Adsorption of carboxylate on calcium carbonate (10 (1)over-bar 4) surface: Molecular simulation approach

Abstract

The wettability of the hydrophilic calcium carbonate surface is altered by the adsorption of amphiphilic carboxylate compounds forming an oleophilic layer on the surface. In this study, we characterize the adsorption of carboxylates such as benzoate and stearate on the calcium carbonate (10 (1) over tilde 4) surface using density functional theory (DFT) and molecular dynamics (MD) simulations. From our DFT computations using PBE-D3 method, the binding energy of a carboxylate adsorbed on the calcium carbonate in water phase is calculated to be -29.45 kcal/mol, which is utilized to develop a new set of force field parameters for molecular simulations. The optimal adsorption density of the carboxylates on the carbonate surface is determined using the newly developed force field: the adsorption of benzoate shows two probable adsorption densities at 20.20 angstrom(2)/molecule and 40.40 angstrom(2)/molecule, while the stearate adsorption has a single optimum at 20.20 angstrom(2)/molecule, which is in a good agreement with the experimental results. Lastly, through performing the steered molecular dynamics simulations to characterize the potential of mean force for the desorption of the carboxylate molecules from the calcium carbonate surface, the binding free energy is calculated as -148 kcal/mol in the presence of oil phase. This indicates that due to the stability of the carboxylate monolayer on calcium carbonate, the spontaneous desorption of carboxylate molecule from the calcium carbonate surface in nature is not likely. (C) 2015 Elsevier B.V. All rights reserved.