Lennard-Jones Walls Research Articles

Monitoring local order in the liquid-X interface

ABSTRACT We offer a comparison of two different characterizations of favored local structure in an inhomogeneous liquid, using as vehicles simulations of the interfaces between a Lennard-Jones liquid and: (i) a flat Lennard-Jones wall, (ii) the 100 face of a face-centred cubic Lennard-Jones crystal, and (iii) a Lennard-Jones vapor. The interfacial density distributions in (i) and (ii) have oscillations extending several particle diameters into the bulk liquid, while in (iii) it monotonically decays from the bulk liquid to the vapor. We search for transient ordered fluctuations using the Aperture Cross Correlation Function (ACCF) of the system. Particles in the in the two layers closest to the surface of the FCC-liquid interface exhibit a four-fold mirroring of the surface structure; no structure is found elsewhere in this or the other interfaces. The absence of transient ordered structure in the liquid–vapor interface yields a different picture of that interface than derived from the cluster analysis reported by Godonoga et al. (Mol. Phys., 109 (7–10), 1393–1402 (2011)). We argue the advantages of using the ACCF to characterize transient ordered fluctuations as being both experimentally accessible in favourable cases and not reliant on a pre-assumed list of possible structures.

Nanoconfinement in Slit Pores Enhances Water Self-Dissociation.

We investigate the self-dissociation of water that is nanoconfined between the sheets of a realistic layered mineral, FeS mackinawite, as well as between Lennard-Jones walls via abinitio simulations. By comparing it with the same reaction in bulk water under various thermodynamic conditions, we show that such strong two-dimensional confinement between hard surfaces greatly enhances the self-dissociation process of water-thus increasing its ionic product K_{w} due to nanoconfinement. In addition to providing free energies, we analyze in detail the underlying dielectric properties in terms of dipole moment distributions, and thus the polarity of the liquid, as well as local polarization fluctuations as quantified by dielectric tensor profiles perpendicular to the lamella.

Molecular Dynamics Study on Proton Transport in Supported Nafion Ionomer Thin Films on Lennard-Jones Walls

The catalyst layer, one of the components of membrane electrode assembly in polymer electrolyte fuel cells, was modeled by molecular dynamics methods, and proton transport in ionomer thin films was simulated to elucidate the effect of surface wetting interaction on the ionomer thin film morphology and proton transport property. Ionomer thin film properties are considered to be affected by the surface property of supported carbon due to their nano-scale thickness (4-10 nm thick). One of the surface properties is surface wetting interaction basically attributed to the physical or chemical property of surfaces.In this study, modeled Nafion thin films with 4 nm and 10 nm thick were generated on the two ideal LJ smooth surfaces which have different strength of interactions to focus on the effect of the surface wetting interaction on the ionomer thin film morphology and proton transport. Protons exist as hydronium ions in ionomer thin films for chemical stability and are transported by the two transport mechanisms which are the vehicle mechanism (just move as hydronium ions) and the Grotthuss mechanism called proton hopping. It is common that proton transport model is restricted to only the vehicle mechanism when using classical molecular dynamics method, however, our proton transport model includes the Grotthuss mechanism by using aTS-EVB method (anharmonic two-state empirical valence bond method) in order to make proton transport model more reproducible.From the analysis in the ionomer thin films with 4 nm thick, surface wetting interaction was found to affect the orientation of sulfonic group near the surface. The orientation is well-ordered on the higher hydrophobic surface, and the sulfonic groups point to the gas phase direction, while it is randomly distributed on the lower hydrophobic surface, which make differences in the water cluster connectivity and size in ionomer thin films between the two surfaces. Proton transport property was evaluated by the self-diffusion coefficient. The proton self-diffusion coefficient in ionomer thin films was indicated to be improved on higher hydrophobic surface probably because higher hydrophobic surface makes ideal water cluster structures for proton transport in ionomer thin films.As compared to the ionomers with the different film thickness, the proton self-diffusion coefficient in ionomer thin films on higher hydrophobic surface was indicated to decrease in the increase of the film thickness, while that on lower hydrophobic surface was indicated to increase slightly. This result is expected to come from the confinement and interface effects which can be remarkable when the thickness decrease. The ionomer morphology on higher hydrophobic surface has advantages for proton transport probably thanks to the effects as observed in the ionomer with 4 nm thick, while that on lower hydrophobic surfaces has disadvantage. Since the effects can be weaker with the increase in the film thickness, the proton self-diffusion coefficient in ionomer thin films on higher hydrophobic surface was found to decrease with the increase in the film thickness, while that on lower hydrophobic surface was found to increase slightly.

