Molecular Dynamics Simulation Studies Of The Non-phosphorus Scale And Corrosion Inhibitors

Non-phosphorous water treatment agent has an important application in water treatment and phosphate-free of agents is the way of the future. In this paper, the interaction models between common non-phosphorus scale and corrosion inhibitors and scale crystals with and without wate were simulated. The binding energies, deformation energies and pair correlation functions from molecular dynamics (MD) simulations with and without water were calculated and compared. The mechanisms of inhibiting scale and corrosion of the inhibitors have been investigated, and the essence of solvent effect on scale and corrosion inhibition has been revealed. The results obtained can provide a theoretical guidance to developing new scale and corrosion inhibitors. The whole work can be divided into two parts:The first part focused on the inhibition mechanism of non-phosphorus scale inhibitors with MD method. The interaction models between polymer inhibitors Polyacrylic acid (PAA), hydrolyzed polymaleic anhydride (HPMA), polyepoxysuccinic acid (PESA), and polyaspartic acid (PASP) and scale crystal surfaces including the (104) and (110) surfaces of calcite (CA) crystal, the (001) and (020) surfaces of anhydrite (AD) crystal, and the (001) and (110) surfaces of hydroxyapatite (HA) crystal with and without the present of water were constructed, respectively. The resulted48models simulated with COMPASS force field and NVT-MD method one by one.4polymer inhibitors can inhibit the growth of calcium scale. They deform obviously with and without when they interact with scale crystals. The results of models with water are closer to the experimental data. The pair correlation functions (PCFs) g(r)Total,g(r)Ca(Crystal)-o(-C=O)、 g(r)Ca(Crystal)-O(-OH), g(r)Ca(Crystal)-O(Water), g(r))H(Polymer)-O(Water) and g(r)H(Polymer)-O(Crystal) imply the bonding (including H-bond) interactions with and without water are formed between polymer inhibitors and scale crystals in shorter range, while the non-bond interactions are formed in longer range. The interactions between all the species in the models are mainly contributed from the non-bond interaction. Natural bond orbital (NBO) charges of the repeat units of polymer inhibitors were calculated by B3LYP/6-31G*method. The Coulomb interaction is formed between the O atoms of polymer inhibitors and the Ca atoms of scale crystal.For calcium carbonate, the binding energies between4polymer inhibitors and the (104) and (110) surfaces of CA crystal with water have the order of PESA> PASP> HPMA> PAA. The binding energy of the same polymer inhibitor on the (110) surface of CA is greater than that on CA(104). Water molecules decrease the binding energies of the four polymer inhibitors on the surfaces of calcite crystal.For calcium sulphate, the results from MD simulation between polymer inhibitors and AD(001) with water are closer to the experimental data. The sequence of binding energies between4polymer inhibitors and AD (001) and (020) with water is PESA>PASP> HPMA>PAA. The binding energy of the same polymer inhibitor on AD(001) is smaller than that on AD(020). Water molecules weaken the deformations of HPMA and PAA, but aggravate those of PASP and PESA.For calcium phosphate, the results from MD simulation with water between polymer inhibitors and HA (001) are more consistent with the experimental results. The temperature and energy fluctuations of the interaction MD model between polymer inhibitors and HA are severer than those between them and CA, but are almost equivalent to those between them and AD. The binding energies between4polymer inhibitors and the (001) surface of HA with water have the order of HPMA> PASP> PES A> PAA. The binding energy does not vary much between the same polymer and the two surfaces of HA. The results of MD simulations with water are in line with the experimental reports. Water molecules weaken the deformations of HPMA and PES A, but aggravate those of PASP and PAA.These may show that polymer inhibitor interacts with scale crystal surface indirectly through the water molecules, i.e., the water molecules play an important role in investigating the action mechanism of scale inhibitor in solution by MD simulation and can not be ignored when the interaction models are constructed.The second part concentrates on the study of the inhibition mechanism of non-phosphorus corrosion inhibitors to metals with MD method. For carbon steel, the corrosion inhibiton models between hydrolyzed polymaleic anhydride (HPMA), polyepoxysuccinic acid (PESA), and polyaspartic acid (PASP) and the (001) and (110) surfaces of Fe crystal with and without water were simulated with MD method. The results show that.Ebind(001) of the same polymer inhibitor is smaller than Ebind(110)(except PESA). The orders of the binding energies of polymer inhibitors with and without water are PASP> HPMA> PESA, but the binding energy of the former is much smaller than that of the latter. Polymer inhibitor can overcome its intense deformation and closely combine with the face of Fe crystal, and prevent the corrosive medium association with carbon steel, which leads to corrosion inhibition. Because polymer inhibitors can not completely break water molecules and adhere to the surface of Fe crystal, the corrosion inhibition effect is not good. Water molecules affect the interaction between polymer inhibitor and Fe crystal, and affect the deformation of polymer inhibitors, too. From the pair correlation functions, the non-bond interactions are formed between O (inhibitor)-Fe and O (H2O)-Fe. Solvent effect is not negligible in the construction of the model.For the surface of copper, the interactions between6benzotriazole derivatives obtained by modifying benzotriazole with hydroxyl and the (001) surface of Cu2O with and without water were simulated with MD method. The results show that the MD simulation result with water is more consistent with the experiment results. At the same temperature, the sequence of the binding energies of different benzotriazoles with Cu2O (001) in water solution is1-OH-BTA>4-OH-BTA>7-OH-BTA>BTA>5-OH-BTA>6-OH-BTA. Water molecules have an important influence on the interaction between corrosion inhibitors and Cu2O crystal. Because of the difference of molecular structures, the corrosion inhibitions of benzotriazole derivatives are different at various simulation temperatures. Under different temperatures, the order of binding energies of1-OH-BTA with Cu2O(001) is343K>323K>333K, but that of BTA is323K>333K>343K. From non-bond energy and pair correlation functions, the binding energies between corrosion inhibitors and Cu2O crystal are mainly contributed by the Coulomb interaction. Strong adsorption can be raised by the Coulomb interaction between the negatively charged functional groups in corrosion inhibitors and the positive copper ions in the Cu2O(001) face, and further interaction between aggressive media and copper can then be restricted. So the corrosion of copper can be avoided. Chemical bonds or hydrogen bonds are formed in the system of1-OH-BTA/H2O/Cu2O(001). Water molecules can not be ignored in MD simulations, tooAll in all, MD simulations were employed to systematically investigate the relationship between structures and properties of non-phosphorus scale and corrosion inhibitors. The inhibition mechanisms of them are explained, the essences of solvent effect on scale and corrosion inhibition are revealed, and the studies are precursory and originally innovative in front of multi-subject crossing research fields. The research projects assigned by National Natural Science Foundation and Key Laboratory for Ecological-Environment Materials of Jiangsu Province have been successfully completed.