Crystallization of wax esters in oleogels – Relevance of chain length and ester bond position

Abstract

A comprehensive data set is presented to elucidate the crystallization and structure formation behavior of pure wax esters (WEs) and their oleogels. X-ray data (WAXS/SAXS), thermal properties (DSC) and microstructure data (BFM and rheology) are discussed for 23 WEs with total carbon numbers (CN) between 24 and 48. Further, the effect of the ester bond position is emphasized. The results clearly show the systematics of WE crystallization. All WEs crystallize in an orthorhombic perpendicular subcell. Crystal lamellae consist of a single molecular layer; depending on the ester bond position, these are orthogonal (ΔCN = +2 and −4) or inclined (62.5°). With increasing CN, the heat of fusion (Δh^f) increases linearly, the melting point temperature (T^SL) asymptotically. For the same CN, non-symmetric WEs show reduced values of the caloric properties. For identical absolute values of ΔCN, i.e. + 2 and −2, the orthogonal arrangement yields higher Δh^f and T^SL. Comparing pure WEs and oleogels reveals essentially identical systematics, though WE crystallization in oleogels seems to diverge from ideal solubility. The microstructure shows little dependence on the orientation of the lamellae to the methyl end plane. In general, increasing CN results in a more clearly defined crystal habit with larger crystals and comparatively small gel rigidities (G*_max). For the same CN, increasing ΔCN results in less ordered structures with shorter edges and larger G*_max values. The data indicate that the caloric properties are more influenced by orthogonal or tilted arrangement. In contrast, the microstructure observations can be better explained by kinetic aspects during crystallization.