Molecular theory on dielectric constant at interfaces: a molecular dynamics study of the water/vapor interface

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down. Some computational issues pertinent to Ewald calculations of the dielectric properties are also discussed.     

Molecular dynamics simulation of liquid water confined inside graphite channels: dielectric and dynamical properties

Electric and dielectric properties and microscopic dynamics of liquid water confined between graphite slabs are analyzed by means of molecular dynamics simulations for several graphite-graphite separations at ambient conditions. The electric potential across the interface shows oscillations due to water layering, and the overall potential drop is about -0.28 V. The total dielectric constant is larger than the corresponding value for the bulklike internal region of the system. This is mainly due to the preferential orientations of water nearest the graphite walls. Estimation of the capacitance of the system is reported, indicating large variations for the different adsorption layers. The main trend observed concerning water diffusion is 2-fold: on one hand, the overall diffusion of water is markedly smaller for the closest graphite-graphite separations, and on the other hand, water molecules diffuse in interfaces slightly slower than those in the bulklike internal areas. Molecular reorientational times are generally larger than those corresponding to those of unconstrained bulk water. The analysis of spectral densities revealed significant spectral shifts, compared to the bands in unconstrained water, in different frequency regions, and associated to confinement effects. These findings are important because of the scarce information available from experimental, theoretical, and computer simulation research into the dielectric and dynamical properties of confined water.     

液态水的分子动力学模拟

用分子动力学(MD)模拟方法在150~376K的温度范围内对液态水的微正则系统进行了研究。考察了液态水的结构及其性质。模拟采用了由从头算得出的柔性水-水相互作用势MCYL。对时间和空间的平均得出了液态中水分子几何构型及温度改变所引起的液态水结构变化。对径向分布函数gOH, gOO, gHH及配位数的分析表明, 在所考察的温度范围内, 每个水分子与相邻分子形成的氢键数为2~3, 水分子在参与的2个氢键中同时作为授受体。结合对振动谱的研究表明在低温时液态水形成的网络结构可能随温度的升高而形成小的簇结构。     

Molecular dynamics simulation of planar elongational flow at constant pressure and constant temperature

Molecular dynamics simulations of liquid systems under planar elongational flow have mainly been performed in the NVT ensemble. However, in most material processing techniques and common experimental settings, at least one surface of the fluid is kept in contact with the atmosphere, thus maintaining the sample in the NpT ensemble. For this reason, an implementation of the Nose-Hoover integral-feedback mechanism for constant pressure is presented, implemented via the SLLOD algorithm for elongational flow. The authors test their procedure for an atomic liquid and compare the viscosity obtained with that in the NVT ensemble. The scheme is easy to implement, self-starting and reliable, and can be a useful tool for the simulation of more complex liquid systems, such as polymer melts and solutions.     

Molecular dynamics simulation of cellobiose in water

The conformational behavior of cellobiose was studied by molecular dynamics simulation in a periodic box of waters. Several different initial conformations were used and the results compared with equivalent vacuum simulations. The average positions and rms fluctuations within single torsional conformations of cellobiose were affected only slightly by the solvent. However, water damped local torsional librations and transitions. The conformational energies of the solute and their fluctuations were also sensitive to the presence of solvent. Intramolecular hydrogen bonding was weakened relative to that observed in vacuo due to competition with solvating waters. All cellobiose hydroxyl groups participated in intermolecular hydrogen bonds with water, with approximately eight hydrogen bonds formed per glucose ring. The hydrogen bonding was predominantly between water hydrogens and solute hydroxyl oxygens. Intermolecular hydrogen bonding to ring and bridge oxygens was seldom present. The diffusion coefficients of both water and solute agree closely with experimental values. Water interchanged rapidly between the solvating first shell and the bulk on the picosecond time scale. © 1993 John Wiley & Sons, Inc.     

