodium dimer, singly ionized helium trimer and lithium cyanide. The simulations were performed within a real-space, real-time, implementation of time-dependent density functional theory (TDDFT). High harmonic generation, Coulomb explosion and laser induced photo-dissociation are observed. The scheme also describes non-adiabatic effects, such as the appearance of even harmonics for homopolar but isotopically asymmetric dimers, even if the ions are treated classically. This TDDFT-based method is reliable, scalable, and extensible to other phenomena such as photoisomerization, molecular transport and chemical reactivity.Received: 15 October 2003PACS: 33.80.Gj Diffuse spectra; predissociation, photodissociation - 33.80.Wz Other multiphoton processes     

Shear-induced undulation of smectic-$\mathsf{A}$: Molecular dynamics simulations vs. analytical theory

Experiments on a variety of systems have shown that layered liquids are unstable under shear even if the liquid layers are planes of constant velocity. We investigate the stability of smectic-A like liquids under shear using Molecular Dynamics simulations and a macroscopic hydrodynamic theory (including the layer normal and the director as independent variables). Both methods show an instability of the layers, which sets in above a critical shear rate. We find a remarkable qualitative and reasonable quantitative agreement between both methods for the spatial homogeneous state and the onset of the instability.Received: 2 October 2003, Published online: 9 March 2004PACS: 61.30.Dk Continuum models and theories of liquid crystal structure - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 05.70.Ln Nonequilibrium and irreversible thermodynamics - 83.10.Rs Computer simulation of molecular and particle dynamics - 83.50.Ax Steady shear flows, viscometric flowTh.Soddemann : Present address: Rechenzentrum der Max-Planck-Gesellschaft, Boltzmannstrasse 2, 85748 Garching, Germany     

Quantum effect induced reverse kinetic molecular sieving in microporous materials

We report kinetic molecular sieving of hydrogen and deuterium in zeolite rho at low temperatures, using atomistic molecular dynamics simulations incorporating quantum effects via the Feynman-Hibbs approach. We find that diffusivities of confined molecules decrease when quantum effects are considered, in contrast with bulk fluids which show an increase. Indeed, at low temperatures, a reverse kinetic sieving effect is demonstrated in which the heavier isotope, deuterium, diffuses faster than hydrogen. At 65 K, the flux selectivity is as high as 46, indicating a good potential for isotope separation.     

Status of experiments probing the dynamics of water in confinement

In many relevant situations, water is not in its bulk form but is instead attached to some substrates or filling some cavities. We shall call water in the latter environment confined or interfacial water as opposed to bulk water. This confined water is essential for the stability and function of biological macromolecules. In this review paper, we present the more recent up to date account of the dynamics of confined water as compared with that of bulk water. Various techniques are used to study the dynamics of confined water. Among them, quasi-elastic and inelastic neutron scattering is a powerful tool to study translational and rotational diffusion as well as vibrational density of states of confined water. Various examples involving water confined in porous media, adsorbed on surface of ionic crystals, in the presence of organic solutes and at the surface of biological molecules are presented. The combined effects of the hydration level and the temperature on the retardation of the water molecules motions are discussed on the basis of phenomenological models as well as of power law fits based on the Mode Coupling Theory.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.12.Ex Neutron scattering (including small-angle scattering) - 66.30.Pa Transport properties of condensed matter (nonelectronic): Diffusion in nanoscale solids     

Rotational dynamics and friction in double-walled carbon nanotubes

We report a study of the rotational dynamics in double-walled nanotubes using molecular dynamics simulations and a simple analytical model that reproduces the observations very well. We show that the dynamic friction is linear in the angular velocity for a wide range of values. The molecular dynamics simulations show that for large enough systems the relaxation time takes a constant value depending only on the interlayer spacing and temperature. Moreover, the friction force increases linearly with contact area and the relaxation time decreases with the temperature with a power law of exponent -1.53+/-0.04.     

Water alignment and proton conduction inside carbon nanotubes

First-principles molecular dynamics simulations have been carried out to investigate the structure, electronic properties, and proton conductivity of water confined inside single-walled carbon nanotubes. The simulations predict the formation of a strongly connected one-dimensional hydrogen-bonded water wire resulting in a net electric dipole moment directed along the nanotube axis. An excess proton injected into the water wire is found to be significantly stabilized, relative to the gas phase, due to the high polarizability of the carbon nanotube.