Leonardo Chiappisi

Complexes of oppositely charged polyelectrolytes and surfactants – recent developments in the field of biologically derived polyelectrolytes

We review the work done on complexes between biopolyelectrolytes such as ionically modified cellulose or chitosan and oppositely charged surfactants. Around equimolarity of the charges one typically observes precipitation but for other mixing ratios one may form long-time stable complexes, where structure and rheology depend on the mixing ratio, total concentration and the molecular constitution of the components. In addition, it may be the case that the structures are formed under non-equilibrium situations and therefore depend on the preparation path. The binding is shown to occur cooperatively and the micelles present often retain their shape irrespective of the complexation. However, the rather stiff biopolyelectrolytes may lead to an interconnection between different aggregates thereby forming a network with the corresponding rheological properties. In general, the structure and the properties of the aggregates are rather versatile and correspondingly one can create a wide range of different surfactant–biopolyelectrolyte systems by appropriately choosing the composition. This is very interesting as it allows for formulations with a large range of tuneable properties with ecologically friendly polyelectrolytes for many relevant applications.

Co-assembly in chitosan–surfactant mixtures: thermodynamics, structures, interfacial properties and applications

In this review, different aspects characterizing chitosan-surfactant mixtures are summarized and compared. Chitosan is a bioderived cationic polysaccharide that finds wide-ranged applications in various field, e.g., medical or food industry, in which synergistic effects with surfactant can play a fundamental role. In particular, the behavior of chitosan interacting with strong and weak anionic, nonionic as well as cationic surfactants is reviewed. We put a focus on oppositely charged systems, as they exhibit the most interesting features. In that context, we discuss the thermodynamic description of the interaction and in particular the structural changes as they occur as a function of the mixed systems and external parameters. Moreover, peculiar properties of chitosan coated phospholipid vesicles are summarized. Finally, their co-assembly at interfaces is briefly reviewed. Despite the behavior of the mentioned systems might strongly differ, resulting in a high variety of properties, few general rules can be pointed out which improve the understanding of such complex systems.

Chitosan/alkylethoxy carboxylates: a surprising variety of structures.

In this work, we present a comprehensive structural characterization of long-term stable complexes formed by biopolycation chitosan and oppositely charged nonaoxyethylene oleylether carboxylate. These two components are attractive for many potential applications, with chitosan being a bioderived polymer and the surfactant being ecologically benign and mild. Experiments were performed at different mixing ratios Z (ratio of the nominal charges of surfactant/polyelectrolyte) and different pH values such that the degree of ionization of the surfactant is largely changed whereas that of chitosan is only slightly affected. The structural characterization was performed by combining static and dynamic light scattering (SLS and DLS) and small-angle neutron scattering (SANS) to cover a large structural range. Highly complex behavior is observed, with three generic structures formed that depend on pH and the mixing ratio, namely, (i) a micelle-decorated network at low Z and pH, (ii) rodlike complexes with the presence of aligned micelles at medium Z and pH, and (iii) compacted micellar aggregates forming a supraaggregate surrounded by a chitosan shell at high Z and pH. Accordingly, the state of aggregation in these mixtures can be tuned structurally over quite a range only by rather small changes in pH.

Enzymatically cross-linked hyperbranched polyglycerol hydrogels as scaffolds for living cells.

Although several strategies are now available to enzymatically cross-link linear polymers to hydrogels for biomedical use, little progress has been reported on the use of dendritic polymers for the same purpose. Herein, we demonstrate that horseradish peroxidase (HRP) successfully catalyzes the oxidative cross-linking of a hyperbranched polyglycerol (hPG) functionalized with phenol groups to hydrogels. The tunable cross-linking results in adjustable hydrogel properties. Because the obtained materials are cytocompatible, they have great potential for encapsulating living cells for regenerative therapy. The gel formation can be triggered by glucose and controlled well under various environmental conditions.

Toward Bioderived Intelligent Nanocarriers for Controlled Pollutant Recovery and pH-Sensitive Binding

The pH-triggered formation of supramolecular complexes between the cationic biopolysaccharide chitosan and an environmentally friendly anionic surfactant is exploited for the formulation of selective and controlled-recovery systems. A strong advantage of this system is the very small pH range in which the binding/release process takes place. Because of this high pH responsiveness, chitosan-surfactant complexes are employed for the sequestration of various compounds by binding or releasing them from the complexes. In particular, the selective recovery of a model hydrophobic pollutant in the presence of a hydrophilic one is presented. The process is highly selective and effective, with more than 90% of the hydrophobic dye and ca. 10% of the hydrophilic dye recovered. Furthermore, the method can be extended to the selective recovery of metal ions, and in both cases, the original surfactant and chitosan mixture can be recovered, thereby rendering this an efficient and sustainable process. These showcase experiments depict quite different scenarios in which pH-responsive fully biodegradable polysaccharide-surfactant complexes can be employed and may substitute synthetic products in various fields, e.g., wastewater treatment, cosmetics, and agriculture, thereby yielding environmentally improved approaches.

