Schering Preis 1997
Dr. Cosima Stubenrauch
Université Paris Sud, Laboratoire de Physique des Solides, Orsay, Frankreich
Microemulsions with alkylglucosides from a macroscopic and a NMR-spectroscopic point of view
Solubilizing large quantities of water and oil by means of surfactants in so-called microemulsions is becoming increasingly important in both technology and research. For a large-scale application of microemulsions in the cosmetic and pharmaceutical industries it is imperative that, along with other requirements that have to be met, the microemulsions are not toxic. With respect to the above-mentioned applications the microemulsions examined in the thesis at hand can be regarded as model systems. As an appropriate surfactant an alkylglucoside has been used; these surfactants are synthesized on the basis of renewable raw materials, namely fatty alcohols and saccharides.
Moreover, the geraniol which has been used as a cosurfactant is a frequently used natural perfume oil. A very interesting application of microemulsions in the pharmaceutical industry is, for example, the solubilization of strongly hydrophobic fluoroalkanes used as short-time blood plasma substitutes to maintain a patient's oxygen supply. Furthermore, microemulsions can be used as "nanotablets," i.e. tablets of nanometer sizes, by means of which drugs can be released specifically.
Microemulsions are thermodynamically stable, optically transparent mixtures of water and oil, i.e. of two components, which are immiscible in the absence of a surfactant. The macroscopically homogeneous mixtures are submicroscopically heterogeneous - the two solvents form so-called domains. The size of these domains, which are separated by a monomolecular surfactant layer, is in the range of 1-100 nm. The domains could be isolated droplets in a continuous phase, i.e. oil droplets in water (o/w-microemulsion) or water droplets in oil (w/o-microemulsion).
If both water and oil form spatially continuous structures one speaks of bicontinuous microemulsions having a spongelike structure. These different microstructures can be obtained, for example, by varying the nature and the composition of the mixed (surfactant/cosurfactant) layer. In the thesis at hand it has been shown how the composition of the interfacial mixed layer affects the structure of the oil and water domains. The latter, i.e. the microstructure, has been studied by means of a modern NMR-technique which allows us to measure the self-diffusion coefficients of all four components of the microemulsion. In this technique the displacement of nuclear spins in a controlled magnetic field gradient is monitored. The decisive information which one obtains by self-diffusion NMR studies is to distinguish between discrete droplets and bicontinuous structures.
The self-diffusion coefficients of the solvent molecules (oil and water) give direct insight into connectivity of the domains. The results of the thesis at hand demonstrate that the curvature of the interfacial mixed layer is determined by the ratio of surfactant to alcohol in the layer and that a transition from discrete oil-droplets to discrete water-droplets via a bicontinuous structure can be obtained by changing this ratio. It is the first time that this relation has been demonstrated in a quantitative manner for a quaternary microemulsion system. Although the aim of the present work was predominantly to investigate the microstructure and the physico-chemical properties of a four component microemulsion system the results are of great interest for the formulation of new, non-toxic microemulsions. Owing to the favourable properties of alkylglucosides an increasing economic importance of these surfactants is to be expected.