Theoretical Studies On Selective Adsorption And Accumulation Behavior Of Self-assembled Monolayers

Since the alkyl thiols self-assembled monolayers （SAMs） on Au surface wasfirst obtained in1983by Nuzzo and Allara, its adsorption mechanism and practicalapplication are widely concerned in the scientific community, and the research on theadsorption of SAMs are getting more. The application of SAMs in the daily life isbecoming more and more widely with the development of self-assembled technology,which not only can be used for metal corrosion, but also can be used in application ofthe biosensor, now it has a new development in the environmental governance. It canonly speculate the macro adsorption mechanism in experimental study ofself-assembled monolayers, while the use of computer simulation technology canexplain the selective adsorption mechanism of self-assembled monolayers surface fromthe molecular level or even the atomic level. The research results can be summarizedas follows:1. Simulated self-assembled film of C18alkane molecules was formed on thesurface of Si （111） by molecular dynamics method, modified the surface ofself-assembled monolayer through six different head groups （-CH₃,-C=C,-OCH₃,-CN,-NH2,-COOH）. This paper emphatically studies the internal structure and surfaceproperties of the SAM formed on the surfaces of Si （111） after modification by the sixdifferent head groups. Analyzing SAM atom density peaks and the surface area ofthese six kinds of different head groups, conclusions are following:（1） Hydrogen bond can be formed between the three kinds of hydrophilic headgroups （-CN,-NH2,-COOH）, which makes their effection on the internal structure ofSAM is greater than the three kinds of hydrophobic head groups （-CH₃,-C=C,-OCH₃）.Especially complex hydrogen mesh structure can be formed between adjacent headgroups in the SAM with COOH head group, which makes its effection on the internalstructure of SAM is greatest.（2） The film thickness and the included angle between the C18alkane chains andthe Si surface substrate of the SAM with three hydrophilic head groups （-CN,-NH2,-COOH） is generally less than the SAM with three hydrophobic head groups （-CH₃,-C=C,-OCH₃）.（3） The SAMs surface with three hydrophilic head groups （-CN,-NH2,-COOH）are getting more smooth, flat due to the interactions of hydrogen bonding between the hydrophilic head groups. While the SAMs surface with three hydrophobic head groups（-CH₃,-C=C,-OCH₃） are getting more wrinkled.2. The wettability of four flat and four fold self-assembled monolayers withdifferent head groups （-CH₃,-C=C,-CN,-COOH） are studied with a combinationmethod of Molecular Dynamics and Quantum Chemistry, the results show that: thecoverage change of water on the self-assembled monolayer surface with-CH₃and-C=C head groups is not obvious before and after the simulation, which illustrates thatthe increase of corrugation degree has little effect on the wetting of two hydrophobicself-assembled monolayer surface. As to the two kinds of self-assembled monolayerwith-CN head group, the combination of two alkyl chains and water molecules isthrough a bridge structure, and it can not form a multi-water bridge structure. Theincrease of corrugation degree has not change this combination way before and afterthe simulation, so the increase of corrugation degree has little effect on the surfacecoverage. As to the two kinds of self-assembled monolayer with–COOH head group, adouble water bridge structure is formed between its two alkyl chains and watermolecules, water molecules can not penetrate deeply into the alkyl chains since thedistance between the two head groups is shorter. While with the increasement ofcorrugation degree, the distance between the adjacent hydrophilic head groups isgetting longer, which makes the vertical distance between the two head groups isbigger, and this distance offers space for the down adsorption of water molecules.Therefore multi-water bridge structure will be formed between the two head groups,which increases the adsorption of water molecules on the SAM surface and enhancesthe surface wettability of self-assembled monolayer.3. The binding energies between three hydrophilic head groups and the twoconformations of2-chlorophenol are calculated with DFT method, via compared thebinding energis of the three hydrophilic head groups with2-chlorophenol,2-chlorophenol with water molecule and the combination of three hydrophilic headgroups with water molecule, we know that the binding energy of-NH2head group witha2-CP is not only bigger than the binding energy of-NH2head group with watermolecule, but also bigger than the binding energy of2-CP with water molecule, then itcan be speculated that SAM with-NH2head group may become potentialchlorophenols decontamination material to enrich and separate the chlorophenols in theaqueous solution.