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A series of novel dimeric pyridinium surfactants has been synthesised and the effects of a semi-flexible p-xylyl spacer and flexible, polyethylene glycol spacers have been studied. The nature of the spacer determines solubility and aggregation behaviour in two- and three-dimensional systems. Some of these insoluble compounds form two-dimensional, rigid-condensed structures at the air–water interface, while others form liquid-analogue monolayers. Whereas the latter compounds become soluble after exchange of the counterions, the former remain insoluble. The aggregation behaviour of Langmuir layers was, inter alia, investigated by Brewster angle microscopy. The micellisation behaviour of diluted aqueous solutions of soluble surfactants was primarily investigated by conductometric measurements and thermodynamic parameters of aggregation have been deduced with respect to the spacer length.

Metallo-supramolecular self-assembly has yielded a plethora of discrete nanosystems, many of which show competence in capturing guests and catalyzing chemical reactions. However, the potential of low-molecular bottom-up self-assemblies in the development of structured inorganic materials has rarely been methodically explored so far. Herein, we present a new type of metallo-supramolecular surfactant with the ability to stabilize non-aqueous emulsions for a significant period. The molecular design of the surfactant is based on a heteroleptic coordination cage ( CGA-3 ; CGA = Cage-based Gemini Amphiphile), assembled from two pairs of organic building blocks, grouped around two Pd(II) cations. Shape-complementarity between the differently functionalized components generates discrete amphiphiles with a tailor-made polarity profile, able to stabilize non-aqueous emulsions, such as hexadecane-in-DMSO. These emulsions were used as a medium for the synthesis of spherical metal oxide microcapsules (titanium oxide, zirconium oxide, and niobium oxide) from soluble, water-sensitive alkoxide precursors by allowing a controlled dosage of water to the liquid-liquid phase boundary. Synthesized materials were analyzed by a combination of electron microscopic techniques. In situ liquid cell transmission electron microscopy (LC-TEM) was utilized for the first time to visualize the dynamics of the emulsion-templated formation of hollow inorganic titanium oxide and zirconium oxide microspheres.

Self-assembled, porous coordination cages with a functional interior find application in controlled guest inclusion/release, drug delivery, separation processes, and catalysis. However, only few studies exist that describe their utilization for the development of self-assembled materials based on their 3-dimensional shape and external functionalization. Here, dodecyl chain-containing, acridone-based ligands (LA) and shape-complementary phenanthrene-derived ligands (LB) are shown to self-assemble to heteroleptic coordination cages cis-[Pd2(LA)2(LB)2]4+ acting as a gemini amphiphile (CGA-1; Cage-based Gemini Amphiphile-1). Owing to their anisotropic decoration with short polar and long nonpolar side chains, the cationic cages were found to assemble into vesicles with diameters larger than 100 nm in suitable polar solvents, visualized by cryo-TEM and Liquid-Cell Transmission Electron Microscopy (LC-TEM). LC-TEM reveals that these vesicles aggregate into chains and necklaces via long-range interactions. In addition, the cages show a rarely described ability to stabilize oil-in-oil emulsions.

We present a realization of a fast interfacial Marangoni microswimmer by a half-spherical alginate capsule at the air-water interface, which diffusively releases water-soluble spreading molecules (weak surfactants such as polyethylene glycol (PEG)), which act as "fuel" by modulating the air-water interfacial tension. For a number of different fuels, we can observe symmetry breaking and spontaneous propulsion although the alginate particle and emission are isotropic. The propulsion mechanism is similar to soap or camphor boats, which are, however, typically asymmetric in shape or emission to select a swimming direction. We develop a theory of Marangoni boat propulsion starting from low Reynolds numbers by analyzing the coupled problems of surfactant diffusion and advection and fluid flow, which includes surfactant-induced fluid Marangoni flow, and surfactant adsorption at the air-water interface; we also include a possible evaporation of surfactant. The swimming velocity is determined by the balance of drag and Marangoni forces. We show that spontaneous symmetry breaking resulting in propulsion is possible above a critical dimensionless surfactant emission rate (Peclet number). We derive the relation between Peclet number and swimming speed and generalize to higher Reynolds numbers utilizing the concept of the Nusselt number. The theory explains the observed swimming speeds for PEG-alginate capsules, and we unravel the differences to other Marangoni boat systems based on camphor, which are mainly caused by surfactant evaporation from the liquid-air interface. The capsule Marangoni microswimmers also exhibit surfactant-mediated repulsive interactions with walls, which can be qualitatively explained by surfactant accumulation at the wall., 21 pages, 14 figures

We present a new system based on alginate gels for the encapsulation of a ferrofluid drop, which allows us to create millimeter-sized elastic capsules that are highly deformable by inhomogeneous magnetic fields. We use a combination of experimental and theoretical work in order to characterize and quantify the deformation behavior of these ferrofluid-filled capsules. We introduce a novel method for the direct encapsulation of unpolar liquids by sodium alginate. The addition of polar alcohol molecules allows us to encapsulate a ferrofluid as a single phase. This encapsulation method increases the amount of encapsulated magnetic nanoparticles resulting in high deformations and offers possible applications of capsules as actuators, switches, or valves in confined spaces like microfluidic devices. We determine both elastic moduli of the capsule shell, Young's modulus and Poisson's ratio, by employing two independent mechanical methods, spinning capsule measurements and capsule compression between parallel plates. We then show that the observed magnetic deformation can be fully understood from magnetic forces exerted by the ferrofluid on the capsule shell if the magnetic field distribution and magnetization properties of the ferrofluid are known. Using an iterative solution scheme that couples a finite element / boundary element method for the magnetic field calculation to the solution of the elastic shape equations, we achieve quantitative agreement between theory and experiment for deformed capsule shapes using the Young modulus from mechanical characterization and the surface Poisson ratio as a fit parameter. This detailed analysis confirms the results from mechanical characterization that the surface Poisson ratio of the alginate shell is close to unity, that is, deformations of the alginate shell are almost area conserving.

