Phase science of surfactants

Recent major developments that have taken place in surfactant phase science may be summarized in five points. First, the equilibrium vesicle phase formed by catanionic surfactant mixtures in the very dilute region has been investigated for several systems which also form several mesophases in the concentrated region. Coexistence of two reversed hexagonal phases and first order vesicle→micelle phase transition have been demonstrated. Second, by manipulating the hydrophilic-lipophilic balance and molecular geometry of many surfactant systems, the changes in observed phase behaviour serve to increase our knowledge of aggregation processes in general and the structures of the biréfringent intermediate phases, sponge phase and dilute lamellar phase in particular. Third, the lyotropic nematic and defective mesh-type lamellar phases are readily formed by fluorinated surfactants; the structures of the two phases, which are still debatable, have been studied by several techniques. The first observation of a lower consolute loop along the dilute lamellar phase was reported for the fluorinated anionic surfactant systems. Fourth, the valency of counterions has been shown to have dramatic effects on the aggregation and phase behaviour of ionic surfactants. Finally, surfactant-protein systems form novel gel phases and locations of phases provide information concerning the contribution of electrostatic and hydrophobic effects to surfactant-protein interactions.