Crystallization of n-Hexadecane inside emulsion droplets studied by X-ray Scattering

Department of Advanced Materials Science, University of Tokyo* Hiroshima University** National Food Research Institute, Japan***
â—‹Yuya Shinohara* Tadashi Takamizawa* Satoru Ueno** Isao Kobayashi*** Mitsutoshi Nakajima*** Yoshiyuki Amemiya*

Crystallization inside emulsion droplets now attracts attention due to its possibility to control the polymorph inside the droplets. Recent study suggests that the interfacial heterogeneous nucleation at the oil-water interface have large effects on the process of crystallization in oil-in-water (O/W) emulsion droplets and that a transient rotator phase of n-Hexadecane (C16), so called rotator phase, play a vital role of crystallization of C16 inside the emulsion droplets. In the present study, we investigated the effect of the droplet size and the type of hydrophobic part of surfactant on the crystallization of C16 in O/W emulsions by using combined measurement of time-resolved two-dimensional (2D) small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS) and DSC. It was observed that the crystallization into the rotator phase and stable triclinic crystal were greatly influenced by the hydrophobic base of the surfactant and the size of droplet. Effects of the oil-water interface on the crystallization were also studied by microbeam 2D SAXS-WAXS. By scanning microbeam x-ray (5 um x 5 um) over a crystallized droplet, we investigated the orientation of crystals in a single emulsion droplet. The result shows that the C16 molecules align their axes parallel to the hydrophobic base of the surfactant, and that the difference of the hydrophobic part of surfactant leads to a change of crystal size in droplets. These results also strongly suggest that the surfactant at oil-water interface play a precursor role for the crystallization.