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Polymers 2014, 6(5), 1414-1436; doi:10.3390/polym6051414
Article

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

1
, 2
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 and 1,2,*
Received: 27 March 2014; in revised form: 9 May 2014 / Accepted: 9 May 2014 / Published: 16 May 2014
(This article belongs to the Special Issue Polyelectrolytes 2014)
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Abstract: 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.
Keywords: polyelectrolyte complexes; associative phase separation; complexation; coacervation; precipitation; charge regulation polyelectrolyte complexes; associative phase separation; complexation; coacervation; precipitation; charge regulation
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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MDPI and ACS Style

Jha, P.K.; Desai, P.S.; Li, J.; Larson, R.G. pH and Salt Effects on the Associative Phase Separation of Oppositely Charged Polyelectrolytes. Polymers 2014, 6, 1414-1436.

AMA Style

Jha PK, Desai PS, Li J, Larson RG. pH and Salt Effects on the Associative Phase Separation of Oppositely Charged Polyelectrolytes. Polymers. 2014; 6(5):1414-1436.

Chicago/Turabian Style

Jha, Prateek K.; Desai, Priyanka S.; Li, Jingyi; Larson, Ronald G. 2014. "pH and Salt Effects on the Associative Phase Separation of Oppositely Charged Polyelectrolytes." Polymers 6, no. 5: 1414-1436.


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