| Surfactant Liquid Crystals: Soft Soap, a Piece of Cake and Surfactant "Rafts" |
| School of Chemical Engineering & Analytical Science, University of Manchester PO Box 88, Manchester, M60 1QD, UK Surfactant liquid crystals (mesophases) occur very commonly in many formulated products and during their processing. Indeed, they are more common than surfactant solutions, but are rarely recognised by users. The most common structures (cubic, hexagonal, lamellar, "gel", "intermediate") are now well-established, as is the link between surfactant chemical structure and mesophase architecture. This lecture will start with an overview of the common mesophase structures, including why they form. Some examples of liquid crystals in common products will be given. There are very simple rules that can be used to predict the liquid crystals formed by any water-soluble surfactants. The second part of the lecture will describe some recent results. Linear alkylbenzene sulphonate is the worldfs largest volume synthetic detergent and is the main component in many detergent products. The liquid crystal phases formed by LAS are complex. Some pure LAS isomers have an upper consolute loop within the lamellar phase region where dilute and concentrated phases coexist. Quite remarkably, purified commercial LAS forms multiple lamellar phases in water. This must involve "lateral phase separation" of the different LAS isomers in a manner similar to the formation of lipid "rafts" in biological membranes. Commercial LAS in water shows this behaviour to a much smaller extent, but when sodium sulphate is present (as in many industrial processes) the phenomenon is even more widespread. The mechanisms responsible for this behaviour will be discussed. |