Theoretical Study On Wettability Properties Of Lanthanum Oxide And Its Modified Surfaces

In recent years,super-hydrophobic materials have attracted much attention in many fields such as anticorrosion,fogging and self-cleaning due to their special wettability.Rare earth oxides are widely used in the preparation and modification of super-hydrophobic materials for the special distribution of extra-nuclear electrons and their intrinsic hydrophobicity.Many materials based on rare earth oxides have been developed and their application fields have also been expanded.The study on the wettability mechanism of rare earth oxides is the theoretical foundation for the modification and further application of this kind of materials.It has been proved experimentally that the surface hydrophobicity of rare earth oxides is influenced by the lattice orientation,surface element ratio of different valence states,oxygen vacancies of surface,annealing temperature and so on.However,due to the lack of research and understanding at the atomic level on the surfaces of rare earth oxides,the relationship between the various factors is unclear.Therefore,it is urgent to study and discuss the wettability mechanis m of rare earth oxides at atomic level deeply.In view of this,based on the density functional theory?DFT?and taking lanthanum oxide?La₂O₃?as the research system.The surface wettability,the possible hydrolys is dissociation in the wetting behavior and its influence on the hydrophobic properties were studied in theoretically.The intrinsic hydrophobicity mechanism was revealed at atomic level,and the effect of surface reaction on hydrophobic stability was investigated.In addition,the influence mechanism and principle of organic chain length and functio na l groups during the super-hydrophobic modification of lanthanum oxide surface were also studied.The main results are as follows:Firstly,the optimal adsorption model for water molecules on La₂O₃?001?surface is H₂O_W-La.The weak hydrogen bonds between hydrogen atoms and surface oxygen atoms were formed in water.The orientation of adsorption is parallel to the base plane,and the adsorption energy is-0.362 eV.The simulated water contact angle?WCA?of the surface multi-layer water molecule adsorption model is 103.7°,which shows hydrophobic.Compared with other hydrophilic oxide surfaces,the concentration of La adsorption site s on lanthanum oxide surface is lower,which limits the covering of water molecules on the solid/liquid interface.At the same time,the oxygen-hydrogen bonds of the adsorption surface are parallel to the substrate,which reduce the intermolecular interactions between water molecules.These results reduce the surface energy and make the surface of lanthanum oxide have intrinsic hydrophobicity.The transition state calculation shows that the dissociation of water molecules on the lanthanum oxide surface forms two kinds of hydroxyl groups,with a dissociation energy barrier is 0.210 eV.The WCA simula tio n value of the hydroxylation surface was 94.6°,which indicated that the adsorption sites of surface water molecules were increased after the hydroxylation of lanthanum oxide surface.Besides,the presence of hydroxyl groups enhanced the interactions between surface and water molecules.The adhesion between solid and liquid interfaces was also increased,thus decreases the hydrophobic properties of the surface.Although the lanthanum oxide surface is still in the hydrophobic state after hydroxylation,the stability of lanthanum oxide surface is poor due to the obvious influence of hydroxylation.It is necessary to further modify the surface of lanthanum oxide for hydrophobic applicatio n to improve the stability of the performance.Secondly,by means of molecular dynamics,the super-hydrophobic mechanism of rare earth oxide modified surface and its influencing factors were studied,concentrating on the length of carbon chain and the reaction groups.The results show that the modification of carbon chain could improve the hydrophobicity of lanthanum oxide surface and achieve stable super-hydrophobicity,and the functional groups mainly affect the adsorption stability of surface.There is a convergence relationship between the thickness of carbon chain layer and hydrophobicity,while after the chain length is nine,the change is not obvious.The length of the carbon chain affects the coverage of organic matter.Increasing the length of the carbon chain will enhance the interactions within the molecular layers,and there is a certain competitive relationship with the interfac ia l reaction energy.Therefore,the carboxylic acid organic has certain advantages over other kinds of organic compounds for strong interfacial interactions.