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Investigation of the Microenvironment in Nonionic Reverse Micelles Using Methyl Orange and Methylene Blue as
Abstract:
The microenvironment in nonionic reverse micelles of Triton X-100, n-hexanol, and water in cyclohexane is investigated by using methyl orange (MO) and methylene blue (MB) as absorption probes. Information about the states of water in the polar core of the reverse micelles is obtained through the solvatochromic behavior of MO and the sensitivity of the complex formation between MB and Triton X-100 in reverse micelles to 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 amounts of water are compatible with the proposal that there are three types of water present in the polar core: the primary bound water, which interacts directly with the ethylene oxide (EO) groups of the surfactant to form a primary hydration shell, the second bound water, which is 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 resembles the bulk water. It is found that there is an equilibrium between 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 lead to the equilibrium shift from bound MB to free MB. In the Triton X-100 reverse micellar system studied, the secondary bound water is present at about W = 1.8 and water pools begin to form when the water content reaches W = 5.3. The strong interaction between MB and nonionic surfactants, which results in the complex formation, and its sensitivity to the state of water in the polar core are found to be also present in the reverse micelles of either Triton X-114 or C12E4 in cyclohexane. Copyright 1998 Academic Press. Copyright 1998Academic Press