The energetics of forming equilibrated bilayer vesicles

On the basis of detailed model calculations we demonstrate that the free energy required to form an equilibrated spherical bilayer vesicle is made up of two main contributions: the (size-independent) work of bending the constituent monolayers and the work of stretching the bilayer that is determined by the planar bilayer tension. A major part of the bending free energy arises due to a reduction of the overall hydrocarbon chain volume upon forming a spherically shaped bilayer shell out of a planar monolayer, while keeping the total hydrocarbon/water contact area fixed and the chemical potentials constant. This contribution is only partially counterbalanced by a sum of contributions due to curvature-dependent molecular free energies of various origins. The overall free energy of bending a pure (charged) SDS bilayer into a vesicle is found to be quite large (similar to 90kT). However, admiring with a long-chain alcohol brings it down substantially and is thus crucial for spontaneous vesicle formation to occur.