Dielectric properties of water: a molecular dynamics study on the effects of molecule count and cutoff radius.

CONTEXT: Currently, the study of systems confined within various materials, such as graphene and graphene oxide, for diverse applications such as water desalination, metal separation from water, battery cells, and high-efficiency capacitors, is very common. Among these systems, water is a prominent subject of investigation. Understanding the impact of the number of molecules and the optimal cutoff radius is essential in order to determine the parameters. This study investigates the dielectric properties of H 2 O in relation to both the number of molecules and the cutoff radius within the simulation cell. We employ the flexible FBA/ ϵ water model, known for its improved accuracy in reproducing water's properties across various thermodynamic states. We range from 60 to 1372 molecules and use cutoff radius from 6 to 1.2 nm, encompassing a wide range of calculations to assess their impact on the simulation results, including critical properties like the dielectric constant, density, and phase change enthalpy. METHODS: Computational theoretical calculations were performed by solving the equations of motion through molecular dynamics (MD) simulations in the liquid phase. These simulations were conducted using the isothermal-isobaric (NPT) ensemble. The primary software used to solve these equations is GROMACS, and own programs for creating the initial cell and performing the analysis.

Carbon dioxide adsorption on a modified zeolite with sodium dodecyl sulfate surfactants: A molecular dynamics study.

Molecular dynamics simulations are carried out to study adsorption of Carbon dioxide (CO2) in a zeolite modified with anionic surfactants at different gas concentrations. Sodium dodecyl sulfate (SDS) surfactant is used and simulations at different SDS concentrations were conducted. The results show that adsorption of the gas is influenced by the amount of SDS on the zeolite surface. In addition, gas retention inside and outside the solid is observed and it strongly depends on the free sodium ions in the zeolite. The most favorable adsorption takes place at low CO2 concentrations with few SDS molecules. Adsorption was studied in terms of density profiles and pair correlation functions and strong interactions of the CO2 molecules with the sodium ions were observed.

Prediction of experimental properties of CO2: improving actual force fields.

Most of the existing classical CO2 models fail to reproduce some or many experimental properties such as surface tension, vapor pressure, density, and dielectric constant at difference thermodynamic conditions. Therefore, it we propose a new computational model to capture better structural, dynamical, and thermodynamic properties for CO2. By scaling the Lennard-Jones parameters and point charges; three target properties, static dielectric constant, surface tension, and density, were used to fit actual experimental data. Moreover, by constructing a flexible model, effects of polarization might be included by variations of the dipole moment. Several tests were carried out in terms of the vapor-liquid equilibria, surface tensions, and saturated pressures showing good agreement with experiments. Dynamical properties were also studied, such as diffusion coefficients and viscosities at different pressures, and good trends were obtained with experimental data.

Sodium Chloride, NaCl/ϵ: New Force Field.

A new computational model for sodium chloride, the NaCl/ϵ, is proposed. The force field employed for the description of the NaCl is based on a set of radial particle-particle pair potentials involving Lennard-Jones (LJ) and Coulombic forces. The parametrization is obtained by fitting the density of the crystal and the density and the dielectric constant of the mixture of the salt with water at a diluted solution. Our model shows good agreement with the experimental values for the density and for the surface tension of the pure system, and for the density, the viscosity, the diffusion, and the dielectric constant for the mixture with water at various molal concentrations. The NaCl/ϵ together with the water TIP4P/ϵ models provide a good approximation for studying electrolyte solutions.

Direct Coexistence Methods to Determine the Solubility of Salts in Water from Numerical Simulations. Test Case NaCl.

The solubility of NaCl, an equilibrium between a saturated solution of ions and a solid with a crystalline structure, was obtained from molecular dynamics simulations using the SPC/E and TIP4P-Ew water models. Four initial setups on supersaturated systems were tested on sodium chloride (NaCl) solutions to determine the equilibrium conditions and computational performance: (1) an ionic solution confined between two crystal plates of periodic NaCl, (2) a solution with all the ions initially distributed randomly, (3) a nanocrystal immersed in pure water, and (4) a nanocrystal immersed in an ionic solution. In some cases, the equilibration of the system can take several microseconds. The results from this work showed that the solubility of NaCl was the same, within simulation error, for the four setups, and in agreement with previously reported values from simulations with the setup (1). The system of a nanocrystal immersed in supersaturated solution was found to equilibrate faster than others. In agreement with laser-Doppler droplet measurements, at equilibrium with the solution the crystals in all the setups had a slight positive charge.

Non-polarizable force field of water based on the dielectric constant: TIP4P/ε.

The static dielectric constant at room temperature and the temperature of maximum density are used as target properties to develop, by molecular dynamics simulations, the TIP4P/ε force field of water. The TIP4P parameters are used as a starting point. The key step, to determine simultaneously both properties, is to perform simulations at 240 K where a molecular dipole moment of minimum density is found. The minimum is shifted to larger values of µ as the distance between the oxygen atom and site M, lOM, decreases. First, the parameters that define the dipole moment are adjusted to reproduce the experimental dielectric constant and then the Lennard-Jones parameters are varied to match the temperature of maximum density. The minimum on density at 240 K allows understanding why reported TIP4P models fail to reproduce the temperature of maximum density, the dielectric constant, or both properties. The new model reproduces some of the thermodynamic and transport anomalies of water. Additionally, the dielectric constant, thermodynamics, and dynamical and structural properties at different temperatures and pressures are in excellent agreement with experimental data. The computational cost of the new model is the same as that of the TIP4P.