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

Editorial: 25th Meeting of the European Colloid and Interface Society, September 04-09, 2011 in Berlin

Editorial: 25th Meeting of the European Colloid and Interface Society, September 04-09, 2011 in Berli

Formation of Well-Defined Vesicles by Styrene Addition to a Nonionic Surfactant and Their Polymerization Leading to Viscous Hybrid Systems

Self-assembled structures in aqueous solutions can be fixed by polymerization after adding hydrophobic monomers and can thereby be used as templates which allow to substantially alter the properties of these systems. In this work, we started from a self-assembled micellar system consisting of the nonionic surfactants tetradecyldimethylamine oxid and Pluronic L35 to which styrene was added as a polymerizable monomer. Interestingly, it was observed that styrene induces a transition from micelles to well-defined vesicles in a similar manner as a typical cosurfactant. The structural transition of the aggregates upon styrene addition as well as the structures formed after initiating a polymerization reaction were investigated by means of turbidity, dynamic and static light scattering, small-angle neutron scattering, and rheology measurements. Especially the scattering results confirmed the interesting effect of styrene on the mesoscopic structure and showed a structural evolution from rod-like micelles for low styrene concentrations to vesicles at intermediate styrene amounts, and then finally the formation of microemulsion droplets for high styrene content. Their polymerization of the vesicles again leads to a shape change to wormlike, polymerized aggregates, whose presence then results in rather viscous systems. In contrast, the microemulsions with higher styrene content then are templated and retain their size after polymerization, thereby leading to nanolattices

Understanding the Formation of Anisometric Supraparticles: A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface

Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop–air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation. This leads to a buildup of a stiff fumed silica mantle of ∼20 μm thickness that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles

Understanding the Formation of Anisometric Supraparticles: A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface

Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop–air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation. This leads to a buildup of a stiff fumed silica mantle of ∼20 μm thickness that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles

Quantitative Description of Temperature Induced Self-Aggregation Thermograms Determined by Differential Scanning Calorimetry

A novel thermodynamic approach for the description of differential scanning calorimetry (DSC) experiments on self-aggregating systems is derived and presented. The method is based on a mass action model where temperature dependence of aggregation numbers is considered. The validity of the model was confirmed by describing the aggregation behavior of poly­(ethylene oxide)-poly­(propylene oxide) block copolymers, which are well-known to exhibit a strong temperature dependence. The quantitative description of the thermograms could be performed without any discrepancy between calorimetric and van 't Hoff enthalpies, and moreover, the aggregation numbers obtained from the best fit of the DSC experiments are in good agreement with those obtained by light scattering experiments corroborating the assumptions done in the derivation of the new model

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

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 <i>Z</i> (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 <i>Z</i> and pH, (ii) rodlike complexes with the presence of aligned micelles at medium <i>Z</i> and pH, and (iii) compacted micellar aggregates forming a supraaggregate surrounded by a chitosan shell at high <i>Z</i> 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

Shaping Vesicles–Controlling Size and Stability by Admixture of Amphiphilic Copolymer

The production of structurally well-defined unilamellar vesicles and the control of their stability are of utmost importance for many of their applications but still a largely unresolved practical issue. In the present work we show that by admixing small amounts of amphiphilic copolymer to the original components of a spontaneously vesicle-forming surfactant mixture we are able to control the self-assembly process in a systematic way. For this purpose we employed a zwitanionic model system of zwitterionic TMDAO and anionic LiPFOS. As the copolymer reduces the line tension of the intermediately formed disks, this translates directly into a longer disk growth phase and formation of correspondingly larger vesicles. By this approach we are able to vary their size over a large range and produce vesicles of extremely low polydispersity. Furthermore, the temporal stability of the formed vesicles is enhanced by orders of magnitude in proportion to the concentration of copolymer added. This is achieved by exerting kinetic control that allows engineering the vesicle structure <i>via</i> a detailed knowledge of the formation pathway as obtained by highly time-resolved SAXS experiments. Synthesis of such very well-defined vesicles by the method shown should in general be applicable to catanionic or zwitanionic amphiphiles and will have far reaching consequences for controlled nanostructure formation and application of these self-assembled systems

Structure and Dynamics of Networks in Mixtures of Hydrophobically Modified Telechelic Multiarm Polymers and Oil in Water Microemulsions

The structural and dynamical properties of oil-in-water (O/W) microemulsions (MEs) modified with telechelic polymers of different functionality (e.g., number of hydrophobically modified arms, <i>f</i>) were studied by means of dynamic light scattering (DLS), small-angle neutron scattering (SANS), and high frequency rheology measurements as a function of the polymer architecture and the amount of added polymer. For this purpose, we employed tailor-made hydrophobically end-capped poly­(<i>N,N</i>-dimethylacrylamide) star polymers of a variable number of endcaps, <i>f</i>, of different alkyl chain lengths, synthesized by the reversible addition–fragmentation chain transfer method. The addition of the different end-capped polymers to an uncharged ME of O/W droplets leads to a large enhancement of the viscosity of the systems. SANS experiments show that the O/W ME droplets are not changed upon the addition of the polymer, and its presence only changes the interdroplet interactions. The viscosity increases largely upon addition of a polymer, and this enhancement depends pronouncedly on the alkyl length of the hydrophobic sticker as it controls the residence time in a ME droplet. Similarly, the high frequency modulus <i>G</i>₀ depends on the amount of added polymer but not on the sticker length. <i>G</i>₀ was found to be directly proportional to <i>f</i> – 1. The onset of network formation is shifted to a lower number of stickers per ME droplet with increasing <i>f</i>, and the network formation becomes more effective. Thus, the dynamics of network formation are controlled by the polymer architecture. The effect on the dynamics seen by DLS is even more pronounced. Upon increasing the polymer concentration, slower relaxation modes appear that become especially pronounced with increasing number of arms. The relaxation dynamics are correlated to the rheological relaxation, and both are controlled by the polymer architecture