Effects of counterions and co-ions on foam films stabilized by anionic dodecyl sulfate.

The influence of counterions and co-ions on the stability and thickness of foam films stabilized by anionic dodecyl sulfate (DS(-)) has been studied in a thin film pressure balance. Particularly, the effect on the properties of foam films of (i) the counterions Li(+), Na(+), and Cs(+) of DS(-) and (ii) monovalent inorganic salts added to sodium dodecyl sulfate solutions is considered. Generally, addition of salt improved the stability of the foam films. As a second order, an increasing ionic size led to an increased adsorption, which in the case of cations gave thinner and less stable films and in the case of anions led to thicker and more stable films. Hence, an effect of anions was observed though the film surfaces were already negatively charged by the anionic DS(-), leading to the conclusion that adsorption of anions to the film surface is governed by ion specific rather than electrostatic interactions. At a fixed surfactant and varying salt concentration, a maximum in film thickness could be identified at a salt concentration well below the surfactant concentration. We anticipate that (i) at low salt concentration salt mainly affects the charging of a film interface, whereas (ii) at high salt concentration salt mainly affects the screening of the electrostatic repulsion between the two interfaces of the film.

Ion distribution in polyelectrolyte multilayers with standing-wave X-ray fluorescence.

Absolute ion concentration and its profile across polyelectrolyte multilayer films were studied. The films were prepared by alternating adsorption of polyanions and polycations from aqueous solution. Standing-wave X-ray fluorescence was used to map the ion profile. The well-studied multilayer system PSS/PAH was investigated, and bromide ions were used as probe entities. The results show that the sign of the charge of the outermost layer and the washing procedure after finishing the preparation have a decisive effect on the ion concentration and the ion profile. Multilayers with PSS as the outermost layer contain fewer bromide ions than the PAH-terminated multilayers. Exposure to water washes the ions out, but even after 6 h of washing, not all of the bromide ions had been removed.

Formation and dielectric properties of polyelectrolyte multilayers studied by a silicon-on-insulator based thin film resistor.

The formation of polyelectrolyte multilayers (PEMs) is investigated using a silicon-on-insulator based thin film resistor which is sensitive to variations of the surface potential. The buildup of the PEMs at the silicon oxide surface of the device can be observed in real time as defined potential shifts. The influence of polymer charge density is studied using the strong polyanion poly(styrene sulfonate), PSS, combined with the statistical copolymer poly(diallyl-dimethyl-ammoniumchloride-stat-N-methyl-N-vinylacetamide), P(DADMAC-stat-NMVA), at various degrees of charge (DC). The multilayer formation stops after a few deposition steps for a DC below 75%. We show that the threshold of surface charge compensation corresponds to the threshold of multilayer formation. However, no reversion of the preceding surface charge was observed. Screening of polyelectrolyte charges by mobile ions within the polymer film leads to a decrease of the potential shifts with the number of layers deposited. This decrease is much slower for PEMs consisting of P(DADMAC-stat-NMVA) and PSS as compared to PEMs consisting of poly(allylamine-hydrochloride), PAH, and PSS. From this, significant differences in the dielectric constants of the polyelectrolyte films and in the concentration of mobile ions within the films can be derived.

Long-range interactions between soft colloidal particles in slit-pore geometries.

Combining theoretical and experimental techniques, we investigate the structure formation of charged colloidal suspensions of silica particles in bulk and in spatial confinement (slit-pore geometry). Our focus is to identify characteristic length scales determining typical quantities, such as the position of the main peak of the bulk structure factor and the period of the oscillatory force profile in the slitpore. We obtain these quantities from integral equations/SANS experiments (bulk) and Monte Carlo simulations/colloidal probe-AFM measurements (confinement), in which the theoretical calculations are based on the Derjaguin-Landau-Verwey-Overbeck (DLVO) potential. Both in bulk and in the slitpore, we find excellent qualitative and quantitative agreement between theory and experiment as long as the ionic strength chosen in the DLVO potential is sufficiently low (implying a relatively long-ranged interaction). In particular, the bulk properties of these systems obey the widely accepted density scaling of xi proportional to phi(-1/3). On the other hand, systems with larger ionic strengths and, consequently, more short-ranged interactions do not obey such power law behavior and rather resemble an uncharged hard-sphere fluid, in which the relevant length scale is the particle diameter.