We provide a C/C++ software for the shape analysis of deflated elastic capsules in a pendant capsule geometry, which is based on an elastic description of the capsule material as a quasi two-dimensional elastic membrane using shell theory. Pendant capsule elastometry provides a new in-situ and non-contact method for interfacial rheology of elastic capsules. Given an elastic model of the capsule membrane, pendant capsule elastometry determines optimal elastic moduli by fitting numerically generated axisymmetric shapes optimally to an experimental image. For each digitized image of a deflated capsule elastic moduli can be determined, if another image of its undeformed reference shape is provided. Within this paper, we focus on nonlinear Hookean elasticity because of its low computational cost its wide applicability, but also discuss and implement alternative constitutive laws. For Hookean elasticity, Young's surface modulus (or, alternatively, area compression modulus) and Poisson's ratio are determined; for Mooney-Rivlin elasticity, the Rivlin modulus and a dimensionless shape parameter are determined; for neo-Hookean elasticity, only the Rivlin modulus is determined, using a fixed dimensionless shape parameter. Comparing results for different models we find that nonlinear Hookean elasticity is adequate for most capsules. If series of images are available, these moduli can be evaluated as a function of the capsule volume to analyze hysteresis or aging effects depending on the deformation history. An additional wrinkling wavelength measurement allows the user to determine the bending modulus, from which the layer thickness can be derived. We verify the method by analyzing several materials, compare the results to available rheological measurements, and review several applications. We make the software available under the GPL license at github.com/jhegemann/opencapsule., Comment: The source code of our pendant capsule elastometry software is freely available at https://github.com/jhegemann/opencapsule

An X-ray reflectivity study on the adsorption behavior of human apolipoprotein A1 (apoA1) at hydrophilic and hydrophobic interfaces is presented. It is shown that the protein interacts via electrostatic and hydrophobic interactions with the interfaces, resulting in the absorption of the protein. pH dependent measurements at the solid/liquid interface between silicon dioxide and aqueous protein solution show that in a small pH range between pH 4 and 6, adsorption is increased due to electrostatic attraction. Here, the native shape of the protein seems to be conserved. In contrast, the adsorption at the liquid/gas interface is mainly driven by hydrophobic effects, presumably by extending the hydrophobic regions of the amphipathic helices, and results in a conformational change of the protein during adsorption. However, the addition of differently charged membrane-forming lipids at the liquid/gas interface illustrates the ability of apoA1 to include lipids, resulting in a depletion of the lipids from the interface.

In this article we investigated the deformation of alginate capsules in magnetic fields. The sensitivity to magnetic forces was realised by encapsulating an oil in water emulsion, where the oil droplets contained dispersed magnetic nanoparticles. We solved calcium ions in the aqueous emulsion phase, which act as crosslinking compounds for forming thin layers of alginate membranes. This encapsulating technique allows the production of flexible capsules with an emulsion as the capsule core. It is important to mention that the magnetic nanoparticles were stable and dispersed throughout the complete process, which is an important difference to most magnetic alginate-based materials. In a series of experiments, we used spinning drop techniques, capsule squeezing experiments and interfacial shear rheology in order to determine the surface Young moduli, the surface Poisson ratios and the surface shear moduli of the magnetically sensitive alginate capsules. In additional experiments, we analysed the capsule deformation in magnetic fields. In spinning drop and capsule squeezing experiments, water droplets were pressed out of the capsules at elevated values of the mechanical load. This phenomenon might be used for the mechanically triggered release of water-soluble ingredients. After drying the emulsion-filled capsules, we produced capsules, which only contained a homogeneous oil phase with stable suspended magnetic nanoparticles (organic ferrofluid). In the dried state, the thin alginate membranes of these particles were rather rigid. These dehydrated capsules could be stored at ambient conditions for several months without changing their properties. After exposure to water, the alginate membranes rehydrated and became flexible and deformable again. During this swelling process, water diffused back in the capsule. This long-term stability and rehydration offers a great spectrum of different applications as sensors, soft actuators, artificial muscles or drug delivery systems.

In a series of experiments, we investigated the non-linear rheological properties of aqueous solutions of entangled wormlike micelles (WLMs) in steady-state shear flow and in large amplitude oscillating shear (LAOS) experiments. On grounds of their monoexponential stress relaxation properties, we studied semi-dilute solutions of the cationic surfactants cetylpyridinium chloride (CPyCl) or cetyltrimethylammonium bromide (CTAB) after addition of different amounts of sodium salicylate. The rheological data of these networks of WLMs were systematically compared with the numerically calculated results of the one-mode Giesekus constitutive equation. It turned out that the viscous resistance and the first normal stress difference, measured in steady-state shear flow, start-up, and relaxation experiments, were accurately predicted by the one-mode Giesekus model. In rheological tests, where we applied large oscillating shear amplitudes (LAOS), the transient shear stress could also approximately be described by means of the Giesekus model. The non-linear oscillating first normal stress difference, however, showed large deviations in respect to the theoretical predictions. These discrepancies between different rheological experiments, which we observed in oscillating and stationary flow, pointed to the existence of flow instabilities, which occurred in the LAOS regime. These, more complicated rheological processes, were induced by shear-banding and/or the presence of flow-induced phase transitions, which can occur in oscillatory and stationary shear. The non-linear phenomena, discussed in this article, are of general importance, and they can be equally observed in entangled solutions of flexible macromolecules.

