| Self-assembled films are ordered and dense molecular assemblies formed by the spontaneously chemical adsorption of the active molecules on solid surface. They are the thermodynamically steady system. When the solid is exposed to the solution or gas with the active molecules, self-assembled films can be formed automatically. They do not need the environments of no water or vacuum and not need any special instrument. Self-assembled films are ideal models for studying some complex phenomena concerning about the surfaces, since their structures are orderly and can be designed flexibly. They have the widely applications in many fields, such as biochemistry, nano-materials, metal corrosion and sensors. Because of their densely packed and stable structures, self-assembled films play an important role in the field of surface modification, especially in the field of the inhibitive corrosion of metals.Copper and iron are important metals which are widely used in buildings, chemistry, electric industry, and etc. But their chemical property is very active, and they tend to be oxygenated when exposed to air or water. Thus, the corrosion inhibition of copper and iron has become an important research subject. The technique of self-assembly is practically and theoretically advantageous in the protection of metals. From the aspect of corrosion inhibition of metals, it is very necessary to choose some appropriate organic compounds to self-assemble on the metal surface.The purpose of our research is to find some proper self-assembled systems as the barrier layers which can efficiently insulate the metal from the external surroundings. Phosphates were selected to prepare self-assembled films on copper and iron surfaces in order to protect the metals against corrosion. The inhibitive effect was measured by electrochemical methods, such as electrochemical impedance spectroscopy and polarization curves. The surface analytical methods, such as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) were used to confirm the action of adsorption by detecting the elements or the group in the organic molecules adsorbed on the metal surface. In order to find a better way for corrosion inhibition of metal, we have investigated the effects of some factors, such as concentration and immersion time on the inhibitive efficiency (IE).An external magnetic field was first applied during the process of self-assembly. Results show that the addition of the magnetic field can improve the quality of the self-assembled films markedly. With the increase of field strength, the IE increases. The reason may be that the external magnetic field makes the polar TEP molecules range more orderly and causes the film denser.Moreover, a kind of surfactant, cetyltrimethyl ammonium bromide (CTAB) was used to prepare the mixed films with TEP. Results show that the mixed films have the better quality than the films made of the single molecules.Quantum chemical calculations can provide some information about the process of adsorption of molecules on the metal surface at the molecular level. The results of quantum chemical calculations are consistent with the results of the experiments. Quantum chemical calculations can not only explain the results of experiment, but also be guidance for selecting or synthesizing the proper compounds for self-assembled films with lower toxicity. The main contents and results of this dissertation can be summarized as the following:1. Self-assembled films of triethyl phosphate (TEP) and triphenyl phosphate (TPP) on the copper surfaceCopper electrode was immersed in 7 mol dm-3 HNO3 to obtain a fresh surface. Then it was immersed in 1×10-3 mol dm-3 TEP ethanol solutions or TPP ethanol solutions, respectively. After the designed time, TEP or TPP films were formed on the copper electrode. Results of electrochemical impedance spectroscopy (EIS) show that the charge transfer resistance (Rct) increases with the increase of the immersion time. At the initial period (1 h), Rct of the films increases quickly. The coverage (θ) of TEP gets to 81.0% and that of TPP gets to 85.4%. During the subsequent period, 8 goes on increasing, but the increasing rate is not as fast as before. Form 1 h to 4 h, and to 12 h,θof TEP gets to 90.2%, 93.8% andθof TPP gets to 88.1%, 96.5%. But when the immersion time is 24 h,θof TEP gets to 85.3% and that of TPP gets to 93.1%. At this time, 9 decreases instead. At the same immersion time, IE of TPP is higher than that of TEP. The reason is that there are planar phenyl groups in TPP molecules and phenyl groups can occupy larger space, which can cover more surface area of copper electrode. Compositions of the films were analyzed with X-ray photoelectron spectroscopy (XPS). The appearance of peaks of elements can confirm the adsorption of TEP on copper. The Milliken charge distribution of TEP and TPP molecules were