Crystallization and Polymorphism of 1,3‐Acyl‐Palmitoyl‐rac‐Glycerols

Abstract Crystallization and melting behavior, small‐angle X‐ray scattering, X‐ray powder diffraction and infra‐red absorbance were measured for nine 1,3‐acyl‐palmitoyl‐ rac ‐glycerols (1,3‐acetoyl‐, ‐butyroyl‐, ‐hexanoyl‐, ‐octanoyl‐, ‐decanoyl‐, ‐lauroyl, ‐myristoyl‐ and ‐oleoyl‐palmitoyl‐ rac ‐glycerol and 1,3‐dipalmitoyl‐glycerol). All but one of the prepared 1,3‐diacylglycerols (1,3‐DAG) were β‐stable with 1,3‐acetoyl‐palmitoyl‐ rac ‐glycerol being the exception (β′‐stable). Small‐angle X‐ray scattering indicates that molecules in β‐tending diacid 1,3‐DAG adopt a herringbone‐type configuration similar to monoacid 1,3‐DAG. In this configuration acyl chains of the same length associate and regular chain‐end matching between terminal methyl groups delineate lamellae. In contrast, molecules in crystalline 1,3‐acetoyl‐palmitoyl‐ rac ‐glycerol are oriented similar to those of 1(3)‐monoacylglycerol. Interestingly, DSC curves indicate five of the nine diacid compounds have meta‐stable forms—suggesting these forms are quite common for diacid 1,3‐DAG. Meta‐stable forms are observed in the melting curve when the difference in length between acyl chains is large (1,3‐acetoyl‐, ‐butyroyl‐ and ‐hexanoyl‐palmitoyl‐ rac ‐glycerol), and in the crystallization curve when the difference is moderate (1,3‐decanoyl‐ and ‐lauroyl‐palmitoyl‐ rac ‐glycerol).