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Inorganic surfactants with multifunctional heads

Final Report Summary - I-SURF (Inorganic surfactants with multifunctional heads)

Surfactants are molecular compounds with dipolar architecture containing a water-compatible moiety, the so-called head group, and at least one water-incompatible moiety as side-chain(s). Surfactants are mass products, which are used on an everyday-basis in science, technology and at home. Their function is to decrease the energy of interfaces of all kinds. Thus, they can be used to solubilize dirt, to form droplets in emulsions or to stabilize nanoparticles. However, surfactants also occur in biological systems in the form of lipids as an essential constituent of the cellular membrane. In addition, surfactants fascinate by their ability to form aggregates such as micelles, vesicles and liquid crystals depending on exposure to certain solvents and depending on concentration. In other words, the predominant feature of surfactants is amphiphilicity/ surface activity.
The main goal of the project was the creation of a new set of surfactants equipped with at least one additional function beyond interfacial activity. Ultimately, we seek unique compounds with cooperative properties achieved by a synergistic combination of amphiphilicity with this function. We try to achieve this goal via the synthesis of surfactants with new and advanced head groups. Because pure organic compounds do not offer the full spectrum of materials properties, we have specialized on integrating inorganic constituents as head groups. This class of surfactants are ISURFs, and we have just started to explore it. Three cases should illustrate, how we extended the limits of current surfactant science.
A classic problem in catalysis is that substrates often possess different compatibility in different solvents, typically an aqueous phase and an organic phase. Thus, in phase-transfer catalysis one uses a catalyst plus a surfactant-like additive for bridging the phases. Consequently, it is straightforward to think of surfactants, which are catalysts at the same time. Imagine, a surfactant would not only remove dirt, but it would actually destroy it. Inspired by this idea, we have developed surfactants with catalyst moieties as head-groups. The novel surfactants can either form bonds (C-C cross-coupling catalysis) or they can cleave bonds (Lewis-acid catalysis). An elegant way for boosting the catalytic activity by light is by making and using so-called bolaform amphiphiles with two head groups. One is the catalyst, the other can bind specifically to surfaces, for instance of optically active nanoparticles.
Semiconductors are extremely useful among others in energy technology (photovoltaics, photocatalysis, batteries). Whereas a semiconductor is commonly a crystalline solid-state material, such as titanium dioxide, a conventional surfactant is an electrically insulating, molecular species. Inspired by the vast possibilities for semiconducting amphiphiles, we developed systems with redox-active head groups combined to conducting, pi-conjugated side chains. Such novel semiconductor surfactants can have numerous features of a leaf. They are biocompatible, form membranes, absorb visible light leading to a cascade of electron-transfer reactions and finally the photo-conversion of carbon dioxide (CO2).
The amphiphilic property of a surfactant is a constant. This is why, stimuli-responsive systems are highly attractive, which in state 1 would form micelles or emulsions and in state 2 would show no aggregates any more. In addition, one would like to apply the necessary trigger for the change of state as simple as possible, with high reversibility and without the consumption of energy. These are the reasons, why we have developed magnetic ISURFs by incorporation of appropriate metal complexes in the surfactant's architecture, respectively its head group. The advantage of magnetism is, the applied force is not shielded by water. Weak fields such as created by commercial permanent magnets are sufficient to cause a significant reaction and a dynamic instead of a static behavior.