INVESTIGATION OF THE MICROENVIRONMENT IN NONIONIC REVERSE MICELLES USING METHYL-ORANGE AND METHYLENE-BLUE AS ABSORPTION PROBES

The microenvironment in nonionic reverse micelles of Triton X-100, n-h exanol, and water in cyclohexane is investigated by using methyl orang e (MO) and methylene blue(MB) as absorption probes. Information about the states of water in the polar core of the reverse micelles is obtai ned through the solvatochromic behavior of MO and the sensitivity of t he complex formation between MB and Triton X-100 in reverse micelles t o the state of water in the polar core. The results obtained from the spectra of MO and MB in the TX-100 reverse micelles with different amo unts of water are compatible with the proposal that there are three ty pes of water present in the polar core: the primary bound water, which interacts directly with the ethylene oxide (EO) groups of the surfact ant to form a primary hydration shell, the second bound water, which i s next to the primary hydration shell of the EO groups and bound less tightly, and the free water, which is present in the water pool and re sembles the bulk water. It is found that there is an equilibrium betwe en two states of MB in the reverse micelles: the bound MB and the free MB, and the presence of secondary bound water and free water will lea d to the equilibrium shift from bound MB to free MB. In the Triton X-1 00 reverse micellar system studied, the secondary bound water is prese nt at about W = 1.8 and water pools begin to form when the water conte nt reaches W = 5.3. The strong interaction between MB and nonionic sur factants, which results in the complex formation, and its sensitivity to the state of water in the polar core are found to be also present i n the reverse micelles of either Triton X-114 or C12E4 in cyclohexane. (C) 1998 Academic Press.