Surfactants are molecules of enormous scientific and technological importance, which are widely used as detergents, emulsifiers, and for the preparation of diverse nanostructures. Their fascinating ability to form self-organized structures, such as micelles or liquid crystals, originate from their amphiphilic architecture-a polar head group linked to a hydrophobic chain. While almost all known surfactants are organic, a new family of surfactants is now emerging, which combines amphiphilic properties with the advanced functionality of transition-metal building blocks, for example, redox or catalytic activity and magnetism. These hybrid surfactants exhibit novel self-organization features because of the unique size and electronic properties of the metal-containing entities.
Magnetic moustaches: Inorganic surfactants (I-SURFs) with head groups containing Dy(3+) undergo a hierarchical self-organization cascade controlled by magnetic interactions. The resulting aggregates are shaped like dumbbells with frayed, moustache-like ends.
The
solvent-mediated ability for molecularly encoded self-assembly
into states of higher order (micelles, lyotropic liquid crystals)
embodies the basis for many applications of surfactants in science
and society. Surfactants are used frequently in recipes for nanoparticle
synthesis. Because ordinary surfactants comprise insulating constituents
(alkyl groups as side-chains and charged organic heads), such nanostructures
are wrapped in an electrically inactive barrier, and this is a large
disadvantage for future developments in nanotechnology. Implications
of micelles with electrically conducting walls made from either “metallic”
or “semiconducting” surfactants are huge, also in other
areas such as nanoelectrocatalysis or micellar energy storage. We
cross this frontier by replacing not only the hydrophilic chain but
also the hydrophilic head by electronically conducting entities. We
report the synthesis of surfactants with oligo para-phenylene-ethynylene as a π-conjugated side-chain attached
to a redox-active, inorganic polyoxometalate cluster as charged head.
It is proven that electronic communication between head and tail takes
place. Hybridization on the molecular level leads to the emergence
of advanced surfactant features such as semiconductor properties (Egap = 2.6 eV) in soft lyotropic systems (micelles,
liquid crystals).
An ideal material for the storage of electrical energy is characterized by high specific energy and high specific power at the same time, which is a task of enormous difficulty. The so‐called redox flow battery is a highly promising approach. This new energy storage technology is based on two half‐cells containing dissolved electrochemically active species. Compared to conventional, static accumulators it is not only engineered in a unique way but also needs a tailor‐made basis of chemical materials. Therefore, many different redox‐active materials are being investigated. However, research is focused mainly on the redox properties, not taking possible synergistic effects arising from self‐assembled structures into account. Here, a novel surfactant is presented containing an electroactive polyoxometalate (POM) head connected to anthraquinone (AQ) as the relevant electron reservoir via a π‐conjugated chain. When organized into micelles, electrons put on the POM corona “slide” into their depot inside the micellar core until needed. Cyclic voltammetry proves the high reversibility and stability of this system, which therefore can be regarded as micellar energy storage.
Nonequilibrium states of matter are
arousing huge interest because
of the outstanding possibilities to generate unprecedented structures
with novel properties. Self-organizing soft matter is the ideal object
of study as it unifies periodic order and high dynamics. Compared
to settled systems, it becomes vital to realize more complex interaction
patterns. A promising and intricate approach is implementing controlled
balance between attractive and repulsive forces. We try to answer
a fundamental question in surfactant science: How are processes like
lyotropic liquid crystals and micellization affected, when headgroup
charge becomes so large that repulsive interactions are inevitable?
A particular challenge is that size and shape of the surfactant must
not change. We could realize the latter by means of new hybrid surfactants
with a heteropolyanion head [EW11O39]n− (E = PV, SiIV, BIII; n = 3, 4, 5). Among the unusual self-assembled
structures, we report a new type of micelle with dumbbell morphology.
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