Driving Forces for Oppositely Charged Polyion Association in Aqueous Solutions: Enthalpic, Entropic, but Not Electrostatic

Driving forces for association between oppositely charged biological or synthetic polymers in aqueous solution have long been identified as electrostatic in origin. This attraction is broken down into an entropic component, due to loss of counterions, and an enthalpic component, stemming from Coulombic attraction between opposite charges. While the balance between entropic and enthalpic contributions shifts according to the conditions, the presence of exotherms or endotherms on mixing, though small, are viewed as signatures of Coulombic interactions which support theories of polyelectrolyte association rooted in continuum electrostatics. Here, a head-to-head comparison is made between mechanisms based on electrostatics and those based on specific ion pairing, or ion exchange. Using a Hofmeister series of counterions for a common polycation, poly­(diallyldimethylammonium), enthalpy changes on association with poly­(styrenesulfonate) are shown to derive from changes in water perturbation, revealed by Raman scattering studies of water O–H vibrations. The free energy for complexation is almost completely entropic over all salt concentrations

Janus Nanofilms

To make a two-dimensional Janus object, the perfluorinated anionic polyelectrolyte Nafion was adsorbed to the surface of ultrathin films of polyelectrolyte complex. Nafion changed the wetting characteristics of the polyelectrolyte multilayer (PEMU) of poly­(diallyldimethylammonium) and poly­(styrenesulfonate) from hydrophilic to hydrophobic. PEMUs assembled on aluminum substrates and terminated with Nafion could be released by exposure to alkali solution, producing free-floating films in the 100 nm thickness regime. Water contact angle measurements showed a strong difference in hydrophilicity between the two sides of this Janus film, which was further characterized using atomic force microscopy and X-ray photoelectron spectroscopy (XPS). XPS revealed different fluorine contents on both sides of the PEMU, which could be translated to a Nafion gradient through the film. Fourier transform infrared spectroscopy showed the Nafion-containing films were much more resistant to decomposition by high salt concentration

The Polyelectrolyte Complex/Coacervate Continuum

Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly­(styrenesulfonate) (PSS) and poly­(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation

Saloplastic Macroporous Polyelectrolyte Complexes: Cartilage Mimics

Complexes of sodium poly(4-styrenesulfonate) (NaPSS) and poly(diallyldimethylammonium chloride) (PDADMAC) were formed on mixing equimolar solutions in high salt concentration. Under ultracentrifugal fields, the complex precipitates were transformed into compact polyelectrolyte complexes (CoPECs), which showed extensive porosity. The mechanical properties of CoPECS make them attractive for bioimplants and tissue engineering applications. Free NaPSS chains in the closed pores of CoPECs create excess osmotic pressure, which controls the pore size and contributes to the mechanical resistance of the material. The mechanical properties of CoPECs, modulated by the ionic strength of the doping medium, were studied by uniaxial tensile testing and the stress−strain data were fit to a three-element Maxwell model which revealed at least two regimes of stress relaxation

Static and Dynamic Solution Behavior of a Polyzwitterion Using a Hofmeister Salt Series

Polymers made from zwitterionic repeat units (bearing no net charge) have intriguing solution properties, especially in contrast to polyelectrolytes, such as an apparent indifference to salt concentration. These polyzwitterions (PZs) have come under renewed scrutiny because of their use in high performance antifouling coatings. Here, an amidosulfobetaine polymer was used to shed light on the complex and poorly understood response of PZ solution conformation to ionic strength. A Hofmeister anion series NaX, where X = SO₄^2–, Cl^–, Br^–, NO₃^–, ClO₄^–, and SCN^–, provided a systematic way to tune PZ/ion interactions. A consistent picture of PZ conformation emerged, where the role and location of counterions (how they pair with the polymer chain) depend on their position in the Hofmeister series. At least four regimes of PZ conformation/interaction as a function of ionic strength were observed, the last showing no change in coil size (hydrodynamic radius) as a function of ionic strength for all monovalent salts in the concentration range 0.6–4 M. Hydrophobic (less hydrated) anions ClO₄^– and SCN^– yielded a clear minimum in coil size at lower [NaX], whereas PZ in solutions of hydrophilic ions SO₄^2– and Cl^– showed only a hint of the much-discussed “anti-polyelectrolyte” expansion of PZ with increasing [NaX]. Static light scattering results, when analyzed using Stockmeyer’s theory of scattering from multicomponent systems, revealed that NaX is associated with PZ with a corresponding increase in apparent molecular weight. Static light scattering measurements at low [NaX] show solution ions are excluded from PZ coils dressed with hydrophobic NaX. Dynamic light scattering in salt-free solutions at elevated temperatures revealed substantial chain stiffening of PZ, thought to be caused by nearest-neighbor interactions between zwitterion groups. DLS yielded a fast mode in these salt-free solutions, ascribed to soliton-like transport of waves of associated zwitterionic groups along the PZ backbone

