(468e) The Role of Charge Density in Polyelectrolyte-Micelle Coacervation

In solution, electrostatic complexation may drive oppositely-charged macro-ions to undergo an associative phase separation process termed complex coacervation. Two liquid phases form as a result: a complex coacervate phase – the dense, polyelectrolyte-rich liquid— and a dilute phase, the supernatant. The widespread applications of coacervates for biomolecular stabilization, controlled delivery, and personal care products have resulted in extensive analysis of the phase behavior and properties of polyelectrolyte-micelle systems. However, much of the work to date has focused primarily on characterizing the phase behavior of specific polyelectrolyte-micelle systems rather than using predictive design rules for a broader range of systems. The phase behavior of coacervate systems is governed by electrostatics: a 1:1 charge ratio between the macro-ions is expected for coacervation. However, polyelectrolyte-micelle coacervates typically involve mixed-micelles, consisting of one charged species and one neutral species, which introduces an additional variable Y: the micellar charge fraction. The critical micellar charge fraction Y_c, where complexation is induced, is the typical means to characterize polyelectrolyte-micelle systems and has been shown in the literature to be a function of polymeric and micellar chemistries.

Work by Dubin and co-workers have shown that the steric accessibility of micelle surface charges affects the observed composition for Y_c, with a greater degree of steric hindrance shifting Y_c to higher levels.¹ We hypothesize that the charge densities of both the polycation and the micelle affect coacervate phase behavior, which we can test by varying both the charge density of the polymeric species and by using steric exclusion to decrease the apparent charge density of the micelles. Specifically, we utilize a combination of turbidimetry coupled with optical microscopy to characterize the phase behavior of a series of cationic random co-polymers of varying charge densities with a panel of anionic mixed-micelles with different hydrophilic head group sizes. Preliminary results support our hypothesis: for a given mixed-micelle, we saw a positive shift in Y_c with decreasing polymer charge density. Similarly, for a given polymer we observed that increasing levels of steric exclusion correlated with increases in Y_c; we believe these steric effects functionally decrease the micellar surface charge density. We are now applying electrophoretic light scattering to quantify the relationship between the zeta potentials ζ of each cationic co-polymer and the critical micellar charge fraction Y_c of its corresponding polymer-micelle complex. Our goal is to establish design rules that take into consideration the head group chemistry of surfactant micelles and the effective charge density of each co-polymer to accelerate the design of new materials for complex coacervate-based applications.

1. Fan, Y., Kellermeier, M., Xu, A. Y., Boyko, V., Mirtschin, S., & Dubin, P. L. (2017). Modulation of Polyelectrolyte–Micelle Interactions via Zeta Potentials. Macromolecules, 50(14), 5518–5526. doi: 10.1021/acs.macromol.7b00584