Charged Lennard-Jones Particles Research Articles

Investigation of structural, dynamics, and dielectric properties of an aqueous potassium fluoride system at various concentrations by molecular dynamics simulations

Potassium-ion-based batteries have emerged as promising alternatives to traditional lithium-ion batteries for energy storage systems due to their affordability, wide accessibility and comparable chemical characteristics to lithium. This study employs molecular dynamics simulations to explore the physical phenomena of potassium fluoride in aqueous solutions. The interatomic interactions were defined using the OPLS-AA force field, while the SPC/E water model and ions were represented as charged Lennard?Jones particles. The simulations were conducted across concentrations ranging from 0.1 to 1.0 mol kg-1. The insights derived from this investigation provide valuable understanding into the behaviour of KF electrolytes and their potential utility in energy storage systems. A comprehensive comprehension of the impact of KF electrolyte concentration on structural, dynamic and dielectric properties is pivotal for the design and optimization of potassium-ion batteries, as well as other electrochemical devices leveraging KF-based electrolytes. This research significantly contributes to the ongoing endeavours aimed at developing efficient and economically viable energy storage solutions that transcend the confines of traditional lithium-ion batteries.

Molecular dynamics simulations and FTIR spectroscopy investigations on the hydration, transport, and dielectric properties of the NaF(aq) system at various concentrations

Sodium fluoride has many applications in various fields, such as dentistry, geochemistry, and rechargeable battery technology. Indeed, the determination of the hydration structures, dynamics, and dielectric properties of the NaF electrolyte in aqueous solutions is a crucial element in order to optimize its performance and predict its intervention mechanisms in industrial processes. In this study, we used the molecular dynamics approach to study the NaF(aq) binary system at different concentrations ranging from 0.1 to 1.0 mol.kg−1 using the OPLS-AA force field combined with the extended simple point load water model (SPC/E). Thus, the ions were represented as charged Lennard-Jones particles. The radial distribution functions (RDF) have been calculated to analyze the hydration shell structures of different ion pairs. The microstructures, self-diffusion coefficient, and dielectric constant were determined by molecular dynamics at various concentrations. Moreover, our FTIR spectroscopy data confirm the results obtained from molecular simulations.

Molecular dynamics simulations of Lithium Fluoride aqueous solutions: Effects of ion concentration on the structural and dynamical properties at T=300 K

Lithium fluoride aqueous system play an important role in a variety of chemical engineering, energy, biochemistry and environmental processes. Lithium salts are also widely used in conventional electrolytes for making lithium-ion batteries. Computer simulation of lithium in fluoride aqueous solution has an important tool in understanding the structural and dynamical characteristics of ionic complexes. In this investigation, the structural and dynamical properties of supersatured LiF systems have been studied by molecular dynamics simulations at different molalities range from 0.05 up to 2.00 mol.Kg-1 using extended simple point charge (SPC/E) water model and the ions which are modeled as charged Lennard-Jones particles. Molecular dynamics simulations return highly complex data. The cartesian positions of each atom of lithium chloride aqueous solution are recorded at every time step of the trajectory. Therefore, the analysis of data requires to calculate the radial distribution functions (RDFs) describing the structure of the hydration shells around the ions in solutions and the hydration number. The structural properties of the water and Li+ and F- ions, such as the coordination number, interparticle distance, self-diffusion coefficient and dielectric constant are strongly depending on the molality and chemical nature of counterions.

MD simulations of charged binary mixtures reveal a generic relation between high- and low-temperature behavior.

Experimental studies of the glassy slowdown in molecular liquids indicate that the high-temperature activation energy E∞ of glass-forming liquids is directly related to their glass transition temperature Tg. To further investigate such a possible relation between high- and low-temperature dynamics in glass-forming liquids, we analyze the glassy dynamics of binary mixtures using molecular dynamics simulations. We consider a binary mixture of charged Lennard-Jones particles and vary the partial charges of the particles and, thus, the high-temperature activation energy and the glass transition temperature of the system. Based on previous results, we introduce a phenomenological model describing relaxation times over the whole temperature regime from high temperatures to temperatures well inside the supercooled regime. By investigating the dynamics of both particle species on molecular and diffusive length scales along isochoric and isobaric pathways, we find a quadratic charge dependence of both E∞ and Tg, resulting in an approximately constant ratio of both quantities independent of the underlying observable, the thermodynamic ensemble, and the particle species, and this result is robust against the actual definition of Tg. This generic relation between the activation energy and the glass transition temperature indicates that high-temperature dynamics and the glassy slowdown are related phenomena, and the knowledge of E∞ may allow us to approximately predict Tg.