The behavior of magnetically responsive aqueous Fe(III) surfactant solutions at liquid interfaces is analyzed. Such surfactants attracted much attention, because of the ability to manipulate interfaces by magnetic fields without any use of magnetic nanoparticles. A detailed analysis of the surface properties proves that the mixing of paramagnetic electrolyte solution with anionic, cationic and nonionic surfactants yields the similar magnetic response and no effect of the surfactant charge can be observed. We conclude that the observed magnetic shiftability of interfaces is caused by a combination of the paramagnetic behavior of the bulk liquid and a reduction of the surface tension. Thus, this work gives an alternative interpretation of the properties of "magnetic surfactants" compared to the ones claimed in the literature.

Structural and mechanical properties of molecularly thick polysiloxane membranes were studied on different liquid subphases to investigate the impact of the subphase's pH value on the cross-linking process. The lateral structure of these films was studied in-situ by grazing incidence diffraction while torsions pendulum experiments reveal the response of the system to mechanical stress. The results show a hindered cross-linking on acidic subphases. At alkaline and neutral pH conditions the cross-linking process was not effected. The data revealed that the degree of polymerization can be tuned by regulating the subphase's pH value, which opens the opportunity to build complex polysiloxane membranes in a controlled manner.

BACKGROUND Bilberries (Vaccinium myrtillus L.) have been suggested to have preventive properties against diseases associated with oxidative stress such as colon cancer or inflammatory bowel diseases. Therefore the gastrointestinal tract is regarded as a potential target for prevention. In this study the antioxidative properties of a commercially available anthocyanin-rich bilberry extract (BE) were investigated in comparison with four different BE-loaded microcapsule systems. As markers to describe the antioxidant status in this cellular system, intracellular reactive oxygen species (ROS) levels, oxidative DNA damage and total glutathione (tGSH) levels were monitored. RESULTS Incubations with the BE-loaded capsule systems showed an increase in cellular glutathione levels and reduction of ROS levels at high BE concentrations (100-500 µg mL(-1) ) and a positive effect on the formation of DNA strand breaks (5-10 µg mL(-1) BE). The biological properties of BE-loaded pectin amide core-shell capsules, whey protein matrix capsules and coated apple pectin matrix capsules were comparable to those of the non-encapsulated BE. CONCLUSION Overall, the BE and the encapsulated BE types tested have antioxidative activity under the studied assay conditions in terms of the prevention of oxidative DNA damage, the reduction of intracellular ROS and the enhancement of cellular tGSH.

Elastic capsules, prepared from droplets or bubbles attached to a capillary (as in a pendant drop tensiometer), can be deflated by suction through the capillary. We study this deflation and show that a combined analysis of the shape and wrinkling characteristics enables us to determine the elastic properties in situ. Shape contours are analyzed and fitted using shape equations derived from nonlinear membrane-shell theory to give the elastic modulus, Poisson ratio and stress distribution of the membrane. We include wrinkles, which generically form upon deflation, within the shape analysis. Measuring the wavelength of wrinkles and using the calculated stress distribution gives the bending stiffness of the membrane. We compare this method with previous approaches using the Laplace-Young equation and illustrate the method on two very different capsule materials: polymerized octadecyltrichlorosilane (OTS) capsules and hydrophobin (HFBII) coated bubbles. Our results are in agreement with the available rheological data. For hydrophobin coated bubbles, the method reveals an interesting nonlinear behavior consistent with the hydrophobin molecules having a rigid core surrounded by a softer shell.

This study focuses on the preparation and characterization of magnetic switchable thin iron oxide–polymer films. In a series of experiments, the formation and growth of iron oxide under ultrathin polysiloxane layers was controlled by changing the concentration of iron ions in the aqueous subphase or by varying the residence time of ammonia in the gas phase above the liquid sample. The growth of the combined film structures is studied in situ by interfacial rheology, optical microscopy, and x-ray scattering experiments and ex situ by scanning electron microscopy. Different stages of iron oxide aggregation, from a very thin layer of amorphous iron oxide with thickness of a few nanometers up to micrometer thick coatings of crystalline maghemite (γ-Fe2O3) were investigated. The specific interactions between the inorganic iron oxide and the polymer membranes cause the creation of new composite materials which are sensitive to magnetic forces.

Microscopic capsules made from polysaccharides are used as carriers for drugs and food additives. Here, we use NMR microscopy to assess the permeability of capsule membranes and their stability under different environmental conditions. The results allow us to determine the suitability of different capsules for controlled drug delivery. As a measure of the membrane permeability, we monitor the diffusion of paramagnetic molecules into the microcapsules by dynamic NMR microimaging. We obtained the diffusion coefficients of the probe molecules in the membranes and in the capsule core by comparing the measured time dependent concentration maps with numerical solutions of the diffusion equation. The results reveal that external coatings strongly decrease the permeability of the capsules. In addition, we also visualized that the capsules are stable under gastric conditions but dissolve under simulated colonic conditions, as required for targeted drug delivery. Depending on the capsule, the timescales for these processes range from 1 to 28 h.

