Self-Assembly of the Bio-Surfactant Aescin in Solution: A Small-Angle X-ray Scattering and Fluorescence Study

This work investigates the temperature-dependent micelle formation as well as the micellar structure of the saponin aescin. The critical micelle concentration (cmc) of aescin is determined from the concentration-dependent autofluorescence (AF) of aescin. Values between cmcaescin,AF(10 degrees C) = 0.38 +/- 0.09 mM and cmcaescin,AF(50 degrees C) = 0.32 +/- 0.13 mM were obtained. The significance of this method is verified by tensiometry measurements. The value determined from this method is within the experimental error identical with values obtained from autofluorescence (cmcaescin,T(WP)(23 degrees C) = 0.33 +/- 0.02 mM). The structure of the aescin micelles was investigated by small-angle X-ray scattering (SAXS) at 10 and 40 degrees C. At low temperature, the aescin micelles are rod-like, whereas at high temperature the structure is ellipsoidal. The radii of gyration were determined to approximate to 31 angstrom (rods) and approximate to 21 angstrom (ellipsoid). The rod-like shape of the aescin micelles at low temperature was confirmed by transmission electron microscopy (TEM). All investigations were performed at a constant pH of 7.4, because the acidic aescin has the ability to lower the pH value in aqueous solution.