Polyelectrolyte-surfactant complex: phases of self-assembled structures

TL;DR

This study uses computer simulations to explore how ionic surfactants and polyelectrolytes form various self-assembled structures, revealing transitions from cylindrical complexes to spherical micelles driven by electrostatic and hydrophobic interactions.

Contribution

It introduces a detailed simulation model to analyze the phase behavior of polyelectrolyte-surfactant complexes, highlighting the structural transitions influenced by interaction strengths.

Findings

Formation of cylindrical complexes at low hydrophobicity and electrostatic strength.

Transition to spherical micelles with increased hydrophobic attraction.

Non-monotonic shape dependence on Coulomb interaction strength.

Abstract

We study the structure of complexes formed between ionic surfactants (SF) and a single oppositely charged polyelectrolyte (PE) chain. For our computer simulation we use the ``primitive'' electrolyte model: while the polyelectrolyte is modeled by a tethered chain of charged hard sphere beads, the surfactant molecules consist of a single charged head bead tethered to a tail of tethered hard spheres. A hydrophobic attraction between the tail beads is introduced by assuming a Lennard-Jones potential outside the hard-sphere diameter. As a function of the strengths of both the electrostatic and the hydrophobic interactions, we find the following scenario: switching on and increasing the electrostatic forces first leads to a stretching of the PE and then by condensation of SF to the formation of a complex. For vanishing hydrophobic forces this complex has the architecture of a molecular bottle-brush cylindrically centered around the stretched PE molecule. Upon increasing the hydrophobic attraction between the SF tails, a transition occurs inverting this structure to a spherical micelle with a neutral core of SF tails and a charged corona of SF heads with the PE molecule wrapped around. At intermediate hydrophobicity there is a competition between the two structures indicated by a non-monotonic dependence of the shape as function of the Coulomb strength, favoring the cylindrical shape for weak and the spherical micellar complex for strong interaction.