Study on Micellar Structure and Intermicellar Interactions in Ionic Micellar Solutions

Of the methods available to study systems of colloidal particles, those based on scattering and on transport properties measurements have proved to be very useful. At the limit of infinite dilution the data of these techniques depend only on the geometry of the individual particles. On the other hand, for more concentrated systems, the data interpretation becomes complicated because of particle correlation. Normally information on size and shape of colloidal particles are extracted by extrapolating to infinite dilution the experimental data. However this extrapolation is impossible in ionic micelle solutions since these micelles grow by varying either amphiphile concentration or ionic strength. A comparative analysis of scattering and/or diffusion coefficient measurements offers a useful method to quantify the particle correlation effect on these techniques. In this way reliable micelle hydrodynamic radii and information on particle interaction potential are obtained without extrapolating the data to infinite dilution. In this frame an approach based on the integrated study of Dynamic Light Scattering (DLS), 1H-Pulse-Gradient-Spin-Echo NMR (PGSE-NMR) and Small Angle X-Ray Scattering (SAXS) data was tested on some bile salt micellar solutions. Solutions at different ionic strength were analyzed. Different models to represent the intermicellar interactions were used. Approximate equations and Monte Carlo simulations were applied to estimate the particle correlation effect on scattering and diffusivity measurements. Reliable information on micellar geometries was obtained. A check of the validity of the models was performed by discussing the SAXS, DLS and PGSE-NMR data agreement.