Frequency and field-dependent response of confined electrolytes from Brownian dynamics simulations

Abstract

Using Brownian dynamics simulations, we investigate the effects of confinement, adsorption on surfaces and ion-ion interactions on the response of confined electrolyte solutions to oscillating electric fields in the direction perpendicular to the confining walls. Nonequilibrium simulations allow to characterize the transitions between linear and nonlinear regimes when varying the magnitude and frequency of the applied field but the linear response, characterized by the frequency-dependent conductivity, is more efficiently predicted from the equilibrium current fluctuations. To that end, we (rederive and) use the Green--Kubo relation appropriate for overdamped dynamics, which differs from the standard one for Newtonian or underdamped Langevin dynamics. This expression highlights the contributions of the underlying Brownian fluctuations and of the interactions of the particles between them and with external potentials. While already known in the literature, this relation has rarely been used to date, beyond the static limit to determine the effective diffusion coefficient or the DC conductivity. The frequency-dependent conductivity always decays from a bulk-like behavior at high frequency to a vanishing conductivity at low frequency due to the confinement of the charge carriers by the walls. We discuss the characteristic features of the crossover between the two regimes, most importantly how the crossover frequency depends on the confining distance and the salt concentration, and the fact that adsorption on the walls may lead to significant changes both at high- and low-frequencies. Conversely, our results illustrate the possibility to obtain information on diffusion between walls, charge relaxation and adsorption by analyzing the frequency-dependent conductivity.

Finding Infinities in Nanoconfined Geothermal Electrolyte Static Dielectric Properties and Implications on Ion Adsorption/Pairing

“…Therefore, dielectric screening increases, not decreases, with ion concentration, contrary to conclusions based on ϵ̅ ⊥ computed using MD simulations. , In those works the nanoslit confinement likely also leads to sufficient equilibrium sampling of ions in the z -direction, but either ϵ ⊥ ( z ) are not reported or the slits are too narrow to draw conclusions about divergences (Section S6, Figure S6; see Section S2 for brief discussions of dielectric sampling times − ). Note that nominally “equilibrium” molecular theories have been applied to calculate ϵ r , yielding results similar to MD predictions in the MHz plateau region if the long-time ionic response that causes divergences is omitted …”

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

“…We now examine three timescales frequently considered in the dynamics of ions in confined electrolytes obtained with analytical theories. 29,[31][32][33]35,72 Since they rely on an implicit solvent description, a common diffusion D for cations and ions, as well as mean-field theory and a simplified representation of the interface, we limit ourselves to examining qualitative trends with salt concentration rather than how well these time scales compare quantitatively with our simulations. The first timescale is These timescales are reported in Fig.…”

“…Figure5: Comparison of relevant characteristic times as a function of salt concentration from simulations (symbols) and analytical theories (dashed lines) 29,[31][32][33]72. τ Q is calculated as the integral of the normalized QACFs (see Eq.…”

Diffusion of Water Molecules on the Surface of Silica Nanoparticles─Insights from Nuclear Magnetic Resonance Relaxometry

“…The mechanism of molecular motion in the vicinity of surfaces is a subject of intensive investigations. Majority of the studies are theoretical, i.e., by means of molecular dynamics simulations. − Experimental means revealing the mechanism of molecular motion are very limited. With this respect, nuclear magnetic resonance (NMR) relaxometry is a unique method, revealing not only the time scale of dynamical processes but also their nature (mechanisms).…”