Polymorphism in Single-Acid Triglycerides of Positional and Geometric Isomers of Octadecenoic Acid

The polymorphism of 25 glycerol trioctadecenoates with double bonds ranging from ~4-~ I 7 was investigated by differential scanning calorimetry. Triglycerides with cis bonds in odd positions ~7-~13 exhibited three intermediate melting (13'-) forms, but those with cis bonds in even positions, except cis ~4, lacked 13'-forms. Among the trans compounds, only ~ 11, 13, and 14 showed 13'-forms. The cis and trans ~5 triglycerides were unusual, because they readily assumed low melting (0:-) forms that were not easily converted to high melting (/3-) forms. I3-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 (~Hf) for l3-forms, likewise, fluctuated with double bond position but nonuniformlY; trails ~6 had the highest ~Hf (43 cal/g), cis ~12 the lowest (21 cal/g).