Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

10B/2004G282
2+Hydration Structure around Cu Restricted in Reverse Micelle of AOT
Takahiro OHKUBO*1, Kazuyuki ANDO2, Hideki SAKAI1,2, Masahiko ABE1,2
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

1

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, which
concentration is 50 mM, 100 mM, and 150 mM,respectively. Here, we fixed w values as 2 at which mean
diameter of reverse micelles determined by the dynamic
light scattering method were 3.5 nm. 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. These
results show that a mean hydration number and distance
between a Cu ion and an oxygen atom of water molecule
for the NSNs are 2.4±0.3 and 0.197±0.003 nm,respectively. Here, the hydration number for NSN is lessthan that of bulk phase, while the distance between a Cu
ion and a water molecule is similar to each other. Theseresults strongly indicate that such a hydration structurearound Cu ion in reverse micelle is strongly affected by

the restriction effect, especially the geometrical restriction
from the molecular assembly of AOT.

Cu NSN 150mM

Cu NSN 100mM| FT(r) |Introduction
EXAFS technique is quite useful to obtain thehydration structure around a metal ion. Up to now, weapplied EXAFS technique to study nano-scale ionicsolution, which we named nanosolution (NSN) of RbBr,
confined in the slit-shaped nanospaces of activated carbon
fiber (ACF), showing hydration anomalies in thepreceding papers [1-4]. 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 is described in this reportas a function of concentration of dioctyl sulfosuccinate
sodium salt (aerosol OT; AOT), which form reverse
micelles in aqueous solution.

Cu NSN 50mM

CuSO4 (Bulk)

0.00.20.4

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.
rCu-O / nm σ2 / 10-5 nm2 ∆E0 / eVmM

CN50

2.60.197

2.8

-4.4

100

2.4

0.197

2.8-5.8

150

2.3

0.197

2.8

-5.8

Bulk

6*

0.197

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).
[3] T. Ohkubo et al., Aust. J. Chem. 56, 1013 (2003).
[4] T. Ohkubo et al., Physica Scr. T115, 685 (2005).
* tkohkubo@rs.noda.tus.ac.jp

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