Non-Ionic Binary Surfactant Systems and Microemulsions as Model Systems for Thermal Diffusion Studies

Thermal diffusion (or the Soret effect) describes the diffusion of matter in the presence of atemperature gradient. Although this process is studied and applied since more than 100 years theunderlying molecular mechanism in liquids is still far from being understood. Colloidaldispersions are versatile model systems to study the thermal diffusion behavior of large particlesin a solvent. However, the synthesis of colloidal particles with identical morphology is acomplex task. On the other hand aqueous surfactant solutions and microemulsions are promisingsystems to study the Soret effect. Compared to colloidal particles the aggregates formspontaneously and do not require any additional stabilisation. Furthermore the radius and theshape of the diffusing aggregates, as well as their interfacial tension can be systematicallyadjusted via the variation of the composition and temperature. In order to address someimportant aspects of the thermal diffusion behavior three different types of self-assembledsurfactant systems were formulated and systematically studied: (i) At first the role of the ionicdye Basantol® Yellow 215, which causes an unexpected two-mode signal in the classicalThermal Diffusion Forced Rayleigh Scattering (TDFRS) experiment [Ning et al. Progr ColloidPolym Sci (2006) 133: 111-115], is examined using the system H2O - C12E6. Systematic phasebehavior studies and small angle neutron scattering experiments (SANS) proved that the dye isincorporated into the aggregates like an ionic co-surfactant, influencing not only the thermaldiffusion behavior but also the overall properties of the systems. These results strongly suggestthat one should refrain from the use of surface-active dyes in TDFRS experiments onself-assembled systems. (ii) To study the dependence of the Soret coefficient ST on the radius ofthe aggregates and the slope of the interfacial tension, both being controversially discussed in theliterature, different microemulsions of the type H2O - C12E5 -n-alkane were formulated thatallow for an isothermal study of these dependencies. Correlating the results of the systematicSANS and interfacial tension measurements with the Soret coefficient ST it was found that withinthe measurement range ST depends almost linearly on the droplet radius, the slope of theinterfacial tension as well as on the product of both quantities. (iii) In the last part systematicsurface tension and TDFRS measurements were performed in binary aqueous n-alkyl glucoside(CiGj) solutions to study the thermal diffusion behavior around the critical micelle concentration(cmc). The obtained results clearly show that the Soret coefficient exhibits an abrupt change atthe cmc. To sum up, this work shows that aqueous surfactant systems and microemulsions areeminently suited to elucidate some of the underlying molecular mechanism of the Soret effect. von Datta, Sascha