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Published August 14, 2018 | public
Journal Article

Salt Partitioning in Complex Coacervation of Symmetric Polyelectrolytes


We perform a general thermodynamic analysis for the salt partitioning behavior in the coexisting phases for symmetric mixtures of polycation and polyanion solutions. We find that salt partitioning is determined by the competition between two factors involving the ratio of the polyelectrolyte concentration in the coacervate phase to that in the supernatant phase and the difference in the exchange excess chemical potential Δμ_(ex) -- the excess chemical potential difference between PE segments and small ions -- between the coexisting phases. The enrichment of salt ions in the coacervate phase predicted by the Voorn−Overbeek theory is shown to arise from its neglect of chain connectivity in the excess free energy which results in Δμ_(ex) = 0 under all conditions. We argue that chain connectivity in general leads to a finite value of Δμex, which decreases with increasing PE concentration. Explicit calculations using theories that include the chain connectivity correlations -- a simple liquid-state theory and a renormalized Gaussian fluctuation theory -- show nonmonotonic behavior of the salt-partitioning coefficient (the ratio of salt ion concentration in the coacervate phase to that in the supernatant phase): it is larger than 1 at very low salt concentrations, reaches a minimum at some intermediate salt concentration, and approaches 1 at the critical point. This behavior is consistent with recent computer simulation and experimental results.

Additional Information

© 2018 American Chemical Society. Received: April 9, 2018. Revised: June 30, 2018. Publication Date (Web): July 18, 2018. We thank Matthew Tirrell for sending ref (29) prior to publication. This work was conducted jointly by King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, and California Institute of Technology (Caltech) under a collaborative research program in catalysis. Additional support was provided by the Jacobs Institute for Molecular Engineering in Medicine (JIMEM). The authors declare no competing financial interest.

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