Diffusion of Sites versus Polymers in Polyelectrolyte Complexes and Multilayers

It has long been assumed that the spontaneous formation of materials such as complexes and multilayers from charged polymers depends on (inter)­diffusion of these polyelectrolytes. Here, we separately examine the mass transport of polymer molecules and extrinsic sitescharged polyelectrolyte repeat units balanced by counterionswithin thin films of polyelectrolyte complex, PEC, using sensitive isotopic labeling techniques. The apparent diffusion coefficients of these sites within PEC films of poly­(diallyldimethylammonium), PDADMA, and poly­(styrenesulfonate), PSS, are at least 2 orders of magnitude faster than the diffusion of polyelectrolytes themselves. This is because site diffusion requires only local rearrangements of polyelectrolyte repeat units, placing far fewer kinetic limitations on the assembly of polyelectrolyte complexes in all of their forms. Site diffusion strongly depends on the salt concentration (ionic strength) of the environment, and diffusion of PDADMA sites is faster than that of PSS sites, accounting for the asymmetric nature of multilayer growth. Site diffusion is responsible for multilayer growth in the linear <i>and</i> into the exponential regimes, which explains how PDADMA can mysteriously “pass through” layers of PSS. Using quantitative relationships between site diffusion coefficient and salt concentration, conditions were identified that allowed the diffusion length to always exceed the film thickness, leading to full exponential growth over 3 orders of magnitude thickness. Both site and polymer diffusion were independent of molecular weight, suggesting that ion pairing density is a limiting factor. Polyelectrolyte complexes are examples of a broader class of dynamic bulk polymeric materials that (self-) assemble via the transport of cross-links or defects rather than actual molecules

Zwitteration As an Alternative to PEGylation

A direct, head-to-head comparison of the efficacy of a zwitterionic versus a poly(ethylene glycol), PEG, coating in preventing protein adsorption to silica and aggregation of silica nanoparticles is presented. The same siloxane coupling chemistry was employed to yield surfaces with similar coverages of both types of ligand. Nanoparticle and planar surfaces were challenged with salt, serum, lysozyme, and serum albumin at 25 and 37 °C. While both types of surface modification are highly effective in preventing protein adsorption and nanoparticle aggregation, the zwitterion provided monolayer-type coverage with minimal thickness, whereas the PEG appeared to yield a more three-dimensional coating. The mechanism for adsorption resistance is thought to be based on preventing ion pairing between protein and surface charges, which releases counterions and water molecules, an entropic driving force enough to overcome a disfavored enthalpy of adsorption

Spin-Coated Polyelectrolyte Coacervate Films

Thin films of complexes made from oppositely charged polyelectrolytes have applications as supported membranes for separations, cell growth substrates, anticorrosion coatings, biocompatible coatings, and drug release media, among others. The relatively recent technique of layer-by-layer assembly reliably yields conformal coatings on substrates but is impractically slow for films with thickness greater than about 1 μm, even when accelerated many fold by spraying and/or spin assembly. In the present work, thin, uniform, smooth films of a polyelectrolyte complex (PEC) are rapidly made by spin-coating a polyelectrolyte coacervate, a strongly hydrated viscoelastic liquidlike form of PEC, on a substrate. While the apparatus used to deposit the PEC film is conventional, the behavior of the coacervate, especially the response to salt concentration, is highly nontraditional. After glassification by immersion in water, spun-on films may be released from their substrates to yield free-standing membranes of thickness in the micrometer range

Flipped Polyelectrolyte Multilayer Films: Accessing the Buried Interface

Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making free-standing multilayers of poly­(diallyldimethylammonium) (PDADMA) and poly­(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield “nanosandwiches” of nonstoichiometric PEMU between layers of stoichiometric PEMU