Molecular Dynamics Study on Proton Transport in Supported Nafion Ionomer Thin Films on Lennard-Jones Walls

Molecular dynamic simulations were carried out to investigate the effects of wettability of carbon supports and the thickness of thin films on proton transport. The simplified Lennard-Jones (LJ) wall model was used as the support model, which allows us to reproduce different hydrophobic walls by just tuning the LJ parameters. The thickness dependence was investigated in the observed thickness range in a catalyst layer: 4 and 10 nm. The proton transport property was evaluated in terms of self-diffusion coefficients (D H+) and water cluster structures formed in the thin films. The obtained D H+ indicated that the D H+ increases with increasing hydrophobicity of the wall. It is also indicated that, for the high hydrophobic wall case, the D H+ decreases with increasing the thickness of thin films, while, for the low hydrophobic wall case, the D H+ is almost the same. It was confirmed that these trends were associated with the water structures formed in the thin films, i.e., water-layer-like structures are formed when the D H+ are larger.

Molecular dynamics study of the effect of wettability of the carbon support on proton transport in Nafion ionomer thin films

Molecular dynamics simulations were carried out to elucidate the effect of the wettability of the carbon support used for Nafion ionomer thin films on proton transport in the ionomer, which is related to the power density of polymer electrolyte fuel cells. The Lennard-Jones wall model was used as the support model for the ionomer to generate two different hydrophobic walls: the high hydrophobic wall (H-wall) and the low hydrophobic wall (L-wall). The proton transport model, including the Grotthuss mechanism, was used to express real proton transport phenomena (In early work, we confirmed that it well reproduces the experimentally measured proton self-diffusion coefficient in a Nafion membrane.) The obtained proton self-diffusion coefficient (DH+) indicated that the DH+ for the H-wall case is larger than for the L-wall case. This is related to the morphology of the films. For the H-wall case, the sulfonic groups that form part of the hydrophilic Nafion side chains were confirmed to be oriented in the direction to the wall in the upper side of the film and opposite from the wall in the lower side of the film, which can lead to the alignment of Nafion molecules and also create lamellar water structures in the film. It was also confirmed that such water structures have better cluster connectivity and larger cluster size, meaning that they serve as better proton transport pathways.

Multilayer Adsorption of Methane and Chloromethane on the Molybdenum (100) Surface

Using all-atom molecular dynamics simulations, the multilayer adsorption of two single-carbon alkane analogues—methane (CH4) and chloromethane (CH3Cl)—in the liquid phase on a metallic substrate (molybdenum (100) surface) has been studied to elucidate differences in the adsorbed film structure as a function of the overall film thickness, system temperature, substrate mobility, and pairwise substrate interaction potential (Lennard-Jones 12-6 versus embedded atom method versus Lennard-Jones 9-3 “wall” potential, used as a basis for comparison). Simulations suggest a clear predilection to well-ordered packing arrangements dictated by the adsorbate molecular geometry when adsorbed on the flat, featureless Lennard-Jones wall, but adsorption on the atomistic molybdenum substrate exhibits a more complex structure and orientation influenced by the adsorbate molecular geometry but also dependent on the presence or lack of small variations in surface structure caused by substrate mobility and interatomic potential. The result illustrates differences in packing geometry and surface binding energy on very noticeable scales despite the magnitude of surface vibrations allowed.

Molecular dynamics simulations of the interaction between polyhydroxylated compounds and Lennard-Jones walls: preferential affinity/exclusion effects and their relevance for bioprotection

