Nonequilibrium transitions in thermotropic phases of eicosenoic acid methyl esters
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Methyl esters ofcis-5-eicosenoic (5-EAME) andcis-11-eicosenoic (11-EAME) acids from the seed oil ofLimnanthes alba (Meadowfoam) exhibit a degree of thermotropic polymorphism unobserved with shorter and longer chainlength monoenic methyl esters. 5-EAME freezes at 264 K and melts at 266 K if cooled no lower than 215 K. 11-EAME freezes at 239 K and melts at 255 K if cooled at no lower than 240 K. Solids cooled to lower temperatures undergo phase transformation to highermelting polymorphs (274 K, 5-EAME; 262 K, 11-EAME), and samples often exhibit double melting endotherms. Quantities of each high-melting phase vary with time at temperatures below characteristic initiation temperatures. Highly temperature-sensitive phase conversions suggest low temperature nucleation, followed by crystal growth and conversion, as reheating allows molecular motion. Once formed, both high-melting phases melt with essentially the same melting entropy. Thermodynamic and kinetic analyses imply that differences exhibited by the isomeric esters derive from aliphatic configuration distal to the double bond.
Key WordsDifferential scanning calorimetry fatty ester polymorphism Limnanthes Meadowfoam
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