Structure, thermodynamic and kinetic signatures of a synthetic polyelectrolyte coacervating system

- The PAA/PDADMAC polyelectrolyte pair is a model coacervating system featuring various complex structures according to the initial molar charge ratio.
- The structures formed are at equilibrium owing to a relatively low binding energy between PAA and PDADMAC.
- True soluble polyelectrolytes complexes exist in a defined range of physicochemical conditions.
- Nanoparticles of PAA/PDADMAC complex are probably the precursors of coacervation.
- The coacervation is an extremely fast process (< 1 s) featuring a distinctive kinetic signature evidenced by stopped-flow experiments.

While many studies on coacervation have targeted biomacromolecules, we review in this article the key structure, thermodynamic and kinetic features of a fully synthetic coacervating system based on polyacrylic acid (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC) oppositely charged polyelectrolytes at pH 10, where PAA chains are fully deprotonated. Among the main points of interest, we can highlight (i) the presence of polyelectrolyte complex (PEC) nanoparticles that, unexpectedly, coexist with a certain amount of coacervate droplets in a large range of compositions, even far from stoichiometry; (ii) the fact that these PEC nanoparticles are likely precursors of the coacervation occurring at stoichiometry; (iii) the formation of soluble PECs only in a certain range of physicochemical conditions; (iv) the equilibrium properties of the system; (v) and last but not least a distinctive kinetic signature at stoichiometry evidenced by a peak in light scattering at very short times (~ 100 ms). Some of these results can be rationalized on the basis of weak interaction unfolding between oppositely charged PAA and PDADMAC chains as revealed by microcalorimetry measurements.

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