Preparation and characterization of microemulsions formulated with PEG fatty acid glycerides

Microemulsions are thermodynamically stable and homogeneously dispersed systems formed spontaneously by water, oil, surfactants, and co-surfactants at specific proportions under specific conditions. Microemulsions can not only dissolve certain active ingredients to achieve protection and slow release, but also effectively promote their transdermal absorption, further improving the bioavailability due to the nanoscale of the emulsion droplets. In this study, microemulsions formulated with surfactant polyethylene glycol (PEG) fatty acid glycerides were prepared and the effects of oil property, the amphiphilicity of surfactant, the types of co-surfactants, the main/co-surfactant mixing ratio (K_m) as well as the temperature, pH, ionic strength of the systems on the phase behavior of the studied system were discussed in the framework of the pseudo-ternary phase diagram. The results indicate that lower polarity oils are more likely to form microemulsions constructed by PEG fatty acid glycerides. The formation of microemulsion depends much on the amphiphilicity of surfactant; PEG fatty acid glycerides with higher hydrophile-lipophile balance (HLB) facilitate the formation of microemulsions. The shorter the chain length of co-surfactant (alcohols), the easier it is for the formation of microemulsion. For alcohols with a certain number of carbon chains, the fewer the hydroxyl groups, the easier it is to form microemulsions. This study identifies an optimal K_m of 2:1. Based on the fact that PEG fatty acid glycerides are nonionic surfactants, the prepared microemulsions have a good tolerance to pH fluctuations (pH 4-9) and keep stable within a wide temperature range, which enable the microemulsions to be applied in variable environments. The higher concentration salt inhibits microemulsion formation in some degree though the lower concentration salt has no obvious effect on the formation of microemulsion. Furthermore, the types of microemulsions in the single-phase region were further identified, and the microstructure of microemulsions formulated with PEG fatty acid glycerides was investigated by transmission electron microscopy (TEM). Typical spherical oil-in-water (O/W) microemulsions, uniformly distributed water-in-oil (W/O) microemulsions, and the characteristic network structure of bicontinuous microemulsions were observed. On this basis, the water-soluble nicotinamide was singled out to study the transdermal behavior of substance in oil-in-water microemulsion and in traditional emulsion. The results indicate that the substance in microemulsion have better transdermal property compared with conventional emulsion. The present work provides not only fundamental information for the preparation of microemulsions formulated with PEG fatty acid glycerides but also the reference for its application in practice.