Corrosion inhibitors: physisorbed or chemisorbed?

Adsorption mode of corrosion inhibitors is often determined by criteria based on the standard free energy of adsorption, with values greater than −20 kJ/mol attributed to physisorption and values smaller than −40 kJ/mol attributed to chemisorption. Several arguments are presented herein to show that these are not very reliable criteria to distinguish between physisorption and chemisorption. The most notable among them is that chemisorption may involve bond-breaking and bond-making that result in a rather "weak" standard adsorption free energy (or enthalpy). For this reason, more reliable criteria are recommended that are readily available in first-principle computational modeling studies. The strong molecule–surface interaction, involved in chemisorption, should also be detectable by spectroscopy.