The Influence of Ion Pair Formation on the Phase Behavior of Polyelectrolyte Solutions

Abstract

In this paper we calculate the phase diagrams of polyelectrolyte solutions taking into account the possibility of ion pair formation between counterions and ions on polymer chains with their subsequent aggregation in multiplets. Multiplets play a role of reversible physical crosslinks between different polymer chains. Furthermore, we assume that the dielectric constant, and hence the binding energy of ion pairs and multiplets, depends on the volume fraction of polymer in the solution. We have shown that ion pairing and multiplet formation leads to instability of the homogeneous phase, so that at some conditions the phase separation can proceed even in good solvent. Another qualitatively new feature of the behavior of polyelectrolyte solutions is the emergence of a triple point on the phase diagram. The effect of the solvent polarity and the polarity of the dry polymer on the general form of the phase diagram is studied in detail.

A new equation of state of a flexible-chain polyelectrolyte solution: Phase equilibria and osmotic pressure in the salt-free case

“…This effect has been described in several theoretical works [17][18][19]21,[25][26][27] and has also been shown in Monte-Carlo simulations 20 . However, to the best of our knowledge theoretical models correctly predicting critical parameters of such a purely electrostatics driven phase transition are still missing.…”

“…In work 27 the contribution of the correlation attraction was described at the level of mean-spherical approximation (MSA) that allowed to obtain a good agreement with simulation results for critical temperature but underestimated the critical monomer concentration. Secondly, most theoretical approaches [17][18][19]21,25,26 ignore the effects of the monomer concentration fluctuations in the regime of semi-dilute polymer solution. However, these effects, as was already mentioned above, should be important for solutions of polyelectrolytes as well as for neutral polymers 1,12 .…”

“…However, these effects, as was already mentioned above, should be important for solutions of polyelectrolytes as well as for neutral polymers 1,12 . In order to evaluate the contribution of the non-electrostatic interactions to the total free energy of the solution the authors usually resort to some mean-field type approximations [17][18][19]21,25,26 where the fluctuation effects are fully ignored 1,9,10 . Finally, all the above mentioned theoretical models are based on the implicit assumption that contributions of electrostatic and non-electrostatic interactions are independent.…”

Charge regularization in phase separating polyelectrolyte solutions

“…All of the theories [1][2][3][4][5][6][7][8][9][10][11][12][13][14] of coexistence of polyelectrolyte phases, formulated so far, with varying levels of approximations, have the common key assumption that each polyelectrolyte chain maintains the same constant effective charge for all coexisting phases at all allowed ranges of temperature, polyelectrolyte concentration, and salt concentration. This assumption cannot be correct in view of the large body of literature, based on theoretical arguments, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] computer simulations, [31][32][33][34][35][36][37][38] and experiments, [39][40][41][42][43][44][45][46][47]…”

Control of Gel Swelling and Phase Separation of Weakly Charged Thermoreversible Gels by Salt Addition

“…non-fixed backbone charge have been considered in the context of non-geling, weakly charged polymers 18 , as well as for charged chemical gels. Many of the properties described in those works are likely to carry over to the case of weakly charged thermoreversible gels 22 . We have also previously reported that addition of doubly ionizing monomers to the polymer backbone (of LCST type) induces features in the swelling and phase separation behavior that might quantitatively different from those of equivalent systems with double amount of singly ionizable monomers 11 .…”