Dielectric response of a polar fluid trapped in a spherical nanocavity

TL;DR

This study uses Molecular Dynamics simulations to analyze how a polar fluid confined in spherical nanocavities responds to external dielectric environments, revealing insensitivity in structure but strong dependence in dynamic relaxation and permittivity.

Contribution

It provides detailed insights into the static and dynamic dielectric properties of confined polar fluids, highlighting the influence of external permittivity on relaxation dynamics and permittivity profiles.

Findings

Density and orientational order are insensitive to external permittivity.

Permittivity profiles oscillate and are weakly dependent on external permittivity.

Dynamic relaxation of dipole moments varies significantly with external permittivity.

Abstract

We present extensive Molecular Dynamics simulation results for the structure, static and dynamical response of a droplet of 1000 soft spheres carrying extended dipoles and confined to spherical cavities of radii $R=2.5$, 3, and 4 nm embedded in a dielectric continuum of permittivity $\epsilon' \geq 1$. The polarisation of the external medium by the charge distribution inside the cavity is accounted for by appropriate image charges. We focus on the influence of the external permittivity $\epsilon'$ on the static and dynamic properties of the confined fluid. The density profile and local orientational order parameter of the dipoles turn out to be remarkably insensitive to $\epsilon'$. Permittivity profiles $\epsilon(r)$ inside the spherical cavity are calculated from a generalised Kirkwood formula. These profiles oscillate in phase with the density profiles and go to a ``bulk'' value $\epsilon_b$ away from the confining surface; $\epsilon_b$ is only weakly dependent on $\epsilon'$, except for $\epsilon' = 1$ (vacuum), and is strongly reduced compared to the permittivity of a uniform (bulk) fluid under comparable thermodynamic conditions. The dynamic relaxation of the total dipole moment of the sample is found to be strongly dependent on $\epsilon'$, and to exhibit oscillatory behaviour when $\epsilon'=1$; the relaxation is an order of magnitude faster than in the bulk. The complex frequency-dependent permittivity $\epsilon(\omega)$ is sensitive to $\epsilon'$ at low frequencies, and the zero frequency limit $\epsilon(\omega=0)$ is systematically lower than the ``bulk'' value $\epsilon_b$ of the static primitivity.