title: Molecular Understanding on the Underwater Oleophobicity of Self-Assembled Monolayers: Zwitterionic versus Nonionic.
abstract: 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° 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 on the effect of different alkyl ammonium ions on mixed-charged SAMs reveals that the underwater OCAs are >143.6° 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.
authors.affiliation: School of Chemistry and Chemical Engineering, Guangdong Provincial Key Laboratory for Green Chemical Product Technology, South China University of Technology , Guangzhou 510640, China.
authors.forename: Gang
authors.identifier: 0000-0002-0672-1656
authors.initials: G
authors.lastname: Cheng
authors.affiliation: School of Chemistry and Chemical Engineering, Guangdong Provincial Key Laboratory for Green Chemical Product Technology, South China University of Technology , Guangzhou 510640, China.
authors.forename: Mingrui
authors.initials: M
authors.lastname: Liao
authors.affiliation: School of Chemistry and Chemical Engineering, Guangdong Provincial Key Laboratory for Green Chemical Product Technology, South China University of Technology , Guangzhou 510640, China.
authors.forename: Daohui
authors.initials: D
authors.lastname: Zhao
authors.affiliation: School of Chemistry and Chemical Engineering, Guangdong Provincial Key Laboratory for Green Chemical Product Technology, South China University of Technology , Guangzhou 510640, China.
authors.forename: Jian
authors.identifier: 0000-0002-3033-7785
authors.initials: J
authors.lastname: Zhou