Effects of emulsifiers on crystallization behavior of lipid crystals in nanometer-size oil-in-water emulsion droplets

We observed the crystallization and polymorphic transformation processes of palm stearin (PS) dispersed in nanometersize oil-in-water emulsion droplets (nanometer-size droplets, average diameter 120 +/- 30 nm) using DSC and in-situ synchrotron radiation X-ray diffraction (SR-XRD) techniques. The nanometer-size emulsion droplets were prepared by using four types of highly hydrophilic polyglycerine fatty-acid mono-esters, in which a polar moiety was made of 10 polymerized glycerins (decaglycerine, 10G) and nonpolar moieties were lauric, myristic, palmitic, and stearic fatty acids. When decaglycerin monolaurate (10GlL) was employed, crystallization temperatures (T-c's) of high-melting and low-melting fractions of PS were 8 and 2 degrees C, which were decreased from 31 and 3 degrees C for the bulk PS. However, the T-c of the high-melting fraction of PS was increased as the fatty-acid moiety was changed to myristic (10G1M), palmitic (10G1P), and stearic (10G1S) acids; in particular, the T-c became 38 degrees C with 10G1S. Furthermore, the crystallization of the high-melting fraction with 10G1M, 10G1P, and 10G1S exhibited only wide-angle SR-XRD patterns without small-angle SR-XRD patterns, indicating,that these crystals formed thin films in the nanometer-size emulsion droplets that did not diffract small-angle SR-XRD patterns. We did not observe these features in our previous work on PS crystallization in bulk and micrometer-size emulsion droplets (Sonoda, et al. J. Am. Oil Chem. Soc. 2004, 81, 365). It was assumed that the freezing of a high-melting emulsifier may act as a template for nucleation of a high-melting fraction of PS, whose effects are more remarkable in nanometer-size emulsion than in micrometer-size emulsion, due to the tight packing of the hydrophobic region of the interfacial emulsifier membrane of the nanometer-size emulsion.