Nile red probing for sphere-to-rod-to-wormlike micelle transition in aqueous surfactant solution

Nile red (phenoxazone-9) was used as a fluorescence probe to detect the sphere-to-rod-to-wormlike micellar transition in the aqueous mixtures of cetyltrimethylammonium bromide and sodium salicylate or potassium bromide, and also in the aqueous solution of a Gemini surfactant, ethanediyl-α,ω-bis(dimethyl dodecyl ammonium bromide). The results showed the Ia/Ib of Nile red (the fluorescence intensity ratio of the twisted intramolecular charge transfer band to the planar locally excited band) was a more sensitive index than the Ia (the fluorescence intensity of the twisted intramolecular charge transfer band) to represent the micellar transition. This was attributed to the special molecular structure of Nile red that has a large conjugated ring and the can form hydrogen bond with water molecules at the electron acceptor group. This led to sensitive response of the twisted intramolecular charge transfer state of Nile red to the change of microenvironment.