Structural and dynamical properties simulations of potassium fluoride aqueous system at various temperatures from 298.15 K to 358.15 K

The energy storage systems are dominated by lithium–ion based chemistry. Currently, the focus has shifted to its nearest neighbours and extensive studies have been carried out on sodium and potassium ion-based batteries. Incorporation of potassium fluoride into solar cells (CIGS) improves the conversion efficiency by increasing the majority carrier concentration. In this study, we present results from molecular dynamics simulations of water solutions and clusters containing potassium fluoride with the interatomic interactions described by OPLS-AA force field for understanding the mechanisms of the physical phenomena of these ionic complexes. The structural and dynamical properties of KF solutions have been studied at various temperatures using TIP4P water model and the ions which are modeled as charged Lennard-Jones particles. The analysis of the simulation results showed that the temperature has decisive effects on the physicochemical properties of the KF electrolyte in aqueous solutions.

Molecular dynamics simulation of lithium fluoride in aqueous solutions at different temperatures 300 K – 360 K

Lithium metal is one of the most promising anodes for rechargeable batteries due to its large capacity, but its performance is plagued by high chemical reactivity, forming an unstable Li–electrolyte interface. Lithium fluoride has been recently touted as a promising material to improve this interface. Computer simulation of lithium in fluoride aqueous solution has an important tool in understanding the structural and dynamical characteristics of ionic complexes. In this investigation, the structural and dynamical properties of supersatured LiF systems have been studied by molecular dynamics simulations at different temperatures range from 300 K up to 360 K using SPC/E water model and the ions which are modeled as charged Lennard-Jones particles. The cartesian positions of each atom of lithium chloride aqueous solution are recorded at every time step of the trajectory. Therefore, the analysis of data requires to calculate the radial distribution functions (RDF) describing the structural and dynamical properties of the water and Li+ and F-ions, such as the coordination numbers, interparticle distances, self-diffusion coefficients and dielectric constants at various temperatures.

Comparative Modeling of Ions and Solvent Properties in Ca-Na Montmorillonite by Atomistic Simulations and Fluid Density Functional Theory

AbstractMolecular dynamics (MD) simulations provide an accurate description of the mineral–fluid interface from the perspective of the atomistic level taking into account all atom interactions. This simulation approach is computationally expensive if applied to large molecular systems. Classical Fluid Density Functional Theory (f-DFT) delivers structural and thermodynamic information at comparatively small computational costs. Numerous applications of f-DFT for electrolytes neglect an explicit consideration of solvent. In this work, an unrestricted three-component model (3CM) of f-DFT was applied, which incorporates Lennard-Jones (LJ) attractions for the description of the short-range interactions of fluid–fluid and fluid–wall rather than the hard sphere repulsions, named DFT/LJ-3CM. The DFT/LJ-3CM model considers ions as charged LJ particles and treats solvent molecules as neutral LJ particles. To validate the performance of the DFT/LJ-3CM, the f-DFT calculations were compared with atomistic simulations for montmorillonite (Mnt) with various hydrated states in electrolyte solutions. This benchmarking was used to assess critically the advantages and limitations of the f-DFT model. The calibrated DFT/LJ-3CM model for Na and Ca Mnt was applied to calculate cation selectivity for the ion exchange equilibrium with effective ion radius and swelling behavior of Mnt. The predictions of the DFT/LJ-3CM model were found to be in good agreement with the atomistic simulations and experimental data under a wide range of conditions. At the same time, the DFT calculations were 3–4 orders of magnitude faster than conventional MD simulations. Thus, the DFT/LJ-3CM model can be a computationally effective alternative to atomistic simulation in providing structural and thermodynamic properties of fluid–clay mineral interfaces. The DFT/LJ-3CM model provides a robust approach, which can be used for upscaling in reactive transport simulators and modeling ion migration taking place under more complex thermo-chemo-hydro-mechanical conditions.

Self-diffusion coefficients and orientational correlation times in aqueous NaCl solutions: Complementarity with structural investigations

The dynamical properties of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0–6 m) at ambient conditions are characterized by molecular dynamics (MD) simulations. MD simulations are performed with a flexible SPC water model as a solvent, while the ions are treated as charged Lennard–Jones particles. In this paper, attention has been focused on the self-diffusion coefficients ( D i ) of ions and water molecules and on orientational correlation time of water molecules. It is found that the self-diffusion coefficients decrease with ion concentration. Moreover, the self diffusion coefficients of sodium and chloride at higher salt concentrations are very comparable which may be due to the formation of clusters of these ions. The deduced rotational dynamics speeds up as the salt concentration increases. Some complementarities between dynamical properties and structural ones, recently obtained, are carried out.