Bilberries (Vaccinium myrtillus L.) and their major polyphenolic constituents, anthocyanins, have preventive activities inter alia against colon cancer and inflammatory bowel diseases. However, anthocyanins are sensitive to environmental conditions; thus their bioavailability in the gastrointestinal tract is an important determinant of their in vivo activity. In the study reported here, the potential benefits of encapsulating an anthocyanin rich bilberry extract (BE) on anthocyanin stability were investigated. Nonencapsulated BE and three different BE loaded microcapsule systems were incubated in simulated gastric fluid (SGF) and fed state simulated intestinal fluid (FeSSIF). After exposure to these media, released anthocyanins were identified and quantified by HPLC with UV/Vis detection. Although a rapid release of anthocyanins was observed within the first 20 min, encapsulation of anthocyanins doubled the amount of available anthocyanins after 150 min of incubation. These results illustrate the ability of encapsulation to inhibit early degradation of anthocyanins in the intestinal system.

Liquid-filled pectinate capsules have a large potential for the controlled and site-specific delivery of liquid drugs. Earlier studies have shown that pure pectinate capsules can store drugs only for a few minutes. Here, we show that the retention time can be extended to several hours by coating the capsules with the natural resin shellac. A bilberry extract containing anthocyanins with promising therapeutic properties was used as model drug to characterize the permeability of the capsules by in vitro drug release measurements. Characterizing the structure of the double-layered capsule membranes by NMR microscopy, we optimized the capsule production by adjusting the pH-value in the coating process and the gelation time of the pectinate hydrogel layer. A comparison of the layer thicknesses with drug release measurements reveals that capsules with the thinnest shellac layers provide the best entrapment. Additional squeezing experiments show that the shellac layer makes the capsules also mechanically more stable.

Calcium alginate beads are enclosed in a wide range of products including food, pharmaceuticals, and cosmetic formulations. The biopolymer matrix is often used to stabilize active ingredients and to provide a controlled release under well-defined conditions. In this context, it is of high interest to study the magnetic-induced attraction, elongation, and rupture of capsules or beads. In this work, we synthesized new types of magnetic switchable alginate beads. The magnetic sensitivity was achieved by incorporation of magnetic nanoparticles (MNPs) within the alginate gel. We measured the mechanical properties of single alginate beads in squeezing experiments, the evaporation of water and the magnetic sensitivity by stimulation of these beads in external fields. In all these measurements, the alginate and the nanoparticle concentration were systematically varied. We could show that the incorporation of MNPs generates a magnetic response of the beads and reduces the evaporation of water but has no influence on the mechanical stability of the beads during compression. Calculations of the shear modulus by means of the squeezing data result in good agreement in comparison to the shear moduli measured by rheological frequency sweep tests. With scanning electron microscopy, we could analyze the molecular structure of such composite systems, and we observed a homogeneous distribution of the MNPs within the gel matrix.

Vesicles can be found in many applications like drug delivery or as models for cell membranes. It is often necessary to produce vesicles which are easy to adjust in size and which can be filled with different types of ingredients. In this publication we used phase transfer techniques in to form well defined vesicles. The synthesis of these particles occurred in three different steps. First, a water phase was covered by an oil phase containing surfactants. A water-in-oil emulsion was then added to the oil phase. In the third step the phase transfer was stimulated by sedimentation or centrifugation processes. In a series of experiments we measured the vesicle sizes and encapsulation efficiencies. Giant vesicles, formed by sedimentation processes had typical sizes between 1–10 μm. Smaller vesicles between 100–500 nm were observed after centrifugation processes. With both methods we could produce vesicles with encapsulation amounts about 10%.

In this paper we report on the synthesis of a new family of nonionic dendritic amphiphiles that self-assemble into defined supramolecular aggregates. Our approach is based on a modular architecture consisting of different generations of hydrophilic polyglycerol dendrons [G1-G3] connected to hydrophobic C(11) or C(16) alkyl chains via mono- or biaromatic spacers, respectively. All amphiphiles complex hydrophobic compounds as demonstrated by solubilization of Nile Red or pyrene. The structure of the supramolecular assemblies as well as the aggregation numbers are strongly influenced by the type of the dendritic headgroup. While the [G1] amphiphiles form different structures such as ringlike and fiberlike micelles, the [G2] and [G3] derivatives aggregate toward spherical micelles of low polydispersity clearly proven by transmission electron microscopy (TEM) measurements. In the case of the biaromatic [G2] derivative, the structural persistence of the micelles allowed a three-dimensional structure determination from the TEM data and confirmed the aggregation number obtained by static light scattering (SLS) measurements. On the basis of these data, molecular packing geometries indicate a drastic mass deficit of alkyl chains in the hydrophobic core volume of spherical micelles. It is noteworthy that these highly defined micelles contain as little as 15 molecules and possess up to 74% empty space. This behavior is unexpected as it is very different from classical detergent micelles such as sodium dodecyl sulfate (SDS), where the hydrophobic core volume is completely filled by alkyl chains.

