Arie de Keizer

Complex coacervate core micelles

In this review we present an overview of the literature on the co-assembly of neutral-ionic block, graft, and random copolymers with oppositely charged species in aqueous solution. Oppositely charged species include synthetic (co)polymers of various architectures, biopolymers - such as proteins, enzymes and DNA - multivalent ions, metallic nanoparticles, low molecular weight surfactants, polyelectrolyte block copolymer micelles, metallo-supramolecular polymers, equilibrium polymers, etcetera. The resultant structures are termed complex coacervate core/polyion complex/block ionomer complex/interpolyelectrolyte complex micelles (or vesicles); i.e., in short C3Ms (or C3Vs) and PIC, BIC or IPEC micelles (and vesicles). Formation, structure, dynamics, properties, and function will be discussed. We focus on experimental work; theory and modelling will not be discussed. Recent developments in applications and micelles with heterogeneous coronas are emphasized.

Adsorption of the protein bovine serum albumin in a planar poly(acrylic acid) brush layer as measured by optical reflectometry.

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.

Biodegradability and change of physical characteristics of particles during anaerobic digestion of domestic sewage.

At the high-rate anaerobic treatment of domestic sewage, both biological and physical processes play an important role. Therefore, the anaerobic biodegradability of raw, paper-filtered and membrane-filtered sewage and black water has been investigated in batch experiments. Additionally, the effect of anaerobic digestion on physical characteristics, like particle size, surface tension and zeta-potential, of the present particles is studied. The biodegradability of domestic sewage and black water at 30 degrees C is almost similar (71-74%). Moreover, a high methanogenesis of the colloidal fraction in domestic sewage (86 +/- 3%) is achieved, showing that the low removal of colloidal particles in continuous high-rate anaerobic reactors is due to low physical removal rather than biodegradability. The lowest biodegradability is demonstrated for the dissolved fraction (62%). The results show that after anaerobic digestion the average radius of particles with diameter < 4.4 and < 0.45 microns increased for domestic sewage, while it decreased for black water. Part of the surface-active components in domestic sewage is not biodegraded during anaerobic batch digestion, as indicated by the development of the surface tension. The negative zeta-potential of all particles hardly changes during digestion, showing that colloidal interactions were not affected by anaerobic digestion.

Spontaneous symmetry breaking: formation of Janus micelles

We describe the preparation and solution properties of Janus micelles, i.e., non-centrosymmetric nanoparticles with compartmentalized shells, viaco-assembly of two fully water-soluble block copolymers. They consist of a mixed core of poly(N-methyl-2-vinyl pyridinium iodide) (P2MVP) and poly(acrylic acid) (PAA), and a shell segregated into two sides, consisting of poly(ethylene oxide) (PEO) or poly(acryl amide) PAAm. These Janus particles form spontaneously and reversibly, i.e., association, dissocation, and reassociation can be carefully controlled via parameters, such as polymer mixing fraction, solution pH, and ionic strength. Dynamic (polarized and depolarized) and static light scattering, cryogenic transmission electron microscopy, small angle neutron and X-ray scattering, and two-dimensional nuclear magnetic resonance spectroscopy are used to monitor the micelle formation and to characterize the micellar structure. The Janus particles were found to be ellipsoidal, with a cigar-like overall shape and a disc-like core. This peculiar morphology is driven by the delicate interplay between two opposing forces: an attraction between the oppositely charged core blocks and a subtle repulsion between the water-soluble, neutral corona blocks.

Temperature responsive complex coacervate core micelles with a PEO and PNIPAAm corona.

In aqueous solutions at room temperature, poly( N-methyl-2-vinyl pyridinium iodide)- block-poly(ethylene oxide), P2MVP 38- b-PEO 211 and poly(acrylic acid)- block-poly(isopropyl acrylamide), PAA 55- b-PNIPAAm 88 spontaneously coassemble into micelles, consisting of a mixed P2MVP/PAA polyelectrolyte core and a PEO/PNIPAAm corona. These so-called complex coacervate core micelles (C3Ms), also known as polyion complex (PIC) micelles, block ionomer complexes (BIC), and interpolyelectrolyte complexes (IPEC), respond to changes in solution pH and ionic strength as their micellization is electrostatically driven. Furthermore, the PNIPAAm segments ensure temperature responsiveness as they exhibit lower critical solution temperature (LCST) behavior. Light scattering, two-dimensional 1H NMR nuclear Overhauser effect spectrometry, and cryogenic transmission electron microscopy experiments were carried out to investigate micellar structure and solution behavior at 1 mM NaNO 3, T = 25, and 60 degrees C, that is, below and above the LCST of approximately 32 degrees C. At T = 25 degrees C, C3Ms were observed for 7 < pH < 12 and NaNO 3 concentrations below approximately 105 mM. The PEO and PNIPAAm chains appear to be (randomly) mixed within the micellar corona. At T = 60 degrees C, onion-like complexes are formed, consisting of a PNIPAAm inner core, a mixed P2MVP/PAA complex coacervate shell, and a PEO corona.

