Abstract
Complex coacervation is an electrostatically-driven phase separation phenomenon that is utilized in a wide range of everyday applications and is of great interest for the creation of self-assembled materials. Here, we utilized turbidity to characterize the effect of salt type on coacervate formation using two vinyl polyelectrolytes, poly(acrylic acid sodium salt) (pAA) and poly(allylamine hydrochloride) (pAH), as simple models for industrial and biological coacervates. We confirmed the dominant role of salt valence on the extent of coacervate formation, while demonstrating the presence of significant secondary effects, which can be described by Hofmeister-like behavior. These results revealed the importance of ion-specific interactions, which are crucial for the informed design of coacervate-based materials for use in complex ionic environments, and can enable more detailed theoretical investigations on the role of subtle electrostatic and thermodynamic effects in complex coacervation.
Highlights
Electrostatically-driven self assembly can be tuned through a variety of parameters, including the chemical nature of the charged species, the size or length of the charged particle or molecule, particle shape, polydispersity, charge density, pH, and ionic strength [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
We systematically examine the effects of various salts on the complex coacervation of two vinyl polyelectrolytes, poly(acrylic acid sodium salt) and poly(allylamine hydrochloride)
The most direct variable for controlling coacervate formation is the charge stoichiometry, or the molar ratio of polycation to polyanion, in the system. Both of the polymers used in this study are weak polyelectrolytes; the pKa for poly(acrylic acid sodium salt) (pAA) is around pH 4.5, while the pKa for poly(allylamine hydrochloride) (pAH) is around pH 8.5 [9]
Summary
Electrostatically-driven self assembly can be tuned through a variety of parameters, including the chemical nature of the charged species, the size or length of the charged particle or molecule, particle shape, polydispersity, charge density, pH, and ionic strength [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The overall formation of complex coacervates is strongly dependent upon solution conditions (i.e., pH and ionic strength), and is driven through a combination of attractive electrostatic forces and entropically-favorable molecular rearrangements, where the loss of configurational entropy caused by the electrostatic interaction of oppositely-charged polyelectrolytes is counterbalanced by the release of small, bound, counter-ions from the polyelectrolyte salts into solution [2,6,14,36,37,38,39,40,41] We systematically examine the effects of various salts on the complex coacervation of two vinyl polyelectrolytes, poly(acrylic acid sodium salt) (pAA) and poly(allylamine hydrochloride) (pAH) By using this type of well-controlled model system, we eliminate effects that could be attributable to differences in the interaction of the polymer backbone with the environment, or differences in polymer hydrophobicity resulting from different side-chain lengths. We discuss the relationship between salt effects and polyelectrolyte charge density by examining the consequences of varying pH
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