MOLECULAR ARRANGEMENTS OF FATTY-ACIDS AND CHOLESTEROL AT LIQUID GRAPHITE INTERFACE OBSERVED BY SCANNING-TUNNELING-MICROSCOPY

We have directly imaged organic molecules such as fatty acids and cholesterol adsorbed onto a graphite substrate using a scanning tunneling microscope (STM). The arrangements of pure fatty acids such as myristic, palmitic, stearic, arachidic, behenic and elaidic acids are essentially similar except that STM images change systematically with the length of the alkyl chain. The STM images attained atomic resolution and as a result we observed the individual hydrocarbon groups in the alkyl chains. In the direction parallel and perpendicular to the molecular axis, the molecules on the graphite substrate form dimers via hydrogen bonding between carboxyl groups and a superstructure with a period of four or five molecules, respectively. The image of a monolayer of a binary mixture of fatty acids adsorbed onto the substrate reflects two different lengths of alkyl chains. A monolayer of cholesterol molecules was imaged for the first time.