Estimating standard adsorption Gibbs energy from corrosion inhibition efficiencies: A case of multilayer adsorption

In corrosion inhibition studies, the standard adsorption Gibbs energy is often estimated from c / θ versus c plots, where c is the inhibitor concentration and θ is the surface coverage. In doing so, inhibition efficiency ( η ) is assumed to be a good approximation of surface coverage, i.e., θ ≈ η . Consequently, adsorption is effectively treated as monolayer adsorption, since inhibition efficiency is, by definition, confined to values below one. However, there is no fundamental physical reason why adsorption on flat surfaces should be limited to a monolayer. For this reason, a model that maps multilayer surface coverage to inhibition efficiency is developed based on a few reasonable physical assumptions. It is shown that multilayer adsorption reduces the slope of the c / η versus c curve—widely interpreted as the c / θ curve due to the θ ≈ η approximation—and leads to a more pronounced curvature compared to the monolayer case. While the bending at low concentrations resembles that caused by repulsive lateral interactions, the overall curvature remains a distinct feature of multilayer adsorption. However, it is argued that in experiments—where only a limited number of data points are typically obtained—these features are often not distinctive enough to allow multilayer identification. This helps explain why the θ ≈ η approximation, which implicitly assumes monolayer adsorption, has been so widely accepted and used. • A new model is developed that maps inhibition efficiency ( η ) to multilayer surface coverage ( θ ). • Multilayer adsorption increases curvature and decreases the slope of the c / θ vs. c plot. • The multilayer c / η vs. c curve arches above the c / η = c line at low concentrations ( c ), and follows it at higher concentrations. • The c / η vs. c plot is typically insufficiently distinctive to identify multilayer adsorption from experimental data. • The adsorption Gibbs energy of the first adsorbed layer can be deduced from the c / η vs. c plot.