Positron annihilation studies of some odd-n fatty acids

Positron lifetime measurements have been made as a function of temperature in solid samples of fatty acids having chain lengths of 15, 17, 19, 21, and 23 carbon atoms in order to complement a previous study of even-n fatty acids (1). The well-known solid-solid transitions occurring several degrees below the melting point were clearly observed in the positron data. Good agreement with published values (2) for the transition temperatures was obtained. Comparison of the lifetime data for odd-n acids with that obtained for even-n acids reveals a pronounced odd-even effect, especially in the dependence of the long lifetime intensity I3 on chain length. These variations, which are similar to the melting point alternation of even-n and odd-n fatty acids, are attributed to free-volume variations in this homologous series