Abstract
The classical Voorn-Overbeek thermodynamic theory of complexation and phase separation of oppositely charged polyelectrolytes is generalized to account for the charge accessibility and hydrophobicity of polyions, size of salt ions, and pH variations. Theoretical predictions of the effects of pH and salt concentration are compared with published experimental data and experiments we performed, on systems containing poly(acrylic acid) (PAA) as the polyacid and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) or poly(diallyldimethyl ammonium chloride) (PDADMAC) as the polybase. In general, the critical salt concentration below which the mixture phase separates, increases with degree of ionization and with the hydrophobicity of polyelectrolytes. We find experimentally that as the pH is decreased below 7, and PAA monomers are neutralized, the critical salt concentration increases, while the reverse occurs when pH is raised above 7. We predict this asymmetry theoretically by introducing a large positive Flory parameter (= 0.75) for the interaction of neutral PAA monomers with water. This large positive Flory parameter is supported by molecular dynamics simulations, which show much weaker hydrogen bonding between neutral PAA and water than between charged PAA and water, while neutral and charged PDMAEMA show similar numbers of hydrogen bonds. This increased hydrophobicity of neutral PAA at reduced pH increases the tendency towards phase separation despite the reduction in charge interactions between the polyelectrolytes. Water content and volume of coacervate are found to be a strong function of the pH and salt concentration.
Highlights
When aqueous solutions of oppositely charged polyelectrolytes are mixed together, they often phase separate into distinct coacervate/precipitate and supernatant phases, rich and dilute in polymer, respectively [1]
We model a representative polyelectrolyte complexation experiment, wherein aqueous solutions of oppositely charged polyelectrolytes prepared at the same pH and salt concentration are mixed together
For ≫ 1, the entropic penalty of forming a polyion complex is minimal and the enthalpic gain on complexation dominates, leading to large magnitude of Satisfactory agreement is found between the theory and experiment using ω (PDMAEMA) = 1.8, ω (PAA) = 1.2, and = 0.26 nm, but at small is slightly underestimated by the theory
Summary
When aqueous solutions of oppositely charged polyelectrolytes are mixed together, they often phase separate into distinct coacervate/precipitate and supernatant phases, rich and dilute in polymer, respectively [1]. The resulting “dense phase” is referred as a coacervate or a precipitate, depending on its visual appearance and water content [2,3,4]. Coacervates have high water content and are reminiscent of a viscous liquid. Precipitates, on the other hand, are solid-like structures with less water content and are reminiscent of a polymer glass. Polyelectrolyte complexation is a general term used to describe phase separation in solutions of oppositely charged polyelectrolytes, where the resulting dense phase can be a coacervate/precipitate or a cloudy phase
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