| Self-assembly of bolaamphiphiles in water:Formation of nanoparticles, nanofibers, and hydrogels. |
| Amphiphilic molecules with a polar group and a hydrophobic alkyl chain form, when put into water, a variety of well-known lyotropic phases, which have been extensively studied. The most common ones are the normal and inverted micellar phases, the normal and inverted hexagonal phases consisting of cylinders packed in a hexagonal lattice, and the lamellar phases, where bilayers of the amphiphilic molecules are separated by water layers. Amphiphilic molecules with two polar headgroup connected by one long alkyl chain are called bolaamphiphiles [1]. Depending on the size of the polar groups in relation to the cross-sectional area of the connecting alkyl chain, these bolaamphiphiles form either the already known lyotropic phases described above or a completely new type of aggregate structure, namely very long and stiff nanofibers with a diameter similar to the length of the molecules [2,3]. These nanofibers lead to a gelling of the aqueous solution. The gel behavior is caused by a physical cross-linking of the nanofibers via hydrophobic contacts and be the entanglement of the long and stiff nanofibers. The nanofibers can be converted to micellar-like aggregates upon heating. This process is connected with a "melting" of the hydrocarbon chains and can be followed by a variety of physico-chemical methods, such as DSC, FT-IR, WAXS, SANS, ESR, and electron microscopy [3-6]. The re-formation of the nanofiber network at low temperature is time and concentration dependent. Variation of the length of the hydrocarbon chain and the size of the headgroup leads to a change of the existence range of these nanofibers and micellar aggregates. By mixing bolalipids with different chain lengths the temperature of the fiber-micelle transition can be fine-tuned to a desired value [7]. Variation of the chemical structure of the bolalipid headgroup leads to pH-sensitive hydrogels, when a dissociable proton is present. The nanofibers can also be used for binding Au-nanoparticles leading to linear arrays of Au-nanoparticles [8] and for templating to produce networks of hollow silica tubes.
Acknowledgements: This work was supported by grants from the Deutsche Forschungsgemeinschaft
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