The assembling of magnetic nanoparticles in ordered structures as well as the preparation of very thin magnetic switchable polymer membranes is an important aim in many technical fields. We studied the influence of γ-Fe2O3 nanoparticles on the polymerization process and on the properties of the poly(organosiloxane)/nanoparticle-composite layer by surface rheological measurements, surface pressure/area (π/A) isotherm measurements, and Brewster angle microscopy. The adsorption process dynamics were studied by X-ray reflectivity and surface potential measurements. The results confirm the presence of attractive electrostatic interactions between the partial negatively charged monolayer and the positively charged nanoparticles. For further investigations, we prepared Langmuir–Blodgett layers of these polymer-nanoparticle composite and investigated them by atomic force microscopy and UV-Vis spectroscopy. We found that the concentration of nanoparticles was very low and the particles were mainly arranged below the polymer layer.

In this publication we describe a new self-association process, which leads to the formation of ultra-thin alginate layers at the interface between oil and water. The water phase contains a highly dilute solution of sodium alginate. These macromolecules are negatively charged and they are not surface active. The oil phase contains a small concentration of positively charged surfactants. At the interface between oil and water, the cationic surfactants tend to form complexes with the negatively charged alginate polyelectrolytes in the aqueous solutions. This leads to striking adsorption processes of the solved polysaccharide molecules at the oil-water interface. Upon the addition of calcium ions, a cross-linking process sets in and one obtains the thin viscoelastic membranes, which are anchored at the interface between oil and water. The thickness of these membranes is of the order of 0.2 mm. Similar structures can also be formed by solving positively charged Gemini surfactants in the oil phase. In this case, the cationic surfactant molecules induce the adsorption processes of alginate macromolecules, and they also act as cross-linking compounds. In a series of experiments, we measured the surface rheological properties of these ultra-thin alginate membranes. The results of these investigations point to the presence of electrostatically stabilized membranes. Special interest was given to the influence of the guluronate content of the alginates, which is important for the cross-linking mechanism according to the egg-box model. Finally, this article finishes with the discussion of the proposed building mechanisms of these membranes.

Due to their pronounced viscoelastic properties polymer microcapsules are able to undergo mechanical deformations when they are stimulated by external signals. The aim of our work was to prepare new types of electric switchable microcapsules based on the interfacial polycondensation of octadecyltrichlorosilane (OTS) and to investigate their deformation behavior in externally electric fields. In a series of experiments we explored the influence of different additives on the deformation behavior of the capsules in electric fields as well as on the shear rheological properties of the polysiloxane networks at the planar interface. We could show that the additives had a significant influence on the rheological membrane properties, but we observed only small changes in the deformation of the whole capsules.

Vesicles are of great interest as drug delivery system or models for cell membranes. For many applications, it is necessary to produce vesicles which are unilamellar, monodisperse, easy to adjust in size, and which can be filled with various types of active compounds. In a series of experiments, we produced giant vesicles with dimension of several millimeters by phase transfer processes. This new technique allowed synthesizing defined vesicles with lipid and surfactant membranes. The preparation of these aggregates occurred in two steps. First, we filled some amount of water into a cuvette and covered this liquid with an oil phase. Surfactants or lipids were solved either in the water or the oil phase. In the second step, a water droplet filled with methyl blue and saccharose was formed with a syringe in the oil phase. Due to density difference, the water droplet passed the plane oil/water interface and during this process it was transformed into a vesicle. The giant liposomes, thus formed, showed a high sensitivity against variations of the osmotic pressure, and their stability reached from seconds to hours. Due to the phase transfer process, the vesicle membranes often contained incorporated lenses of oil. If this hydrophobic liquid was released from the membrane, the vesicles decayed into smaller liposomes with a broad particle size distribution.

In a series of experiments, we studied the interfacial activity of aromatic aliphatic molecules with rigid gemini-like structures at the interface between toluene and water. These molecules, called clips and tweezers, have rigid central benzene or naphthalene spacer-units, each substituted with two polar groups as well as two rigid aromatic side walls. They can serve as host molecules and selectively bind a variety of electron-deficient aromatic and aliphatic guest molecules. In different experiments, we compared the interfacial tensions with the calculated hydrophilic-lipid-balance (HLB) values of these molecules. The measured interfacial tensions depend as much on the HLB values as on the geometric structure of the water insoluble molecules. The concentration dependence of the surface tension gave evidence for the formation of inverse micellar aggregates, which were formed in the oil phase above a well-defined value of the bulk concentration. The presence of aggregates in the organic liquid could also be investigated by dynamic light scattering measurements. We observed typical diameters of the inverse micellar aggregates in the order of 5.6 nm, and the critical micelle concentrations (cmc's) coincided well with the results of interfacial tension measurements. From the surface excess in the vicinity of the cmc, we calculated the space occupied by a single clip molecule on the self-assembled monolayer. The observed molecular surface area was in agreement with the effective molecular diameters of the molecules. In additional experiments, we could also show that complexes with aromatic guest molecules such as 1,2-4,5-tetracyanobenzene (TCNB) led to a reduction of the amphiphilic clip properties.

