Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

7C/2003G292, 7C/2004G109, 12C/2005G217

Solvation structures of bromide ions at the air/solution interface and in the bulk
Mayumi HOSHIDE1, Hajime TANIDA2, Kaoru KASHIMOTO3, Takanori TAKIUE3,Makoto ARATONO3, Toru OZEKI4, Iwao WATANABE*1
1
Osaka Prefecture University, 2-1 Daisen, Sakai, Osaka 590-0035
2
SPring-8/JASRI, Mikazukio, Sayo, Hyogo 679-5148
3
Kyusyu University, Hakozaki 6-10-1, Higashiku Fukuoka 812-8581
4
Hyogo University of Teacher Education, 942-1 Shimokume, Yashiro, Kato, Hyogo 673-1494

Experiments-1
The Br-K edge XAFS spectrum of a 100mmol kg
NaBr solution was recorded at BL12C by using thetransmission method. The TR-XAFS was performed at
BL7C for the surface of the same solution.
Results and Discussion
Fig. 1 shows the χ spectra for Br ions in the bulk of a-1
100mM kg NaBr solution (B-Br) (red line) and at itssolution surface (S-Br) (black line). The curve fitting
analyses on the χ spectra indicate that the Br-O (in Br ···H-O-H) bond length is shorter for S-Br. The hydrationnumbers are similar each other or that for S-Br is slightlysmaller. It is usually observed that smaller coordination
number results in shorter coordination distance.
Since the BL7C station provides a horizontallypolarized X-ray, the TR-XAFS method detects thecoordinating atoms on the horizontal plane strongly but
not the vertically coordinating atoms. Fig.1 also shows
one of the χ spectra obtained from Br K-edge TRXAFS for the DTA-Br aqueous solution surface (greenline). As reported previously [1], there are two kinds ofBr ions at the surface of surface active DTA-Br solution,+
one strongly contacting with the DTA positive head andhaving smaller hydration number (DTA-Bound Br ), andthe other weakly contacting to the cation head and having
larger hydration number similar to the bulk value of 6--(Free Br ). The green line pertains to the Free Br . The
bond length for Free Br is shorter than that for S-Br andits hydration number is larger. This result is in accordancewith a model in which Free Br is surrounded by watermolecules on the horizontal plane fixed by neighbour+
DTA head groups at side, thus the hydration number
detected by the TR-XAFS becomes large, but due to theloss of some of hydration water molecules, possibly on
the top position of Br anion, the bond length becomesshort.

1k3χ

Introduction
Many solution chemists are now making efforts toknow whether halide anions come to the top layer of theaqueous solution on the contrary to the classicalelectrostatic model which predicts that the ions arerepelled from the surface. If the halide anions prefer thesolution surface, their solvation structure at the surface
may not be the same as that in the bulk. This issue can bestudied by the XAFS experiment.
Total-reflection XAFS (TR-XAFS) spectroscopy
applied to the solution surface provides detailed
information about the solvation structure of thesubstances at the air/solution interface. A study on
hydration structures of Br ions at the air/dodecyltrimethylammonium bromide (DTA-Br) aqueous solution
interface has been reported [1]. In the present report, thedifference in the solvation structure of Br ions is
discussed by comparing the XAFS results from the TRXAFS and the normal transmission XAFS.

0-1

234-15k/Å
Fig.1 EXAFS χ spectra.

Br ionsrBr-O /nmin the NaBr bulk solution
on the NaBr solution
surfaceon the DTA-Br solution#surface

0.327

hydration
number6.00.323

5.60.318

7.2-#-Table 1. The results of EXAFS analysis. Br ions weakly+
bound to DTA cation at the surface.
Reference[1] P F Activity Report 2004 #22 Part B (2005)* iwaowata@center.osaka-wu.ac.jp

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