Anomalous diffusivity and electric conductivity for low concentration electrolytes in nanopores

Abstract

We apply equilibrium and nonequilibrium molecular dynamics simulations to study the dynamic properties of electrolytes in nanopores. The realistic primitive model and the restrictive primitive model widely used in the statistical mechanics of liquid-state theory are applied to model the electrolytes. The electrolytic ions are immersed in water, treated in this work as either a dielectric continuum ignoring the size of solvent molecules or a macroscopic dielectric continuum (polar property) plus neutral soft spheres, and the aqueous electrolyte is put in a confined space. To simulate a condition mimicking closely processes of practical interest and yet maintaining the simulation computationally manageable, we consider an infinitely long and uncharged cylindrical tube. The equilibrium property of the self-diffusion coefficient D and the nonequilibrium property of electric conductivity sigma are computed in terms of electrolyte concentration, particle size, and cylindrical pore radius. The simulation results for the continuum solvent restrictive primitive model and continuum solvent primitive model show normal behavior for D versus pore radius R at ionic concentration 0.1M -i.e., D decreases with decreasing R -display an R independence of D at a certain threshold concentration and undergo an anomalous increase in D with reducing R at a lower value 0.025 M. The mechanism of the anomaly at the ionic concentration 0.025 M was sought for and interpreted in this work to arise from the energetic and entropic factors. Our simulated data of sigma at this same concentration follow the same trend as D. To delve further into the transport properties, we perform simulation studies for the discrete solvent primitive model and make a detailed analysis of the characteristic of the ion radial density functions. Comparison of the latter functions with those in the continuum solvent primitive model sheds light on the simulated diffusion coefficient within the context of discrete solvent primitive model which is about two orders of magnitude less. This difference in D is naturally attributed to the solvent effect. Similar disparities were reported by others for the discrete and continuum restrictive primitive models.

Molecular simulation study of temperature effect on ionic hydration in carbon nanotubes

“…The diffusion of the ion in the narrow nanopore is considerably slower than in the bulk solution. Chan and his co-workers [18][19][20] also found the incomplete hydration of the ions in the nanopores by the equilibrium MD and non-equilibrium MD simulations. The simulation studies showed that the energy cost of constraining a hydrated K + within a nanopore is smaller than the cost of constraining the hydrated Na + .…”

MD Simulations on the Transport Behaviors of Mixed Na+ and Li+ in a Transmembrane Cyclic Peptide Nanotube under an Electric Field

“…Introduction of an Electric Field. In this work, nonequilibrium molecular dynamics (NEMD) 29,30 were performed to study the transport behaviors of Na + and Li + through the transmembrane CPNT of 8×(WL) 4 /POPE under an external electric field, which was applied along the positive direction of the z-axis, with strengths of 0.1, 0.2, and 0.3 V nm −1 , respectively. MD simulation tests indicate that the POPE bilayer may be destroyed gradually by one or more water columns, causing the system to fall apart under an electric field E ≥ 0.4 V nm −1 .…”

Permeation of polyelectrolytes and other solutes into the pore spaces of water-swollen cellulose: A review

“…Suppressed diffusion of solute molecules within very small capillaries also can be predicted based on molecular dynamics simulations. Thus, Lai et al (2004) predicted that electrolytes would have a slower diffusion rate by as much as two orders of magnitude within a fine porous material.…”