Stability of complex coacervate core micelles containing metal coordination polymer

We report on the stability of complex coacervate core micelles, i.e., C3Ms (or PIC, BIC micelles), containing metal coordination polymers. In aqueous solutions these micelles are formed between charged-neutral diblock copolymers and oppositely charged coordination polymers formed from metal ions and bisligand molecules. The influence of added salt, polymer concentration, and charge composition was investigated by using light scattering and cryo-TEM techniques. The scattering intensity decreases strongly with increasing salt concentration until a critical salt concentration beyond which no micelles exist. The critical micelle concentration increases almost exponentially with the salt concentration. From the scattering results it follows that the aggregation number decreases with the square root of the salt concentration, but the hydrodynamic radius remains constant or increases slightly. It was concluded that the density of the core decreases with increasing ionic strength. This is in agreement with theoretical predictions and is also confirmed by cryo-TEM measurements. A complete composition diagram was constructed based on the composition boundaries obtained from light scattering titrations.

Complex Coacervate Core Micelles from Iron-Based Coordination Polymers

Complex coacervate core micelles (C3Ms) from cationic poly(N-methyl-2-vinyl-pyridinium iodide)-b-poly(ethylene oxide) (P2MVP(41)-b-PEO(205)) and anionic iron coordination polymers are investigated in the present work. Micelle formation is studied by light scattering for both Fe(II)- and Fe(III)-containing C3Ms. At the stoichiometric charge ratio, both Fe(II)-C3Ms and Fe(III)-C3Ms are stable for at least 1 week at room temperature. Excess of iron coordination polymers has almost no effect on the formed Fe(II)-C3Ms and Fe(III)-C3Ms, whereas excess of P2MVP(41)-b-PEO(205) copolymers in the solution can dissociate the formed micelles. Upon increasing salt concentration, the scattering intensity decreases. This decrease is due to both a decrease in the number of micelles (or an increase in CMC) and a decrease in aggregation number. The salt dependence of the CMC and the aggregation number is explained using a scaling argument for C3M formation. Compared with Fe(II)-C3Ms, Fe(III)-C3Ms have a lower CMC and a higher stability against dissociation by added salt.

Adsorption of Anionic Surfactants in a Nonionic Polymer Brush: Experiments, Comparison with Mean-Field Theory, and Implications for Brush−Particle Interaction

The adsorption of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) in poly(ethylene oxide) (PEO) brushes was studied using a fixed-angle optical flow-cell reflectometer. We show that, just as in solution, there is a critical association concentration (CAC) for the surfactants at which adsorption in the PEO brush starts. Above the critical micelle concentration (CMC) the adsorption is found to be completely reversible. At low brush density the adsorption per PEO monomer is equal to the adsorption of these surfactants in bulk solution. However, with increasing brush density, the number of adsorbed surfactant molecules per PEO monomer decreases rapidly. This decrease is explained in terms of excluded volume interactions plus electrostatic repulsion between the negatively charged surfactant micelles. Experimentally, a plateau value in the total adsorption is observed as a function of grafting density. The experimental results were compared to the results of an analytical self-consistent field (aSCF) model, and we found quantitative agreement. Additionally, the model predicts that the plateau value found is in fact a maximum. Both experiments and model calculations show that the adsorption scales directly with the polymerization degree of the polymers in the brush. They also show that an increase in the ionic strength leads to an increase in the adsorbed amount, which is explained as being due to a decrease in the electrostatic penalty for the adsorption of the SDS micelles. The adsorption of SDS micelles changes the interactions of the PEO brush with a silica particle. This is illustrated by atomic force microscopy (AFM) measurements of the pull-off force of a silica particle from a PEO brush: at high enough PEO densities, the addition of SDS leads to a very strong reduction in the force necessary to detach the colloidal silica particle from the PEO brush. We attribute this effect to the large amount of negative charge incorporated in the PEO brush due to SDS adsorption.

Ultradense Polymer Brushes by Adsorption

Standing room only: Dense polymer brushes can be prepared by adsorbing a diblock copolymer comprising a neutral block and a polyelectrolyte block to an oppositely charged polyelectrolyte brush (see picture). The density of the resulting neutral brush is determined by charge compensation, leading to brush densities well over 1 nm(-2). The diblock copolymer can be desorbed by changing the solution conditions.

Redox responsive molecular assemblies based on metallic coordination polymers

Redox responsive molecular self-assemblies are very important in a wide range of applications including bioengineering, nanotechnology, and medicine. We demonstrate here a redox responsive micellar system based on metal–ligand coordination and electrostatic interaction. The micelles are spontaneously formed in the mixed solutions of a block polyelectrolyte and an oppositely charged iron–bisligand coordination polyelectrolyte, where both Fe2+ and Fe3+ ions are applicable. Switching between these two oxidation states does not trigger any decomposition of the micelles, but changes the charge composition and the magnetic properties of the system, which enables the uptake of extra, oppositely charged species into the micelles. Moreover, this process triggers spectral and morphological changes of the micelles. These results open a new vista for making redox responsive smart molecular assemblies.