Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

4A/2004G227

Orientation of n-Hexadecane molecules in a single emulsion droplet revealed bymicrobeam two-dimensional SAXS-WAXS measurement
Yuya
SHINOHARA*1, Satoru UENO2, and Yoshiyuki AMEMIYA1
1
The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2
Hiroshim University, Higashi-Hiroshima, Hiroshima 739-8528, Japan
Introduction
Alkanes are among the most basic components of soft
materials; nevertheless the precise mechanism ofcrystallization has not been clarified. Metastable phase ofn-alkanes, rotator phase, is a key for the understanding of
the crystallization. Recently, the existence of the rotatorphase in C16 [1] in emulsion droplets was experimentallycertified [2], while the precise mechanism ofcrystallization through the rotator phase remains unsolved.
For the study of the structural inhomogeneity in a single
droplet, scanning microbeam X-ray scattering is apromising tool. In this study, we used scanningmicrobeam 2D-SAXS-WAXS in order to investigate
crystal orientations in a single droplet.
ExperimentalThe sample used was C16 droplets in O/W emulsions.
Samples of C16 with > 99 % purity were purchased from
Sigma Chemicals and no further purification steps weretaken. The O/W emulsion droplets were made bymicrochannel emulsification technique [3], by which we
obtained large droplets with nearly monodisperse size
distribution. We added 1 wt.% concentration of emulsifier
before emulsification.
Experiments were performed at BL-4A. The X-ray
wavelength was 1.54 Å and the beam size was around 5µm × 5 µm. A large aperture X-ray Image Intensifier (270mmφ) coupled X-ray CCD detector [4] was used as adetector. The sample-to-detector distance was around 160
mm. Various positions in a droplet were irradiated by themicrobeam X-ray and 2D-SAXS-WAXS image wassimultaneously recorded.
Results & Discussion
Figure 1 shows the result of scanning microbeam
SAXS-WAXS. A droplet with a diameter of around 50µm was scanned by the microbeam X-ray. The SAXSWAXS image from the center part of the droplet showedisotropic scattering pattern, while the SAXS-WAXS fromthe upper side oil-water interface and from the lateral sideoil-water interface showed anisotropic scattering pattern.
There is a decisive difference between the scatterings
from the upper and the lateral sides of interface. In theSAXS-WAXS image from the upper interface, SAXS isobserved in the vertical direction, and WAXS is observedin the horizontal direction. On the other hand, the SAXSWAXS image from the lateral side interface was rotatedby 90°. These results show that the C16 molecules aligntheir axes parallel to the hydrophobic base of the176

Users' Report

Fig. 1: Microbeam SAXS-WAXS images from threepositions in a droplet. The central image is obtained by
optical microscope. Black circle in scattering patterns
corresponds to the scattering from Myla films used as
the windows of the sample cell. Scattering inside thecircle corresponds to the SAXS, which is from lamellar
spacing, and scattering outside the circle corresponds tothe WAXS, which is from the packing of the molecules.
surfactant. This finding strongly suggests that thesurfactant at oil-water interface plays a precursor role forthe crystallization.
References[1] E. B. Sirota and A. B. Herhold, Science, 283, 529,
(1999).
[2] Y. Shinohara et al., Phys. Rev. Lett. 94, 097801,(2005).
[3]T. Kawakatsu et al., J. Am. Oil Chem. Soc. 74, 317(1997).
[4] Y. Amemiya et al., Rev. Sci. Instrum. 66, 2290, (1995)* shinohara@x-ray.k.u-tokyo.ac.jp