Loading of alpha-tocopherol in a nonionic microemulsion: Phase behavior and structural characteristics

Abstract

In this research, we present a systematic analysis of events inside single phase region of a nonionic microemulsion (ME) during incorporation of water and α-tocopherol (AT, a hydrophobic payload). The formulation was developed using α-tocopherol polyethylene glycol succinate, ethanol and oleic acid, respectively, as surfactant, cosurfactant and oil phase. Phase boundary was constructed using water titration method. Behavior of the formulation was studied at two points within the phase boundary. These points carried identical surfactant-cosurfactant mixture (Smix) but different oil content (labeled as DL1: 5% oil and DL2: 10 % oil). These compositions were sequentially challenged with water (in 5 % increments) and examined using dynamic light scattering, small angle neutron scattering and rheological techniques. Contrary to the reports on MEs composed with ionic components, we did not observe any significant effect of AT solubilization upon formulation’s microstructure or phase boundary at a dose of 15 mg/mL. However, swollen aggregates appeared as secondary peaks in light scattering data during water addition. We attribute it to reduction in Smix – oil ratio. As a result of this, oleic acid molecules were exposed to surfactant-poor aqueous phase. Similar observations were recorded at higher oil level. Rheological studies indicated towards weak association among hydrated droplets as a result of which the dispersion exhibited shear-thinning behavior. Despite these changes, the formulation could retain its isotropicity. The information on microstructural changes can be valuable in maximizing the payload inside microemulsion formulations.