Molecular Understanding on the Underwater Oleophobicity of Self Assembled Monolayers: Zwitterionic versus Nonionic

Molecular dynamics simulations are conducted to investigate the underwater oleophobicity of self-assembled monolayers (SAMs) with different head groups. Simulation results show that the order of underwater oleophobicity of SAMs is methyl < amide < oligo(ethylene glycol) (OEG) < ethanolamine (ETA) < hydroxyl < mixed-charged zwitterionic. The underwater oil contact angles (OCAs) are <133 degrees for all nonionic hydrophilic SAMs, while the mixed-charged zwitterionic SAMs are underwater superoleophobic (OCA can reach 180). It appears that surfaces with stronger underwater oleophobicity have better antifouling performance. Further study charged SAMs reveals that the underwater OCAs are >143.6 degrees for all SAMs; mixed-charged SAMs containing primary alkyl ammonium ion are likely to possess the best underwater oleophobicity for its strong hydration capacity. It seems that alkyl sulfonate anion (SO3-) is more hydrophilic than alkyl trimethylammonium ion (NC3+) for the hydrophobic methyl groups on nitrogen atoms and that the hydration of SO3- in mixed-charged SAMs can be seriously blocked by NC3+. The monomer of SO3- should be slightly longer than that of NC3+ to obtain better underwater oleophobicity in NC3+-/SO3--SAMs. In addition, the underwater oleophobicity of SAMs might become worse at low grafting densities. This work systematically proves that a zwitterionic surface is more underwater oleophobic than a nonionic surface. These results will help for the design and development of superoleophobic surfaces.