Abstract
Polyelectrolyte gels have the capacity to absorb large amounts of multivalent species of opposite charge from aqueous solutions of low ionic strength, and release them at elevated ionic strengths. The reversibility offers the possibility to switch between “storage” and “release” modes, useful in applications such as drug delivery. The review focuses on systems where so-called volume phase transitions (VPT) of the gel network take place upon the absorption and release of proteins and self-assembling amphiphiles. We discuss the background in terms of thermodynamic driving forces behind complex formation in oppositely charged mixtures, the role played by cross-links in covalent gels, and general aspects of phase coexistence in networks in relation to Gibbs’ phase rule. We also briefly discuss a gel model frequently used in papers covered by the review. After that, we review papers dealing with collapse and swelling transitions of gels in contact with solution reservoirs of macroions and surfactants. Here we describe recent progress in our understanding of the conditions required for VPT, competing mechanisms, and hysteresis effects. We then review papers addressing equilibrium aspects of core–shell phase coexistence in gels in equilibrium. Here we first discuss early observations of phase separated gels and results showing how the phases affect each other. Then follows a review of recent theoretical and experimental studies providing evidence of thermodynamically stable core–shell phase separated states, and detailed analyses of the conditions under which they exist. Finally, we describe the results from investigations of mechanisms and kinetics of the collapse/swelling transitions induced by the loading/release of proteins, surfactants, and amphiphilic drug molecules.
Highlights
Charged polymer networks have the ability to swell by absorbing water from the surroundings.The hydrogels created are soft materials with both intriguing and useful properties
We describe the results from investigations of mechanisms and kinetics of the collapse/swelling transitions induced by the loading/release of proteins, surfactants, and amphiphilic drug molecules
This review focusses on phase transitions and phase coexistence in systems of polyelectrolyte gels interacting with species of opposite charge to the network, such as surfactant micelles, peptides, or multivalent salt ions, where core–shell morphologies frequently appear [16]
Summary
Charged polymer networks have the ability to swell by absorbing water from the surroundings. This review focusses on phase transitions and phase coexistence in systems of polyelectrolyte gels interacting with species of opposite charge to the network, such as surfactant micelles, peptides, or multivalent salt ions, where core–shell morphologies frequently appear [16]. In such systems, depletion of the interacting species from the solution takes place during the collapse transition unless the gel is in contact with a very large solution volume. The purpose of the present article is two-fold. (1) To review papers about phase transition mechanisms and driving forces behind phase separation in polyelectrolyte gels induced by amphiphiles and proteins. (2) To highlight specific research problems that our lab has focused on recently and how we have addressed them
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