Zwitterionic diazeniumdiolates of the form RN[N(O)NO(-)](CH(2))(2)NH(2) (+)R, where R=CH(3) (1), (CH(2))(3)CH(3) (2), (CH(2))(5)CH(3) (3), and (CH(2))(7)CH(3) (4) were synthesized by reaction of the corresponding diamines with nitric oxide. Spectrophotometrically determined pK(a)(O) values, attributed to protonation at the terminal oxygen of the diazeniumdiolate group, show shifts to higher values in dependence of the chain lengths of R. The pH dependence of the decomposition of NO donors 1-3 was studied in buffered solution between pH 5 and 8 at 22 degrees C, from which pK(a)(N) values for protonation at the amino nitrogen, leading to release of NO, were estimated. It is shown that the decomposition of these diazeniumdiolates is markedly catalyzed by anionic SDS micelles. First-order rate constants for the decay of 1-4 were determined in phosphate buffer pH 7.4 at 22 degrees C as a function of SDS concentration. Micellar binding constants, K(SM), for the association of diazeniumdiolates 1-3 with the SDS micelles were also determined, again showing a significant increase with increasing length of the alkyl side chains. The decomposition of 1-3 in micellar solution is quantitatively described by using the pseudo-phase ion-exchange (PIE) model, in which the degree of micellar catalysis is taken into account through the ratio of the second-order rate constants (k(2m)/k(2w)) for decay in the micelles and in the bulk aqueous phase. The decay kinetics of 1-3 were further studied in the presence of cosolvents and nonionic surfactants, but no effect on the rate of NO release was observed. The kinetic data are discussed in terms of association to the micelle-aqueous phase interface of the negatively charged micelles. The apparent interfacial pH value of SDS micelles was evaluated from comparison of the pH dependence of the first-order decay rate constants of 2 and 3 in neat buffer and the rate data obtained for the surfactant-mediated decay. For a bulk phase of pH 7.4, an interfacial pH of 5.7-5.8 was determined, consistent with the distribution of H(+) in the vicinity of the negatively charged micelles. The data demonstrate the utility of 2 and 3 as probes for the determination of the apparent pH value in the Stern region of anionic micelles.

We studied the adsorption of gamma-Fe 2O 3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of gamma-Fe 2O 3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed zeta potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (pi/ A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.

In a series of experiments, we studied the dynamic properties of aqueous surfactant solutions of cetyltrimethylammonium bromide (CTAB) at conditions after adding different amounts of sodium salicylate (NaSal). The aggregates, present in these solutions, are elongated, wormlike micelles, which tend to form entanglement networks. The viscoelastic, gel-like samples were analyzed by means of static, dynamic, and electrophoretic light scattering techniques. We separately investigated the effects of surfactant concentration and added salt on intermicellar interactions. The electrostatic interactions between the anisometric micelles were analyzed by considering the effective dimensions of the aggregates. We calculated the Debye-Huckel lengths from experimental data of the osmotic second virial coefficient and from the diffusion second virial coefficient. It turned out that the results were in good agreement with theoretically estimated values. We also measured the zeta potential and intensity of scattered light in a large range of different salt concentrations keeping the CTAB concentration constant. We observed an isoelectric point and charge reversal of the threadlike micelles at an excess salicylate concentration of about 100 mM. The observed decrease of the zeta potential points to striking processes of counterion condensation. In these solutions, the salicylate ion acts as a cosurfactant, due to its discrepancy between polar and hydrophobic groups. We also detected a simple linear correlation between the zeta potentials and the Debye screening lengths of the surfactant solutions.

Microcapsules are nowadays applied in a wide range of products including food, pharmaceuticals, and cosmetic formulations. Because of their pronounced viscoelastic properties, polymer microcapsules are able to undergo mechanical deformations when they are stimulated by external signals. The elongation of capsule membranes, up to bursting processes, is important for the controlled release of active ingredients under well-defined conditions. Their application as quick and reliable control release systems affords a detailed knowledge of their rheological and mechanical properties. The aims of our work were to prepare new types of magnetic switchable microcapsules and to investigate their deformation behavior in externally imposed magnetic fields. The magnetic sensitivity was achieved by encapsulating magnetic magnetite (Fe3O4) particles within a polyorganosiloxane capsule. We investigated the dynamic response of those capsule suspensions and single capsules to external magnetic forces where aggregation processes, rotational movements, and field-induced deformations in static or rotating fields were observed.

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.

In a series of experiments, we coated iron oxide nanoparticles, which were originally stabilized with lauric acid, with a polymer layer of Octadecyltrichlorosilane (OTS). Characterization of the different coated nanoparticles was accomplished by Static and Dynamic Light Scattering, acoustic spectroscopy, and Atomic Force Microscopy. In various experiments, we systematically investigated the effect of different parameters such as the OTS concentration and iron oxide content on the particle size of the coated nanoparticles. It was recognized that the size of the coated nanoparticles mainly depend on the concentration of OTS (C OTS) measured with respect to the concentration of the iron oxide particles (C mag.). Below a well-defined threshold value of C OTS /C mag, we did not observe any adsorption of OTS on the surface of iron oxide nanoparticles. The particle size of OTS-coated iron oxide nanoparticles increased rapidly at concentration ratios above the threshold concentration and reached a typical plateau value for long periods of time.

