Molecular Understanding on the Underwater Oleophobicity of Self

Jan 25, 2017 - Molecular dynamics simulations are conducted to investigate the underwater oleophobicity of self-assembled monolayers (SAMs) with diffe...
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Molecular understanding on the underwater oleophobicity of self-assembled monolayers: zwitterionic versus nonionic Gang Cheng, Mingrui Liao, Daohui Zhao, and Jian Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03988 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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Figure 1. Simulation cells for different grafting patterns and the structures of monomers. Only the first layer atoms of Si(111) are grafted with monomers. In simulation cells, the gray spheres represent the ungrafted Si atoms; the yellow, blue and red spheres denote grafting sites for neutral hydrophilic, positively charged and negatively charged monomers, respectively. The simulation cells (a), (b) and (c) explicate the grafting percentage of 50%, 37.5% and 25% of the neutral hydrophilic molecules, respectively; whereas the simulation cells (d), (e) and (f) stand for grafting percentage of 50%, 37.5% and 25% of mixed-charged molecules, respectively. As for monomers, gray spheres in the bottom layer mean the grafted Si atoms and C8 alkyl chains are used in all monomers except OEG and ETA systems. The main differences in these structures are the head groups.

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Figure 2. The underwater OCAs and wetting configurations on SAMs.

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Figure 3. Mean HB numbers of SAMs per chain under water. Green bars are for HBs between SAM-water (the left y-axis); while cyan bars are for HBs of SAM-SAM (the right y-axis). A dotted line separates each bar into two parts, which means HBs of different groups.

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Figure 4. The underwater OCAs and wetting configurations on four mixed-charged SAMs

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Figure 5. Structural and dynamic properties of water molecules in the first coordination shells of the head groups for charged monomers: coordination numbers Nco (a) and residence time τre (b) of the SO3- groups with different cations; coordination numbers Nco (c) and residence time τre (d) of NHx groups in alkyl ammonium ions (x=1, 2, 3). In (c) and (d), as no hydrogen atom is directly attached to the nitrogen atom in NC3+, thus no HBs are formed between NC3+ and water molecules.

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Figure 6. Three grafting modes for NC3+-/SO3--SAMs. (a) C9 chain for SO3- monomer and C7 chain for NC3+ monomer; (b) C8 chains for SO3- and NC3+ monomers; (c) C7 chain for SO3- monomer and C9 chain for NC3+ monomer. The three grafting modes distinguish from each other only in the chain length of monomers.

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Figure 7. The underwater OCAs and wetting configurations on NC3+-/SO3--SAMs for three different grafting modes. The three grafting modes are illustrated in Fig 6.

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Figure 8. Snapshots of NC3+-/SO3--SAMs in water with three different grafting modes. The SAMs are shown with CPK, while the water near the SAM are shown in stick mode.

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Figure 9. The underwater OCAs and wetting configurations on SAMs at different grafting percentages. Each row is for the same grafting percentage and each column is for the same SAM.

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Figure 10. Density profiles of DCE molecules and key groups of SAMs along z axis at different grafting percentages of SAMs. The columns represent the SAMs of ETA-, OEG- and NC3+-/SO3--, respectively; while rows indicate the grafting percentage of 50%, 37.5% and 25%, respectively.

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TOC graphic

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Molecular understanding on the underwater oleophobicity of

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self-assembled monolayers: zwitterionic versus nonionic

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Gang Cheng, Mingrui Liao, Daohui Zhao, Jian Zhou*

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School of Chemistry and Chemical Engineering, Guangdong Provincial Key

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Laboratory for Green Chemical Product Technology, South China University of

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Technology, Guangzhou 510640, China

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Address correspondence to J. Zhou.

E-mail: [email protected] Fax: +86 20 87114069; Tel: +86 20 87114069. 1

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Abstract

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Molecular dynamics simulations are conducted to investigate the underwater oleophobicity of

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self-assembled monolayers (SAMs) with different head groups. Simulation results show that the

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order of underwater oleophobicity of SAMs is methyl < amide < oligo (ethylene glycol) (OEG)