Soret effect of n-octyl beta-D-glucopyranoside (C8G1) in water around the critical micelle concentration.

We studied the thermal diffusion behavior of the nonionic surfactant C(8)G(1) (n-octyl beta-D-glucopyranoside) in water for different concentrations between w = 0.25 wt% and w = 2.0 wt% in a temperature range from T = 15 to 60 degrees C using the classical and infrared thermal diffusion forced Rayleigh scattering (TDFRS) setup. The purpose of the present paper is the investigation of the thermal diffusion behavior of surfactant systems around the critical micelle concentration (cmc), which is independently determined by surface tension measurements. In the classical TDFRS, the surfactant solutions show in the presence of a light-absorbing dye a pronounced change of the thermal diffusion coefficient (D(T)) and the Soret coefficient (S(T)) at the cmc. This result agrees with a recent thermal lens study [Santos et al., Phys. Rev. E 2008, 77, 011403], which also showed in the presence of dye a pronounced change of the thermal lens matter signal around the cmc. We found that this change becomes less pronounced, if the dye is absent or a light source is used, which is not absorbed by the dye. At higher concentrations, we observed a temperature-dependent sign change of S(T) as has also been found for solutions of hard spheres at higher concentrations.