Mixed micelles system: equilibrium and kinetics
Lipid-detergent systems are interesting to study, as the two amphiphiles have very different spontaneous curvature, however readily form mixed micelles in solution. These micelles can be shorter cylindrical micelles or long worm-like micelles. For such a system the size of the micelles varies strongly with solute conditions, being dependent on the total amount of amphiphile in solution, as well as, the lipid to detergent ratio in the micelles. Although the broad phase behaviour of such systems has been studied and is relatively well understood, there are still many open questions remaining. Some of the questions that motivated the work presented are: how the length and composition of the micelles varies within the micellar region, and how the micelles grow? The biologically interesting system under study is lecithin and bile salt, where the equilibrium sizes were experimentally determined for different samples within the micellar region. A model, combining the length of the micelles, with the concentrations of lecithin and bile salt in the system is presented, and is used to calculate the composition of the micelles at equilibrium. The kinetics of the growth of the micelles after a pertubation causing a shift in the equilibrium size has not been studied in detail before. The kinetics of the system are studied using a stopped flow setup, which I specifically designed for neutron scattering experiments. The stopped flow setup allows for the measurement to start 200ms after the initial mixing of the two liquids, after dilution of a solution the micelles relax to a new longer length. The kinetics of this relaxation were studied as a function of the initial and final size of the micelles, as well as, the ionic strength of the solution. The micelles were found to grow through coalescence, where the rate of growth seems to be constant for different sizes of micelles and the time taken for the relaxation depends on the difference between the final and initial lengths of the micelles. The rate of growth is strongly influenced through changing the ionic strength of the solution, indicating the importance of an electrostatic barrier to the fusion of micelles.