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

The polymorphism of 25 glycerol trioctadecenoates with double bonds ranging from Δ4-Δ17 was investigated by differential scanning calorimetry. Triglycerides withcis bonds in odd positions Δ7-Δ13 exhibited three intermediate melting (β′-) forms, but those withcis bonds in even positions, exceptcis Δ4, lacked β′-forms. Among thetrans compounds, only Δ11, 13, and 14 showed β′-forms. Thecis andtrans 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 ortrans) alternated depending upon double bond position; an even position correlated with high mp. Heats of fusion (ΔHf) for β-forms, likewise, fluctuated with double bond position but nonuniformly;trans Δ6 had the highest ΔHf (43 cal/g),cis Δ12 the lowest (21 cal/g).