Microstructural transitions in β-carotene loaded nonionic microemulsions upon aqueous phase dilution

In this study, biocompatible formulation of a fully dilutable microemulsion system comprising Tween 60/isopropyl myristate/ethanol/glycerol/water has been accomplished for solubilizing a natural antioxidant, β-carotene. Solubilization capacity of β-carotene in the microemulsion system decreases progressively with increase of aqueous phase weight fraction (Φ). Structural transitions occurring in the free and solubilized microemulsions were investigated and compared via multiple complementary characterization methods. The characterization has been carried out by means of conductivity, viscosity, small angle X-ray scattering (SAXS), surface tension and transmission electron microscopy. The results show that along the selected dilution line, the microemulsion undergoes a continuous transition from water-in-oil (W/O) micelles to oil-in-water (O/W) microemulsion via an ordered bicontinuous structure. The β-carotene loaded has little influence on either the size or the shape of micro-domains or microstructural transitions in the microemulsions. Furthermore, the location of the β-carotene molecules in the microemulsion system varies with the microstructural transition upon dilution of the aqueous phase. The detailed understanding of the structural behavior of the microemuslion systems might be significant for their future applications in drug or nutraceutical delivery.