Interactions between calcium ion and functional groups of organic scale inhibitors in aqueous solutions: an ab initio study

Abstract

The anti-scaling performance of organic inhibitors is closely related to the Ca-trapping capacity of functional groups in polymer frameworks. In this study, we performed static density functional theory calculations and ab initio metadynamics simulations to systematically investigate the association behaviors of Ca²⁺ with carbonate species (CO₃²⁻ and HCO₃⁻) and representative functional groups of organic inhibitors (carboxylate, phosphonate, and sulfonate) in aqueous solutions. By calculating the binding free energies and activation barriers for separating contact ion pairs, we propose a new strategy for predicting the Ca-trapping capacity of organic additives. Simulation results revealed that carboxylate and phosphonate groups possess potential calcium sequestration abilities, while the formation of the calcium–sulfonate pair is almost impossible in aqueous solutions. For the carboxylate group, which possesses a planar structure akin to that of the carbonate species, its presence has less impact on the solvation structure of Ca²⁺. The monodentate and bidentate configurations have similar stabilities for the contact ion pair. However, for the phosphonate and sulfonate groups that possess a trigonal pyramid structure, their association with Ca²⁺ disrupts the symmetry of the coordination structure to a greater extent. The monodentate state is clearly favored over the more tightly coordinated bidentate state. Moreover, since CO₃²⁻ exhibits the strongest affinity for Ca²⁺, a rising CO₃²⁻ concentration at a high pH diminishes the anti-scaling efficiency of all inhibitors. Overall, we offer new insights into the inhibiting effects of organic additives on the initial stage of CaCO₃ formation by analyzing calcium-organic compound interactions. These findings provide theoretical support for the development of more effective scale inhibitors in the industry.