Molecular dynamics simulation is used to investigate the interaction of methanol (MET) and the polyhydroxylated cosolutes (CSLs) ethylene glycol (ETG), glycerol (GLY), glucose (GLU) and trehalose (TRH) in aqueous solution with the surface of rigid Lennard-Jones walls. The walls are designed to represent simplified models for biomolecular (membrane, protein) surfaces and include three functionalisation variants: (i) non-polar (NP, no functionalisation); (ii) semi-polar (SP, surface hydroxyl groups); and (iii) highly polar (HP, positive and negative surface charges). The simulations are performed to investigate, in a simplified context, the preferential affinity/exclusion properties of the different CSLs (compared to water), which are relevant for the phenomenon of bioprotection by polyhydroxylated compounds. The simulations are carried out at three different temperatures (300, 475 and 600 K), and a comparison with simulations involving pure water or the pure liquid CSLs MET, ETG and GLY is also undertaken. In aqueous solution, all CSLs considered evidence preferential affinity (compared to water) for the NP and SP walls. This effect increases in magnitude with increasing CSL size, and is due to the favourable driving force associated with the replacement of multiple water molecules at the wall surface by a single polyhydroxylated CSL molecule (the partial substitution occurring without a significant change in the number of wall-solution hydrogen bonds). The preferential affinity is significantly reduced for the HP wall (for MET, ETG and GLY, preferential exclusion is actually observed). This change is probably related to the higher efficiency of water (compared to the CSLs) in terms of electrostatic solvation (higher dielectric permittivity), and provides an interpretation for the observation that polyhydroxylated CSLs appear to show preferential affinity for the surface of membranes, but preferential exclusion for the surface of proteins.

Molecular simulation of adsorption from dilute solutions.

Adsorption of biomolecules on surfaces is a perennial and general challenge relevant to many fields in biotechnology. A change of the Helmholtz free energy DeltaA takes place when a molecule becomes adsorbed out of a bulk solution. The purpose of our investigations is to explore routes for the calculation of DeltaA by molecular simulations. DeltaA can be obtained both by integration over the mean force on a molecule and via the local density. It turns out that the route via the potential of mean force prevails over the latter due to better consistency. In this work we present results for systems of 1-centre and 2-centre Lennard-Jones mixtures at a 9/3 Lennard-Jones wall.

Structure of Lennard-Jones fluids confined in square nanoscale channels from density functional theory.

The density distribution of Lennard-Jones fluids confined in square nanoscale channels with Lennard-Jones walls has been studied using the nonlocal density functional theory (DFT) based on the Tarazona model. The effect of channel lengths on the density profiles with various chemical potentials was discussed. It was found that there is an apparent layering phenomenon for the confined fluids due to the combining influences of the enhancing solid-fluid interaction and the excluded volume effect. The pronounced density peaks were observed at the corners of square channels due to the strong fluid-solid interactions. The grand canonical ensemble Monte Carlo simulation (GCEMC) was applied to test the nonlocal DFT results. The DFT calculations are in relatively good agreement with the GCEMC simulations. The adsorption isotherms in a series of square channels were evaluated based on the obtained density distributions. The adsorption mechanism within the square pores was investigated. A comparison between the adsorptions of the square pores with those of the corresponding slit-size pores has been given.

Change of Free Energy during Adsorption of a Molecule

A change of the Helmholtz free energy ΔA takes place when a molecule becomes adsorbed out of a bulk solution. The purpose of the paper is to explore routes for the calculation of ΔA from molecular simulations. According to a derivation from the partition function, ΔA is equal to the change of the potential of mean force (PMF) which can be calculated via two different routes. First, the PMF can be obtained as a path integral over the mean force on the solute molecule which can be calculated via a simulation in which the solute molecule is fixed at a given position. Second, the PMF is related to the logarithm of the local density of the solute which can in principle be obtained via a simulation in which the solute molecule moves freely. Simulations are performed for a pure Lennard-Jones fluid and several dilute Lennard-Jones mixtures in contact with a plane Lennard-Jones wall. For the pure Lennard-Jones fluid, both simulation routes yield practically the same result for ΔA. In the case of the dilute solutions, however, simulations do not yield directly reliable local densities of the solute in those cases, in which there is not one very pronounced minimum in ΔA. On the other hand, the path integral over the mean force could be calculated always with reasonable accuracy to yield ΔA. Moreover, via the latter route also the local density of the dilute component can be obtained.

Molecular Dynamics Study of the Structure and Dynamics of Water in Cylindrical Pores

The structure and dynamics of confined water in cylindrical pores have been investigated by molecular dynamics simulations. Both rigid (TIP4P) and flexible (BJH) models have been used. Pore radii between 4.2 and 20 Å have been studied; the pore walls are modeled either as a smooth (10−4) Lennard-Jones wall or as a structured wall consisting of (12−6) Lennard-Jones particles. Polar functional groups on the pore surface are modeled by arrays of point charges. We present results on density and orientational distribution functions and on the water mobility. We observe that water transport through nonpolar pores is fast and dominated by the surface layer, whereas transport in polar pores is slowed down relative to bulk liquid water and occurs preferentially through the center of the pore.

