Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

10B/2004G282

Hydration Structure of Metal Ions Restricted in Reverse Micelle of AOT
Takahiro OHKUBO*1, Kazuyuki ANDO2, Hideki SAKAI1,2, Masahiko ABE1,2
1
Institute of Colloid and Interface Science, Tokyo University of Science, 2641 Yamazaki, Noda,
Chiba 278-8510, Japan
2
Faculty of Science and Technology Tokyo University of Science, 2641 Yamazaki, Noda,
Chiba 278-8510, Japan

Experimental
We synthesized Cu(AOT)2 sample from Cu(NO3)2 andhighly purified AOT (>98%; Aldrich). Also, we cancontrol the diameter of inner water phase formed by AOT
by adjusting the water content (here, concentrated
aqueous solution of CuSO4) against the concentration ofAOT. Here, we denote the water content as w. The reverse
micelles of Cu(AOT)2 having different diameters forwater phase were obtained in heptane as an oil phase.
These samples were sealed in glass cell with Kapton film.
All EXAFS spectra for Cu K-edge (89801.3 eV) wererecorded at BL-10B of Photon Factory, High Energy
Accelerator Research Organization in Tsukuba.
Results and Discussion
We used three kinds of NSN samples and the mean
diameters for the reverse micelles determined by thedynamic light scattering method were 3.5, 4.5, and 7.3
nm for w=2, 4, and 10, respectively. Figure 1 and Table 1
show the Fourier transform of EXAFS oscillation for Cu
K-edge of NSNs and bulk solution and analytical resultsof curve fitting for these samples, respectively. The
results indicated that a mean hydration number anddistance between a Cu ion and an oxygen atom of a watermolecule for the nanosolution are 2.4±0.3 and0.197±0.003 nm, respectively. Here, the hydration
number for NSN is less than that of bulk phase, while thedistance between a Cu ion and a water molecule is similar
to that of bulk solution. These results strongly indicatethat such a hydration structure around Cu ion in reverse
micelle is strongly affected by the restriction effect,

50

Users' Reportespecially the geometrical restriction from the pore wallof AOT.

Cu NSN w =2

Cu NSN w = 4| FT(r) |Introduction
An extended X-ray absorption fine structure (EXAFS)
technique is quite effective to determine the localstructure around a target atom or ion. Then we applied
EXAFS technique to study nano-scale RbBr solution,
which we named RbBr nanosolution (NSN), confined in
the slit-shaped hydrophobic nanospace of activatedcarbon fiber (ACF), showing hydration anomalies in thepreceding papers [1, 2]. Also, NSNs formed in water
nanodroplet surrounded by amphiphilic molecules havephysically and chemically unique properties because of
the size effect of water droplet. An explicit anomaly of
hydration structure for Cu ion dispersed in water droplet
restricted in reverse micelle of dioctyl sulfosuccinate
sodium salt (aerosol OT; AOT) is described in this report.

Cu NSN w = 10CuSO4 (Bulk)

0.0

0.2

0.4

0.6

r / nmFigure 1 Fourier transforms of Cu K-edge EXAFS of Cu
NSN and aqueous solution of CuSO4.

Table 1. Structural parameters obtained by curve fitting.
w2410Bulk

CN
2.4
2.5
2.5
6*

rCu-O / nm σ2 / 10-5 nm2 ∆E0 / eV
0.197
2.8-3.70.197
3.0-5.90.197
3.0-6.00.197
7.1-0.5*: fixed parameter

References[1] T. Ohkubo et al., J. Am. Chem. Soc. 124, 11860(2002).
[2] T. Ohkubo et al., J. Phys. Chem. B 107, 13616 (2003).
* tkohkubo@rs.noda.tus.ac.jp