Ultrasoft primitive model of polyionic solutions: structure, aggregation, and dynamics

TL;DR

This paper introduces an ultrasoft primitive model for polyionic solutions, combining theoretical and simulation methods to explore structure, aggregation, and phase behavior, revealing unique conductor-insulator transitions and tricritical phenomena.

Contribution

It presents a novel ultrasoft core model for polyionic solutions, extending the restricted primitive model by incorporating Gaussian charge distributions and analyzing its phase and dynamic properties.

Findings

Weakly interacting neutral pairs form at low T and density

Sharp conductor-insulator transition line in density-temperature plane

Hints of tricritical behavior similar to 2D Coulomb Gas

Abstract

We introduce an ultrasoft core model of interpenetrating polycations and polyanions with continuous Gaussian charge distributions, to investigate polyelectrolyte aggregation in dilute and semi-dilute, salt-free solutions. The model is studied by a combination of approximate theories (random phase approximation and hypernetted chain theory) and numerical simulations. The calculated pair structure, thermodynamics, phase diagram and polyion dynamics of the symmetric version of the model (the "ultrasoft restricted primitive model" or UPRM) differ from the corresponding properties of the widely studied "restricted primitive model" (RPM) where ions have hard cores. At sufficiently low temperatures and densities, oppositely charged polyions form weakly interacting, polarizable neutral pairs. The clustering probabilities, dielectric behavior and electrical conductivity point to a line of sharp conductor-insulator transitions in the density-temperature plane. At very low temperatures the conductor-insulator transition line terminates near the top of a first order coexistence curve separating a high-density, liquid phase from a low-density, vapor phase. The simulation data hint at a tricritical behavior, reminiscent of that observed of the two-dimensional Coulomb Gas, which contrasts with the Ising criticality of its three-dimensional counterpart, the RPM.