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

10C/2005G171

Micro Structure Analysis of Liquid Crystal
Using X-ray Anomalous Dispersion Effect
Hiroshi ORIHARA*1, Yang-Ho NA1, Kohei AIDA1, Seiji UJIIE2,
Masaaki SUGIYAMA3, Yuji SOEJIMA4, Kazuhiro HARA5
1
Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
2
Faculty of Engineering, Oita University, Oita 870-1192, Japan
3
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
4
Research and Development Center for Higher Education, Kyushu University, Ropponmatsu,Fukuoka 810-8560, Japan
5
Institute of Environmental Systems, Kyushu University, Hakozaki, Fukuoka 812-8581, JapanIntroduction
It may be an interesting problem to determine theposition of molecules doped into substances, giving us theinformation on the interaction between them. For such apurpose, the X-ray anomalous dispersion technique could
be utilized. We can distinguish the dopant and hostmolecules by making an X-ray measurement around theabsorption edge of the dopant with changing the energyof the incident beam. In this report we have made anattempt to determine the position of Cu molecules doped
into a smectic liquid crystal with layered structure.
Experimental
We used poly(oxyethylene) cholesterol (POE)
exhibiting the smectic A phase and copper(II) 4heptylcyclohexylcarboxylate (CuHCC). Measurements
were made in a 2% CuHCC-doped POE. Since the layer
spacing is about 71 Å at room temperature, we used
BL10C for small-angle X-ray scattering (SAXS) in
Photon Factory of KEK. SAXS intensities were measured
as a function of the wavelength around the Cu-K
absorption edge from 1.3762 to 1.3855 Å. The energy ofthe incident X-ray was tuned with a double
monochromator of Si. The scattered X-ray was detectedwith a one-dimensional detector and the intensity wasaccumulated for 150 sec at each step.
Results and discussion
The smectic liquid crystal is a one-dimensional crystal
and so we can observe only the reflections with 00l. In
our experiment, 001 and 002 were observed. Figure 1
shows the wavelength dependence of the integrated
intensity of the 001 reflection. It is clearly seen that theintegrated intensity peaks at the absorption edge. This
result indicates that the dopant molecules are locatedbetween layers, as shown in the following consideration.
For a simple model that Cu atoms are dispersed in thesmectic liquid crystal without correlation, the scattering
intensity is given as2I k = ρ k 2 + 2 f ' ρ k nk + f (n0 + nk2 ) ,(1)
where ρκ and nκ are, respectively, the Fourier transformsof the electron density of the smectic liquid crystal and140

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the averaged number density of the Cu atoms, and ƒ is theatomic form factor of Cu atom. Note that in the smectic
liquid crystal we can take ρκ and nκ as real numbers.
For a small amount of dopant, the third term of r.h.s. ineq. (1) can be omitted. The first term is the contribution
from the host smectic liquid crystal and the second term,
more importantly, expresses the spatial correlation
between the host and the dopant. Around the absorption
edge, the real part, ƒ′, decreases as shown in Fig. 1,
whereas the intensity increases at this point. From theseresults and the second term of eq. (1), ρκnκ(κ is for the001 reflection) should be negative. This means the phasedifference between the fundamental density waves of thehost and the dopant is 180 degrees and therefore thedopant molecules are located between the smectic layers.
We have demonstrated that the X-ray anomalous
dispersion technique is quite useful to determine thepositions of dopant molecules in soft matters.

Fig. 1. Wavelength dependence of the integrated intensity
for the 001 reflection (circles) and the real part ƒ′ (a solidline).
*orihara@eng.hokudai.ac.jp