Adsorption of perfluorocarboxylic acids at the silica surface.

Water molecules may force the hydrophobic perfluoroalkyl chain of perfluorocarboxylic acids (PFCAs) to aggregate at hydrophobic siloxane patches ([triple bond, length as m-dash]Si-O-Si[triple bond, length as m-dash]) of the silica surface despite electrostatic repulsion between surface silanol ([triple bond, length as m-dash]SiO-) and the carboxylate (-COO-) headgroup of PFCA. Acidic pH and background cations promote the adsorption further by reducing the anion-anion Coulomb repulsion.