An integrated study of small-angle X-ray scattering and dynamic light scattering on cylindrical micelles of sodium glycodeoxycholate

Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements were carried out on aqueous micellar solutions of the ionic biological detergent sodium glycodeoxycholate (NaGDC). Apparent diffusion coefficients (D-app) and SAXS spectra of NaGDC 0.1 M solutions at different ionic strengths (0-0.4 M NaCl added) were reported. Moreover, a D-app study as a function of NaGDC concentration c (20-60 mM) in the range 0-0.3 M NaCl was performed. A drastic change of the D-app versus c slope indicates that as the electrolyte concentration is increased, the effect of the repulsive electric double layer is lowered, until a dominance of the van der Waals attraction effect is evidenced at the highest ionic strengths. A comparative analysis of SAXS spectra and D-app data confirms this result. A uniform cylinder was used to model the micelle in the data interpretation. The effect of interactions on SAXS spectra was explored in terms of the decoupling approximation. Restricted to the NaCl concentrations where the Coulombic repulsions predominate, the method of Hayter and Penfold for a charged hard sphere system was used to compute the interparticle structure factor S(q). To compare SAXS and DLS data in the same range, a solution of hard cylinders with larger effective dimensions was also used to represent the micelle samples, thus allowing the determination of the hydrodynamic radii. By increasing the NaCl concentration from 0 to 0.2 M, an almost constant micellar diameter of 32-34 Angstrom and a growing micellar length from 38 to 64 Angstrom were obtained. At the same time, increasing hydrodynamic radii from 16 to 26 Angstrom were estimated.