Molecular dynamics simulation studies of the density profiles of water between (9-3) Lennard-Jones walls

The results of molecular dynamics computer simulations of water laminae confined between two (9-3) Lennard-Jones walls are reported. The interaction potential is identical to the one used in a recent singlet integral equation study [M. J. Booth, D.-M. Duh, and A. D. J. Haymet, J. Chem. Phys. 101, 7925 (1994)]. In the simulation studies the internal pressure, system size, and the treatment of long-range Coulomb interactions has been varied. Large density oscillations similar to the integral equation results have not been observed. Simulated pair correlation functions are found to exhibit only little anisotropy near the interface.

Wetting transitions at a solid–fluid interface

Nonlocal density functional free energy theories are used to investigate the nature of the wetting transition of a Lennard-Jones fluid near a Lennard-Jones wall. In particular, the wetting behavior is investigated for argon films on a solid carbon dioxide substrate for which the Lennard-Jones parameters are known. In accordance with previous predictions of local density functional theories, the transition is found to be of first order. The locus of the prewetting line of the Ar–CO2 system is calculated. Density profiles obtained from the Tarazona and generalized hard rod models are both in qualitative agreement with Monte Carlo simulations of Finn and Monson, although Tarazona’s model performs better quantitatively.

Computer simulations of water and aqueous electrolyte solutions at interfaces

The results of computer simulations of water and aqueous electrolyte solutions in contact with smooth and corrugated walls are reviewed. Oxygen atom density profiles have been calculated by various authors from simulations using different water models in contact with hard walls, Lennard-Jones type walls of various powers, and polarizable walls where the polarizability is described by image charges. These results for smooth walls are compared with the density profiles obtained for the vicinity of a corrugated platinum crystal. There is general agreement that the dipole vectors of the water molecules near the walls are preferentially oriented parallel to the interface. Dipole distribution functions are discussed with respect to the electrostatic surface potential drop. The inhomogeneities and anisotropies of water—water pair correlation functions near smooth and corrugated walls are presented in detail. The hydration shells of ions at interfaces differ from those in the bulk solutions as demonstrated by a simulation of a 2.2 molal Lil solution between Lennard-Jones walls. Finally, the self-diffusion coefficients, the hindered translational motions of the water molecules parallel and perpendicular to smooth and corrugated walls, and the librational motions are discussed, calculated from the simulations with the help of the corresponding velocity autocorrelation functions.

A molecular dynamics study of an aqueous LiI solution between Lennard-Jones walls

An MD simulation of a 2.2 m aqueous LiI solution between 12-6 Lennard-Jones walls has been performed which extended over 19 ps at an average temperature of 283 K. The basic tetragonal cell contained 200 ST2 water molecules and eight ions of each kind described as Lennard-Jones spheres with an elementary charge at the center. The effect of the walls on the hydration shell of the ions is investigated by means of ion–water radial distribution functions. Anisotropic water pair correlation functions are reported in order to show specific structural features near the walls. Changes in translational and rotational motions of the water molecules near the walls are discussed by means of velocity and angular velocity autocorrelation functions and their spectral densities, respectively.

A molecular dynamics study of water between Lennard-Jones walls

An MD simulation of 216 ST2 water molecules between 12-6 Lennard-Jones walls has been performed which extend over 20 ps at an average temperature of 287 K. The oxygen atom density profile is reported the influence of the walls on the orientation of the water molecules on the self-diffusion coefficient have been investigated The results are compared with those from MC and MD simulations of similar systems.

The interfacial density profile of a Lennard-Jones fluid in contact with a (100) Lennard-Jones wall and its relationship to idealized fluid/wall systems: A Monte Carlo simulation

Using the Monte Carlo simulation method, we study the interfacial liquid density profile for several systems; Lennard-Jones (L-J) fluid/L-J (100) solid fcc face, L-J fluid/L-J (10–4) wall, L-J fluid/’’Boltzmann averaged’’ wall, L-J fluid/hard wall, and hard-sphere fluid/Boltzmann averaged wall. Upon comparing the simulation results, we conclude that the Boltzmann averaged wall is a good effective one-dimensional potential for a (100) structured wall, and that the attractive interactions between the fluid atoms significantly influence the interfacial fluid density profile.