Solvational perturbations were employed to selectively tune the aggregational preferences of insulin at 60 degrees C in vitro in purely aqueous acidic solution and in the presence of the model co-solvent ethanol (EtOH) (at 40%(w/w)). Dynamic light scattering (DLS), thioflavin T (ThT)-fluorescence, Fourier transform infrared (FTIR) and atomic force microscopy (AFM) techniques were employed to characterize these pathways biophysically with respect to the pre-aggregational assembly of the protein, the aggregation kinetics, and finally the aggregate secondary structure and morphology. Using cell viability assays, the results were subsequently correlated with the cytotoxicity of the insulin species that form in the two distinct aggregation pathways. In the cosolvent-free solution, predominantly dimeric insulin self-assembles via the well-known amyloidogenic pathway, yielding exclusively fibrillar aggregates, whereas in the solution containing EtOH, the aggregation of predominantly monomeric insulin proceeds via a pathway that leads to exclusively non-fibrillar, amorphous aggregates. Initially present native insulin assemblies as well as partially unfolded monomeric species and low molecular mass oligomeric aggregates could be ruled out as direct and major cytotoxic species. Apart from the slower overall aggregation kinetics under amorphous aggregate promoting conditions, which is due to the chaotropic nature of high EtOH concentrations, however, both pathways were unexpectedly found to evoke insulin aggregates that were cytotoxic to cultured rat insulinoma cells. The observed kinetics of the decrease of cell viabilities correlated well with the results of the DLS, ThT, FTIR and AFM studies, revealing that the formation of cytotoxic species correlated well with the formation of large-sized, beta-sheet-rich assemblies (>500 nm) of both fibrillar and amorphous nature. These results suggest that large-sized, beta-sheet-rich insulin assemblies of both fibrillar and amorphous nature are toxic to pancreatic beta-cells. In the light of the ongoing discussion about putative cytotoxic effects of prefibrillar and fibrillar amyloid aggregates, our results support the hypothesis that, in the case of insulin, factors other than the specific secondary or quarternary structural features of the various different aggregates may define their cytotoxic properties. Two such factors might be the aggregate size and the aggregate propensity to expose hydrophobic surfaces to a polar environment.

A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L(-1) and 1000 mmol L(-1). X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration.

Upon incorporation of magnetic nanoparticles (mNPs) into gels, composite materials called ferrogels are obtained. These magneto-responsive systems have a wide range of potential applications including switches and sensors as well as drug delivery systems. In this article, we focus on the properties of calcium alginate capsules, which are widely used as carrier systems in medicine and technology. We studied the incorporation of different kinds of mNPs in matrix capsules and in the core and the shell of hollow particles. We found out that not all particle–alginate or particle–CaCl2 solution combinations were suitable for a successful capsule preparation on grounds of a destabilization of the nanoparticles or the polymer. For those systems allowing the preparation of switchable beads or capsules, we systematically studied the size and microscopic structure of the capsules, their magnetic behavior and mechanical resistance.

Vesicles or liposomes are of great interest as drug delivery system or simple model for cell membranes. In biological environments vesicles are capable of transporting messenger molecules in high concentrations within a cell. For industrial applications, it is necessary to produce vesicles which are unilamellar, monodisperse, easy to adjust in size, and which can be filled with various types of active compounds. Particularly the defined filling of these tiny compartments has not yet been brought to a large scale. Our research project within the DFG-priority program 1273 (Colloid Process Engineering) was focused on a new method, which can easily be used for the continuous production of such colloidal particles. Moreover, the novel approach allows us to use a large variety of incorporated ingredients. The high encapsulation efficiency in addition with the flexible synthesis facilitates the utilization as a drug carrier system. On grounds of the interesting structure, consisting of an unilamellar surfactant shell, which is swollen with oil, and an enclosed aqueous reservoir (core), the produced colloidal particles may alternatively be denoted as a special case of water-in-water-emulsions. The synthesis of these particles occurred in three steps. First, a water phase was covered by an oil phase containing surfactants or lipids. A water-in-oil emulsion or microemulsion was then added to the oil phase. In the third step the phase transfer of aqueous droplets from the oil phase into the underlying water phase was stimulated by sedimentation, flow, electric forces or centrifugation processes. During this phase transition a small amount of the organic solvent was entrapped in the ultra-thin membranes and influenced the properties of the filled, vesicular structures. The thin layer of organic solvents reduced the diffusion processes from the core of the vesicles into the surrounding water phase. This might be of special advantage for the encapsulation of water soluble ingredients as drugs or other interesting compounds. It also offers the opportunity, to store oil soluble substances in the swollen membranes of the vesicles. On the other hand the thin oil layers surrounding the vesicles induced creaming processes and influenced the stability of these aggregates. For all applied experimental techniques we systematically measured the encapsulation capacity, the size of the filled vesicles, the amount of entrapped oil within the membranes and the stability of these aggregates. It turned out, that the jet-stream and the electrospray technique provided the best results concerning long-term stability, vesicle production and encapsulation efficiency. Due to the broad spectrum of different applications, we could use the phase-transfer process for the production of tailor-made, filled and swollen vesicles, which showed interesting properties.

