Modelagem computacional da interação entre o íon de cálcio (II) e espécies inibidores de incrustação: compreendendo a reação anti-incrustação

Abstract

The deposition of scaling salts, such as calcium carbonates (CaCO3), in production pipelines is a very challenging problem for the oil and gas industry, occurring as a consequence of the physicochemical conditions in oil wells. In order to remedy the oil industries, use chemical species as scale inhibitors to prevent deposition. The understanding of the molecular mechanism of action of inhibitors is not very clear. Therefore, this thesis presents a computational modeling of the interaction between scale inhibitors and calcium cations (Ca2+), in order to provide details at the molecular level about the mechanistic action of anti-scalants. The computational results shown in this thesis are based on calculations that combine density functional methods with the implicit solvation model. The thesis was divided into three different studies, involving the calculation of the interaction energy between the Ca2+ ion(s) and inhibitory species, namely: (i) the hexametaphosphate (HMF) anion; (ii) the monomer of the poly(4-styrene-sulfonicacid-co-maleic-acid) [P(SSMA)] copolymer and the surfactant sodium dodecyl sulfate (DSS); and (iii) monomers of the carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC) polymers. In all studies, the interaction energies between the inhibitor species and the Ca2+ ion were compared with those formed by Ca2+ with the carbonate (CO32–) and bicarbonate (HCO3–) anions. In (i) the complexes formed by the interaction between the Ca2+ ions and the HMF coordination sites are thermodynamically favorable. Furthermore, the pKa values of the (de)protonation process for hexametaphosphoric acid (HPO3)6 were calculated, namely: pKa_1 = –1.2; pKa_2 = –1.0; pKa_3 = +3.7; pKa_4 = +5.6; pKa_5 = +10.4; and pKa_6 = +12.0. The values obtained by the theoretical calculations show good agreement with values of +6.2 and +9.2 obtained experimentally. In (ii) the theoretical results suggest that the interaction energies between the monomeric species of P(SSMA) and Ca2+ ions are thermodynamically favorable if the coordination occurs through the carboxylate groups. However, the interaction energy between SDS and Ca2+ ion is unfavorable in terms of Gibbs energy. Furthermore, the cooperative interaction of the surfactant DSS and P(SSMA) with the Ca2+ ion(s) can result in an exergonic complex. Finally, in (iii) the results of the calculations of interaction energy between Ca2+ and CMC and HEC monomers suggest that the Ca2+ cation interacts more strongly with CMC than with HEC. This is because while the CMC monomer has carboxylate groups to interact with the cation, the HEC has only polar (non-anionic) groups. Furthermore, for this system, the interaction energy of the monomers with a CaCO3 cluster was calculated. Theoretical results suggest that the CMC monomer can replace the position of a CO32–ion in the cluster, but the HEC monomer does not have this ability.