Disjoining pressure isotherms of free-standing liquid films (foam films) consisting of different polyelectrolyte/ surfactant combinations are measured in a thin film pressure balance (TFPB). In dependence of the charge of polyelectrolyte and surfactant, a transition from an electrostatically stabilized common black film (CBF) to a sterically stabilized Newton black film (NBF) can be induced in some cases while for other polyelectrolyte/ surfactant combinations the film is a CBF up to several thousands of pascals. The thinner NBF is less stable, and the film breaks after a few minutes. An exchange of the polymers by monomers leads to the same kind of film as that for the respective polymer, while the addition of, for example, simple salt leads always to a transition from CBF to NBF. The typical stratification of polyelectrolyte/surfactant films is not observed in monomer/surfactant films.

Pyrene labels are used as a molecular probe to investigate the rearrangement of polyelectrolytes confined in a free-standing aqueous film of progressively decreasing thickness. Monitoring the spectroscopic changes of covalently labeled poly(acrylic acid) in a surfactant-stabilized film, we followed the effects of confinement down to a film thickness of 15 nm. Already upon film formation at thicknesses of 50 nm, a marked increase of the excimer/monomer ratio was found irrespective of the degree of charge of the polymer. Hence, the majority of polyelectrolytes is located within regions of locally elevated polymer density. Surface tension measurements showed these regions to be located at the interfaces in case of uncharged polyelectrolytes. In charged polyelectrolytes, stratification of the film is observed and accompanied by an additional rise of the excimer/monomer ratio, suggesting that charged polyelectrolytes reside in a few distinct sites within the film.

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The research topics of our group are in general from the field of physical chemistry of colloidal systems. Within this rather wide layout a large variety of quite different questions and systems are tackled, where the common bridging factor is the aim of understanding the properties of colloidal systems based on their mesoscopic structure and dynamics, which in turn are controlled by their molecular composition. With such an enhanced understanding of the correlation between mesoscopic structure and the macroscopic properties the goal then is to employ this knowledge in order to formulate increasingly complex colloidal system with correspondingly more variable and interesting functionalities. From this general context of investigations, some representative systems and questions that have been studied in recent time by us are covered in this text.They comprise the phase behaviour and the structures formed in solutions of surfactants and amphiphilic copolymers. Once these static properties are known, we also have a high interest in the dynamic properties and the kinetics of morphological transitions as they are observed under non-equilibrium conditions, since they are frequently encountered in applications. A key property of amphiphilic molecules is their ability to solubilise sparingly soluble compounds thereby forming microemulsions or nanoemulsions, where the ability to form such systems depends strongly on the molecular architecture of the amphiphiles. By turning to polymeric amphiphiles the concept of surfactants and their architecture can be extended largely towards more versatile structures, more complex self-assembly and much larger length and time scales. Another direction is the surfactant assisted formation of nanoparticles or mesoporous inorganic materials. By combining copolymers with other polymers, copolymers, colloids, or surfactants – for instance via electrostatically driven co-assembly – one may then form increasingly complex colloidal aggregates. By doing so one is able to control rheological properties or develop complex delivery systems, whose properties can be tailor-made by appropriate choice of the molecular build-up. This striving towards well controlled complexity achieved by means of self- and co-assembly then leads to increasingly more functional systems and is the key direction for future research activities in our group.