Molecular dynamics simulation of the water|nitrobenzene interface

The structure and dynamics of the neat water|nitrobenzene liquid|liquid interface are studied at 300 K using molecular dynamics computer simulations. The water is modeled using the flexible SPC potential, and the nitrobenzene is modeled using an empirically determined nitrobenzene potential energy function. Although nitrobenzene is a polar liquid with a large dielectric constant, the structure of the interface is similar to other water|non-polar organic liquid interfaces. Among the main structural features we describe are an enhancement of interfacial water hydrogen bonds, the specific orientation of water dipoles and nitrobenzene molecules, and a rough surface that is locally sharp. Surface roughness is also characterized dynamically. The dynamics of molecular reorientation are shown to be only mildly modified at the interface. The effect due to the polarizable many-body potential energy functions of both liquids is investigated and is found to affect only mildly the above results.     

Molecular dynamics simulation of liquid water: hybrid density functionals

The structure, dynamical, and electronic properties of liquid water utilizing different hybrid density functionals were tested within the plane wave framework of first-principles molecular dynamics simulations. The computational approach, which employs modified functionals with short-ranged Hartree-Fock exchange, was first tested in calculations of the structural and bonding properties of the water dimer and cyclic water trimer. Liquid water simulations were performed at the state point of 350 K at the experimental density. Simulations included three different hybrid functionals, a meta-functional, four gradient-corrected functionals, and the local density and Hartree-Fock approximations. It is found that hybrid functionals are superior in reproducing the experimental structure and dynamical properties as measured by the radial distribution function and self-diffusion constant when compared to the pure density functionals. The local density and Hartree-Fock approximations show strongly over- and understructured liquids, respectively. Hydrogen bond analysis shows that the hybrid functionals give slightly smaller average numbers of hydrogen bonds than pure density functionals but similar hydrogen bond populations. The average molecular dipole moments in the liquid from the three hybrid functionals are lower than those of the corresponding pure density functionals.     

Brownian dynamics simulation of a model simple electrolyte in solvents of low dielectric constant

Brownian dynamics simulations are performed to investigate the ionic transport of model simple electrolytes, in which ions are interacting with each other through the repulsive core and Coulombic interactions. The equivalent conductivity and self-diffusion coefficient show minima as the function of the number density of ions when the dielectric constant of the solvent is low. Although the minimum of the former is in harmony with various experiments, no experiment has ever been reported on that of the latter. The analysis of time-dependent transport coefficients reveals that the presence of the minima is ascribed to the slow dynamics, rather than to static association models. The inclusion of a model function that resembles the short-range part of the potential of mean force induced by solvent affects the transport coefficients qualitatively, which suggests the importance of solvent-induced potential of mean force in the conduction mechanism of electrolytes in solvents of low dielectric constant.     

Conductance of Cs+ ion in water: Molecular dynamics simulation

The continuum theory of Hubbard-Onsager predicts for the friction coefficients the following behavior: >0 and /P<0. In contrast to Hubbard-Onsager theory, experimental observations on Cs⁺ ion in water show that at low temperatures <0 and /P>0. To explain the observed behavior of Nakahara et al. proposed the passage through cavities (PTC) mechanism. We performed a molecular dynamics computer simulation to determine if the PTC mechanism is responsible for the observed behavior of . No passage through cavities was observed. Molecular dynamics computer simulations were performed on Cs⁺ ion in water at temperature of 268 K and densities of 1.00 and 1.083 g-cm^–3. Our results indicate that the observed behavior of for Cs⁺ ion is related to the difference in the reorientation times of water molecules in the solvation shell and in the bulk.     

Molecular flexibility effects upon liquid dynamics

Simulation results for the diffusive behavior of polymer chain/penetrant systems are analyzed. The attractive range and flexibility of simple chain molecules were varied in order to gauge the effect on dynamics. In all cases, the dimensionless diffusion coefficient, D*, is found to be a smooth, single-valued function of the packing fraction, eta. The functions D*(eta) are found to be power laws with exponents that are sensitive to both chain stiffness and particle type. For a specific system type, the D*'s for both penetrant and chain-center-of-mass extrapolate to zero at the same packing fraction, eta0. This limiting packing fraction is interpreted to be the location of the glass transition, and (eta0-eta), the distance to the glass transition.     

Molecular dynamics simulation of liquid water between two walls

A molecular dynamics simulation, lasting ≈25 ps, has been performed with 150 ST2 water molecules between two quasi-hard repulsive walls, at a temperature of 302 K. A number of static and dynamic properties have been computed as a function of the distance from the walls, showing that water near the walls is in general more “ordered” than in the bulk, and that this bulk water behaves like ordinarv liquid ST2 water.     

