Yuk Wai Tang

Diffusivity and conductivity of a primitive model electrolyte in a nanopore

Equilibrium and non-equilibrium molecular dynamics simulations are applied to obtain the diffusion coefficient and electric conductivity of ions in dilute electrolytes confined in neutral cylindrical pores. The electrolyte is described with the restricted primitive model and the wall of the pore is modelled as a soft wall. The equilibrium molecular dynamics simulations show that the axial diffusion coefficient of ions decreases with increasing confinement. For a fixed pore radius the diffusion coefficient decreases with increasing number density of the ions. The current response of the system to an applied electric field is maintained at constant temperature by Gaussian isokinetic equations of motion, and at constant concentration by periodic boundary conditions with recycling of ions in the axial direction. The electric conductivity is calculated from the current density and the electric field applied for different pore sizes. In contrast to the trend in diffusivity, conductivity increases slightly in smaller pores. For a very small pore, however, conductivity is lower than the bulk, because oppositely charged ions moving in opposite directions under the electric field cannot avoid collisions with each other in a narrow channel.

Theoretical investigations of the vapour-liquid equilibrium and dielectric properties of dipolar Yukawa fluids in an external field

In a low field approximation, using the dipolar Yukawa fluid model (in mean spherical approximation as a reference system) a consistent field-dependent free energy expression is proposed for the calculation of the vapour-liquid equilibrium of polar fluids in an applied electric field. A perturbation theory high field approximation expression of the free energy is also proposed to study the field-dependent properties of fluids. In the high field approximation, equations for the field-dependent polarization and for the nonlinear dielectric constant (or Piekara constant) are also predicted. It has been discussed that our approximations are appropriate to describe the vapour-liquid-like phase equilibria and the magnetization curves of magnetic fluids.

Non-equilibrium molecular dynamics simulation study of the frequency dependent conductivity of a primitive model electrolyte in a nanopore

The frequency dependence of electrical conductivity in a 0.1 molar univalent restricted primitive model electrolyte confined in cylindrical pores is studied by non-equilibrium molecular dynamics simulations. At high frequencies, conductivity is independent of pore size and approaches the zero value limit. The phase lag is independent of pore size and approaches the value π/2 at high frequency. At low frequencies, the conductivity is relatively constant and approaches the zero frequency (dc) conductivity value. For pores with radius smaller than 3 times the ion diameter, severe confinement effects lead to different low frequency behaviour. In these very small pores, axial collisions increase at low frequency and lead to much lower conductivity and a negative phase shift. The current response in severely confined electrolytes can be analogous to an LRC circuit with resonance at a characteristic frequency.

Ion transport in simple nanopores

Equilibrium and non-equilibrium molecular dynamics (EMD and NEMD) simulations are reported for the study of ion transport in an infinite long cylindrical nanopore. Results are compared for 3 models of electrolytes including the restricted primitive model (RPM), the solvent primitive model (SPM), and the extended simple point charge model (SPC/E). In EMD simulations, the mean square displacements are used to yield diffusion coefficients. Conductivity can be obtained through the Nernst–Einstein relation. Current and conductivity are calculated directly in NEMD simulations in which an external field is present along the pore axis. The effects of confinement on the ion transport are studied for the 3 model electrolytes. Comparing the EMD results and the NEMD results show that the Nernst–Einstein relation fails for the 3 models of electrolytes in very narrow nanopores. In addition to direct current NEMD simulations, alternate current (AC) NEMD simulations are performed to investigate the frequency dependence of ion transport. Towards high frequencies, a pore-size independent behavior is observed with vanishing conductivity and a phase lag approaching 90°. The effect of confinement is more evident at low frequencies and an electrical capacitor like behavior is observed in the narrowest pores, as indicated by the conductivity, the phase lag and the Cole–Cole plot. The narrowest pores show a combined reactance–resistance–capacitance (LRC) character and a maximum conductivity can be seen at the resonance frequency.

The Dot and Line Method: A Long Range Correction to Coulomb Interaction in a Cylindrical Pore

The charge line (CL) method had been used in the past to represent the periodic charges in Monte Carlo simulations of ions in a cylindrical pore. In this method, there exists a possible singularity when the edge of the image line overlaps with an ion in the central cylinder. This singularity is more problematic for molecular dynamics when the force is evaluated. Molecular dynamics simulations with the CL method have not been reported in the literature. By replacing the first section of the image charge line with an image point, we show that the CL method can be improved and be applicable in the molecular dynamics simulation of electrolytes in a cylindrical geometry. The modified method is demonstrated to be effective by simulations of a high packing primitive model electrolyte, representing the state of a molten salt.

Forces between charged surfaces in a solvent primitive model electrolyte

Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the components of the force between parallel charged surfaces in an electrolyte. The solvent primitive model (SPM) was used to investigate the effect of neutral hard sphere solvent particles on the force between the surfaces. The effects of particle size, wall charge density, charge valency of the electrolyte, and the exclusion of neutral hard sphere are discussed. When solvent particles are considered, the total force between the charged surfaces is always repulsive, even for divalent counterions. This is different from the earlier conclusion reached with a restricted primitive model electrolyte. The repulsive force decreases in going from monovalent counterions to divalent counterions.