Scanning tunneling microscopy studies of the self-assembly of carboxylic esters on graphite: Linear distortion and multiple adsorption structures

Self-assembled monolayers of carboxylic esters (stearic acid palmityl ester, lauric acid palmityl ester, and lauric acid behenyl ester) on graphite were investigated using scanning tunneling microscopy. All three esters, which are bent at the carboxylic group in the gas phase, are distorted into a straight-chain shape upon self-assembly on graphite. This results from optimizing the adsorption energy by matching the adsorbate molecular chain with the graphite substrate lattice periodicity. In all the formed lamellae, the long alkyl chain of the ester always aligns with the long chain of the adjacent molecule. Steric repulsion of the carbonyl group pointing perpendicularly to the neighboring molecule weakens the interaction of the molecular chain-trough angles of 73, 61, and 49degrees in addition to the 90degrees angle typical of n-alkane monolayers. This results from a shifting of 1/2, 1, or 3/2 units from the adjacent molecule in a lamella. The relatively weak interaction between ester molecules and substrate lattice also results in the formation of zigzag patterns with domain-domain angles of 145, 133, and 122degrees, respectively. The structures of esters adsorbed on HOPG indicate, contrary to what might be expected, that physisorbed molecular adsorbates do not necessarily have the same geometry as in the gas phase.