Water molecules may force the hydrophobic perfluoroalkyl chain of perfluorocarboxylic acids (PFCAs) to aggregate at hydrophobic siloxane patches (Si–O–Si) of the silica surface despite electrostatic repulsion between surface silanol (SiO⁻) and the carboxylate (–COO⁻) headgroup of PFCA. Acidic pH and background cations promote the adsorption further by reducing the anion–anion Coulomb repulsion.