Molecular Mechanism Of Underwater Oleophobicity Of Self-assembled Monolayer Surfaces

Materials with underwater superoleophobicity have been the research spot for their broad potential applications,including self-cleaning,oil/water separation,resistance-reduction,anti-contamination,micro-liquid control and in-liquid electrowetting.Presently,lots of materials with underwater superoleophobicity have been artificially synthesized,but the wetting mechanism is still unknown.Computer simulation helps people understand the intrinsic wetting mechanism of oleophobic materials,thus making theoretical guidance in the development of functionalized materials.The self-assembled monolayer(SAM)is easy to characterize and can adjust its surface chemistry via terminal functional groups,which is suitable for the fundamental research.This work will explore the underwater oleophobic properties of SAM surfaces by all-atom and coarse-grained molecular dynamics simulations.Firstly,the effect of surface chemistry on underwater oleophobicity of SAMs was studied by all-atom molecular dynamics.Simulation results show that the order of underwater oleophobicity of SAMs is methyl(CH3-)< amide(NH2-CO-)< oligo(ethylene glycol)(OEG-)< ethanolamine(ETA-)< hydroxyl(OH-)< mixed-charged SAMs of quaternary ammonium ions(NC3+-)and sulfonate anion(SO3-)(NC3+-/SO3--SAMs);zwitterionic surface is more underwater oleophobic than a nonionic surface.For nonionic hydrophilic SAMs,it indicates that both the ordered structure and the hydrogen bonding(HB)between the SAMs and water determine the underwater oleophobicity of SAMs,and that the HB between SAM itself is disadvantaged to the underwater oleophobicity.At lower grafting densities,the underwater oleophobicity of SAMs might become worse,and the underwater oil contact angle(OCA)for OEG-SAM is less than 83.2° but mixed-charged NC3+-/SO3--SAMs still maintain the OCA of 123.2°.Secondly,the effect of different alkyl ammonium ions and chain length on mixed-charged SAMs was explored by all-atom and coarse-grained molecular dynamics simulations.All underwater OCAs are > 143° for mixed-charged SAMs with different alkyl ammonium ions as the cation and SO3-as the anion.Mixed-charged SAMs containing primary alkyl ammonium ion possess the best underwater oleophobicity for its strong hydration capacity.The monomer of SO3-should be slightly longer than that of NC3+ to obtain better underwater oleophobicity in NC3+-/SO3--SAMs.The coarse-grained molecular simulation results show that,when the volume is similar for the cation and anion in mixed-charged SAMs,positively charged group being toward the outside of SAMs will be beneficial to the underwater oleophobicity.Finally,the structure-property relationship in the underwater oleophobicity of betaine SAMs was investigated by coarse-grained molecular dynamics simulations.With positively charged group toward the outside of SAMs,the self-association interaction is weaker and the hydration layer near the SAMs surface is denser and more stable,thus the underwater oleophobicity is improved.Increasing the chain length between positively and negatively charged groups will lead to the terminal beads bending to the inner of SAMs,resulting in the reduction of underwater oleophobicity of betaine SAMs;raising the hydrophilicity of the chain between positively and negatively charged groups will strengthen the underwater oleophobicity by a small degree.In addition,increasing the electrolyte layer thickness can evidently enhance the hydration layer and underwater oleophobicity.