Micropolarity of Reversed Micelles: Comparison between Anionic, Cationic, and Nonionic Reversed Micelles

The interface polarity of benzyl-n-hexadecyldimethylammonium chloride (BHDC) cationic reversed micelles in benzene was studied and compared with those of the anionic systems of sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) in benzene and hexane. The nonionic reversed micelles of the poly(oxyethylene)n dodecyl ethers type surfactants Brij 30 in benzene, cyclohexane, and decane and C12E5 in hexane, heptane, and decane were also investigated. 1-Methyl-8-oxyquinolinium betaine (QB) was used as optical probe. The influence of surfactant concentration and W = [H2O]/[surfactant] were analyzed as a function of the absorption bands shifts in the visible, B1, and in the UV, B2, and the absorbances ratio of these bands. At W = 0 the polarity sensed in the micelle interfaces of AOT-hexane and AOT-benzene is entirely similar while for BHDC-benzene is quite smaller. The detected interaction between the QB and BHDC may favor the entrance of QB into the oil side of the micelle interface thus sensing a less polar environment. At W >/= 10 once the polar heads of the surfactants are completed hydrated, the anionic as well as the cationic surfactants show the same behavior. The polarity of Brij 30 and C12E5 in the aliphatic hydrocarbons is comparable. However at W = 0 their micropolarity is higher than that of AOT-hexane, showing that the formation of hydrogen bond between QB and the free OH groups of these surfactants is the major factor affecting the microenvironment. In all the studied systems even at the maximum possible W, the polarity sensed by QB never reaches the value of pure water, that is, never reaches the bulk water phase prevailing in the micellar core.