Molecular self-assembly of substituted terephthalic acids at the liquid/solid interface: investigating the effect of solvent.

Self-assembly of three related molecules - terephthalic acid and its hydroxylated analogues - at liquid/solid interfaces (graphite/heptanoic acid and graphite/1-phenyloctane) has been studied using a combination of scanning tunnelling microscopy and molecular mechanics and molecular dynamics calculations. Brickwork-like patterns typical for terephthalic acid self-assembly have been observed for all three molecules. However, several differences became apparent: (i) formation or lack of adsorbed monolayers (self-assembled monolayers formed in all systems, with one notable exception of terephthalic acid at the graphite/1-phenyloctane interface where no adsorption was observed), (ii) the size of adsorbate islands (large islands at the interface with heptanoic acid and smaller ones at the interface with 1-phenyloctane), and (iii) polymorphism of the hydroxylated terephthalic acids' monolayers, dependent on the molecular structure and/or solvent. To rationalise this behaviour, molecular mechanics and molecular dynamics calculations have been performed, to analyse the three key aspects of the energetics of self-assembly: intermolecular, substrate-adsorbate and solvent-solute interactions. These energetic characteristics of self-assembly were brought together in a Born-Haber cycle, to obtain the overall energy effects of formation of self-assembled monolayers at these liquid/solid interfaces.