A search for the prewetting line

Abstract

This paper describes efforts to locate the prewetting line in a binary liquid system (isopropanol–perfluoromethylcyclohexane) at the vapor–liquid interface. We placed tight upper bounds on the temperature separation (0.2 K) between the prewetting line and the line of bulk liquid phase separation. We did not detect the prewetting line in systems at equilibrium. Experimental signatures indicative of the prewetting line occurred only in nonequilibrium situations. Several theories predict that the adsorption of one of the components (the fluorocarbon, in this case) at the liquid–vapor interface should increase abruptly, at a temperature sightly above the temperature at which the mixture separates into two liquid phases. A regular solution calculation indicates that this prewetting line should have been easily detectable with the instruments used in this experiment. Significant features of the experiment are: (1) low-gradient thermostatting, (2) in situ stirring, (3) precision ellipsometry from the vapor–liquid interface, (4) high resolution differential index of refraction measurements using a novel cell design, and (5) computer control.