STM observations of exchange dynamics at the solid-liquid interface using a molecular tracer

A molecular tracer experiment has been performed to observe exchange of mol ecules between phases at the solid-liquid interface. Mixtures of long-chain molecules were adsorbed on a graphite surface from solution. The thioether hexadecyl sulfide (CH3(CH2)(15))(2)S was coadsorbed with sterically simila r molecules hexadecyl ether (CH3(CH2)(15))(2)O or tritriacontane C33H68 The se molecules cocrystallize but are readily distinguishable in STM images du e to strong contrast between sulfur atoms and methylene or oxygen groups. T hioether molecules are seen to continually enter and leave the monolayer ev en in well-ordered crystalline regions. Molecules reside on the surface for periods of several seconds'. Despite continual replacement of molecules th e images remain well-defined, indicating that each exchange is completed wi thin a few milliseconds.