Future directions in physiochemical modeling of the thermodynamics of polyelectrolyte coacervates

Abstract

We review theories of polyelectrolyte (PE) coacervation, which is the spontaneous association of oppositely charged units of PEs and phase separation into a polymerdense phase in aqueous solution. The simplest theories can be divided into "physicsbased" and "chemistry-based" approaches. In the former, PEs are treated as charged, long-chain, molecules, defined by charge level, chain length, and chain flexibility, but otherwise lacking chemical identity, with electrostatic interactions driving coacervation. The "chemistry-based" approaches focus on the local interactions between the species for which chemical identity is critical, and describe coacervation as the result of competitive local binding interactions of monomers and salts. In this article, we show how these approaches complement each other by presenting recent approaches that take both physical and chemical effects into account. Finally, we suggest future directions toward producing theories that are made quantitatively predictive by accounting for both long range electrostatic and local chemically specific interactions.

Effects of Salt on Phase Behavior and Rheological Properties of Alginate–Chitosan Polyelectrolyte Complexes

“…The absence of transition from solid–liquid to liquid–liquid phase separation in our system can be attributed to several reasons. The longer and semiflexible chains of the alginate and chitosan can resist the conformational change of the polymers. − We also hypothesize that the sodium alginate chains are less susceptible to structural changes with the addition of NaCl as the polymer is already under the influence of Na + ions. , In a previous study, it has been shown using isothermal titration calorimetry that the addition of NaCl does not significantly affect the change of enthalpy of binding . Further, the slight hydrophobic nature of the chitosan chain can also play a role, as it reduces the hydration level. , A previous work from Schlenoff and co-workers has shown that the association between the polyelectrolytes in solid PECs is stronger with increasing hydrophobicity in the system .…”

“…The longer and semiflexible chains of the alginate and chitosan can resist the conformational change of the polymers. − We also hypothesize that the sodium alginate chains are less susceptible to structural changes with the addition of NaCl as the polymer is already under the influence of Na + ions. , In a previous study, it has been shown using isothermal titration calorimetry that the addition of NaCl does not significantly affect the change of enthalpy of binding . Further, the slight hydrophobic nature of the chitosan chain can also play a role, as it reduces the hydration level. , A previous work from Schlenoff and co-workers has shown that the association between the polyelectrolytes in solid PECs is stronger with increasing hydrophobicity in the system . Specifically, the complexes obtained from poly­(allylamine hydrochloride) and poly­(acrylic acid sodium salt) did not show complete dissolution even at the maximum solubility concentration (6 M) of NaCl due to the hydrophobicity of the PEs. ,, All these factors, in addition to the osmotic effect, control the phase behavior of our system. , Correspondingly, Figure and the characterization of PECs discussed below capture the change in structure and properties with changing NaCl concentration.…”

“…49 Further, the slight hydrophobic nature of the chitosan chain can also play a role, as it reduces the hydration level. 46,47 A previous work from Schlenoff and co-workers has shown that the association between the polyelectrolytes in solid PECs is stronger with increasing hydrophobicity in the system. 51 Specifically, the complexes obtained from poly(allylamine hydrochloride) and poly(acrylic acid sodium salt) did not show complete dissolution even at the maximum solubility concentration (6 M) of NaCl due to the hydrophobicity of the PEs.…”

Polyelectrolyte-multivalent molecule complexes: physicochemical properties and applications

“…62,63 Complex coacervation was also studied theoretically. 64,65 The first model that was able to phenomenologically capture the phase behavior of complex coacervates is the Voorn-Overbeek theory. 66,67 Essentially, the Voorn-Overbeek theory describes the mixing free energy of the system as a combination of the translational entropy within the Flory-Huggins theory framework, and the electrostatic attraction using the Debye-Hu ¨ckel theory.…”

Salt-Dependent Phase Re-entry of Weak Polyelectrolyte Complexes: from Associative to Segregative Liquid–Liquid Phase Separation

“…Numerous other approaches have emerged to address the limitations of the original V–O theory, including liquid state theories, – scaling theories, – and field theoretic simulations. , These approaches have been developed as complementary methods to overcome the deficiencies of the V–O theory. Recent reviews have provided comprehensive summaries and comparisons of the advancements made in polyelectrolyte theory. ,, …”