Cluster Formation in Polyelectrolyte−Micelle Complex Coacervation

Abstract

The temperature-induced liquid-liquid phase transition (complex coacervation) of a polycation-anionic/nonionic mixed micelle system was examined over a range of macroion concentrations and polycation molecular weights (MW) using turbidimetry and dynamic light scattering (DLS). DLS revealed a progressive increase in complex/aggregate size with temperature up to the phase transition at T(φ), followed by splitting of these clusters into respectively smaller and larger particles. We present two explanations: (1) large (200-400 nm) clusters (soluble aggregates) are necessary and sufficient coacervation precursors, and (2) the process of coacervation itself is accompanied by the expulsion of smaller aggregates to form submicrometer droplets. Although a reduction in T(φ) for higher MW appears to be correlated with larger clusters, in support of model 1, the opposite correlation between cluster size and T(φ) is seen upon isoionic dilution. We conclude that enhanced coacervation and increased cluster size at high polymer MW arise independently from increased intercomplex attractive forces. Dilution, on the other hand, leads to diminished cluster size, whereas the decrease in T(φ) on dilution is a reflection of coacervate self-suppression, previously observed for this system. The splitting of clusters into large and small species near T(φ) is explained by macroion disproportionation, as proposed by Shkolvskii et al for DNA condensation. We demonstrate and explain a similar phenomenon: broadening of the phase transition by an increase in cluster polydispersity, resulting from an increase in surfactant polydispersity.

Intermolecular Interactions in Polyelectrolyte and Surfactant Complexes in Solution

“…The CAC of neutral polymer-surfactant complexes decreases with increasing temperature [40,42] due to a decrease in hydrophilicity with increased temperature, as demonstrated for polyethylene oxide (PEO) with the cationic surfactant hexadecyltrimethylammonium chloride (HTAC). For highly-charged polyelectrolyte systems, there is an influence of temperature, however the system is more strongly influenced by the electrostatic interactions, which have only a minor temperature dependence [3,37,43,44,45].…”

The Giant SALR Cluster Fluid: A Review

“…However, as the mechanism of cluster formation via a balance between long-range repulsion and short-range attraction, although not widely appreciated, is now firmly established, it is timely to revisit the problem of complex coacervation from this perspective. Indeed, the behaviour of polyion-surfactant mixtures has already been discussed in this light 70 , but here we focus exclusively on oppositely-charged polyion mixtures that form complex coacervates.…”

“…Moreover, Kizilay et. al 70 emphasize the propensity of complex coacervates to form from small near-neutral complexes of oppositely-charged polyions. This provides a clue as to how complex coacervates might be modelled in terms of the SALR model.…”

Review on plant protein–polysaccharide complex coacervation, and the functionality and applicability of formed complexes

“…Weiß et al 78 observed a temperature dependency for the system of hydroxypropyl methylcellulose phthalate, where formed coacervates precipitated at a critical temperature. Kizilay et al 79 explained that at a critical temperature the formed clusters first grow in size, followed by separation into larger and smaller colloidal clusters due to macroion disproportionation. Dubin et al 80 also observed this critical temperature for both dilute and concentrated polyelectrolyte mixtures.…”