Surfactant gel (Lβ) phases are very common, occurring in products such as fabric and hair conditioners. They are excellent emulsion stabilisers, finding numerous applications in creams and lotions. The gel phase has a very similar architecture to a lamellar phase, consisting of surfactant bilayers. But, the major difference is that the surfactant tails are in a rigid all-trans configuration (almost) rather than having liquid-like molecular configurations with a mixture of gauche- and trans- states. The controlling molecular feature in the formation of gel phases is the alkyl chain. Alkanes (CnH2n+2) with a chain length n>22 pass through a “rotator” phase on melting of the crystal, rather than undergoing a direct crystal→liquid transition. Similarly, surfactants with a chain length n>ca.7 can form a gel phase, provided that the head group has an appropriate structure. Longer chain di-alkyl surfactants (n>12) are the best known examples of gel phase systems. Membrane lipids commonly fall into this category. In this presentation a short review of the surfactant structure requirements for gel phase formation will be given, together with a description of gel phase structures. Recently, we have carried out density and other measurements on phospholipids gel phases that demonstrate significant non-equilibrium behaviour. There is a strong variation in this behaviour with surfactant chain length. A very marked “pretransitional-gel formation” appears to occur in the lamellar phase just above the gel/lamellar transition temperature. It is unlikely that any gel phases formed from phospholipids are in a local free-energy minimum (equilibrium) state. For mixtures of commercial surfactants (and membrane lipids) the gel/lamellar transition occurs over a temperature range rather than at a specific temperature. Under these conditions it appears that gel/lamellar regions coexist on a relatively short distance scale, perhaps corresponding to gel (and lamellar) “patches”.
School of Chemical Engineering and Analytical Sciences, University of Manchester