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Published December 27, 2016 | public
Journal Article

Salting-Out and Salting-In of Polyelectrolyte Solutions: A Liquid-State Theory Study


We study the phase behavior of polyelectrolyte (PE) solutions with salt using a simple liquid-state (LS) theory. This LS theory accounts for hard-core excluded volume repulsion by the Boublik–Mansoori–Carnahan–Starling–Leland equation of state, electrostatic correlation by the mean-spherical approximation, and chain connectivity by the first-order thermodynamic perturbation theory. We predict a closed-loop binodal curve in the PE concentration-salt concentration phase diagram when the Bjerrum length is smaller than the critical Bjerrum length in salt-free PE solution. The phase-separated region shrinks with decreasing Bjerrum length, and disappears below a critical Bjerrum length. These results are qualitatively consistent with experiments, but cannot be captured by the Voorn–Overbeek theory. On the basis of the closed-loop binodal curve and the lever rule, we predict three scenarios of salting-out and salting-in phenomena with addition of monovalent salt into an initially salt-free PE solution. The Galvani potential—the electric potential difference between the coexisting phases—is found to depend nonmonotonically on the overall salt concentration in some PE concentration range, which is related to the partition of the co-ions in the coexisting phases. Free energy analysis suggests that phase separation is driven by a gain in the electrostatic energy, at the expense of a large translational entropy penalty, due to significant counterion accumulation in the PE-rich phase.

Additional Information

© 2016 American Chemical Society. Received: October 5, 2016; Revised: November 23, 2016; Publication Date (Web): December 12, 2016. 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. The authors gratefully acknowledge the support provided by KFUPM and Caltech. J.W. acknowledges partial financial support from the US National Science Foundation (Grant No. NSF-CBET-820 0852353). The authors declare no competing financial interest.

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