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

4A/2004G227

Orientation of n-Hexadecane molecules in a single emulsion droplet studied bymicrobeam two-dimensional SAXS measurement
Tadashi TAKAMIZAWA1, Yuya SHINOHARA*1, Satoru UENO2, and Yoshiyuki AMEMIYA1
1
The University, of Tokyo, Kashiwa, Chiba 277-8561, Japan
2
Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, Japan
Introduction
Recent study on the crystallization of the n-Hexadecane(C16) inside oil-in-water (O/W) emulsion droplets using
small-angle x-ray scattering (SAXS) and wide-angle xray scattering (WAXS) showed that the crystallization
behavior inside the emulsion droplets by the surfactant aswell as that at the oil-water interface [1,2]. Especially, theappearance of the transient phase [3] has a relationshipbetween the forms of hydrophobic base of the surfactant.
These results suggest that the crystallization inside thedroplets originates from the interfacial heterogeneous
nucleation at the oil-water interface [4]. However,detailed mechanism of crystallization inside a droplet is
not clarified, because the SAXS measurement so far wasperformed over the ensemble of many droplets. In thepresent study, we used a microbeam x-ray and measured
SAXS at various positions in a single emulsion droplet inorder to study the local structure of crystals in a droplet.
Experimental
The 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. Surfactants were Tween 40 (polyoxyethelene
sorbitan monolpalmitate: C62H122O26), Tween 80(polyoxyethelene sorbitan monooleate: C64H124O26). TheO/W emulsion droplets were made by microchannel
emulsification technique [5], by which we obtained large
droplets with nearly monodisperse size distribution. The
average diameter of sample was 32.6 µm. Experiments
were performed at BL-4A. The X-ray wavelength was
1.54 Å and the beam size was around 5 µm * 5 µm. A
position of the x-ray beam on sample was monitored by
an optical microscope. An X-ray CCD detector coupledwith an X-ray Image Intensifier [6] was used as the SAXS
detector. The sample-to-detector distance was around 650
mm in SAXS.

direction is narrower in Tween 80. Furthermore, thescatterings are observed at all the positions of the droplet
with Tween 80. These results suggest that the size ofcrystals in a droplet with Tween 40 is larger than that withTween 80. This difference due to the surfactant would bea key for the understanding of crystallization mechanism
in a single emulsion droplet.

Fig. 1: Optical microscope image of emulsion with Tween
40 and the long axis orientations (represented by whitearrow) of C16. The area surrounded by red frame
represents the region scanned by a microbeam X-ray. No
arrow is drawn in regions where no scattering wasobserved. The diameter of the droplet is around 50 µm
References[1] Y. Shinohara et al., Phys. Rev. Lett., 94, 097801(2005).
[2] Y. Shinohara et al., PF Act. Rep. 2004 Part A, 9 (2005).
[3] E. Sirota and A. Herhold, Science, 283, 529 (1999).
[4] S. Ueno et al., Cryst Growth Des. 3, 6 (2003).
[5] T. Kawakatsu et al., J. Am. Oil Chem. Soc. 74,317(1997).
[6] Y. Amemiya et al., Rev. Sci. Instrum. 66, 2290, (1995).
*shinohara@x-ray.k.u-tokyo.ac.jp

Results & Discussion
Figure 1 shows the optical microscope image of acrystallized emulsion with Tween 40 and the long axis
orientations of C16 obtained by SAXS. Two features areobserved; one is that the observed orientation of the longaxis is perpendicular to the oil-water interface, and theother is that scattered x-rays are not observed at someparts of emulsion. The former feature is almost the samewith the case of the droplets with Tween 80, but the sizeof region where the long axes (arrows) show the same114Users' Report