Optical microscopy and nuclear magnetic resonance studies of mesophases formed at compositions between hexagonal and lamellar phases in sodium n-alkanoate + water mixtures and related surfactant systems
The mesophase structures formed in sodium dodecanoate + water mixtures have been investigated using n.m.r. spectroscopy and polarising microscopy. Four different regions were indicated by optical microscopy, while 2H and 23Na n.m.r. measurements detected only three mesophases separated by first-order phase boundaries. From the n.m.r. data we conclude that the two mesophases with highest surfactant concentrations both consist of bilayers. One is the normal lamellar (Lα) phase whilst the second, containing less surfactant and occurring over limited temperature and composition ranges, has very thin bilayers. We suggest that within this phase there is a hydrogen-bonded water network linking the bilayers. The phase formed at lowest surfactant concentration is hexagonal (H1). To account for the optical microscope and n.m.r. data, we propose that as concentration is increased the H1 phase gradually changes to a deformed hexagonal lattice (H1d). The H1d structure and the second lamellar phase are termed ‘intermediate’ phases.
Optical microscopy has been used to investigated the occurrence of intermediate phases in other C8-C18 sodium and potassium alkanoates with even chain numbers. These phases are found with the C12 to C18 sodium soaps whilst the C8 and C10 sodium soaps form a cubic phase (V1). The C10 derivative also forms one intermediate phase (probably H1d). Broadly similar phase behaviour was found for the potassium soaps except that two intermediate phases and a V1 phase occur with the C10 compound, while with the C12 derivative the intermediate phases are transformed to V1 at higher temperatures. The same technique has been used to investigate mesophases of sodium n-alkyl sulphates. A single intermediate phase only forms with C10 to C14 sodium alkyl sulphates. For the hexyl derivative a V1 phase occurs, while the intermediate phase of the C8 homologue changes to V1 on heating. Only H1 and Lα occur for sodium oleyl sulphate.
Intermediate phases only appear to form with longer-chain ionic surfactants. Literature data indicate that only V1 phases occur with non-ionic and zwitterionic surfactants. There is a brief discussion of possible explanations for this.