Polyelectrolytes in Salt Solutions: Molecular Dynamics Simulations

We present results of the molecular dynamics simulations of salt solutions of polyelectrolyte chains with number of monomers N = 300. Polyelectrolyte solutions are modeled as an ensemble of bead–spring chains of charged Lennard-Jones particles with explicit counterions and salt ions. Our simulations show that in dilute and semidilute polyelectrolyte solutions the electrostatic induced chain persistence length scales with the solution ionic strength as I–1/2. This dependence of the chain persistence length is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing the salt concentration as R ∝ I–1/5. This is in agreement with the scaling of the chain persistence length on the solution ionic strength, lp ∝ I–1/2. In semidilute solution regime at low salt concentrations the chain size decreases with increasing polymer concentration, R ∝ cp–1/4, while at high salt concentrations we observed a weaker dependence of the chain size on th...

DNA Melting in Slit Pores: A Reaction Density Functional Theory

A reaction density functional theory (R-DFT) is developed for chemical reactions in confined space by integrating reaction thermodynamics and DFT for chain fluids. The theory is applied to investigate DNA melting in slit pores, with nucleotides represented by coarse-grained charged Lennard-Jones particles. Three types of slit pores are considered for DNA melting: repulsive pore, attractive pore, and under electric field. In repulsive pores, the melting temperature increases slightly with reducing pore width, and the increase magnitude is nearly the same for DNA of different chain lengths. The double-strand DNA (dsDNA) and single-strand DNA (ssDNA) are located in the slit center, particularly for long DNA due to the effect of configuration entropy. In attractive pores, the melting temperature increases with increasing wall-fluid interaction. The DNA chains are preferentially adsorbed near the slit walls with a strong wall-fluid interaction. Under electric field, the melting temperature increases slightly and is more distinct for shorter DNA.

Rouse Dynamics of Polyelectrolyte Solutions: Molecular Dynamics Study

We performed molecular dynamics simulations of dilute and semidilute polyelectrolyte solutions without hydrodynamic interactions to study Rouse dynamics of polyelectrolytes. Polyelectrolyte solutions are modeled by an ensemble of bead−spring chains of charged Lennard-Jones particles with explicit counterions. The simulations were performed for partially and fully charged polymers with the number of monomers N = 25−373. We show that the simulations of the Rouse dynamics give qualitatively similar results to the experimentally observed dynamics of polyelectrolyte solutions. Our simulations showed that the chain relaxation time depends nonmonotonically on polymer concentration. In dilute solutions, this relaxation time exhibits very strong dependence on the chain degree of polymerization, τ ∼ N3. The chain relaxation time decreases with increasing polymer concentration of dilute solutions. This decrease in the chain relaxation time is due to chain contraction induced by counterion condensation. In the semidi...

Molecular Dynamics Simulations of Dendritic Polyelectrolytes with Flexible Spacers in Salt Free Solution

We present the results of molecular dynamics simulations of dendritic polyelectrolytes in dilute salt-free solutions. The dendritic polyelectrolytes are modeled as an ensemble of regular-branched bead-spring chains of neutral and charged Lennard-Jones particles with explicit counterions. A wide range of molecular variables of the dendritic polyelectrolytes such as generation number, spacer length, and charge density were considered in the simulations. The effect of dendrimer size on relaxation time, the conformation of spacers, and the size dependence of the dendrimer on molecular variables are discussed and compared with a Flory type theory. The osmotic coefficients of the dilute dendritic polyelectrolyte solutions, as well as the profiles of monomers and counterions, are calculated directly from the simulations. Our simulation results show that the inner spacers of the dendrimers are extensively stretched, and the size dependence on the molecular weight deviates from the scaling prediction that assumes a Gaussian elasticity of the spacer.