Molecular dynamics simulation of a water/metal interface

First results of a molecular dynamics study of a water/metal interface, lasting 3.3 ps at an average temperature of 294 K, are reported. The basic periodic box contains 216 water molecules and a crystal slab of 550 platinum atoms with (100) surface planes. A combination of a Lennard-Jones potential between centers of mass and a Coulomb potential arising from dielectric interactions of the water charge distribution with the metal is employed for the water-wall interaction, the ST2 model for the water-water, and a nearest-neighbour harmonic potential for the platinum-platinum interactions. Considerable adsorption at the interface together with a drastic change of the water structure is observed.     

Molecular dynamics simulation of the association of nonpolar spheres in water

We apply molecular dynamics (MD) simulations to the study of the association of nonpolar spheres of effective radii between 1.6 and 6.1 A dissolved in water. The constrained MD method is used to calculate the potential of mean force (PMF) of the interaction between spheres. The depth of the potential of mean force increases with increasing radius of the nonpolar sphere. Our results suggest that the PMF is largely governed by size or entropic effects, and that energetic effects associated with the breaking or distortion of hydrogen bonds are of minor importance.     

The dielectric constant of water: influence of the quadrupole moment

We examine the suggestion that the very high dielectric constant of water is associated with the absence of a significant component of the quadrupole tensor along the direction of the dipole moment. We find that although ?_zz ≈ 0, there is a large “effective axial” moment ?^eff_zz.     

Molecular dynamics simulation of the PEO sulfonic acid anion in water

Atomistic molecular modeling has been used to study the sulfonic acid anion of poly(ethylene oxide) (PEO sulfonic acid anion) in vacuum and a polymer electrolyte system consisting of the PEO sulfonic acid anion in water. The vibrational spectra of the molecules were simulated by the local mode method and found to be in good agreement with the experimental IR and Raman spectra. The structure of PEO sulfonic acid anion was studied in vacuum and water and compared to the structure of an isolated PEO sulfonic acid in vacuum. The simulated value for the root mean square end-to-end distance for the PEO sulfonic acid anion was 22 Å in vacuum and 12 Å in water. The root mean square radius of gyration of the PEO sulfonic acid anion was 8.4 Å in vacuum and 5.6 Å in water. The PEO sulfonic acid anion was randomly coiled in water and in an extended shape in vacuum.     

Molecular dynamics simulation of the renin inhibitor H142 in water

Summary H142 is a synthetic decapeptide designed to inhibit renin, an enzyme acting in the regulation of blood pressure. The inhibiting effect of H142 is caused by a reduction of a-Leu-Val-peptide bond (i. e. C(=O)-NHCH₂-NH). The conformational and dynamical properties of H142 and its unreduced counterpart (H142n) was modelled by means of molecular dynamics simulations. Water was either included explicitly in the simulations or as a dielectric continuum. When water molecules surround the peptides, they remain in a more or less extended conformation through the simulation. If water is replaced by a dielectric continuum, the peptides undergo a conformational change from an extended to a folded state. It is not clear whether this difference is a consequence of a too short simulation time for the water simulations, a force-field artifact promoting extended conformations, or if the extended conformation represents the true conformational state of the peptide. A number of dynamic properties were evaluated as well, such as overall rotation, translational diffusion, side-chain dynamics and hydrogen bonding.     

Molecular dynamics simulation of pressure-driven water flow in silicon-carbide nanotubes

Many properties of silicon carbide (SiC) nanotubes, such as their high mechanical strength and resistance to corrosive environments, are superior to those of their carboneous counterparts, namely, carbon nanotubes (CNTs) and, therefore, SiC nanotubes can be a viable alternative to CNTs in a variety of applications. We employ molecular dynamics simulations to examine flow of water in SiC nanotubes and to study the differences and similarities with the same phenomenon in the CNTs. The simulations indicate that SiC nanotubes always provide larger flow enhancements than those reported for the CNTs. Moreover, a given flow enhancement in SiC nanotubes requires an applied pressure gradient that is at least an order of magnitude smaller than the corresponding value in a CNT of the same size.