The interactions of water with the polar headgroups in the
reversed micelles of sodium bis(2-ethylhexyl)
sulfosuccinate (AOT) and sodium bis(2-ethylhexyl) phosphate
(NaDEHP) have been studied by means of
Fourier transform infrared spectroscopy (FT-IR). O−H stretching
bands of water in both systems varied
with water content (W
0, the molar ratio of water
to surfactant). When W
0 increased from 1 to
20, the O−H
stretching frequency shifted from 3493 to 3416
cm-1 in AOT formed reverse micellar system
and from 3385
to 3417 cm-1 in NaDEHP system, approaching
the frequency of the bulk water. As a response to
water
solubilization, frequency decreases of sulfonate and phosphate
stretching mode in AOT and NaDEHP reverse
micellar solutions have also been observed, indicating hydrations of
the two surfactants. Results showed
that the water molecules encapsulated in reversed micelles are similar
to that in the bulk form when W
0 >
16
in AOT system and W
0 > 12 in NaDEHP system.
In addition, the splitted peaks of the asymmetric
sulfonate
stretching band were found shifting to lower frequencies upon
hydration, due to the weakening of the sodium−sulfonate interaction after hydration and an associated increase in
their spatial separation.
The states and structure of the solubilized water in reversed micelles and microemulsions of sodium
bis(2-ethylhexyl) sulfosuccinate (AOT) and of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane
have been characterized by FT-IR and NMR spectroscopic parameters. According to the four-component
hydration model, the free, anion-bound, bulklike, and cation-bound water are present in reversed micelles
and both of the water-in-oil (W/O) microemulsions formed by AOT and the bicontinuous microemulsions
formed by NaDEHP in n-heptane. The observed chemical shifts (δ) of the water protons from NMR spectra
were expressed as the weighted average of the in-core anion-bound, bulklike, and cation-bound water.
Chemical shifts δ for individual components were evaluated from molar fractions, which were obtained
by deconvolution of the O−H stretching vibrational absorption bands, and the observed δ. Results show
that in W/O microemulsion of AOT in n-heptane and bicontinuous microemulsion of NaDEHP in n-heptane,
the chemical shifts for individual components exhibit constant values, indicating stable microstructure
for the given species, which can be considered as the criterion of either W/O or bicontinuous microemulsions.
Results also show that in reversed micelles of both AOT and NaDEHP, the O−H bond strength and thereby
the microstructure of different hydration species vary with water content, which can be explained by the
interaction between electrical double layers. In transition of reversed micelles to microemulsions, the
microstructures of water molecules transform from the variable state to a stable state.
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