The Mechanism Of Hydration And Hydrophobic Improvement Of Montmorillonite In Aqueous Solutions:a Quantum Mechanics/Molecular Dynamics Simulation Study

The treatment of coal slime water is an important link in the coal preparation technology and process.High mudding coal slime water contains fine particles of clay minerals,such as montmorillonite,kaolinite,and illite.The presence of these clay minerals would bring adverse effects on the settlement and dewatering of coal slime water,where the effect of the montmorillonite is the most significant.The interfacial properties of montmorillonite and the interaction of water molecules,metal ions and hydrophobic modification reagent with montmorillonite particles were studied based on the quantum chemical/molecular dynamics simulation.The hydrophobic flocculation sedimentation test,including supernatant transmittance,floc morphology,contact angle and adsorption amount of reagent,were carried out.These works were aimed at the understanding of the interaction mechanism and realization of the interfacal regulate and control laws.The paper contains the following research works and the main conclusions:?1?According to the DFT simulation results of montmorillonite surface properties and the adsorption of water molecule on surface,the surface oxygen six-membered ring?SOR?has obvious electronegativity,which is the stable adsorption site of metal equilibrium cation or positively charged groups of organic molecule.Metal cations are adsorbed onto the montmorillonite?001?surface mainly through electrostatic interaction;the adsorption energy of bivalent cations was larger than that of monovalent cations;the adsorption energy on montmorillonite with the substitution of Si⁴⁺ by Al³⁺ in tetrahedral layer was larger than that with the substitution of Al³⁺ by Mg²⁺ in octahedral layer.The metal equilibrium Na+ cation and surface a atoms on the Na-MMT?001?basal surfaces are active sites by nucleophilic and electrophilic attacks,respectively.The-SiOH and-AlOH2 groups of montmorillonite?010?edge surface are most likely to subjected to the deprotonation reaction while the-AlOH and-SiOAl-groups are most likely to subjected to the deprotonation reaction.Single water molecule is adsorbed on Na-MMT?001?basal surface mainly through electrostatic interaction between Na+ and water O atom,and is adsorbed on Na-MMT?010?edge surface through the hydrogen bond interaction of water H or O with surface-OH or-OH2 groups,where the adsorption energy are-62.71 kJ/mol and-82.97kJ/mol,respectively.After the adsorption of more than three water molecules,the competition between "water-water","water-ion" and"water-surface" would appear and more water molecules are bonded to Na+ which has the trend of moving away from the surface adsorption site.?2?The MD simulation results of water molecule adsorbed on montmorillonite surface show that when the fraction of water coverage increase from 2/3 ML to 4/3 ML,the restriction of surface for Na+ cations decrease so that Na+ cations move from surface to the bulk water,and the water coordination structure of Na+ converts from inner-sphere to outer-sphere.When the fraction of water coverage increased from 2/3 ML to 8/3 ML,the first,second and third water layers gradually form on the interface.?3?The DFT simulation results of hydrophobic modification agents adsorbed on montmorillonite surface show that the alkyl amine cations are adsorbed on the surface oxygen six-membered ring?SOR?of montmorillonite mainly through electrostatic interaction and hydrogen bond interaction of N-H…O.The adsorption energy of long chain C12 alkyl amine are-312.43 kJ/mol,-265.18 kJ/mol,-257.30 kJ/mol and-220.54 kJ/mol,respectively,and in the order of:dodecyl primary amine>dodecyl secondary amine>dodecyl tertiary amine>dodecyl quaternary amine.The adsorption energy of C12?18 alkyl primary amine cations and quaternary ammonium cations increase with the increasing of the length of the carbon chain.When the length of the carbon chain increases to Ci6,the adsorption energy did not increase any more.?4?The MD simulation results of hydrophobic modification agents adsorbed on montmorillonite/water interface show that all the anionic surfactant?dodecyl carboxylic acid molecules and anions?and the molecular form of cationic surfactant?dodecyl primary amine molecules?can move away from the intital adsorption position to agglomerate each other,that is to say,these surfacants have no hydrophobic modification effects.While the cation form of dodecyl amine has the effect of hydrophobic modification,because the polar head groups could be steadily adsorbed on the surface,in turn,the non polar carbon chains toward the aqueous solution and stay together through the hydrophobic association interaction.The solution pH plays an important role in the hydrophobicity modification of cationic surfactants.When the pH is about 8,the amine ions that adsorbed on the surface just neutralize the surface negative charge,and the remaining surface space was occupied by the amine molecules,where the hydrophobic modifier could form closer,ordered semi-micellar structure.In this case the effect of hydrophobic modification was best.The greater the concentration of surfactant and the length of carbon chain,the better the hydrophobicity modification effect of montmorillonite surface.However,when the concentration exceeds 1ML?one surface oxygen six-membered ring was replaced by one surfactant structure?,and when the carbon chain length of is more than C₁₆,the hydrophobicity modification effect decreased.?5?The results of contact angle and adsorbing capacity test show that the size order was dodecyl primary amine>dodecyl secondary amine>dodecyl tertiary amine>dodecyl quaternary ammonium.The contact angle and adsorption amount of montmorillonite increase with the increasing of dodecylamine concentration,and do not change when above 1×10^-3mol/L.They first increased and then decreased with the pH value increasing from about 4 to about 10,reaching the maximum at pH = 8.The contact angle increased as the carbon chain C_n grows,but no longer changes when larger than C₁₆.The experimental results of floc morphology,contact angle and adsorption are in good agreement with the theoretical results.Therefore,the theoretical calculation can provide theoretical support for the selection and design of hydrophobicity modifier of mineral interface and the control of solution environment.