Variable-temperature scanning tunneling microscopy has been used to observe molecular dynamics in the vicinity of order-disorder transitions in alkane monolayers at the interface between graphite and organic fluids. In crystalline lamellae adsorbed from solution a reversible increase in longitudinal molecular motion with increasing temperature is observed, until the amplitude becomes comparable to the lamella width and the order disappears. A two-dimensional smectic and a columnar phase are observed at the interface with a neat melt and a solution of a particularly long-chain alkane, respectively.

• Received 21 February 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1395