Differential scanning calorimetry of single acid triglycerides: Effect of chain length and unsaturation

Abstract

The polymorphism of 13 single acid triglycerides with acyl group chain lengths ranging from 16–22 was studied by differential scanning calorimetry. In contrast to the singleβ′-form generally attributed to such triglycerides, at least two intermediate endotherms were found for most samples between the least stable (α) and most stable (β) polymorphs. For saturated triglycerides, two versions of the familiar “tuning fork” model meet theβ′-form requirement of alternate fatty acid chains in planes perpendicular to each other. The detection of three intermediate endotherms for triolein, tri-cis-1 1-octadecenoin and trierucin (and possibly also trilinolein) may be rationalized by assuming that the segments of polymethylene chains on either side of double bonds may zigzag in different planes. Four exceptions for which no evidence was found forβ′-forms are tri-cis-6-octadecenoin, tri-cis-6-hexadecenoin, tri-trans-6-octadecenoin and trielaidin. Three of these exceptions contain Δ6-acyl groups and have in common segments with even numbers of methylene groups on either side of the double bonds. These same three triglycerides also have a shorter than usual polymethylene segment between the ester linkages and the double bonds and a longer than usual distance from the double bonds to the terminal methyl groups. Tri-trans-6-octadecenoin and trielaidin are exceptional in still another way. Only they and are exceptional in still another way. Only they and trierucin exhibited significant nonconformity with an empirical relationship between melting points and heats of fusion ofβ-forms. Otherwise, all points in a plot of the former physical constants vs. the latter closely fit a smooth curve, the positive slope of which gets larger as the X-axis values, i.e., melting points, increase.