Formation and characterization of ordered bicontinuous microemulsions.

Ordered bicontinuous microstructures formed in a fully water-dilutable, pseudoternary unique nonionic microemulsion were obtained and characterized. The concentrate contained a mixture of triacetin/d-alpha-tocopherol acetate/ethanol/Tween 60. Upon dilution, the concentrate was transformed from a reversed micellar system to oil-in-water microemulsion droplets. The transformation occurred through an intermediate phase of ordered bicontinuous structures. The factors that governed the construction of this unique phase, and its physical and structural properties, were characterized in detail. The techniques used included small angle X-ray scattering (SAXS), self-diffusion and quantum filtered NMR, differential scanning calorimetry, rheology measurements, electrical conductivity, and dynamic light scattering. This mesophase displays microemulsion properties along with some characteristics of lyotropic liquid crystals (but is not a mixture of the two). Similar to microemulsions, the structures were transparent and spontaneously formed and exhibited thermodynamic stability. Yet, unlike microemulsions, they showed short-range order at room temperature. Additionally, the microstructures exhibited non-Newtonian flow behavior, characteristic of lamellar structures. The bicontinuous ordered microemulsions were obtained upon heating (to 25 degrees C) from the lamellar phase existing at low temperatures (5 degrees C). The main feature governing the bicontinuous mesophase formation was the amphiphilic nature of oil blends composed of d-alpha-tocopherol acetate and triacetin. The oils functioned as cosurfactants, altering the packing parameter of the surfactant and leading to the construction of bicontinuous structures with short-range order. These unique structures might have drug or nutraceutical delivery advantages.