Surfactant gel (L_β) phases routinely form metastable states with long relaxation times to the equilibrium structure

The gel (L_β) phase is commonly observed on cooling a lamellar phase of longer chain amphiphiles. A review of gel/lamellar transition enthalpies for a wide range of di-alkylphosphatidyl cholines (lecithins) suggests that a large number of different alkyl chain arrangements are possible for the gel phase. Furthermore, the application of Ostwald's law of stages indicates that the initial gel state formed on cooling is very likely to be a non-equilibrium metastable state. Experimental evidence from surfactant-specific electrode studies and rheology for the hexadecyltrimethylammonium bromide/hexadecanol (CTAB/C₁₆OH) gel phase system is presented to demonstrate that slow equilibration effects are observed in practise. The detailed molecular organisation within gel phases has yet to be elucidated, and will be the subject of intensive effort over the next few years.