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

15A/2004G075

Observation of transient rotator phase of n-Hexadecane in emulsion droplet by
two-dimensional SAXS-WAXS-DSC
Yuya SHINOHARA*1, Satoru UENO2,1 Kentaro KAYASHIMA1, Yasuo
OGAWA1, Tadashi
1
TAKAMIZAWA , and Yoshiyuki AMEMIYA
1
TheUniversityof 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. For n-Hexadecane (C16), although
Sirota and Herhold first observed the transient-rotatorphase in bulk samples [1], it was detected in only 9 out of
85 measurements. Recently, the existence of the rotatorphase in C16 was experimentally certified [2], while theprecise mechanism of crystallization through the rotatorphase remains unsolved. In the present study, we have
performed 2D-SAXS-WAXS-DSC in order to investigate
the structural change at the crystallization.
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. 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. The average diameter of sample
was 30 µm.
Experiments were performed at BL-15A. The X-raywavelength was 1.50 Å. A large aperture X-ray Image
Intensifier (270 mmφ) coupled X-ray CCD detector [4]was used as a detector. The sample-to-detector distance
was around 160 mm and both SAXS and WAXS weresimultaneously measured. We performed time-resolved
SAXS-WAXS measurements together with Differential
Scanning Calorimetry (DSC). Samples were heated to ~10 °C above the melting temperature and the cooled to -8°C with a cooling rate of 2 °C/min. The exposure time foreach image was 1.5 s and an interval between theexposures was 3 s.
Results & Discussion
Figure 1 shows the relationships between the thermal
data and SAXS-WAXS intensity from the C16 emulsion
during crystallization [2]. These data are obtained byazimuthally averaging SAXS-WAXS intensity. It isshown that both SAXS and WAXS from the rotator phaseappear at 14.0 °C and disappears at 10.8 °C. The second
weak peak of DSC at 10.8 °C corresponds to the sudden
increase of scattering intensity from the triclinic phaseand the disappearance of scattering from the rotator phase.

Fig. 1: (upper) DSC and time evolution of scattering
intensity profiles from the rotator phase (SAXS 1st),
triclinic structure, and halo during cooling of C16
emulsion droplets. (Lower) An expanded view of themarked portion of the upper figure. The scattering
intensity profiles from the rotator phase are shown [2].
These results suggest that at this cooling rate, thetransition from the rotator phase to the triclinic phaseoccurs at this temperature and that at the same time therapid crystallization from the liquid oil into the triclinic
phase begins. This situation is very similar to the wellknown surface freezing in alkanes and expected as a key
for further understanding of the 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

Users' Report175