Phase transitions between vesicles and micelles driven by competing curvatures

We present a model explaining the phase transition between cylindrical micelles and vesicles (bilayers) in a mixed dilute solution of phospholipids and surfactants. The model predicts a first-order transition between micelles and vesicles, which depends on the relative concentration of the two components. The phase transition boundaries are calculated as a function of the specific areas of the two components, their spontaneous curvatures and elastic moduli. The transition is driven by a very large difference in spontaneous curvatures between lipid and surfactant. The free energy takes into account the entropy of mixing as well as the curvature energy. Our predictions are in qualitative agreement with experiments. For mixtures of octyl glucoside (surfactant) and phosphatidylcholine (phospholipid), we obtain good agreement with experimental data.