CHARACTERIZATION OF LECITHIN-TAURODEOXYCHOLATE MIXED MICELLES USING SMALL-ANGLE NEUTRON-SCATTERING AND STATIC AND DYNAMIC LIGHT-SCATTERING

We have used small-angle neutron scattering (SANS) to probe the structure and interparticle interactions of lecithin-taurodeoxycholate mixed micelles. The data are fit to a core-shell model that provides the micelle composition and dimensions. The effects on the scattering spectra of electrostatic and excluded-volume interactions are explored in terms of the decoupling approximation (Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 2461) and the random phase approximation (Shimada, T.; Doi, M.; Okano, K. J. Chem. Phys. 1988, 88, 2815). We found the TDC-lecithin micelles are cylindrical particles with an average cross-sectional radius of 26.7 +/- 0.4 Angstrom. The core-shell structure is found to be an appropriate model for the highly hydrated micelles. The micelle length increases dramatically with an increase in added electrolyte, but not with decreasing concentration as previously reported. The SANS data analysis shows that particles in 0.05 M NaCl grow by less than 15% with a 3-fold decrease in the total surfactant concentration. This is in contrast to the simple interpretation of dynamic light scattering of the same samples that shows an apparent doubling of the micelle length with the same decrease in surfactant concentration. This discrepancy is attributed to neglect of the thermodynamic and hydrodynamic interactions in the analysis of the dynamic light scattering data. Corrections for the thermodynamic interactions are determined from the static data and applied to the interpretation of dynamic light scattering measurements. The strength of hydrodynamic and entanglement interactions is also discussed in relation to existing models for both semidilute polymer solutions and spherical particles.