Volume Transition and Phase Coexistence in Polyelectrolyte Gels Interacting with Amphiphiles and Proteins

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.

Phase Behavior of Salt-Free Polyelectrolyte Gel-Surfactant Systems

Ionic surfactants tend to collapse the outer parts of polyelectrolyte gels, forming shells that can be used to encapsulate other species including protein and peptide drugs. In this paper, the aqueous phase behavior of covalently cross-linked polyacrylate networks containing sodium ions and dodecyltrimethylammonium ions as counterions is investigated by means of swelling isotherms, dye staining, small-angle X-ray scattering, and confocal Raman spectroscopy. The equilibrium state is approached by letting the networks absorb pure water. With an increasing fraction of surfactant ions, the state of the water-saturated gels is found to change from being swollen and monophasic, via multiphasic states, to collapsed and monophasic. The multiphasic gels have a swollen, micelle-lean core surrounded by a collapsed, micelle-rich shell, or a collapsed phase forming a spheroidal inner shell separating two micelle-lean parts. It is shown that the transition between monophasic and core-shell states can be induced by variation of the osmotic pressure and variation of the charge of the micelles by forming mixed micelles with the nonionic surfactant octaethyleneglycol monododecylether. The experimental data are compared with theoretical predictions of a model derived earlier. In the calculations, the collapsed shell is assumed to be homogeneous, an approximation introduced here and shown to be excellent for a wide range of compositions. The theoretical results highlight the electrostatic and hydrophobic driving forces behind phase separation.

Single Microgels in Core-Shell Equilibrium: A Novel Method for Limited Volume Studies

The interactions of cationic surfactant dodecyltrimethylammonium bromide and cationic protein cytochrome c with anionic polyacrylate microgels have been investigated in microscopic liquid droplets by means of a micropipette technique at ionic strength 0.01 M and pH 8. Experiments on single microgels in solutions of limited amounts of the surfactant provide the first evidence of microgels in a stable biphasic core-shell state with the surfactant partitioned to the shell. Under the same conditions, the protein is found to distribute uniformly in the microgels. Quantitative data in the form of swelling and binding isotherms are presented and compared with literature data for macrogels and with predictions of a recent gel theory. Theory is found to be in semiquantitative agreement with the experiments. The importance of polyion-mediated attractions between the protein molecules is analyzed theoretically and proposed to explain the continuous but highly cooperative binding isotherms.

Polyelectrolyte-surfactant complexes formed by poly[3,5-bis(trimethylammoniummethyl)4-hydroxystyrene iodide]-block-poly(ethylene oxide) and sodium dodecyl sulfate in aqueous solutions

Formation of polyelectrolyte-surfactant (PE-S) complexes of poly[3,5-bis(trimethylammoniummethyl)-4-hydroxystyrene iodide]-block-poly(ethylene oxide) (QNPHOS-PEO) and sodium dodecyl sulfate (SDS) in aqueous solution was studied by dynamic and electrophoretic light scattering, small-angle X-ray scattering (SAXS), atomic force microscopy, and fluorometry, using pyrene as a fluorescent probe. SAXS data from the QNPHOS-PEO/SDS solutions were fitted assuming contributions from free copolymer, PE-S aggregates described by a mass fractal model, and densely packed surfactant micelles inside the aggregates. It was found that, unlike other systems of a double hydrophilic block polyelectrolyte and an oppositely charged surfactant, PE-S aggregates of the QNPHOS-PEO/SDS system do not form core-shell particles and the PE-S complex precipitates before reaching the charge equivalence between dodecyl sulfate anions and QNPHOS polycationic blocks, most likely because of conformational rigidity of the QNPHOS blocks, which prevents the system from the corresponding rearrangement.

Swelling properties of cross-linked DNA gels

This work represents our contribution to the field of physical chemistry of DNA gels, and concerns the synthesis and study of novel chemically cross-linked DNA gels. The use of covalent DNA gels is a very promising way to study DNA-cosolute interactions, as well as the dynamic behaviour of DNA and cationic compacting agents, like lipids, surfactants and polycations. Manipulating DNA in new ways, like DNA networks, allows a better understanding and characterization of DNA-cosolute complexes at the molecular level, and also allows us to follow the assembly structures of these complexes. The use of responsive polymer gels for targeted delivery of toxic and/or labile drugs has, during the past few years, shown to be a promising concept. The features found in the proposed system would find applications in a broader field of gel/drug interaction, for the development of controlled release and targeted delivery devices.

