SAXS-WAXS study of heterogeneous nucleation of n-alkane crystals in O/W emulsion system

Graduate School of Biosphere Sciences, Hiroshima University, Japan* Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Japan**
○S. Ueno* Y .Shinohara** Y. Amemiya** K. Sato*

Crystallization in encapsulated system, such as vesicle, liposome, cubosome, emulsion etc., has been studied intensively because it is expected that new polymorphs of various soft materials is crystallized in these systems. In this presentation, interfacial heterogeneous nucleation in oil-in-water (O/W) emulsion will be reported by using simultaneous small-angle and wide-angle X-ray diffraction (SAXS-WAXS) measurement. Crystallization of oil phase in O/W emulsion is controlled by a lot of factors such as temperature, oil droplet size, type of emulsifier, ph etc. n-hexadecane-water emulsion system was prepared by high-pressure homogenization techniques. The average droplet size was ~1 μm. Tween 20 was put as an emulsifier. When the high-melting hydrophobic emulsifiers was added to n-hexadecane, the model of oil phase in O/W emulsion, the hexagonal and orthorhombic type polymorphs appeared as new polymorphic forms in O/W emulsion together with triclinic type polymorph which was observed only in bulk system. These new polymorphs would be crystallized on the oil-water interface of oil droplets after the added high-melting hydrophobic emulsifiers crystallized. In other words, the high melting emulsifier plays a role of the template for heterogeneous nucleation and following crystal growth of the new polymorphs. These new polymorphs were observed more than 0.1 wt% concentration of the high-melting emulsifiers. On the other hand, no new polymorph was observed without high-melting hydrophobic emulsifiers added in the oil phase of O/W emulsion. However, when we prepared the larger droplet size, such as 30 or 40 μm, of n-hexadecane droplet in O/W emulsion system, the transient rotator phase, meaning hexagonal phase, was observed without any high-melting emulsifiers in oil droplet of O/W emulsion. This fact suggests that not only the type and concentration of emulsifiers but also the curvature of oil droplet would strongly affect the heterogeneous nucleation and following crystal growth of oil phase in the O/W emulsion system.