Heterogeneous microstructures of spherulites of lipid mixtures characterized with synchrotron radiation microbeam X-ray diffraction

Abstract

We observed microstructures of spherulites made of two triacylglycerols (TAGs) (POP (1,3-dipalmitoyl-2-oleoyl-sn-glycerol) and OPO (1,3-dioleoyl-2-palmitoyl-sn-glycerol)), which exhibit the mixing behavior of a molecular compound (MC) formation at a ratio of POP : OPO = 50 : 50, using synchrotron radiation microbeam (beam area of 5 × 5 μm²) X-ray diffraction (SR-μ-XRD). The spherulites were grown from neat liquid, and from solution containing 50% n-dodecane and 50% POP + OPO. SR-μ-XRD analysis revealed heterogeneous distributions of MC_POP:OPO and component TAGs in every spherulite; TAG compositions in the inner and outer areas differed when the relative ratios of POP and OPO were changed. In the 75POP : 25OPO spherulites, MC_POP:OPO always occupied the inner areas and POP dominated the outer areas as a result of different rates of crystallization of MC_POP:OPO and POP in the spherulite. In contrast, in the 25POP : 75OPO spherulites, the entire area was homogeneously occupied either with almost all OPO or with coexisting MC_POP:OPO and OPO. The microstructures of the spherulites grown from n-dodecane solutions demonstrated basically the same characteristics as those grown from the neat liquid both for 75POP : 25OPO and 25POP : 75OPO. SR-μ-XRD also clarified that the lamellar planes of the crystals are directed parallel to the radial direction from the central to the outer regions of the spherulites.