Dissipative particle dynamics for systems with polar species: Interactions in dielectric media

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 subbeads (the sub-beads can bear charges) kept at a fixed distance, instead of a point-like particle. The interaction forces between such composite beads were studied as well as the relation of the interaction parameters between them to the Flory-Huggins theory; 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 (i.e. the ratio of the force exerted on the charges in vacuum to the corresponding force in the polar liquid) 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 reflecting the presence of the layers of oriented polar molecules surrounding the charges. The latter are presumably the reason for the observed deviations at large distances as well: such oriented molecules surrounding the charges effectively have smaller polarizability compared to the bulk liquid. 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.

DPD Modelling of the Self- and Co-Assembly of Polymers and Polyelectrolytes in Aqueous Media: Impact on Polymer Science

“…A few years later, Gavrilov et al [171] investigated polar systems (formally uncharged) using their DPD method with the electrostatics developed earlier. The polar units were modelled as oppositely charged dumbbells (dipoles) orientated either parallel with respect to the chain (i.e., incorporated into the chain) or perpendicular, i.e., semipendant (one part inside the chain and the other outside).…”

Accurate and Efficient Splitting Methods for Dissipative Particle Dynamics

“…As Shardlow's splitting method, the newly proposed ABOBA method can also be applied in DPD with various fixed conditions [42]; it is worth assessing the performance of the ABOBA method in those settings. It is also worth investigating the performance of various methods in the case of DPD with constraints [21].…”

The effect of explicit polarity on the conformational behavior of a single polyelectrolyte chain

“…In order to take explicit polarity of the species into account, we used a recently reported modification [28][29][30] for DPD. Within that approach, each standard DPD beads is replaced by a "dumb-bell" consisting of two force centers (sub-bead) kept at a fixed small distance d from each other; each sub-bead can carry charge, thus making it possible to simulate beads with dipole moments (see Fig.…”

“…Solvent beads are formed by two sub-beads with the charges and , so they are uncharged and have the dipole moment of . The bulk liquid dielectric permittivity values for different dipole moments were taken from the work 29 . Fig.…”

“…We believe that the reason of these peculiarities is that the mean-field notion of the dielectric permittivity is not entirely accurate when the distance between the charges becomes comparable to the solvent molecule size. Indeed, in the work 28,30 the interaction force between two charges in a polar liquid was studied, and it was shown that the effective dielectric permittivity (i.e. the magnitude of the force decrease upon placing the charges into the polar liquid) is actually distance-dependent.…”