Diffusion of mixed micelles of bile salt-lecithin in amylopectin gels: a Fourier transform infrared microscopy approach

Using Fourier transform infrared (FTIR) microspectroscopy the average translational diffusion coefficients of bile salt-lecithin mixed micelles diffusing in amylopectin gels of varying concentration were measured, on the assumption that the diffusion coefficient was constant during diffusion. This assumption was examined based on quasielastic light scattering (QLS) measurements of the change in the mixed micellar size on dilution. The size increased significantly on dilution with a buffer without the bile salt, whereas the size was almost constant on dilution with a solution of the bile salt in the same buffer. Because bile salt molecules in intermicellar solution (being much smaller) diffuse much faster than the mixed micelles, the mixed micelles in the diffusing front can be treated as if diluted with the bile salt solution. Therefore their size will be constant during diffusion, validating the assumption of a constant diffusion coefficient. Based on this it is possible to show that the micelles diffuse with a diffusion coefficient of ∼ 10−11 m2 s−1, independent (within experimental error) of the amylopectin gel concentration over the range 5%–10% w/w.