Instabilities of Charged Polyampholytes

TL;DR

This paper investigates the size transition of charged polyampholytes from compact to expanded states, revealing that excess charge determines their conformation and draws analogies to charged drop instabilities.

Contribution

It introduces a model linking excess charge to polyampholyte conformation and demonstrates a transition analogous to Rayleigh instability through simulations and theoretical analogy.

Findings

Polyampholyte size depends on excess charge Q.

Transition occurs at Q ≈ q_0 √N, from compact to expanded.

Shape instability similar to charged drop breakup.

Abstract

We consider polymers formed from a (quenched) random sequence of charged monomers of opposite signs. Such polymers, known as polyampholytes (PAs), are compact when completely neutral and expanded when highly charged. We examine the transition between the two regimes by Monte Carlo simulations, and by analogies to charged drops. We find that the overall excess charge, $Q$, is the main determinant of the size of the PA. A polymer composed of $N$ charges of $\pm q_0$ is compact for $Q<Q_c\approx q_0 \sqrt{N}$, and expanded otherwise. The transition is reminiscent of the Rayleigh shape instability of a charged drop. A uniform excess charge causes the breakup of a fluid drop, and stretches out a polymer to a {\it necklace} shape. The inhomogeneities in charge distort the shape away from an ordered necklace.