Association and structure formation in oppositely charged polyelectrolyte-surfactant mixtures

Investigations dealing with association behaviour and structure formation in oppositely charged polyelectrolyte-surfactant mixtures in aqueous solutions are reviewed. Discussion is limited to a selection of vinyl based anionic polyelectrolytes that, when completely ionized, posses the same structural value of the linear charge density parameter. Particular emphasis is placed on the role of polymer chain properties in aggregates with surfactants. Chain characteristics are varied by changing the nature of the charged group, its ionization degree - when possible, the spatial distribution of these groups along the chain, i.e. the tacticity, and the hydrophobic character of other substituents attached to the chain. Quantitative information on the degree of binding in the form of binding isotherms is obtained using surfactant-sensitive membrane electrodes and microstructures of polyelectrolyte-surfactant complexes are determined by synchrotron small angle X-ray scattering. Considerable differences in the degree of binding (including the critical association concentration, CAC, values) and in structures are found. It is concluded that strong interactions in these systems arise from the electrostatic attraction, but this only forms the basis for initial extensive accumulation (anchoring) of surfactant ions in the vicinity of the polyion chain. When this is accomplished, additional specific interactions and effects may come into play. The most powerful of these interactions, the hydrophobic association between the chain and the micelle core, were found in polystyrenesulfonate, PSS, solutions. Other properties are less influential but still lead to CAC values that differ by more than one order of magnitude. These differences are explained by taking into account the chain conformation, flexibility, and hydrophobic character. Specific interactions between PSS and cetylpyridinium, CP, cations result in a soluble non-stoichiometric PSS-CP complex that could be characterized by measuring various solution properties as a function of polymer concentration and degree of complexation. The review is supplemented by including studies of complexation between the spherical fullerene hexamalonate anion and cationic surfactants, which demonstrate a high association tendency with characteristics similar to those found in binding of surfactants by linear polyelectrolytes.

Responsive Polymer Gels: Double-Stranded versus Single-Stranded DNA

Cross-linking of polyelectrolytes such as DNA gives gels that are osmotically highly swollen but contract upon addition of electrolytes and, in particular, upon association of oppositely charged cosolutes with the polyelectrolyte chain. The deswelling behavior of cross-linked DNA gels thus reflects the DNA-cosolute interactions and provides a basis for the development of responsive DNA formulations. Gels of both single- and double-stranded DNA have interesting applications, and a comparison between them provides the basis for understanding mechanisms. Denaturation of cross-linked ds-DNA gels was induced by heating them above the melting temperature and then cooling. This process, studied by fluorescence using ethidium bromide, appeared to be reversible when a heating/cooling cycle was performed. The swelling behavior upon addition of different cosolutes, such as metal ions, polyamines, charged proteins, and surfactants, was investigated for different DNA gel samples, including long and short ds-DNA and long and short ss-DNA. The DNA molecular weight was found to have only a slight effect on the deswelling curves, whereas conformation exhibited a pronounced effect. In general, single-stranded DNA gels exhibited a larger collapse in the presence of cations than did double-stranded DNA. This difference was more pronounced with surfactants than with the other cosolutes investigated. The difference between double- and single-stranded DNA was attributed to differences in linear charge density, chain flexibility, and hydrophobicity. For surfactants with different chain lengths, the swelling behavior displayed by ss-DNA can be interpreted in terms of an interplay between hydrophobic and electrostatic interactions, the latter being influenced by polymer flexibility. Increasing hydrophobicity of the network leads to a decreased critical aggregation concentration (cac) for the surfactant/gel complex, as a result of the strengthened hydrophobic attractive force between the surfactant and the gel chain. The swelling of DNA gels appears to be reversible and to be independent of DNA conformation. Surfactant-induced deswelling of DNA gels under some conditions appears to be quite homogeneous, whereas under other conditions, there is a separation into a collapsed region in the outer parts of the gel sample and an inside swollen part. Such "skin" formation is quite different for ss- and ds-DNA, with ss-DNA giving more pronounced skin formation over a wider range of binding ratio, beta. For example, no macroscopic separation into collapsed and swollen regions was observed at intermediate degrees of binding for ds-DNA gels, whereas a dense surfactant-rich surface phase (skin) was found to coexist with a swollen core network for ss-DNA gels with beta>0.5. One explanation for this difference is the large deformation energy required for the compression of the very stiff ds-DNA chains.