Reinhard Hentschke

Molecular dynamics simulations of ordered alkane chains physisorbed on graphite

Scanning tunneling microscopy (STM) studies at the interface between the basal plane of graphite and organic solutions or melts of long chain alkanes and alkyl derivatives reveal that the molecules order in lamellae with the main molecular axes oriented parallel to the substrate. Here we employ molecular dynamics (MD) simulations to obtain more details on the molecular order and dynamics within the alkane lamellae as a function of density. We find that the orientation of the molecular carbon zigzag planes relative to the graphite is governed by a subtle interplay of packing and entropic effects. In addition, we consider multiple layer adsorption and investigate the rapid loss of order with increasing distance from the interface. Finally, we study the diffusive behavior of an isolated long chain alkane, C350H702, on graphite, which is of interest in the context of STM imaging of isolated macromolecules at interfaces. The sensitive dependence on atomic parameters renders MD simulations a valuable complement...

A molecular-dynamics simulation study of water on NaCl(100) using a polarizable water model

We carry out molecular-dynamics simulations of the water–NaCl(100) interface. The study includes the bulk interface at T=298 K and thin physisorbed films for coverages ranging from 0 to 1.5 at T=140 K. We use an efficient SPC/E based fluctuating charge water model to account for polarization effects. The water model is tested calculating cluster, gas, and liquid phase properties of neat water as well as structural and dynamic properties of solutions containing Na+-, Cl−-, and Ca2+-ions. For the bulk water–NaCl(100) system we analyze the surface induced hydration shell structure as well as residence times and the dipole orientation autocorrelation near the surface. At the low temperature we calculate the coverage dependence of the isosteric heat of adsorption, which is compared to available experimental data, including the coverage dependence of the adsorbate structure and dynamics. We note that our simulations support the formation of the (1×1) superstructure seen in helium scattering experiments.

Auger and resonant neutralization of low energy ions near metal surfaces

Abstract We provide a parameter-free perturbation theory calculation of the probability of charge capture via the direct Auger process to-slow ions scattering from simple metals. The results are sensitively dependent on the surface electron density profile and atomic wavefunction distortion in the near surface region. The time development of the atomic state occupation is examined in the limit of Auger coupling alone. The resonant and Auger transition rates to the 1s state of hydrogen scattered from aluminium, calculated using comparable models, are found to be essentially identical for ion-surface separations exceeding 2a0.

X-ray scattering study and molecular simulation of glass forming liquids: Propylene carbonate and salol

Wide angle x-ray scattering and classical molecular-dynamics simulations were employed to study structural and dynamic properties of two low-molecular weight glass-forming liquids, propylene carbonate, and salol. We observe pronounced changes in the liquid structure as a function of temperature in a wide temperature range bracketing the glass transition temperature. The experimental and simulation results compare the x-ray scattering from the liquid slightly and far above the experimental glass transition temperature, Tgexp. The simulations largely reproduce the characteristic behavior of the experimental x-ray intensities, which is interpreted in terms of clusterlike heterogeneities forming in the liquid as Tgexp is approached.

Molecular dynamics simulation study of the adsorption of chain alkanes from solution onto graphite

We use the molecular dynamics simulation method to study the adsorption of an n alkane from a benzene solution onto the basal plane of graphite. We show that it is possible to simulate the entire adsorption process, i.e., the diffusion of the alkanes to the surface, the penetration of the highly structured solvent layers near the surface, and the physisorption of the chains on the graphite. Specifically, we consider a solution of C14H30 in benzene confined between two graphite surfaces. The alkane chains are initially dispersed near the center of the slit where the solvent is bulklike. During the simulation, the chains are attracted by the surfaces where they adsorb replacing benzene molecules in the structured liquid near the surfaces. The alkane chains adsorb parallel to the graphite plane assuming predominately stretched conformations without tails extending into the solution. The simulation also shows evidence for the onset of self‐organization due to strong lateral interactions between the adsorbed m...

