A MD Study Of The Inhibitory Capability And Mechanism Of Seven Organic Phosphonates Containing Nitrogen

In this dissertation, the functionary mechanisms of seven organic phosphonate inhibitors (AMP, NDP, NTMP, EDTMP, TDTMP, HDTMP and DTPMP) in the indrustrial circulating cooling water system have been investigated by the pH curve method (pHCM) and molecular dynamics (MD) method. The effect of seven phosphonates on the growth of CaCO3has been evaluated by the pHCM. And MD simulation has been used to study the interaction of the objects on three polymorphs of CaCO3crystal. The results are compared with the pHCM conclusions to validate the rationality and dependability of MD computational simulation. Afterwards, the same simulative conditions are used to study the inhibiting effect of the objects on BaSO4, Ca10(PO4)6(OH)2and CaSO4.The experimental results of the pHCM show that the objects at the low concentration except AMP and NDP have a steady and favorable inhibitory effect at the temperature of75±2℃. The greatest inhibition concentrations of the five phosphonates are5μmol/L (NTMP),1μmol/L (EDTMP),0.8μmol/L (TDTMP),0.5μmol/L (HDTMP) and0.2μmol/L (DTPMP), respectively. It is shown that the inhibition effect of CaCO3precipitation is AMP≈NDP<NTMP<EDTMP<TDTMP<HDTMP<DTPMP.The interactions of organic phosphonates on the three polymorphs of CaCO3crystal are simulated by MD method. The results show that the binding energies of calcite, aragonite and vaterite are negative, namely the interaction is exothermic. It also indicates that the strong electrostatic interactions between the oxygen atoms in phosphonic functional groups and the Ca2+ions of crystal surfaces play a dominant role in their adsorption, whereas the van der Waals interactions play a very small role in the non-bonded interaction. The corrected binding energies are obtained to discuss the inhibitor effectiveness:DTPMP> HDTMP> TDTMP> EDTMP> NTMP>>NDP> AMP, according with the previous pHCM experimental data. It proves that the MD modeling method in these conditions is a useful and reasonable tool in the pursuit of an atomistic understanding of interactions between surfaces and additive molecules. The results also clearly indicate the important of the number of phosphonate groups and the length of the backbone methylene chain connecting the two N atoms. In total, the greater the number of the-PO32-groups and the longer the length of the backbone chain, the better the inhibition of precipitation. But the backbone chain effect of aragonite and vaterite is unconspicuous. The Ca-0(P) distances are all close to2.5A in accordance with the Ca-0bond length from experiment. And the strongest adsorption peak at almost2.5A also shows the formation of Ca-0electrovalent bond between the calcium ions of crystal surfaces and the oxygen atoms of phosphonate groups.According to the periodic bond chains (PBC) theory, MD simulation has been employed to study the adsorption of water molecules onto the stepped calcite (011) surface. The results show that the great mass of water molecules are coordinated to the edge calcium ions and the oxygen atoms on the step edge labeled A or at the corner of the surface labeled B. Their interfacial angles are separately129.4°and63.2°. It suggests that the step edge and the corner are the optimal binding sites. Then, the interaction of HDTMP on the stepped surface is considered as an example. It is found that HDTMP closes with the calcium ions at the B site in accordance with the adsorption of water molecules. Based on the above study, the calcite (104) surface is divided into four different stepped surfaces, terminated by either calcium ions or carbonate groups. The results of the interaction of phosphonates with the four stepped surfaces also suggest that the step edge and the corner are the optimal binding sites. Compared with the binding energies of DTPMP on the four surfaces, it is found the stability of the surfaces:Ca> Ca-CO3> CO3-Ca> CO3.The same method and conditions are used to investigate the inhibition mechanism of the seven phosphonates on the surfaces of BaSO4, HAP and CaSO4. The results indicate that the organic compounds except AMP and NDP can close well with all the surfaces. Based on the corrected binding energies, the inhibition effect of organic phosphonates on BaSO4is TDTMP> DTPMP> EDTMP> HDTMP> NTMP> NDP> AMP different with the conclusion of CaCO3. And the effect on HAP and CaSO4is DTPMP> HDTMP> TDTMP> EDTMP> NTMP> NDP> AMP according with CaCO3.The average of Ca-0and Ba-O distances is almost2.3-2.5A which suggests the formation of Ca-0and Ba-O electrovalent bonds.