STRUCTURE OF CUBIC PHASES IN THE TERNARY-SYSTEM DIDODECYLDIMETHYLAMMONIUM BROMIDE WATER HYDROCARBON

We have studied the structure of the ternary cubic phases formed by th e system didodecyldimethylammonium bromide (DDAB)/D2O/hydrocarbon oil by small-angle X-ray scattering (SAXS) and small-angle neutron scatter ing (SANS). Different oils (octane, dodecane, tetradecane, and toluene ) of varying molecular size and degree of penetration into the hydroph obic tail region were used, and systematic trends were observed both i n the phase behavior and structure of the cubic phase. This structure is well-described by the topology of triply periodic minimal surfaces, and either a parallel surface construction in which the surfactant bi layer decorates a minimal surface or one based on a constant mean curv ature family of surfaces can be invoked to describe the data. The diff raction data reveal Bragg reflections which index to well-known cubic structures, and transitions between different cubic symmetries are obs erved on changing the composition of the system. It is found that high ly penetrating oils such as toluene are fully absorbed into the surfac tant region, while nonpenetrating oils such as tetradecane reside larg ely in the middle of the bilayer, thus causing it to swell appreciably on increasing the oil content of the system. The transitions between different cubic symmetries are primarily driven by interfacial curvatu re and occur in a systematic manner with composition. The aqueous volu me fraction is mainly responsible for setting the mean curvature of th e interface, but the interaction of the oil with the surfactant tails is also important in determining the structure through its effect on t he spontaneous curvature. The system adopts that structure which best optimizes the (generally) divergent values of the actual and preferred curvatures at the expense of alternative structures. The observed cub ic structures include triply periodic ones based on the gyroid (G), di amond (D), and the Schwarz-P (P) minimal surfaces. Regions with an unr esolved cubic symmetry as well as biphasic regions with two coexisting cubic symmetries are also found in the phase diagrams. The phase beha vior and structure of ternary cubic phases can thus be accounted for s olely on topological considerations, with the particular composition o f each system merely setting the values of parameters such as curvatur e, lattice constant, etc.