The University of Edinburgh: College of Science and Engineering: School of Physics
Abstract
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