Swelling of Aqueous Lα-Phases by Matching the Refractive Index of the Bilayers with that of the Mixed Solvent

It is shown that Lα-phases in two-phase L1/Lα-regions from surfactants or phospholipids swell to single phase regions when part of the water in the system is replaced by hydrophilic co-solvents, like glycerol or dimethylsulfoxide (DMSO), or by the addition of sugar. The behavior is demonstrated on aqueous phases from non-ionic surfactants and co-surfactants and on lecithin in water. Single Lα-phases are reached in the mixed solvents when the co-solvents content reaches between 50 and 60% and the sugar content about 40%. These Lα-phases are completely transparent, birefringent, and they have a rather low viscosity with a finite structural relaxation time. The loss modulus is always larger than the storage modulus in the whole frequency region between 0.001 and 20 Hz. Turbid, but homogeneously looking phases are obtained and they separate slowly with time when macroscopically separated two-phase samples are mixed again by shaking. The turbidity in the homogeneously looking phases decreases with increasing content of the co-surfactant or sugar. These metastable samples become transparent and stable when the refractive index of the bilayers in the Lα-phase becomes the same as the refractive index of the mixed solvent. The experimental results demonstrate that the matching of the refractive index of the bilayers and the solvent is relevant both for the turbidity and for the swelling of the bilayers in the Lα-phase. The interlamellar spacing between the bilayers is determined by a balance of repulsive forces given by undulations and attractive forces determined by van der Waals forces. The swelling of the Lα-phase is caused by a decrease of the Hamaker constant that controls the attractive forces, and is given by the difference of the refractive indices of the mixed solvent and the bilayers in the Lα-phase. Thus, it is concluded that the change of turbidity and the swelling of the Lα-phase have the same origin, namely the matching of the refractive index of the bilayer system with that of the mixed solvent.