Indomethacin solubilization induced shape transition in CnE7 () nonionic micelles

Shape transitions were examined with regard to the solubilization of the poorly water-soluble drug indomethacin (IMC) in the nonionic surfactants heptaethylene oxide tetradecyl (C14E7) and hexadecyl (C16E7) ethers by means of a dynamic light scattering technique. The cloud points of the pure C14E7 and C16E7 micelles ranged from 58 to 62 °C and from 52.1 to 55.6 °C, respectively, at surfactant concentrations of 1 to 10 mM. The cloud points of IMC-solubilized micelles increased by approximately 1 to 5°. The sizes of the pure C14E7 micelles were 4 to 14 nm at 20 to 40 °C at a concentration of 2 to 20 mM. The apparent hydrodynamic radius (Rhapp) of pure C16E7 micelles varied with temperature and concentration. C16E7 surfactant formed small spherical micelles at 20 and 25 °C at concentrations below 5 mM; the size of the micelles was approximately 5 nm. On the other hand, from 30 to 40 °C and at a higher concentration, C16E7 formed elongated cylindrical micelles, and these elongated micelles entangled or overlapped each other. The Rhapp of the IMC-solubilized C14E7 micelles at 20 to 40 °C and of C16E7 micelles at 20 °C increased compared to that of pure micelles. On the other hand, the cylindrical micelles of C16E7 decreased in size and turned into spherical ones because of the hydrophobicity between the micelles caused by solubilization of IMC. This phenomenon was confirmed by transmission electron microscope (TEM) images.