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-ethylhexylsulfos uccinate (AOT) in benzene and hexane. The nonionic reversed micelles o f the poly(oxyethylene)(n) dodecyl ethers type surfactants Brij 30 in benzene, cyclohexane, and decane and C(12)E(5) 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 absorpti on bands shifts in the visible, B-1, and in the UV, B-2, and the absor bances ratio of these hands. At W = 0 the polarity sensed in the micel le interfaces of AOT-hexane and AOT-benzene is entirely similar while for BHDC-benzene is quite smaller. The detected interaction between th e QB and BHDC may favor the entrance of QB into the oil side of the mi celle interface thus sensing a less polar environment. At W greater th an or equal to 10 once the polar heads of the surfactants are complete d hydrated, the anionic as well as the cationic surfactants show the s ame behavior. The polarity of Brij 30 and C(12)E(5) in the aliphatic h ydrocarbons is comparable. However at W = 0 their micropoalrity is hig her than that of AOT-hexane, showing that the formation of hydrogen bo nd between QB and the free OM groups of these surfactants is the major factor affecting the microenvironment. In all the studied systems eve n at the maximum possible W, the polarity sensed by QB never reaches t he value of pure water, that is, never reaches the bulk water phase pr evailing in the micellar core. (C) 1996 Academic Press, Inc.