A small-angle neutron scattering study of the effects of dilution on particle morphology in mixtures of glycocholate and lecithin

Small-angle neutron scattering measurements have been made to determine the morphology of particles formed in mixed dispersions of lecithin and the bile salt, glycocholate, at a phospholipid to bile salt molar ratio of 0.56 at different total lipid concentrations. At higher lipid concentrations interparticle correlation effects are present, making analysis of the structure difficult. At lower concentrations, however, such effects are not important, and it is seen that the particle morphology is highly dependent on total lipid concentration. As the system is diluted the particles grow in size by elongation along a single axis. The eventual result of this is the formation of long rods with an apparent cross-sectional radius of 27.2(3) Å. On further dilution the rods coalesce to form large sheet-like structures or vesicles. At still lower concentrations the vesicles decrease in size and become more spherical and/or less polydisperse. The initial growth of the particles by the formation of rods is a new observation for this system and is contrary to the predictions of the currently held mechanism for dilution-driven particle growth in these systems. The vesicle formation at low lipid concentrations has been inferred from the results of other work, but the SANS results given here provide the best and most direct evidence for their existence.