On the use of the Langmuir and other adsorption isotherms in corrosion inhibition

In corrosion inhibition studies, the standard adsorption Gibbs energy is often estimated via the linear regression of the Langmuir isotherm in the c/θ=1/K+c form, where both the intercept and the slope are estimated, although the Langmuir isotherm requires the slope of 1. Hence, c/θ=1/K+mc is actually used, where m is the slope. Herein, a theoretical basis for this equation is established. It is demonstrated to be an effective equation that can decently describe various adsorption models and provide relatively accurate estimates of the standard adsorption Gibbs energy, provided surface coverages are reliably determined experimentally. However, any significant deviation from the slope of 1 signals non-Langmuir adsorption due to inter-adsorbate interactions, multi-site adsorption, or surface heterogeneity. Among these three causes, only attractive inter-adsorbate interactions lead to a slope of less than 1.