Atomic Images of Saturated and Unsaturated Fatty Acids at Liquid/Graphite Interface and Difference of Tunneling Currents between Them Observed by Scanning Tunneling Microscopy

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Structures of arachidic acid and elaidic acid molecules adsorbed onto a graphite substrate were investigated using a scanning tunneling microscope. The ordered arrangements of the molecules were observed with atomic resolution. The image of an arachidic acid molecule is composed of nineteen bright spots at the individual hydrocarbon groups in the alkyl chain and a dark region at the carboxyl group. The image of an elaidic acid molecule with an unsaturated hydrocarbon chain is similar to that of an arachidic acid molecule except for a much brighter spot at the double bond in the hydrocarbon chain. As a result, functional groups such as hydrocarbon chains, carboxyl groups and double bonds could be distinguished by the tunnel currents.