Structural investigations of high concentrated aqueous LiCl solutions: X-ray scattering and MD simulations approach

A detailed analysis of aqueous LiCl solution structures is performed by molecular dynamics (MD) simulations and X-ray scattering at two salt concentrations (6 and 7 m) at 298 K. Another run is performed for C = 14 m. MD simulations are carried out with rigid SPC water model, while the ions are treated as charged Lennard-Jones particles. Both MD and X-ray scattering investigations show a significant reorganization of the solvent. The degree of hydrogen bonding of which clearly decreases when compared to pure fluid. MD results also show that the first hydration shell of the cation is greatly distorted at high concentrations whereas the corresponding one of the anion appears nearly invariant. Moreover, the ion-pairing process is accented at highest salt concentration. X-ray data are analyzed to deduce the structure factor and the pair correlation functions of each system. A good concordance is highlighted between experimental and theoretical results. A model of intermolecular arrangement, via MD results, is then proposed to describe the local order in each system, as deduced from X-ray scattering data.

Molecular Dynamics Simulations of Polyelectrolyte Adsorption

We have performed molecular dynamics simulations of polyelectrolyte adsorption at oppositely charged surfaces from dilute polyelectrolyte solutions. In our simulations, polyelectrolytes were modeled by chains of charged Lennard-Jones particles with explicit counterions. We have studied the effects of the surface charge density, surface charge distribution, solvent quality for the polymer backbone, strength of the short-range interactions between polymers and substrates on the polymer surface coverage, and the thickness of the adsorbed layer. The polymer surface coverage monotonically increases with increasing surface charge density for almost all studied systems except for the system of hydrophilic polyelectrolytes adsorbing at hydrophilic surfaces. In this case the polymer surface coverage saturates at high surface charge densities. This is due to additional monomer-monomer repulsion between adsorbed polymer chains, which becomes important in dense polymeric layers. These interactions also preclude surface overcharging by hydrophilic polyelectrolytes at high surface charge densities. The thickness of the adsorbed layer shows monotonic dependence on the surface charge density for the systems of hydrophobic polyelectrolytes for both hydrophobic and hydrophilic surfaces. Thickness is a decreasing function of the surface charge density in the case of hydrophilic surfaces while it increases with the surface charge density for hydrophobic substrates. Qualitatively different behavior is observed for the thickness of the adsorbed layer of hydrophilic polyelectrolytes at hydrophilic surfaces. In this case, thickness first decreases with increasing surface charge density, then it begins to increase.

Local Order in Aqueous NaCl Solutions and Pure Water: X-ray Scattering and Molecular Dynamics Simulations Study

The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O...O and O...H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O...O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.

Molecular dynamics simulation of solution structure and dynamics of aqueous sodium chloride solutions from dilute to supersaturated concentration

Constant-temperature and constant-pressure (NpT) molecular dynamics simulations were performed to study the effects of salt concentration ranging from dilute to supersaturated concentrations on solution structure and dynamical properties of aqueous sodium chloride solutions at 298 K. The rigid SPC/E model was used for water molecules, and sodium and chloride ions were modeled as charged Lennard–Jones particles. Na +–Cl − radial distribution functions showed the presence of contact ion pairs and solvent separated ion pairs. The coordination numbers of Na +–Cl − ion pairs increased with salt concentration up to saturated concentration, although the number of contact ion pairs was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions decreased with salt concentration up to saturated concentration, while that of sodium ion was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions were therefore quite close to each other. The constant number of the contact ion pairs and the almost equality of the tracer diffusion coefficients of both ions would lead to the formation of clusters in supersaturated solutions.

Liquid−Vapor Interfaces of Simple Electrolyte Solutions: Molecular Dynamics Results for Ions in Stockmayer Fluids

The equilibrium and dynamical properties of liquid-vapor interfaces of electrolyte solutions are investigated by employing a simple model where the ions are represented by charged Lennard-Jones particles and the dipolar solvent molecules are characterized by the so-called Stockmayer potential. The technique of molecular dynamics simulation is employed to calculate the density profiles and the orientational structure of the interfaces, the surface tension, the translational and rotational diffusion coefficients, and also the dipole orientational relaxation times of both interfacial and bulk molecules. It is found that the ions prefer to stay in the interior of the solutions and tend to avoid the surfaces. The dipole vectors of the interfacial solvent molecules tend to align parallel to the surfaces. The surface tension shows an increasing trend with increase of ion concentration, and this increase of the surface tension is found to be the net outcome of a decrease of the solvent-solvent contribution and an increase of the ion-solvent and ion-ion contributions. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. The solvent molecules at the interfaces rotate and translate in the parallel direction at a faster rate than that of bulk molecules. The perpendicular diffusion, however, occurs at a slower rate for the interfacial molecules. Also, on increase of ion concentration of the solutions, the dynamical properties of the interfaces are found to change differently from those of the bulk solutions. The equilibrium and dynamical results of the present interfacial solutions are also compared with the results of liquid-vapor interfaces of aqueous NaCI solutions that were reported in an earlier study (Chem. Phys. Lett. 2003, 373, 87).

