Small-angle-scattering study of 12-hydroxystearic physical organogels and lubricating greases

Abstract

We study the structure of the aggregates formed with two 12-hydroxystearic derivatives (acid HSA and Li salt HSLi), respectively, in xylene or hexadecane and decalin. SAXS and preliminary SANS experiments are used. The scattering curves are analyzed by comparison with simple geometrical models. Aggregates are described by infinitely long fibers of squared cross-section. For HSA fibers, the mean radius of gyration of the cross-section is 85 Å, corresponding to a square with 240 Å sides. A solvent and concentration dependence of the fibrillar structure is noticed. Diffraction peaks reveal the short-range order of the internal structure of the fibers or aggregates of fibers in the junction zones of the solid-like gel network. A 47 Å periodicity, approximately twice the hydroxystearic molecular length, supports a bilayered aggregation mechanism. The influence of a non-gelling stearic homologue, as found in industrial lubricating greases, is discussed.