Competitive adsorption, phase segregation, and molecular motion at a solid-liquid interface studied by scanning tunneling microscopy

Binary mixtures of 4'-octyl-4-biphenylcarbonitrile (8CB) and n-tetracontane (C40H82) deposited-as crystalline monolayers from a bulk fluid droplet on a graphite substrate were found to undergo nanometer-scale phase segregatio n into pure 8CB and pure alkane domains. Film morphology, molecular orderin g, and domain motion were studied using scanning tunneling microscopy. Alka ne adsorption was favored in the early stages of film growth, which was fol lowed by a period of gradual annealing in which 8CB displaced alkane molecu les from the surface. The structure and composition of the monolayer show t hat in films deposited from a bulk fluid with a 600:1 molar ratio of 8CB/-t etracontane 8CB adsorption is thermodynamically favored. However, n-tetraco ntane had a significantly larger nucleation rate, leading to high alkane su rface coverage shortly after film formation. Quantitative rate measurements of alkane domain replacement by 8CB have been made and correlated with the microscopic structure of the 8CB-alkane interface.