Oleophobic monolayers

It was found that certain types of polar organic molecules are adsorbed from solutions in non-polar solvents to form well-oriented monolayers on polished solid surfaces. Such monolayers imparted both hydrophobic and oleophobic properties to the polished surfaces of a variety of metallic and non-metallic solids and could be formed from a large variety of solvents, those used ranging from hydrocarbons like hexadecane, mineral oils, benzene, methylnaphthalene and dicyclohexyl to other solvents such as carbon disulfide, carbon tetrachloride, bromobenzene and diphenyloxide. It is shown that the mechanism of the formation of these films on platinum was reversible adsorption from solution and that it definitely was not the accumulation of insoluble films floating at the liquid-air interface as in the Langmuir-Blodgett method. It is concluded that the adsorbed films were made up of almost vertically oriented molecules which were nearly close-packed and attached to the surface through a surface active or polar group. Attention is called to the possibility that such films do not conform to the shape of the solid surface but might bridge over those surface depressions whose areas are not too great compared to the cross-sectional area of the molecules. In order that a compound be able to adsorb as an oleophobic monolayer it appears necessary that its molecular structure be in keeping with the following requirements. First, the molecules of the compound must be capable of approximating close-packed orientation in a monolayer. Second, the surface active or polar group must be located at one extremity of the molecule and one or more methyl groups must be located at the opposite extremity. Finally, the molecules must adsorb to a fiat solid surface with sufficient close-packing so that the outermost portion of the film is essentially a plane surface, densely populated with methyl groups. It was found that compounds whose molecular configurations resemble a long rod or a flat plate, with a polar group attached to one end of the rod or rim of the plate and one or more methyl groups at the opposite end of the rod or rim of the plate, readily satisfy these requirements. Exceptions were found in the case of aliphatic, unbranched polar molecules containing one or more unsaturated bonds, for these did not form oleophobic films. It is suggested that such molecules adsorb on solids at both the polar end group and the unsaturated bonds so that, instead of orienting with their axes vertical to the surface, they are arranged more nearly horizontally. Observations were made to determine the smallest concentrations of each of the various types of oleophobic compounds which would permit the formation of oleophobic monolayers on platinum and pyrex. While weight concentrations of only 10−7 were required for primary aliphatic amines and monocarboxylic acids, roughly 1000 times more was needed for the aliphatic alcohols, esters and ketones and for cholesterol. Some of the relations of these findings to past theoretical and applied research are discussed.