Micelles and nanoparticles as tools to control the occurrence of chemical reactions

Surfactants are known to form, above a given critical concentration, micellar aggregates. Far from constituting just the base of deterging foams, micelles can be used as nano-reactors, which are able to favour reactions that would not occur, or would occur only to a limited extent in aqueous phase or, oppositely, selectively repress some of them. Metal complexation on the micelle surface, enhanced by the presence of a suited hydrophobic ligand, can be the basis of the extraction of metals from water solutions. Moreover, micellar systems can be used as a crude approximation of cellular membranes, to test the biological properties of compounds. Nanoparticles stabilised with non-covalently bound coating agents are dynamic systems, where the stabilising molecule distributes trough an equilibrium between the bound and free-bulk form. Their behaviour bear a physico-chemical resemblance to that of micelles. In recent years we have analysed several metal/ligand systems in micelle/water medium and metal-nanoparticle systems, alone or in the presence of nucleic acids. The surface potential of the micelle is a very critical point. Charged surfactants will give rise to charged micelles that can attract or repulse ions. Hydrogen ions are also subject to this phenomenon, so that the local pH on the micelle differs from the one measured in the bulk. This difference will influence the species distribution of reacting species that can be present in diverse charged forms, thus affecting the extent and also the type of chemical reaction occurring in the micellar phase. The surfactant concentration will also play a role, and this additional parameter should be taken into account when using such systems. The charge and distribution of reactive species between the two phases is also a key factor. In the case of metal complexation of an highly charged porphyrin ring (H2TMpyP4+) with a metal ion in a SDS negatively charged micellar medium: in this case metal complexation is totally repressed. The kinetic features and the charge of the systems can be used to selectively extract a selected metal from a system containing many metals. Also, nanoparticles can be used as tools to induce reactions that will not take place for the free coating agent. For instance studies on the dimethylaminopyridine (DMAP)/DNA system show that DMAP alone does not interact with DNA, whereas DMAP-coated gold-nanoparticles do.