Gelation of microemulsions and release behavior of sodium salicylate from gelled microemulsions

A novel gelled microemulsion was prepared in the presence of the low molecular weight gelator N-stearine-N′-stearyl-l-phenylalanine at a very low concentration. It is completely different from the conventional microemulsion-based gels (MBGs) usually formed by polymeric gelling agents, such as gelatin, agar and κ-carrageenan. The microemulsion consists of i-propyl myristate, Tween 80, propylene glycol and water. The gelled microemulsions showed good thermo-reversibility. The gel-to-sol transition temperature (T_GS) of gelled microemulsion depends upon the concentration of gelator and the composition of the microemulsions. The gelation mechanism was investigated by polarized optical microscopy (POM) and FT-IR. POM images show elongated and strand-like crystallites formed by the aggregation of the gelator, ultimately resulting in the gelation of the microemulsion. FT-IR analysis indicates that intermolecular hydrogen bonds are responsible for the formation of gelator aggregates. Water-soluble sodium salicylate was used as a model drug for the investigation of the release from the gelled microemulsions. The release profiles exhibited a controlled release and followed the first-order release kinetics. The release rates decreased with an increase of the gelator and isopropyl myristate contents. These results reveal potential applications of gelled microemulsion in drug delivery systems.