Charge Density Rules for Polyelectrolyte-Micelle Coacervation

Complex coacervation is a process whereby oppositely-charged macro-ions associatively phase separate into two liquid phases: a macromolecule-rich coacervate phase and a macromolecule-dilute supernatant phase. Complex coacervate applications as microreactors, biodelivery vehicles, and synthetic adhesives have garnered significant interest in a range of charged-colloid systems including polyelectrolyte-micelle complexes. However, much of the work to date has focused primarily on characterizing the phase behavior of individual polyelectrolyte-micelle systems rather than establishing predictive design rules for a broader range of systems. Electrostatic interactions govern coacervate phase behavior, with an expected 1:1 charge ratio between the charged macro-ions. Nonetheless, the mixed-micelle nature of polyelectrolyte-micelle coacervates results in a ternary system that 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.

We hypothesize that the charge densities of both polyelectrolyte and micelle affect phase behavior, which we can test by varying polymeric charge densities and by using steric exclusion to decrease the apparent micellar charge density. Specifically, we use turbidimetry coupled with optical microscopy to characterize the phase behavior of a series of cationic random copolymers of varying charge densities with a panel of anionic mixed-micelles with different neutral 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 accounting for the effective charge density of each macro-ion to accelerate the design of new materials for coacervate-based applications.