Computer Molecular Simulations On The Underwater Oleophobicity Of Zwitterionic Surface Materials

Zwitterionic materials can bind with water molecules by the electrostatic and hydrogen-bond interaction due to their unique molecular structure.As a result,a stable and dense hydration layer can be formed on the material surfaces.Therefore,zwitterionic materials show an excellent hydrophilic and antifouling ability,which are considered as the desirable underwater super-oleophobic materials.However,understanding the underlying wetting mechanism is still missing.In particular,the previous works have demonstrated that the hydrophilicity of zwitterions was closely dependent on their molecular structure.Therefore,it is necessary to explore the mechanism of oleophobicity of zwitterionic materials at the molecular level.The study of the molecular structure-underwater oleophobicity relationship of zwitterionic materials can provide a theoretical guidance for the design and development of a new generation anti-fouling,oil/water separation and underwater self-cleaning products.In this dissertation,molecular dynamics(MD)simulations were performed to investigate the effects of carbon spacer lengths(CSL,the carbon chain between positively and negatively groups in zwitterionic monomers)between the positively and negatively charged groups in zwitterionic monomers,carbon spacer modified with hydroxyl groups,types of positively and negatively charged groups,as well as connection patterns of positively and negatively charged groups on the hydration and underwater oleophobicity of zwitterionic self-assembled monolayers(SAMs).Moreover,the influence of grafting density on the oleophobicity and self-cleaning performance of poly(sulfobetaine methacrylate)(PSBMA)brush was also studied.The results revealed that underlying molecular mechanism on how molecular structure of zwitterion to impact the hydration and underwater oleophobicity of SAMs,as well as the self-cleaning capability of the PSBMA brush.The main contents and findings of the dissertation are as follows:1.The hydration and underwater oleophobicity of sulfobetaine-terminated self-assembled monolayers(SB-SAMs)with different carbon spacer lengths(CSL)between positively and negatively groups of SB molecules were probed by MD simulations.The simulation results show that the hydration of SB-SAMs is positively dependent on the CSL,but the hydration will remain almost the same when CSL?4.The underwater oleophobicity is strengthened and then weakened with the increase of CSL,reaching optimal performance when CSL=3 and the oil contact angle is 150.3°.InNa Cl solution,ions can weaken the self-association between positively and negatively groups of SB molecules,SB-SAMs show better underwater oleophobicity than that in pure water.2.The MD simulations were employed to explore the effect of hydroxylation of carbon spacer on the hydration and underwater oleophobicity of SB-OHn-SAMs.The simulation results elucidate that the addition of hydroxyl groups can weaken the electrostatic interaction between charged groups and water molecules,while the binding ability of SB-OHn-SAMs surface to water molecules can be enhanced.The underwater oleophobicity of SB-OHn-SAMs can be controlled by changing the position of hydroxyl groups in carbon spacer.In detail,when the hydroxyl groups locate in the middle of the carbon spacer,they can form a water bridge with the water molecules around the sulfonate groups and the quaternary amine groups to prevent the interaction between the carbon spacer and the oil molecules;accordingly,this kind of SAMs show the strongest underwater oleophobicity with the oil contact angle and adhesion force being 161.8° and 0.79 nN,respectively.However,when the hydroxyl groups are close to the charged groups,the underwater oleophobicity of SAMs are not changed or even decreased,due to the enhancement of self-association between opposite charged groups or the formation of intramolecular hydrogen bonds.3.The effect of the negatively charged groups,positively charged groups and linkages on the hydration and underwater oleophobicity of different SAMs were studied by MD simulations.The simulation results indicate that the water coordination number of negatively charged groups increase firstly and then decrease with the increase of oxygen atoms,but the hydration structure of the positively charged groups remains almost the same.Sulfonate groups have the largest coordination number of water molecules,about 6.36.The results of oil drop wetting simulation show that the most desirable underwater oleophobicity of SAMs is acquired when the negatively charged group is sulfonate group.The large-sized positively charged groups are responsible for the more coordination numbers of water molecules around the charged groups as well as the stronger binding capacity of SAM surfaces to water molecules.When the positively charged groups are tertiary amine groups and quaternary amine groups,SAMs can show excellent underwater oleophobicity and the oil contact angles being more than 150°.For the linkages between positively charged groups and negatively charged groups,the conformation of linear SAM is very tight and of high regularity.It has stronger hydration and underwater oleophobicity compared with Y-shape SAMs.4.The MD simulations combined with steered molecular dynamics(SMD)were adopted to explore the effect of grafting density on the conformation,hydration,oleophobicity and self-cleaning of the PSBMA brush.As evident from the simulation results,the conformation of the PSBMA brush undertook a transition from the loose and curling state to the tight and straightening state.The hydration analysis shows that the self-association between quaternary amine groups and sulfonate groups is enhanced as the increase of grafting density,resulting in a slight decrease of the coordination number of water molecules around charged groups.However,the residence time of water molecules is significantly prolonged at the interface region of the PSBMA brush,which indicates that the brush surface has stronger binding ability to water molecules and enables a more stable hydration layer.In the process of anti-oil,the repulsive force of the PSBMA brush is strengthened with the increase of grafting density,which is attributed to the strong hydration of the brush surface and the steric hindrance effect between PSBMA chains.This also indicates that the grafting density plays a key role in the self-cleaning performance of the PSBMA brush.If the grafting density is too low,the self-cleaning performance of PSBMA brush will be weakened and the oil will not completely separate from the brush surface.Wheras at high grafting density,the PSBMA brush can perform a self-cleaning process in a short time and oil droplets are thoroughly removed from the brush surface due to its strong hydration.