Properties of films of adsorbed fluorinated acids

Abstract

Monomolecular films of perfluorinated butyric, valeric, caproic, caprylic, capric and lauric acids were prepared by physical adsorption on platinum from their solutions in organic solvents. Their wetting properties for a number of selected liquids were then characterized by contact angle measurements. Regular but small increments in the contact angles were observed with increasing acid chain length. Thus, although perfluorolauric acid monolayers provided the lowest energy surface yet reported, films of even the very short acids were significantly more non-wettable than other types of surfaces, including the fluorinated polymers studied earlier. For the shorter chain acids, monolayer formation was rapid; the two longest chain acids, however, required progressively longer periods of contact between the adsorbing surface and the acid solution for completion of a monolayer exhibiting maximum contact angle. The remarkable liquid-repellency of the perfluoroacid monolayers is caused by the presence of an exposed plane rich in perfluoromethyl groups. Electron diffraction examination of the longer chain acids reveals a molecular orientation consistent with this explanation. Included are surface tension data for several new API hydrocarbons of exceptional purity.