Application of Surface Tension Model for Prediction of Interfacial Speciation of CO₂-Loaded Aqueous Solutions of Monoethanolamine

A thermodynamic platform in combination with experimental surface tension data is used to predict the surface phase speciation of CO₂-loaded aqueous solutions of 30 wt % (%) monoethanolamine (MEA) at 20, 40, and 60 °C from their corresponding bulk phase speciation. The osmotic coefficient dependency of the surface tension of an electrolyte solution has been employed to estimate the surface tension. In order to calculate the osmotic coefficients, the Pitzer equation for the excess Gibbs energy was applied. According to the results of this study, molecular MEA and to some extent protonated MEA and the MEA-carbamate have surface propensity and will accumulate at the surface phase. The carbonate ion shows nearly even distribution between bulk and surface phases, and bicarbonate displays a tendency to remain in the bulk phase that increases with CO₂ loading. The differences in the behaviors of protonated MEA, MEA-carbamate, carbonate, and bicarbonate are due to differences among their sizes and polarizabilities. Using first-principles calculations, the static isotropic dipole polarizabilities for bicarbonate, carbonate, MEA carbamate, and protonated MEA were calculated. The bulk and surface phases' ionic strengths reasonably represent this behavior of ionic species at the surface phase.