Calcium phosphate nanoparticles were prepared by precipitation from aqueous solution. A stable colloid was formed by coating with DNA. The properties of the colloidal dispersion can be adjusted by variation of the inorganic nanoparticles. In particular, the partial substitution of calcium by magnesium or aluminum led to stable dispersions of nanoparticles. By a multi-step precipitation process, it is also possible to include DNA into the particles in order to protect it from intracellular biochemical degradation. The dispersions were analyzed by dynamic light scattering, zeta potential measurements, analytical ultracentrifugation, and transmission electron microscopy. Synthese und Charakterisierung von DNA-funktionalisierten Calciumphosphat-Nanopartikeln Calciumphosphat-Nanopartikel wurden durch Fallung aus wasriger Losung hergestellt. Durch die Beschichtung mit DNA wurde ein stabiles Kolloid erzeugt. Die Eigenschaften der kolloidalen Dispersion konnen durch die Variation der anorganischen Nanopartikel eingestellt werden, wobei insbesondere die teilweise Substitution von Calcium durch Magnesium oder Aluminium die Eigenschaften der Dispersion verbesserte. Der Einschluss von DNA in die Nanopartikel kann durch einen mehrstufigen Fallungsprozess erreicht werden. Dadurch wird die DNA vor dem intrazellularen biochemischen Abbau geschutzt. Die Dispersionen wurden mittels dynamischer Lichtstreuung, Zeta-Potential-Messungen, analytischer Ultrazentrifugation und Transmissionselektronenmikroskopie charakterisiert.

Currently, there is great interest in the development of methods suitable for determining the stoichiometry of biomolecules attached to nanoparticles. We describe the use of the dynamic light-scattering technique (DLS) to determine the stoichiometry of the protein cytochrome P450(BSbeta) attached to CdS and CdSe quantum dots (QDs). The enzyme-conjugated QDs have different diffusion characteristics compared to the QD and enzyme precursors, expressed in their size, scattering intensity as well as zeta-potential values. The significant enhancement of the scattering intensity of QDs observed upon conjugation with the P450(BSbeta) due to the refractive-index increment and the systematic variation in zeta potential resulting from charge neutralization of the anionic QDs by the cationic histidine-tagged P450(BSbeta) have been used for stoichiometry determination.

A rheological study was performed on biofilms of mucoid Pseudomonas aeruginosa strains producing high molecular weight alginate as the major polysaccharide component of the extracellular polymeric substances (EPS). Environmental strain SG81 and clinical strain FRD1 produced O-acetylated alginate, and mutant FRD1153 derived from strain FRD1 was defective in alginate acetylation. Confluent biofilms were prepared by cultivating the bacteria at 36 °C for 24 h on membrane filters placed on Pseudomonas isolation agar. The rheological properties of these pure culture biofilms attached to the membrane filters were analysed using a rotating disc rheometer. The biofilms of all mucoid strains showed viscoelastic properties, with polymer (alginate) cross-linking caused predominantly by physical interactions in the form of entanglements. In dynamic frequency-sweep tests, we observed time-dependent plateau regimens, describing the dynamics of network structures. From these data, we could derive the concentration of elastically effective polymer chains as 5 (±1) x 1019 chains/l (limits are standard deviation) for 24 h old biofilms of P. aeruginosa SG81. Relaxation processes predicted an average lifetime of junction points of the order of 16 (±10) s. Pseudomona aeruginosa FRD1 revealed a more elastic polymer network when compared with the acetylation-defective strain FRD1153, which had a more viscous biofilm structure. With calcium (10 mmol/l) added to the growth medium, enhanced biofilm stability was observed, which was based mainly on Coulomb interactions between divalent calcium ions and the carboxylate groups of alginate, with entanglements making only minor contributions. The calcium-induced increase in polymer network stability was greater in biofilms of P. aeruginosa FRD1 than in the acetylation-defective strain FRD1153. These results indicate that acetyl groups as low molecular weight substituents of alginate polymers strongly influence the rheological behaviour of mucoid P. aeruginosa biofilms and are involved in the stabilization of polymer networks of the EPS matrix in both the absence and the presence of calcium ions.

Molecular clips are able to selectively bind electron deficient aromatic and aliphatic substrates, and these processes are usually investigated in dilute solutions of organic solvents. Caused by discrepancies between polar and hydrophobic groups, molecular clips are surface-active compounds and, in analogy to surfactants, they can form monomolecular films at the water surface. In this publication, we systematically investigated the self-association process and the phase-behaviour of three different molecular clips with the polar head groups -OCH2COOH (a), -OCH2COOEt (b), and -OCONHPh (c) by means of surface-pressure-area-isotherms and Brewster-angle-microscopy (BAM). We observed marked differences for all investigated surface-active compounds. The molecular surface areas of the three clips, determined from pressure-area-isotherms, could be traced back to the molecular diameters of the amphiphilic compounds. In several experiments we investigated the influence of diverse film compression and expansion steps. Hysteresis effects could be explained by different film morphologies. In a series of experiments we could show that the aromatic guest molecule 1,2,4,5-tetracyanobenzene (TCNB), which strongly binds to molecular clips, did not influence the phase diagrams and film structures.

We analyze the deformation and bursting process of nonspherical organosiloxane capsules in centrifugal fields. Measurements were performed in a commercial spinning-drop tensiometer at different values of tube rotation. A theoretical analysis of the mechanics of initially ellipsoidal elastic shells subjected to centrifugal forces is developed where the deformation of the capsule is predicted as a function of the initial geometry and membrane elastic properties. For different types of organosiloxane membranes the Poisson number varies between 0 and 0.9. This phenomenon points to a considerable reduction of the membrane thickness at the onset of mechanical stress. Membrane-breaking processes always initiated at one of the pole ends of the capsules. Such rupture processes can be interpreted in terms of the derived theoretical model.