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

Abstract

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 (L_α) 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 (H_1d). The H_1d 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 H_1d). 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 L_α 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.