Ultra small angle x-ray scattering in complex mixtures of triacylglycerols

摘要

Ultra-small angle x-ray scattering (USAXS) has been used to elucidate, in situ, the aggregation structure of unsheared model edible oils. Each system comprised one or two solid lipids and a combination of liquid lipids. The 3D nano- to micro-structures of each system were characterized. The length scale investigated, using the Bonse-Hart camera at beamline ID-15D at the Advanced Photon Source, ANL, ranged from 300 angstrom-10 mu m. Using the Unified Fit model, level-1 analysis showed that the scatterers were 2D objects with either a smooth, a rough, or a diffuse surface. These 2D objects had an average radius of gyration R-g1 between 200-1500 angstrom. Level-2 analysis displayed a slope between -1 and -2. Use of the Guinier-Porod model gave s approximate to 1 thus showing that it was cylinders (TAGwoods) aggregating with fractal dimension 1 <= D-2 <= 2. D-2 = 1 is consistent with 1D structures formed from TAGwoods, while D-2 = 2 implies that the TAGwoods had formed structures characteristic of diffusion or reaction limited cluster-cluster aggregation (DLCA/RLCA). These aggregates exhibited radii of gyration, R-g2, between 2500 and 6500 angstrom. Level-3 analyses showed diffuse surfaces, for most of the systems. These interpretations are in accord with theoretical models which studied crystalline nano-platelets (CNPs) coated with nano-scale layers arising from phase separation at the CNP surfaces. These layers could be due to either liquid-liquid phase separation with the CNPs coated, uniformly or non-uniformly, by a diffuse layer of TAGs, or solid-liquid phase separation with the CNPs coated by a rough layer of crystallites.