The phase behaviour of lyotropic liquid crystals in linear alkylbenzene sulphonate (LAS) systems

A commercial linear alkylbenzene sulphonate (LAS)/H2O surfactant system was studied to develop understanding of the phase diagram and the important mechanisms determining the behaviour. This is an important area of understanding for formulations. Optical microscopy, differential scanning calorimetry, X-ray scattering and nuclear magnetic resonance were used to study samples between 20 and 75 wt% LAS over a temperature range of 5-80 °C. The system exhibits a broad two-phase (micellar/lamellar) region, which derives from the occurrence of an upper consolute loop within the lamellar phase for single isomer LAS/water mixtures. At low temperature the system appeared to behave as a Gibbs (binary mixture) phase rule material, but on heating unexpected behaviour was observed. There was a change in the swelling behaviour, giving rise to the existence of several concentrated lamellar phases co-existing with a single (more dilute) micellar phase. This was attributed to changes in head-group/counter-ion interactions as a function of composition and temperature. The behaviour is derived from the input of energy overcoming an attractive force between the bilayers, and the different isomers within the system having different properties leading to co-existing bilayers with different compositions. Such behaviour in formulations could result in slow changes with time, giving product instability.