pH and Salt Effects on the Associative Phase Separation of Oppositely Charged Polyelectrolytes

Polymers

Prateek K. Jha¹  Priyanka S. Desai²  Jingyi Li¹ 

[1] Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA; E-Mails:;Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA; E-Mail:

关键词: polyelectrolyte complexes;    associative phase separation;    complexation;    coacervation;    precipitation;    charge regulation;   

DOI  :  10.3390/polym6051414

来源: mdpi

【 摘 要 】

The classical Voorn-Overbeek thermodynamic theory of complexation and phase separation of oppositely charged polyelectrolytes is generalized to account for the charge accessibility and hydrophobicity of polyions, size of salt ions, and pH variations. Theoretical predictions of the effects of pH and salt concentration are compared with published experimental data and experiments we performed, on systems containing poly(acrylic acid) (PAA) as the polyacid and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) or poly(diallyldimethyl ammonium chloride) (PDADMAC) as the polybase. In general, the critical salt concentration below which the mixture phase separates, increases with degree of ionization and with the hydrophobicity of polyelectrolytes. We find experimentally that as the pH is decreased below 7, and PAA monomers are neutralized, the critical salt concentration increases, while the reverse occurs when pH is raised above 7. We predict this asymmetry theoretically by introducing a large positive Flory parameter (= 0.75) for the interaction of neutral PAA monomers with water. This large positive Flory parameter is supported by molecular dynamics simulations, which show much weaker hydrogen bonding between neutral PAA and water than between charged PAA and water, while neutral and charged PDMAEMA show similar numbers of hydrogen bonds. This increased hydrophobicity of neutral PAA at reduced pH increases the tendency towards phase separation despite the reduction in charge interactions between the polyelectrolytes. Water content and volume of coacervate are found to be a strong function of the pH and salt concentration.

【 授权许可】

CC BY   
© 2014 by the authors; licensee MDPI, Basel, Switzerland.