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.
Lanthanide complexes of europium and terbium ions with hydrogen phthalate [Eu(Hphth) 3 and Tb(Hphth) 3 , respectively, Hphth ) hydrogen phthalate] were incorporated into poly(vinyl pyrrolidone) (PVP) matrix. Transmission electron microscopy (TEM) results showed that the lanthanide complexes, with a mean diameter about 8.0 nm, were dispersed homogeneously in the PVP matrix. The excitation of the Ln(Hphth) 3 -PVP system by UV light resulted in a ligand-to-metal energy transfer and the emission spectra of the complexes displayed characteristic luminescence of the central ions. PVP significantly affected the strength of the hypersensitive transitions ( 5 D 0 f 7 F 2 for Eu 3+ , and 5 D 4 f 7 F 5 for Tb 3+ ) of the complexes. For the mixed complexes containing both Eu 3+ and Tb 3+ as central ions, the luminescent intensity was strongly dependent upon the ratio of the lanthanide ions. Trace amount of Eu 3+ quenched the luminescence of the Tb 3+ , whereas the presence of Tb 3+ enhanced the luminescence of the Eu 3+ , thus demonstrating an important feature of energy transfer from terbium to europium in the mixed complexes.
The microstructure of mixed reverse micelles stabilized with surfactants of sodium bis(2-ethylhexyl)sulfosuccinate (aerosol OT, or AOT) and/or sodium bis(2-ethylhexyl)phosphate (NaDEHP) has been studied
by means of Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H NMR),
and electrical conductometry. The O−H stretching band in IR and the chemical shift in 1H NMR of water
solubilized in the reverse micelles change concomitantly with both water content (W
0, the number of solubilized
water molecules per amphipathic molecule) and relative content of surfactants (AOT and NaDEHP). As in
the reverse micelles stabilized with AOT and AOT + NaDEHP (50:50), the maxima of conductivity were
observed in respective conductivity vs water content (κ−W
0) curves below the percolation threshold, while
the conductivity of NaDEHP-formed reverse micelles changed proportionally with water content. The striking
point is that, at a certain water content, the conductivity of the H2O/AOT/NaDEHP/n-heptane system decreases
with increasing temperature from 0 to 55 °C, a rare phenomenon in solution. The changes are attributed to
the hydration of surfactant ions plus the morphology change of surfactant aggregates in respective systems.
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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