Complexity in the self-assembly of bifunctional molecules on HOPG: The influence of solvent functionality on self-assembled structures

Self-assembled monolayers of bifunctional molecules HOOC(CH2)(n)COOH (n = 20, 18, 16, 14, 12, 10), HOOC(CH2)(n)CH2OH (n = 13, 14), and HOCH2(CH2)(14)CH2OH dissolved in octanoic acid were investigated using scanning tunneling microscopy, to understand the self-assembly of bifunctional molecules and the influence of a carboxylic acid solvent on the formation of self-assembled structures on HOPG. In the series of di-acids (HOOC(CH2)(n)COOH), only HOOC(CH2)(20)COOH forms stable coadsorption structures with the solvent octanoic acid. The remaining di-acids form stable single-component monolayers and do not coadsorb with solvent octanoic acid. Coadsorption structures involving mixtures of di-acids were observed. This result suggests that coadsorption with acid solvent or with other di-acids occurs to maximize hydrogen-bond density in the overlayer. A quantitative model based on this concept is proposed. For hetero-bifunctional molecules HOOC(CH2)(n)CH2OH (n = 13, 14), the coadsorption of HOOC(CH2)(14)CH2OH and octanoic acid at the molecular level produces a microscopic mesh made of homogeneously arranged openings with a dimension of similar to 12.5 A x similar to 5.0 A x similar to 1.8 A. For the hetero-bifunctional molecule HOOC(CH2)(13)CH2OH, hydroxyl groups of two adjacent lamellae assemble to form a herringbone geometry, and the two carboxylic acid groups assemble with a straight head-to-head configuration. In addition, a new mixed hydrogen-bonding network of COOH center dot center dot center dot O-H was observed in another self-assembled structure of this molecule. The bifunctional molecule HOCH2(CH2)(14)CH2OH exhibits multiple packing patterns on HOPG via different hydrogen-bonding networks. HOCH2(CH2)(14)CH2OH self-assembles using the H-O center dot center dot center dot O-H network typical of the n-alcohol herringbone structure, forming an asymmetric adsorbate on HOPG. It also forms domains with another hydrogen-bonding network, in which molecules in adjacent lamellae are parallel to each other. This investigation demonstrates the complexity and diversity of self-assembled structures formed from bifunctional molecules on solid surfaces. It also indicates that a solvent with the same functional group as the solute can significantly impact the formation of the self-assembled structures of these bifunctional molecules.