obtained by ab initio calculations. Results show that the charge of O atom of P=O bond is more negative, so TEP and TPP molecules may be adsorbed on the copper surface by O atom of P=O. At the same time, the energies of the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) of TEP and TPP molecules were compared. The energies of HOMO and LUMO of TEP molecule are -10.516 eV and -3.744 eV, respectively. The energies of HOMO and LUMO of TPP molecule are -10.010 eV and -5.904 eV, respectively. Compared with TEP molecule, TPP molecule has the higher energy of HOMO and the lower energy of LUMO, which is the reason that TPP adsorbs on the copper surface more easily. Thus TPP has the better inhibitive effect.2. Self-assembled films of triphenyl phosphate (TPP) and bis-(2-ethylhexyl) phosphate (BEP) on the iron surfaceIron electrode was immersed in TPP and BEP ethanol solutions for designed time to prepare self-assembled films. IE and 9 of the self-assembled films were calculated from the polarization curves and EIS, respectively. Results show that these two kinds of self-assembled films can inhibit the corrosion of iron in 0.5 mol dm-3 H2SO4 efficiently. Results of EIS show that Rct increases with the increase of the immersion time and the concentration of phosphates. As for the self-assembled film formed in 1×10-5 mol dm-3 TPP solutions, Rct increases quickly in the initial 30 min. When it increases to 4 h, Rct gets to the maximum value andθis 78.1% at this time. Thenθremains constant. As for self-assembled films formed in 1×10-3 mol dm-3 TPP solutions, Rct increases quickly in the initial 30 min, but will get to the maximum value only in 2 h. At this time,θis 79.6%. Then it decreases slightly. As for self-assembled films formed in 1×10-5 mol dm-3 or in 1×10-3 mol dm-3 BEP solutions, Rct increases gradually during 24 h. At the same time, the inhibitive effect of self-assembled films formed in 1×10-3 mol dm-3 TPP (BEP) solutions is better than that of self-assembled films formed in 1×10-5 mol dm-3 TPP (BEP) solutions. Polarization curves of iron with and without self-assembled films of TPP or BEP were also used to test the quality of films. Compared with the naked iron electrode, the corrosion potential of the iron electrode with self-assembled films removes in the positive direction. And when the iron electrode is covered with phosphate films, both the cathodic current density and the anodic current density reduce markedly, which indicates that both the anodic and the cathodic electrochemical processes are inhibited. Results of electrochemical measurements show that the self-assembled films have protected the iron electrode against corrosion to some extent. The groups of TPP and BEP molecules were detected by Fourier transform infrared (FT-IR) spectroscopy. Results show that the characterization peaks corresponding to the groups in the compounds were all detected and this confirms the appearance of TPP or BEP on the iron surface. In order to explain the experimental results, molecular simulations were used to provide some information about the process of adsorption of TPP or BEP on iron surface at molecular level. It shows that TPP and BEP molecules tilted at the iron surface after adsorption.3. The effect of the external magnetic field on the self-assemblyCopper electrode was immersed in 1×10-3 mol dm-3 TEP ethanol solutions for 1 h. The magnetic field was first applied during the process of self-assembly. Compared with the self-assembled films formed without the external magnetic field, the films formed under the condition of adding the magnetic field have the better quality. Moreover, IE increases with the increase of magnetic field intensity. When the field intensity increases to 80 mT and to 160 mT, IE increases to 94.2% and 97.6%, respectively. While IE is only 81.0% without the magnetic field. It is presumed that the magnetic field can affect the arrangement of the polar molecules, TER And TEP self-assembled on the copper surface more orderly. So the films become denser and more stable.4. The mixed filmsIn order to improve the quality of self-assembled films, a kind of mixed films made of TEP and cetyltrimethyl ammonium bromide (CTAB) was prepared on the copper surface. First, copper electrode was immersed in 1×10-3 mol dm-3 TEP ethanol solutions for 1 h to form the single self-assembled films. Then TEP films were modified with CTAB. The immersion time in 1×10-3 mol dm-3 CTAB solutions is 1 h or 24 h. Results of electrochemical measurements show that compared with TEP films, R(ct0 of the mixed films increases and the current density reduces.θof the mixed films increases to 99.4%, which is more than 81.0%, that of the single films. These results indicate that the mixed films made of TEP and CTAB have the better corrosion inhibition for copper.
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