Nonequilibrium transitions in thermotropic phases of eicosenoic acid methyl esters

Methyl esters of cis-5-eicosenoic (5-EAME) and cis-11-eicosenoic (11-EAME) acids from the seed oil of Limnanthes alba (Meadowfoam) exhibit a degree of thermotropic polymorphism unobserved with shorter and longer chainlength monoenoic 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 higher-melting 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