Measurement of phase transition free energies in polyelectrolyte-surfactant complexes

When polyelectrolytes are mixed with oppositely charged surfactants at a 1:1 charge ratio, long-range ordered polyelectrolyte-surfactant complexes are formed. In this paper we demonstrate that by using the "Luzzati method" combined with osmotic stress measurements, we were able to determine the polyelectrolyte-surfactant free energy change while transitioning from a Pm3n cubic phase to a hexagonal phase. Assuming that the major contribution to this energy originates from bending energies associated with the structural rearrangements of the surfactants, we determined the bending modulus of the surfactant inside the complex.