Perfluoro-Alcohol-Induced Complex Coacervates of Polyelectrolyte–Surfactant
Mixtures: Phase Behavior and Analysis
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Abstract
Perfluorinated
alcohols and acids such as hexafluoroisopropanol
(HFIP), trifluoroethanol, trifluoroacetic acid, pentafluoropropionic
acid, and heptafluorobutyric acid induce coacervation and phase separation
in aqueous solutions of a wide variety of individual and mixed amphiphiles
[Khaledi Langmuir 2013, 29, 2458]. This paper focuses on HFIP-induced
complex coacervate formation in the mixtures of anionic polyelectrolytes,
such as sodium salt of poly(methacrylic acid) (PMA) or poly(acrylic
acid) (PAA) and cationic surfactants of alkyltrimethylammonium bromides.
In purely aqueous media and over a wide concentration range, mixtures
of PMA and CTAB form the catanionic complex (CTA<sup>+</sup>PM<sup>–</sup>) that is insoluble in water (white precipitate). Upon
addition of a small percentage of HFIP, the mixture goes through phase
transition and formation of two distinctly clear liquid phases. The
phase diagram for the HFIP–PMA–CTAB coacervate system
was studied. The coacervate volume was determined as a function of
system variables such as charge ratio as well as total and individual
concentrations of the system components. These results, combined with
the chemical composition analysis of the separated aqueous top-phase
and coacervate bottom-phase, shed light on the coacervation mechanism.
The results suggest that exchange of counterions and ion-pair formation
play critical roles in the coacervation process. This process facilitated
by HFIP through solvation of the head groups and dehydration of the
hydrophobic moieties of the catanionic complex. Because of the presence
of HFIP, coacervation occurs over a wide range of concentrations and
charge ratios of the oppositely charged polyelectrolyte and surfactant