Ellipsometric Study of the Wetting of Air/Water Interfaces with Hexane, Heptane, and Octane from Saturated Alkane Vapors

Ellipsometric investigations of alkane adsorption from saturated alkane vapor show that hexane and heptane wet the air/water interface with a closed ultrathin film of molecular thickness that is topped with droplets of micrometer dimensions. Octane adsorbs only in a submonolayer film. Brewster angle microscopy and the analysis of the time evolution of the ellipsometric signal of the hexane and heptane adsorption reveal phase transitions; i.e., the alkane films are below their critical temperatures. In the course of the adsorption first the formation of a monolayer with low (gaseous) molecular density is observed. Then a first-order phase transition to a condensed molecular packing occurs. After completion of this condensed monolayer, another transition is observed, either from the molecules lying flat to orienting themselves upright or from a mono- to a bilayer. Depending on the molecular orientation, the molecular packing density in the condensed films is either approximately the same as the liquid bulk density (if the molecules are oriented upright) or it is about 120% of the liquid bulk density (if the molecules lie flat). Ultimately the mono- and/or bilayer is topped with droplets of several micrometers diameter.