From Crab Shells to Smart Systems: Chitosan–Alkylethoxy Carboxylate Complexes

In this work, self-assembly of alkyl ethylene oxide carboxylates and the biopolymer chitosan into supramolecular structures with various shapes is presented. Our investigations were done at pH 4.0, where the chitosan is almost fully charged and the surfactants are partially deprotonated. By changing the alkyl chain length and the number of ethylenoxide units very different water-soluble complexes can be obtained, ranging from globular micelles incorporated in a chitosan network to formation of ordered multiwalled vesicles. The structural characteristics of these complexes can be finely controlled by the mixing ratio of chitosan and surfactant, i.e., simply by the solutions composition. For instance, the vesicle wall thickness can be varied between 5 and 50 nm just by varying the mixing ratio. Accordingly, we expect this system to be an outstanding carrier for hydrophilic compounds with tunable release time option. Moreover, an easy route for preparation of chitosan-based complexes in the solid state with controlled mesoscopic order is presented. This work opens the way to prepare biofriendly materials on the basis of chitosan and mild anionic surfactants which are rather versatile with respect to their structure and properties, allowing for preparation of complexes with highly variable structures in both aqueous and solid phase. Formation of such different structures can be exploited for preparation of carriers, which are able to transport hydrophilic as well as hydrophobic molecules. Furthermore, as chitosan is well known to exhibit antibacterial and anti-inflammatory properties, different applications of these complexes can be indicated, i.e., as drug delivery systems or as coatings for medical implants.

Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers

We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore.

A journey through the phase diagram of a pharmaceutically relevant microemulsion system.

HYPOTHESIS The phase behavior and the properties of water, oil, surfactant, and cosurfactant mixtures depend on the fine balance of different forces, among them the bending energy of the amphiphilic film. Thus, it should be possible to control the structural evolution of nonionic microemulsions by the cosurfactant content and the hydration of the surfactant headgroup. EXPERIMENTS An extensive investigation of the pseudoternary phase diagram of mixtures of water, isopropyl palmitate, polyoxyethylene (10) oleyl ether Brij 97 (C18E10), and butanol is presented for two different cosurfactant concentrations, thereby varying the hydrophilicity of the amphiphile. The studies were performed employing conductometric titrations, differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). FINDINGS A systematic growth of the domain sizes and correlation lengths is observed by increasing the water content in the initial oil/surfactant solution. The formation of a bicontinuous structure, as deduced from conductivity is directly related to the presence of free water unbound to the EO units of the surfactant, as determined by DSC. The experimental results, e.g., the extension of the different phase regions and the mesoscopic structure are discussed on a molecular level, therefore providing a direct link between the composition of the microemulsions and the resulting structure and properties.

Oleylethoxycarboxylate--an efficient surfactant for copper extraction and surfactant recycling via micellar enhanced ultrafiltration.

The nonaoxyethylene oleylether carboxylic acid Akypo RO90 VG, a surfactant with ionic character at high pH and non-ionic character at low pH, has been investigated with respect to copper removal from aqueous streams via micellar enhanced ultrafiltration (MEUF) with subsequent copper and RO90 separation using the method of cloud point extraction (CPE). Almost quantitative Cu2+ removal is obtained in MEUF and more than 90% Cu2+ is separated from RO90 in CPE. The investigation of Cu(2+)/RO90 complexes with small-angle neutron scattering (SANS) shows almost no structural change of RO90 micelles in the presence of Cu2+. These results show the importance of the surfactant head group for optimizing the specific interaction with the ion to be extracted. This optimization and the ability to recycle the surfactant by a temperature variation and using the cloud point phenomenon is an elegant approach to achieve efficient metal ion extraction.

Form factor of cylindrical superstructures composed of globular particles

The scattering form factor arising from N linearly aligned particles, thereby forming a cylindrical superstructure and potentially contained within a cylindrical cage, has been derived. The principal features of the scattering curves are a q−1 regime at mid-q (q is the magnitude of the scattering vector) due to the linear long-range order of the particles and, arising from the repetition of the building blocks, a correlation peak at q ≃ 2π/D (D being the center-to-center distance between the particles). In order to be able to describe real systems also, as they might arise in electrostatically assembled arrays, the effect of polydispersity of particle size and spacing has been evaluated. Finally, the model of aligned particles contained in a cylinder was applied to polyelectrolyte–surfactant complexes. It is shown that, provided that N is sufficiently large, the scattering arising from aligned particles is clearly distinguishable from that of a homogeneous cylinder and therefore the presence of single particles within the complex can be demonstrated. The precise shape of the scattering curves then depends markedly on the contrast conditions between the contained particles, the cylindrical cage and the surrounding medium.