| It's interesting to investigate the self-assembled monolayers (SAMs), which made by some functional molecules with a certain orientation and compact arrangement in the field of solid surface adsorption. SAMs not only changes the surface properties of adhering metal, but prevent corrosion in surroundings. With those features, the technology of SAMs becomes a new way to the field of metal protection. Copper, as an important metal with super electrical performance, is widely used in industrial manufacture. However, copper has been found too easy for oxidation existing in natural environment, which causes immeasurable economic losses. The application for the technology of SAMs on the copper surface has a deeper application value. At present, alkanethiols SAMs has become an important research methods for the protection and functionalization of copper surface. However, the influence law of the structure and the protective performance are not entirely clear in adsorption and assembly process of SAMs. Therefore, this paper takes the copper surface to prepare the corrosion protective film as the target, the effects of solvent properties, operating conditions, and substrate status on the formation and the properties of alkanethiols SAMs on of copper surface are studied by a variety of characterization methods such as electrochemical detection, with theoretical analysis and computational simulation. The effect of the interaction between solvent-solute, solvent-substrate are explored for the kinetics of adsorption and assembly of thiols on copper surface, providing theoretical guidance and technical support for the preparation of SAMs and the functional protection of copper surface.The effect of solvent polarity (ETN) on adsorption and assembling of C18SH on pure copper surface and oxidized copper surface is studied. Seven kinds of common organic solvents are selected in the laboratory:n-hexane, toluene, trichloroethylene, chloroform, acetone, acetonitrile and ethanol. In all of the above solvents, C18SH-SAMs on pure copper surface are in good barrier properties and surface coverage is more than 97.3%. With the increase of solvent polarity, the coverage increases, and the relative quality of SAMs increases. The quality of SAMs on copper is the best in the ethanol solution. For comparison, C18SH adsorbed on copper cannot be directly formed SAMs on the surface since the copper oxide layer are reduced. Compared with high polarity solvents, a limited reduction process of oxidized copper by thiols led to the incompletely formation of monolayers in low polarity solvents. This is due to the generated water on solid-liquid interface is very difficult to dissolve into the low polarity solvent, thus inhibiting the reaction of the thiol and CuO in the solid-liquid interface. However, the water, which is generated by thiol reaction with copper oxide, rapidly dissolves in highly polar solvents and reduction reaction is carried out continuously in highly polar solvents. The formation of SAMs on bottom of copper after complete removal of the oxidized copper layer has the same quality as the formation of SAMs of C18SH directly adsorbed on copper surface.The process of dynamic displacement adsorption of C12SH on copper between n-hexane and ethanol as typical solvent is studied. In initial adsorption stage (t?300s), the formation of the surface coverage of C12SH on copper in n-hexane is significantly higher than that in ethanol. However, the formation of the coverage of C12SH on Cu in ethanol is more than that in n-hexane in the subsequent assembly stage (t?400s). The stable structure of C12SH, n-hexane and ethanol and the adsorption energy of these three kinds of single molecules on Cu (111) surface is used to explain the above phenomena by means of Density functional theory (DFT) method. The adsorption energy of single C12SH molecule on Cu (111) surface (-102.734 kJ·mol-1) is obtained, which belongs to a strong chemical adsorption. And the adsorption energy of single n-hexane on Cu (111) surface is -11.328 kJ·mol-1, which belong to weak physical adsorption. While the adsorption energy of single ethanol on Cu (111) surface is -43.526 kJ·mol-1, it lies between the C12SH and n-hexane. The resistance of C12SH molecule replacement ethanol molecule on copper surface is much higher than that of n-hexane by established physical adsorption model of solute replacement solvent, and it is concluded that the interaction between the solvent and the copper surface is very significant in whole adsorption process of thiol on copper.Using ethanol as solvent, the behavior of adsorption and self-assembly kinetics of n-alkanethiols CH3(CH2)n-1SH (n=10,12,14,16,18) SAMs on copper surface was investigated under different concentration conditions. The adsorption process has two stages, the initial rapid adsorption stage and the subsequent relatively slower assembly stage. In initial adsorption stage, it was found that the adsorption process was consistent with Langmuir kinetics model, no matter changes in chain length and concentration of thiols. The adsorption rate constant ka of thiols on copper surface decreases with the decrease of the thiols concentration of the; ka value decreases significantly with the increase of the thiols chain length. It is found that the adsorption rate constant is determined by the thiols concentration through the modification of ka. In addition, the short chain thiol has a larger diffusion coefficient and diffusion is present in whole process of adsorption of thiols on copper surface. However, the diffusion is not a major factor because of the experimental data only to follow the Langmuir kinetics model. In approach and reach equilibrium, the kinetics follows the first order adsorption-desorption equilibrium model and each desorption rate constant kd are obtained at different concentrations.kd value increased with the increase of the chain length of the chain, and increased with the decrease of the concentration of each thiols, which was consistent with the conclusion that the SAMs had a relatively high defect at low concentrations.The micelle transport adsorption process of C18SH on copper surface is studied in micellar aqueous solution of Triton X-100 surfactant and the effect of mechanical stirring on this adsorption process. Under no stirring conditions, the surface coverage of C18SH on copper is more than 80% before the copper substrate immersed in micellar solution at 120min and the coverage is more than 98.5% since continuous immersion for 10 hours. This adsorption process has four stages and it is in accordance with the second-order diffusion controlled adsorption model. Under the stirring speed below 200 r·min-1, the adsorption rate increases with the increase of stirring speed in the initial adsorption stage of C18SH on copper (t<100min); The change of coverage of the two kinds of stirring speed is gradually close to the unity in the following assembly stage (t>120min), which is in accordance with the first-order diffusion control Langmuir model. When the stirring speed over 300 r·min-1, the initial adsorption rate of C18SH on copper surface can be improved, and subsequent assembly time of thiols SAMs on copper surface can be significantly shortened. The adsorption process is still in accordance with the first-order diffusion control Langmuir model at this time. The stirring can promote the moving speed of effective micelles in the aqueous solution, which can reduce the diffusion boundary layer thickness. The results can promote the adsorption rate of thiols on copper and shorten the assembly time for the formation of SAMs in the micellar aqueous solution.The difference of the structure, electrochemically blocking and wettability in the formation of C18SH and C12SH-SAMs on copper surface are studied by means of EIS, FT-IR and contact angle as a means of detection in TX-100 micellar aqueous solution and ethanol solution. The resistance of C18SH-SAMs and C12SH-SAMs formed in the micellar solution are greater than that of SAMs in ethanol solution, which exhibit the relatively good quality of the formation of SAMs in the micelle solution after copper electrode was immersed in two kinds of solution for 72 hours. The CH2 stretching vibration of C18SH-SAMs is in 2850 cm-1 and 2918 cm-1, and the vibration intensity in ethanol solution decreased significantly because of the relatively high crystallinity of the formation of thiol SAMs in the micellar solution. The contact angle of the SAMs formed in the ethanol solution has obvious hysteresis in virtue of strong hydrophobic interaction between water and alkyl carbon chain of SAMs bring about a relatively more conformational defects of SAMs in the ethanol solution. |
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