Solvent Effects on the Self-Assembly of 1-Bromoeicosane on Graphite. Part I. Scanning Tunneling Microscopy

Self-assembled monolayers of 1-bromoeicosane (BrC20H41) have been investigated at the vacuum-graphite and liquid-graphite interfaces using scanning tunneling microscopy (STM) (Part I) and theory (Part II). Under ultrahigh vacuum conditions at 80 K, STM images show 1-bromoeicosane in a lamellar assembly structure where individual molecules are predominantly arranged with their bromine groups pointed head-to-head with a 66 +/- 3 degrees angle between the lamella direction and the molecular backbone axis. A significant degree of disorder is observed under vacuum conditions, in which head-to-tail defects are interspersed throughout the film. When 1-bromoeicosane monolayers are formed and imaged in equilibrium with solution at similar to 290 K, the molecules again pack with their bromines oriented head-to-head but shift to form a nearly rectangular array. Subtle changes are observed depending on the solvent utilized, with the lamella-backbone angle varying between 81 +/- 3 degrees and 90 +/- 2 degrees, and the corresponding intermolecular spacing ranging from 0.36 +/- 0.02 to 0.46 +/- 0.05 nm. The differences between the vacuum- and solution-based packing, as well as the possibility of two different packing structures occurring in solution, are discussed in light of the joint experimental and theoretical analysis.