Electrostatics-driven shape transitions in soft shells

TL;DR

This study demonstrates that electrostatic interactions can controllably induce shape transitions in soft, charged membranes, such as from spheres to ellipsoids or discs, by tuning environmental salt concentration and surface charge.

Contribution

It introduces a method to manipulate soft shell shapes via electrostatics, supported by simulations and analytical calculations, expanding understanding of membrane shape control.

Findings

Lowering salt concentration induces shape transitions.

Charged shells favor non-spherical shapes energetically.

Shape transitions are robust under charge renormalization.

Abstract

Manipulating the shape of nanoscale objects in a controllable fashion is at the heart of designing materials that act as building blocks for self-assembly or serve as targeted drug delivery carriers. Inducing shape deformations by controlling external parameters is also an important way of designing biomimetic membranes. In this paper, we demonstrate that electrostatics can be used as a tool to manipulate the shape of soft, closed membranes by tuning environmental conditions such as the electrolyte concentration in the medium. Using a molecular dynamics-based simulated annealing procedure, we investigate charged elastic shells that do not exchange material with their environment, such as elastic membranes formed in emulsions or synthetic nanocontainers. We find that by decreasing the salt concentration or increasing the total charge on the shell's surface, the spherical symmetry is broken, leading to the formation of ellipsoids, discs, and bowls. Shape changes are accompanied by a significant lowering of the electrostatic energy and a rise in the surface area of the shell. To substantiate our simulation findings, we show analytically that a uniformly charged disc has a lower Coulomb energy than a sphere of the same volume. Further, we test the robustness of our results by including the effects of charge renormalization in the analysis of the shape transitions and find the latter to be feasible for a wide range of shell volume fractions.