Packing of Br(CH₂)₁₀COOH and Br(CH₂)₁₁COOH on Graphite:  An Odd−Even Length Effect Observed by Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) has been used to study 11-bromoundecanoic acid, Br(CH 2 ) 10 COOH, and 12-bromododecanoic acid, Br(CH 2 ) 11 COOH, adsorbed on the basal plane of highly oriented pyrolytic graphite (HOPG) from phenyloctane solution. The STM tip is immersed in a drop of solution and images molecules at the liquid−solid interface. Atomically resolved images provide a clear picture of the conformation of these two molecules on the graphite surface. Although differing in structure by only a single methylene group, the 11-bromo and 12-bromo species exhibit very different packing structures at the liquid (phenyloctane solution)−solid (graphite) interface. A simple geometric effect is believed to be responsible for the observed packing differences.