Molecular dynamics simulations of the incrustation inhibition by polymeric additives

Abstract

The precipitation of CaCO₃ during washing treatments can be reduced by polymeric additives. This process of incrustation inhibition could be described by modeling the effects upon crystal growth. Molecular dynamics calculations were used. Different conformations of the additives on the substrates developed during the time course of these simulations. The interaction energies of the additives with the substrates and the deformation energies of the additives on the main growth faces of calcite enabled conclusions to be drawn on the efficiency of the additives for incrustation inhibition. In addition, information on the mode of action was derived from the dynamics of the additives on the calcite faces. Although no quantitative results were obtained with the simplified models, trends in efficacy could be uncovered from the differences in the results. An acrylate/maleate copolymer and an oligomaleate were investigated as model systems. The interaction of the two studied additives with the positively charged calcite {1 1̄0} face are significantly stronger than with the neutral calcite {1 0 4} face. Both additives are more mobile on the {1 0 4} face than on the {1 1̄0} face. The dynamics of different polymer chain segments during the course of the molecular dynamics runs show different features. Whereas the carboxy groups at the ends of the chain are very mobile the groups in the middle of the chain are more firmly adsorbed to the crystal surface. Chain segments in the middle of the chain will therefore hinder the crystal growth more effectively than end segments. The additives are deformed during their adsorption to both faces of calcite. The respective deformation energies of the additives are much smaller than the adsorption energies of the additives to the calcite faces. The acrylate–maleate copolymer adsorbs significantly stronger to the calcite faces than oligomaleate. This is due to the higher additive/substrate interaction energies and the larger number of contacts between the carboxy groups of the additives and the Ca²⁺ atoms of the substrate in the case of the acrylate–maleate copolymer, in comparison with oligomaleate. The studied additives can disperse different amounts of CaCO₃ in water. The measured calcium carbonate dispersion capacity is higher for the acrylate–maleate copolymer than the oligomaleate.

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