Molecular orientation and growth mechanism of several fatty acids with different lengths

Abstract

Fatty acids such as palmitic, stearic, arachidic and behenic acid were evaporated on air-cleaved surfaces of KCl in vacuo. These molecules differ in their numbers of carbon atoms from C16 to C22. Evaporated molecules took a lateral and a normal orientation relative to the substrate surface. The surface occupancy of the molecules for each orientation depends both on the molecular length and on the substrate temperature ( T s). As T s increases, then also the number of regions of normal growth hillocks increases over those having a lateral orientation. When T s is constant, longer molecules occupy larger regions of the lateral growth hillocks than those with normal orientation. The curves showing the dependence of the occupancy ratio of the lateral growth hillocks to the normal ones as a function of T s between - 10 and + 45°C resemble each other in the fatty acids containing C16 up to C22 atoms in the molecule. A similar behaviour is observed in the case of the divalent metal salts of the fatty acids having different molecular lengths.