INSOLUBLE PERFLUORINATED MONOLAYERS AND THIN-FILMS ON THE SURFACES OF CORROSIVE ACIDS

Long-chain perfluoropolyethers spread spontaneously on concentrated sulfuric, nitric, and phosphoric acids, forming stable and insoluble films on these acid surfaces. On H2SO4, the fluorinated ethers begin to spread at a concentration of 3.5 M, characterized by steady-state spreading pressures that rise continuously with acid concentration up to 9 mN/m at 17.4 M. Measurements of surface pressure versus area per monomer indicate that the long-chain perfluoroethers preferentially pack lying down on sulfuric acid at monolayer coverage. We find that the spreading pressures correlate most directly with the Hammett acidity of the acid subphase, suggesting that spreading is driven by hydrogen bonding between the acid and the ether oxygen or fluorine atoms of the perfluorinated ether molecules. Proton transfer may also occur at the highest acid concentrations but should play a minor role for the weaker or more dilute acids because of the extremely weak basicity of the fully fluorinated ethers.