Self-assembly of trimesic acid at the liquid-solid interface - a study of solvent-induced polymorphism

A scanning tunneling microscope operated under ambient conditions was utilized to study the self-assembly of trimesic acid (TMA) at the liquid-solid interface. On a graphite substrate, two different open, loosely packed, two-dimensional hydrogen-bond networks were found. Both structures exhibit a periodic arrangement of similar to 1.0 nm wide cavities, which can be used for the co-adsorption of another species (guest) within the cells of this host system. These two polymorphs (chickenwire and flower structures) differ in their molecular packing density and hydrogen-bonding schemes. Using a homologous series of alkanoic acids as solvents, ranging from butyric to nonanoic, selective self-assembly of either the flower or chickenwire forms was achieved on a graphite surface. Solubility of TMA in these acid solvents was found to decrease with increasing chain length, and the longer-chain solvents favored formation of the chickenwire polymorph structure on the surface.