Regine von Klitzing

Internal structure of polyelectrolyte multilayer assemblies

The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.

Disjoining pressure in thin liquid foam and emulsion films - new concepts and perspectives

The present review is a topical survey of the disjoining pressure in thin liquid foam and emulsion films from both the experimental and the theoretical points of view. Section 2 deals with the latest research work on experimental techniques with which the disjoining pressure Π in foam, emulsion, and pseudo-emulsion films can be measured. Although a lot of techniques are available, the question of the origin of the charges at the water/air and the water/oil interfaces of films, which are stabilized by non-ionic surfactants, has not yet been answered. We address this question in section 3, reviewing the latest relevant literature. The relevance of structural forces for the disjoining pressure is outlined in section 4, which focuses on films which are stabilized by surfactant/polyelectrolyte mixtures.

Influence of the ionic strength on the structure of polyelectrolyte films at the solid/liquid interface

Abstract The structure of polyelectrolyte multilayers built up by alternate adsorption of polyanions and polycations is investigated by X-ray reflectivity at the solid/air and neutron reflectivity at the solid/liquid interface. The experiments provide detailed information about the density gradient of polyelectrolyte chains across the film and show the influence of the water content of the film on the internal structure. The polyelectrolyte density is determined by the adsorption conditions (e.g. amount of NaCl) and cannot be changed by addition of salt after adsorption. After drying the film thickness is reduced by 30%.

Temperature, pH, and ionic strength induced changes of the swelling behavior of PNIPAM-poly(allylacetic acid) copolymer microgels

The volume phase transition of colloidal microgels made of N-isopropylacrylamide (NIPAM) is well-studied and it is known that the transition temperature can be influenced by copolymerization. A series of poly( N-isopropylacrylamide- co-allylacetic acid) copolymers with different contents of allylacetic acid (AAA) was synthesized by means of a simple radical polymerization approach. The thermoresponsive behavior of these particles was studied using dynamic light scattering (DLS). Further characterization was done by employing transmission electron microscopy (TEM) and zeta potential measurements. TEM observations reveal the approximately spherical shape and low polydispersity of the copolymer particles. In addition, the measured zeta potentials provide information about the relative surface charge. Since these copolymers are much more sensitive to external stimuli such as pH and ionic strength than their pure PNIPAM counterparts, the volume phase transition was investigated at two different pH values and various salt concentrations. At pH 10 for the copolymer microgels with the highest AAA content, a significant shift of the volume phase transition temperature toward higher values is found. For higher AAA content, a change in pH from 8 to 10 can induce a change in radius of up to 100 nm making the particles interesting as pH controlled actuators.

Packing Density Control in P(NIPAM-co-AAc) Microgel Monolayers: Effect of Surface Charge, pH, and Preparation Technique

In the present paper, thermosensitive coatings are prepared by deposition of P(NIPAM-co-AAc) microgel particles on precoated silicon wafers. The effect of pH, substrate precoating, and preparation technique is studied. The pH value is found to significantly influence the adsorption density, while the substrate surface charge is less important. Hence, the electrostatic contribution of the particle-particle interaction seems to play a more pronounced role for the adsorption density than at least the electrostatic part of the particle-surface interaction. For the latter, also nonelectrostatic contributions like hydrogen bonding and surface roughness play an important role. Immersion of the prepared polyelectrolyte/microgel layers in buffers leads to a reorganization of the adsorbed particles at the surface.

Mixed monolayers of polyelectrolytes and surfactants at the air-water interface

Abstract Dilute mixed solutions of non surface active anionic polymers (polyacrylamide and polystyrene sulfonate) and various surfactants have been studied with several methods: surface tension, ellipsometry, surface and bulk rheology. A strong synergistic lowering of the surface tension is found with cationic surfactants in the concentration range where no appreciable complexation of surfactant and polymer occurs in the bulk solution (as seen from viscosity measurements). A simple adsorption model, in which it is assumed that both polymer and surfactant counterions are expelled away from the surface, is in good agreement with the observations on polyacrylamide sulfonate. The behaviour of the more hydrophobic polystyrene sulfonate is very different and still not understood. The behaviour of this polymer with nonionic surfactants is also different. The connection of foam stability with surface and bulk complexation is discussed.

Versatile Phase Transfer of Gold Nanoparticles from Aqueous Media to Different Organic Media

A novel, simple, and very efficient method to prepare hydrophobically modified gold particles is presented. Gold nanoparticles of different sizes and polydispersities were prepared. The diameter of the gold particles ranges from 5 to 37 nm. All systems were prepared in aqueous solution stabilized by citrate and afterwards transferred into an organic phase by using amphiphilic alkylamine ligands with different alkyl chain lengths. The chain length was varied between 8 and 18 alkyl groups. Depending on the particle size and the alkylamine, different transfer efficiencies were obtained. In some cases, the phase transfer has a yield of about 100%. After drying, the particles can be redispersed in different organic solvents. Characterization of the particles before and after transfer was performed by using UV/Vis spectroscopy, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS) techniques. The effect of organic solvents with various refractive indices on the plasmon band position was investigated.

Forces in foam films containing polyelectrolyte and surfactant

Abstract Foam films of semi-dilute anionic polyelectrolyte solutions containing additionally the cationic surfactant dodecyltrimethylammonium bromide are investigated. The polyelectrolytes used are polystyrene sulfonate (PSS) and acrylamide-acrylamidesulfonate (AAS) as examples of strongly and weakly charged polyions, respectively. The disjoining pressure isotherm results in film stratification well below the critical micelle concentration (CMC) and the critical aggregate concentration (CAC). These pressure oscillations are related to a network of polyelectrolyte chains in the film bulk, and their period to the mesh size. The network can be described by the theory for semi-dilute polyelectrolyte solutions, which is different for the two polyelectrolytes because of different charge densities along the chain. The surfactant has no observable influence on the network. To obtain information about the film interface surface tension measurements at the water–air interface are carried out. In comparison with the pure polyelectrolyte and the surfactant, respectively, the surface tension is much reduced for the mixed systems as a result of the formation of complexes between the surfactant and the polyelectrolyte. The two polymers show different behaviour in saturation of the surface coverage, which leads us to the conclusion that the complexes are different, although the behaviour of the film bulk is not influenced by changes in the interface.

Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles

Plasmons are resonant excitations in metallic films and nanoparticles. For small enough static distances of metal nanoparticles, additional plasmon-coupled modes appear as a collective excitation between the nanoparticles. Here we show, by combining poly(N-isopropylacrylamide) micro- and nanospheres and Au nanoparticles, how to design a system that allows controllably and reversibly switching on and off, and tuning the plasmon-coupled mode.

Film stability control

The factors, which determine the stability of foam films and emulsion films are considered. Many of the recent works in this field deal with complex systems containing macromolecules or solid particles beside the surfactant. The correlation between the stabilizing factors is less understood than in binary surfactant films due to a diversity of interactions between the different compounds.