Microscopic origin of the odd-even effect in monolayer of fatty acids formed on a graphite surface by scanning tunneling microscopy

Monolayers of saturated fatty acids with various chain lengths were adsorbed onto a graphite substrate, then studied using the scanning tunneling microscope (STM). The STM images indicate that in the direction parallel to the molecular axis the molecules form centrosymmetrical dimers via hydrogen bonds between the carboxyl groups. From the symmetry of the STM images, it was found that a unit cell of fatty acids with an even number of carbons is different from that of the odd-numbered series. These phenomena provide evidence for a basic mechanism of the odd-even effects in the arrangement of alkyl chains.