The mesophase structure of surfactants

The binary surfactant system nona (ethylene glycol) mono-( 11 -oxa- 14,18.22,26-tetramethylheptacosylether) in heavy water has been investigated using deuterium nuclear magnetic resonance, optical polarising microscopy and small angle scattering techniques. The phase diagram has been established and the phases encountered exist for the most part from 24 to 60% surfactant by weight. Above and below these concentrations exist isotropic and lamellar phases respectively. The phase diagram features three main phases, a hexagonal phase of circular rods, a lamellar phase and an intermediate phase. The length of the alkyl chain is shown to be the most important factor in stabilising intermediate phases in preference to the bicontinuous cubic phases usually encountered between hexagonal and lamellar phases in shorter chain homologues. A nematic phase has also been observed in the water penetration scan. The intermediate phase is extensive, ranging from 32 to 60% surfactant by weight and is about 5°C wide. It has a body centred tetragonal mesh structure, comprised of layers of surfactant with regularly spaced holes which are also correlated between the layers, the hole of one layer overlapping the intersection of the layer below. As the concentration is decreased, from 60% to 30% (surfactant by weight) this correlation relaxes. The actual form of the aggregate appears to be elliptical rods (ellipticity - 1.7) joined four by four to form the basis of the mesh structure. A simplified version of the model based on circular rods joined by cube junction regions has been used to quantitatively investigate this phase. The results from this crude model produce results very close to those anticipated and produces results to within 2% of those generated using a more sophisticated model. This system is unusual in that the nature of the interaction between aggregates appears to be dominated by a single interaction - 'headgroup overlap force. This arises from the repulsive pressure experienced by the headgroups penetrating from the aggregate interface and also between headgroups on the same aggregate