Monte Carlo simulations of polyampholyte-polyelectrolyte complexes: Effect of charge sequence and strength of electrostatic interactions

We present the results of Monte Carlo simulations of complexation between polyampholyte and polyelectrolyte chains. Polymers are modeled as bead-spring chains of charged Lennard-Jones particles each consisting of 32 monomers. Formation of a polyampholyte-polyelectrolyte complex is driven by polarization-induced attractive interactions. The complex is usually formed at the end of the polyelectrolyte with the polyampholyte chain elongated and aligned along the polyelectrolyte backbone. This complex structure between the polarized polyampholyte chain and the polyelectrolyte leads to maximization of the attractive and minimization of the repulsive electrostatic interactions. The size of a polyampholyte in a complex is usually larger than that of an isolated polyampholyte chain. We also observed that initially collapsed polyampholytes undergo a coil-globule transition by forming a complex. The structure of a polyampholyte-polyelectrolyte complex was analyzed by tail and loop distribution functions. We have found that the number of loops increases while their sizes decrease with the strength of the electrostatic interactions. Polyampholytes with random charge sequence form stronger complexes with polyelectrolytes than those with alternating charge sequence. Polyampholytes with long blocky sequences form a double helix with a polyelectrolyte at sufficiently large values of the Bjerrum length.

Molecular Dynamics Simulations of Polyelectrolyte Solutions: Nonuniform Stretching of Chains and Scaling Behavior

We present the results of molecular dynamics simulations of polyelectrolyte solutions in near-ϑ-solvent conditions for polymer backbone. Polyelectrolyte solutions are modeled as an ensemble of bead-spring chains of charged Lennard-Jones particles with explicit counterions. Simulations were performed for both fully and partially charged polyelectrolyte chains with the number of monomers N varying from 25 to 300 in the range of polymer concentrations covering both dilute and semidilute regime. Polyelectrolyte chains in dilute solutions are nonuniformly stretched with the projection of the linear monomer density onto the end-to-end vector increasing logarithmically from the middle of the chain. The simulation results for chain size dependence on the degree of polymerization at different polymer concentrations are in good qualitative agreement with predictions of the modified scaling model that takes into account nonuniform stretching of polyelectrolyte chains. In semidilute solutions we confirm that the correlation length is inversely proportional to the square root of polymer concentration. By measuring the bond angle correlation function, we have determined that polyelectrolyte chains can be viewed as flexible chains with the persistence length proportional to the correlation length. Our results for the concentration dependence of chain size Re on polymer concentration for the longest chains (N = 187 and N = 300) are approaching the power law c-1/4, predicted by the scaling model of salt-free polyelectrolyte solutions.

Molecular dynamics simulations of aqueous NaCl and KCl solutions: Effects of ion concentration on the single-particle, pair, and collective dynamical properties of ions and water molecules

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations, ranging from 0 M to 4.5 M, to investigate the effects of ion concentration on the single-particle, pair, and collective dynamical properties of aqueous electrolyte solutions. The SPC/E model is used for water and the ions are modeled as charged Lennard-Jones particles. The single-particle dynamics is investigated by calculating the self-diffusion coefficients of ions and water molecules and also the orientational relaxation times. The pair dynamics is studied by evaluating the ion–water residence and water–water hydrogen bond time correlation functions. The relaxation of relative velocity autocorrelation function and the cross velocity correlation function of two hydrogen bonded water molecules are also investigated at varying ion concentration. Finally, we explore the collective dynamical properties by calculating the frequency dependent dielectric function and conductivity. It is found that the self and relative diffusion coeffcients decrease and the orientational relaxation times increase with ion concentration. The residence times of water molecules near ions and also the structural relaxation time of water–water hydrogen bonds show an increasing trend as the ion concentration is increased. The dielectric relaxation time is found to decrease with ion concentration for the solutions investigated here. The static conductivity of concentrated solutions shows significant departure from the Nernst–Einstein behavior due to formation of ion pairs. With an increase of frequency, the conductivity first increases substantially and then decreases at very high frequency. The initial increase of conductivity is attributed to the disruption of ion pairs on application of high frequency electric fields.