| Various synthetic superoleophobic surfaces play important role in different fields.The underwater oil contact angle?OCA?and oil adhesion force?OAF?are often used to describe the underwater oleophobicity of surface/interface.We employed molecular dynamics?MD?simulations and steered molecular dynamics?SMD?simulations to explore the reasons of different OCAs and OAFs on various self-assembled monolayers?SAMs?under water.In most experiments,the underwater oleophobicity of surface/interface can be enhanced by introducing rough structure into hydrophilic surface.However,the theoretical basis behind this strategy has not been clearly revealed.Rough structure was introduced by grafting polyethylene glycol?OEG-SAM?and phosphatidylcholine?PCNC3-SAM?on rough gold substrate,we found that it could not improve underwater oleophobicity of all the hydrophilic SAMs,especially the amphiphilic OEG-SAMs.Moreover,the improvement degrees of underwater oleophobicity on zwitterionic SAMs with different lengths of monomer are different.The relationship between the size of rough structure and the length of monomer in SAMs should be taken into account when introducing roughness.Although the OCAs on SAMs grafted on rough gold substrates are larger than 90°even 150°,in terms of both oil droplet adhesion and the residual rate of oil droplet at the interface,it is found that SAMs with the shortest monomer on the rough gold substrate has the strongest oil self-cleaning capability?the lowest oil droplet adhesion and the residual rate?.On smooth SAMs,the underwater OCA indicates the actual contact area between the droplet and surface.In theoretical understanding of liquid/solid adhesion,the actual contact area between liquid and solid is the most significant parameter to take into account.The order of underwater OCAs on SAMs is OEG-SAM<ETA-SAM<OH-SAM<DOPA-SAM<PO4-@N?CH3?3+-SAM<COO-/N?CH3?3+-SAM;while their order of oil adhesion is COO-/N?CH3?3+-SAM<PO4-@N?CH3?3+-SAM<ETA-SAM<DOPA-SAM<OH-SAM<OEG-SAM.On the whole,the oil adhesion force is correlated with the OCA on the SAMs,a more underwater oleophobic surface is always with a lower oil adhesion force.The ionic surfaces,especially the mixed-charged SAMs,show lower underwater adhesive force and stronger underwater superoleophobicity;there was almost no adsorbed DCE molecule on the SAM when the DCE droplet was pulled away from the bottom.In addition,in the process of oil droplet pulling,the oil contact lines on ionic SAMs gradually shortened and disappeared finally;whereas the oil contact lines on non-ionic SAMs almost kept an initial state even the main part of oil droplet separated from the SAMs.Furthermore,we studied the effects of heterogeneous interfaces on the adhesion and OCA of mixed SAMs.On the mixed SAMs composed of dopamine?DOPA?and phosphorylcholine?PCNC3?in different ratios,their underwater OCAs rise up with the increase of PCNC3component on the surface,and finally reach a peak value of 153.4°.However,their underwater oil adhesion shows an interesting trend:75%DOPA-SAMs>100%DOPA-SAMs>50%DOPA-SAMs>100%PO4-@N?CH3?3+-SAMs>25%DOPA-SAMs.The nonlinear relationship between adhesion force and the proportion of DOPA in mixed SAMs is primarily because of the multiple interactions between the amphiphilic head group of DOPA and the charged group of PCNC3,like the cation-?,?-?and H-bonding interactions.Finally,on the basis of our understanding of mixed charged SAMs and its hydration property,the controllable underwater oil wettability on mixed-charged SAMs has been studied by varying alkyl chain length of-COO-monomer.In CnCOO-/C5N?CH3?3+-SAMs?n=17,16,14,12,10,8,6,4?,when the oleophilic moieties appeared on the outermost,the SAMs showed underwater superoleophilic?OCA of0°?,the oleophilicity declined with the shorter altitude intercept between negative and positive monomers and upside down transformed into underwater oleophobic and superoleophobic?OCA of 172°?states.Overall,when negatively charged groups,instead of positively charged groups,espoused on the outmost position in mixed charged SAMs,the mixed charged SAMs show better underwater oleophobicity,ionic strength acted as a role of promoting underwater superoleophobicity in CmCOO-/C8N?CH3?3+-SAMs?m=8,7,6,5,4,3?.The hydrated ions overwhelmingly strengthened the impact on former hydration layer of pure water formed around the negatively charged groups,and improved the underwater oleophobicity of SAMs to an ideal state effectively. |
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