Dynamic Heterogeneities of Translational and Rotational Motion of a Molecular Glass Former from Computer Simulations

We employ classical molecular dynamics simulations to study the rotational and translational dynamics of propylene carbonate. The homogeneous and heterogeneous contributions to the nonexponential relaxation are studied. The non-Gaussian parameter is found to be strongly correlated with the degree of heterogeneity. Furthermore the coupling between translational and rotational motion is investigated. A dimensionless coupling constant is introduced which is found to increase with increasing time scale and decreasing temperature.

Liquid benzene confined between graphite surfaces. A constant pressure molecular dynamics study

We derive expressions for the internal and external stress tensor for a molecular system which is periodic in two dimensions and confined by an external potential along the third dimension. Based on the external pressure, we propose an extended‐phase‐space constant pressure molecular dynamics simulation method. We apply this method to simulations of liquid benzene confined between two graphite surfaces. The intramolecular interactions are thereby adopted from the amber force field, whereas the liquid‐surface interaction is described in terms of a static external potential acting on the benzene molecules. Our results test the method and provide a molecular picture of the surface induced structure of the liquid as well as its dynamical behavior near the surface. We find that the surface affects the structure of the liquid on a length scale significantly larger than the direct range of the liquid‐surface potential. Additionally, we observe a fast exchange of benzene molecules between the first and second liquid layer adjacent to the surface.We derive expressions for the internal and external stress tensor for a molecular system which is periodic in two dimensions and confined by an external potential along the third dimension. Based on the external pressure, we propose an extended‐phase‐space constant pressure molecular dynamics simulation method. We apply this method to simulations of liquid benzene confined between two graphite surfaces. The intramolecular interactions are thereby adopted from the amber force field, whereas the liquid‐surface interaction is described in terms of a static external potential acting on the benzene molecules. Our results test the method and provide a molecular picture of the surface induced structure of the liquid as well as its dynamical behavior near the surface. We find that the surface affects the structure of the liquid on a length scale significantly larger than the direct range of the liquid‐surface potential. Additionally, we observe a fast exchange of benzene molecules between the first and second liq...

On the origin of dynamic heterogeneities in glass-forming liquids

We employ classical Molecular Dynamics simulations to study the cooperative dynamics of two low molecular weight glass-formers, propylene carbonate and salol. The length scales of dynamic heterogeneities are estimated. After appropriate scaling both glass-formers display a similar temperature dependence of this length scale. Local structural properties like density, regularity, and potential energy are correlated with different local dynamical observables. We find that the dynamic heterogeneities are most strongly related to the local potential energy. To obtain an optimum correlation the local dynamics has to be characterized by the residence time.

Predicting Water Sorption and Volume Swelling in Dense Polymer Systems via Computer Simulation

Atomistic model structures of amorphous polyamide 6 (PA-6) and of an adhesive system consisting of the diglycidyl ether of bisphenol A (DGEBA) as epoxy resin and isophorone diamine (IPD) as a curing agent are generated. For the adhesive, we use a new approach for the generation of the cross-linked polymer networks. It takes into account the chemical reaction kinetics of the curing reaction and, therefore, results in more realistic network structures. On the basis of the corresponding model structures, the equilibrium water content and the swelling ratio of amorphous PA-6 and of the DGEBA+IPD networks are calculated via computer simulation for different thermodynamic conditions. A hybrid method is used combining the molecular dynamics technique with an accelerated test particle insertion method. The results are in reasonable agreement with experiments and, in the case of the PA-6 system, with results obtained via other computer simulation methods.

Swelling of a Model Network: A Gibbs-Ensemble Molecular Dynamics Study

We perform molecular simulations to study the swelling of a model network in contact with different chain-like solvents. The method is Gibbs-ensemble molecular dynamics, which is a molecular dynamics implementation of the concept underlying the Gibbs-ensemble Monte Carlo method. We simulate swelling isotherms for three coarse-grained solvents with one (S1), six (S6), and twelve (S12) centers in the high pressure regime at supercritical temperatures. The simulated swelling isotherms of the S1-solvent exhibit a maximum near the critical pressure which decreases and shifts towards higher pressures with increasing temperature. Based on a modified Flory–Huggins approach, a set of numerically soluble equations is derived in order to reproduce the simulated isotherms. The resulting theoretical isotherms are in very good qualitative agreement with the simulations. Comparison of the different solvents shows also that the swelling ratio decreases when the chain length of the solvent increases.