Dissipative Particle Dynamics for Systems with Polar Species

TL;DR

This paper introduces a novel method for simulating polar species in dissipative particle dynamics by modeling beads as dumb-bells with charges, enabling more accurate representation of dielectric properties and interactions.

Contribution

The work develops a dumb-bell bead model for polar species in DPD, linking dipole moments to dielectric permittivity, and demonstrates the importance of explicit treatment of polar molecules.

Findings

Composite beads behave as point-like at small separations

Dielectric permittivity is affected by charge separation and solvation interference

Explicit treatment of polar species yields more realistic colloidal behavior

Abstract

In this work we developed a method for simulating polar species in the dissipative particle dynamics (DPD) method. The main idea behind the method is to treat each bead as a dumb-bell, i.e. two sub-beads (the sub-beads can bear charges) kept at a fixed distance, instead of a point-like particle. It was shown that at small enough separations the composite beads act essentially as conventional point-like beads. Next, the relation between the bead dipole moment and the bulk dielectric permittivity was obtained. The interaction of single charges in polar liquid showed that the observed dielectric permittivity is somewhat smaller than that obtained for the bulk case at large separation between the charges; at distances comparable to the bead size the solvation shells of the charges start to interfere and oscillations in the observed permittivity occur. Such oriented molecules effectively have smaller polarizability compared to the bulk liquid, so the field of one charge in the vicinity of another charge is reduced not as strong as in the bulk. Finally, we showed why it is necessary to treat the polar species in DPD explicitly instead of implicitly by calculating the local polarizability based on the local species concentrations: the latter leads to the violation of the Newton's third law resulting in simulation artifacts. We investigated the behavior of a charged colloidal particle at an interface of polar and non-polar liquids. We obtained that when the polar molecules are treated explicitly, the charged colloidal particle moved into the polar liquid since it is energetically more favorable for the charged molecules to be immersed in a polar medium; however, within the "implicit polarity" method the colloidal particle is found on top of a "bump" formed by the molecules of the non-polar liquid, which increases the interface area between the liquids instead of decreasing it.