Chain-length dependence of melting of n-alcohol monolayers adsorbed on graphite:: n-hexanol, n-heptanol, n-octanol, and n-nonanol

The structures and melting of n-hexanol, n-heptanol, n-octanol, and n-nonanol monolayers adsorbed on graphite have been studied by x-ray diffraction and compared with our previous results on n-alcohols with shorter chain length. All the monolayers take similar herringbone or related structures consisting of a pair of rows of the molecules in a head-to-head arrangement with OH groups close to each other. These monolayers, except for methanol and n-octanol, show an odd-even effect of carbon number on the melting behavior. The complete monolayers of n-alcohols with odd number of carbon atoms melt via two-dimensional (2D) smectic mesophases, while those of the even-numbered series melt directly into liquids. The odd-even effect on melting is associated with the difference in packing of the terminal CH3. Calculations of a diffraction pattern based on the Debye formula strongly suggest that the 2D smectic mesophase consists of random stacking of a pair of the molecular rows formed by hydrogen bonding and thus is a 2D smectic glass rather than a 2D smectic liquid crystal. The melting mechanism changes for n-octanol and n-nonanol on graphite. (C) 1999 American Institute of Physics. [S0021-9606(99)51339-1].