Ab initio modelling of xanthates adsorbed on Ge and ZnS surfaces

The motivation for this modelling work, in close collaboration with experimentalists, is to contribute to the understanding of xanthate adsorption on ZnS surfaces in the flotation process. Adsorption of xanthates on Ge surfaces is investigated, which also have been studied experimentally in the Agricola Research Centre. Furthermore, results for dibutyldithiophosphates—which are important in the flotation process—are reported. Modelling surface adsorption requires systems including hundreds of atoms. To model such systems at an ab initio level successfully, fast and accurate methods must be used.In the present work we use density functional theory (DFT) combined with pseudopotentials, which is a powerful modelling approach for large chemical systems. The results obtained in an initial study of ethyl and heptyl xanthates and their sodium or potassium salts using all electron Hartree-Fock and DFT calculations are used to validate the pseudopotential method used for modelling adsorption of xanthates on surfaces. Both geometrical and vibrational properties, as well as computer resources needed, are examined and compared. Results obtained using DFT and pseudopotentials are in close agreement with both experimental results and all electron potential calculations, while being obtained much faster. Hence, the pseudopotential approach is chosen to study ethyl/heptyl xanthate adsorbed on Ge and ZnS surfaces. The surfaces are modelled using periodically repeated supercells containing more than one hundred atoms. The geometrical structure and vibrational frequencies are calculated and the modes are described. The results are compared with attenuated total reflection infrared observations. The influence of additional atoms at the surface in the surrounding of the adsorbed xanthate are also investigated.