Wetting of hydrocarbon liquid surfaces by fluorocarbon vapor: A microscopic study

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Original entry: William Bonificio, AP 225, Fall 2009


O. Gang, M. Fukuto, P. Huber and P. Pershan, Wetting of hydrocarbon liquid surfaces by fluorocarbon vapor: A microscopic study, Colloids & Surfaces A 206, 293-297 (2002)

Soft matter keywords

Wetting; Surface; Fluorocarbon; Hydrocarbon; Alkane; X-ray reflectivity; Surface freezing; thin film;


The wetting behavior of a liquid fluorocarbon on a liquid hydrocarbon was examined. At temperatures just above the freezing point of a liquid hydrocarbon there is an effect that occurs called surface freezing, where the surface of the hydrocarbon forms a periodic structure. The researchers discovered that when the hydrocarbon is coated even by the thinnest of fluorocarbon films, this surface freezing effect dissappears. Furthermore, the team researched the wetting effect of this system and discovered that, as expected, complete wetting occurred according to the law d~<math>\Delta T^{-1/3} </math>.

Soft matter discussion

Experimental chamber to control the wetting of two liquids. Each liquid is consecutively put in the reservoir below the substrate. The thickness of the liquid wetting the substrate is controlled by the temperature difference between the substrate and the reservoir. A more detailed explanation of this experimental set up can be obtained from the actual journal article.

Hydrocarbons with long carbon chains are known to have a surface freezing, or SF, effect at temperatures just above the bulk freezing point. The SF occurs when the chains are oriented normal to the surface, resulting in a hexagonal close packed structure with <math>a_{0}</math>~1000<math>\AA</math>.

The researchers aimed to first wet a substrate with the hydrocarbon, and then wet that hydrocarbon with a fluorocarbon. The experimental set up used a chamber that houses the substrate to be wetted, and a reservoir of a wetting liquid. Then by manipulating the activity of the liquid (by way of the vapor pressure using changes in temperature) the substrate is slowly wet by a thin film.

The surface and structure of both films were determined by x-ray reflectivity. The structure of the hydrocarbon alone, with and without SF, was used as the control. Then, small amounts of the fluorocarbon were added. At a thickness of ~7<math>\AA</math> of the fluorocarbon the SF phase of the hydrcarbon o longer existed. The suppression of the SF phase by such a thin layer of fluorocarbon shows the importance of interfacial energy to the SF phase. Even the slightest disturbance to the interface destroyed it.

The researchers continued with the wetting to characterize the wetting properties of the systems. Not surprisingly, the wetting followed the 1/3 power law for classical wetting. This shows that gravity is negligible, and that van der Waals forces dominates at the thicknesses studied. (0 - 500 <math>\AA</math>)