Investigation of the relation between surfactant head group size and formation of different types of lyotropic structures in two surfactants/dye systems

Abstract

Lyotropic phase properties of novel mixtures of some dodecylalkylammonium bromide surfactants (DAABr) in the mixtures of 1-dodecanol (DDeOH)/Sunset Yellow dye (SSY)/water were investigated. DAABr surfactants were chosen as dodecyldimethylammonium bromide (DDMABr), dodecyldimethylethylamonium bromide (DDMEABr) and dodecyltriethylammonium bromide (DTEABr). In lyotropic mixtures, both surfactants were used in the mixtures, i.e., DDMABr/DDMEABr and DDMEABr/DTEABr to control the average number of the methylene (−CH2) groups in their head groups (navg) at the aggregates’ surfaces. By changing the relative concentrations of each surfactant in the mixtures at constant total surfactant and mixture compositions, lyotropic mixtures presenting lamellar, nematic, and hexagonal phases were obtained. The textures and types of lyotropic phases were analyzed by polarizing optical microscopy (POM), also confirmed by Small-angle X-ray scattering data. Uniaxial-to-biaxial nematic phase transitions were determined from the temperature dependence of the birefringences via laser conoscopy. Temperature-navg partial phase diagrams of the mixtures were constructed by combining the POM and laser conoscopy results. Small-angle X-ray scattering (SAXS) was performed to determine the structural parameters. The results indicate that there is a close relation between the stabilization of different lyotropic structures and the average size per surfactant head group (a0) and the navg values.