Near-Critical Phase Behavior in Polyelectrolyte Solutions: Effect of Charge Fluctuations

Abstract

The critical behaviors in polyelectrolyte (PE) solutions are studied by a renormalized Gaussian fluctuation theory. PEs are fully charged linear wormlike chains. The electrostatic interactions are considered in a continuum solvent while ignoring other interactions. The effects of temperature on the criticality are explored in a salt-free polyanion solution, and the effects of salt are explored in a symmetric mixed solution of polycation and polyanion. An unphysical phase coexistence (UPPC), in which a metastable dense phase coexists with an unstable dilute phase, always exists in the phase diagram. In the near-critical region, the UPPC could interfere with the real phase coexistence. The classical critical point is replaced by a “critical line”, giving a flat top for the phase diagram. These behaviors are due to the effect of charge fluctuation in the near-critical region. Our results explain why the mean field approach overestimates the critical salt density by about 10% to experimental data in the study of coacervation between oppositely charged PEs.

Phase Behavior of Partially Charged Polyelectrolyte Solutions with Salt: A Theoretical Study

“…[1] Such ionizable groups can dissociate in a polar solvent such as water, leaving charged groups on polymer chains and releasing Predicting the phase behavior of polyelectrolyte solutions with salt has been one of the challenging problems in polymer physics [30][31][32] and has been the subject of several recent studies. [33][34][35][36][37][38][39] Zhang et al [33] studied the phase behavior of fully-charged polyelectrolyte solutions both in salt-free condition and with added salt using a relatively simple liquid-state (LS) theory but demonstrated promising success in connection with experiments. Their approach is followed in this work but is extended to the more general case of weakly charged polyelectrolyte solutions by incorporating neutral segments to the theory.…”

“…Predicting the phase behavior of polyelectrolyte solutions with salt has been one of the challenging problems in polymer physics [ 30–32 ] and has been the subject of several recent studies. [ 33–39 ] Zhang et al. [ 33 ] studied the phase behavior of fully‐charged polyelectrolyte solutions both in salt‐free condition and with added salt using a relatively simple liquid‐state (LS) theory but demonstrated promising success in connection with experiments.…”

Phase Behavior of Ion-Containing Polymers in Polar Solvents: Predictions from a Liquid-State Theory with Local Short-Range Interactions

“…Such an LS theory has also been applied to study the phase behaviors of concentration-asymmetric mixtures of polycation and polyanion solutions and has also revealed a wealth of interesting and complex phase separations scenarios [57]. Classical density functional theories (cDFT) for charged polymers have also been developed based on a similar framework and have found wide applications in many polyelectrolyte systems [58][59][60][61][62][63][64][65]. A complete theoretical understanding of the solution phase behaviors of charged polymers, however, remains challenging, not only because of the multi-component nature of the system (which, in the simplest case of a salt-free solution of fully charged polymers, consists of solvent, counterions, and charged polymers), but also because of the delicate interplay among various factors, including the translational entropy of each component, excluded volume interactions, chain connectivity, and more importantly the long-range electrostatic interactions.…”

Responsive Properties of Grafted Polyanion Chains: Effects of Dispersion Interaction and Salt

“…In simple electrolyte solutions, the dispersion interactions (DPIs) between the ions and the solvent play a dominant role in determining the thermodynamic properties . The DPIs are particularly important for PEs because the charge fluctuation may be strong . In the salt-free case, molecular dynamics (MD) simulations showed that the DPI can greatly influence the nature of the association between PE chains .…”

“…11 The DPIs are particularly important for PEs 12 because the charge fluctuation may be strong. 13 In the salt-free case, molecular dynamics (MD) simulations showed that the DPI can greatly influence the nature of the association between PE chains. 12 If the solvent affinity is weak, the system is a homogeneous PE solution.…”