Isotopological Supramolecular Networks from Melamine and Fatty Acids

The formation of bimolecular isotopological monolayers comprised of melamine and members of the homologous series of fatty acids extending from pentanoic through tridecanoic acid was studied at the liquid-graphite interface by scanning tunneling microscopy. In these systems, the fatty acids not only serve as solvents, i.e., provider and transport medium for solute molecules, but are also a structure determining part of the self-assembled monolayers. For all solvents, hexagonal structures were observed where the lattice parameter increases linearly with the chain length of the fatty acid from 2.8 nm for pentanoic acid to 4.1 nm for tridecanoic acid. All bimolecular networks exhibit the same topology: the basic unit consists of a melamine hexamer which is stabilized by internal N center dot center dot center dot H-N hydrogen bonds and surrounded by 12 radially arranged fatty acid molecules. The p6 symmetry of the monolayer originates from the melamine core, whereas the lattice parameter is determined by the aliphatic chain length of the fatty acid. For a deeper insight into the relative energetics, all experimentally observed structures were simulated by force-field calculations