Polymorphism in single‐acid triglycerides of positional and geometric isomers of octadecenoic acid

Abstract

Abstract The polymorphism of 25 glycerol trioctadecenoates with double bonds ranging from Δ4‐Δ17 was investigated by differential scanning calorimetry. Triglycerides with cis bonds in odd positions Δ7‐Δ13 exhibited three intermediate melting (β′‐) forms, but those with cis bonds in even positions, except cis Δ4, lacked β′‐forms. Among the trans compounds, only Δ11, 13, and 14 showed β′‐forms. The cis and trans Δ5 triglycerides were unusual, because they readily assumed low melting (α‐) forms that were not easily converted to high melting (β‐) forms. β‐Form mp of compounds in each series ( cis or trans ) alternated depending upon double bond position; an even position correlated with high mp. Heats of fusion (ΔH f ) for β‐forms, likewise, fluctuated with double bond position but nonuniformly; trans Δ6 had the highest ΔH f (43 cal/g), cis Δ12 the lowest (21 cal/g).