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Examining the frictional forces between mixed hydrophobic – hydrophilic alkylsilane monolayers
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10.1063/1.4729312
/content/aip/journal/jcp/136/24/10.1063/1.4729312
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/24/10.1063/1.4729312

Figures

Image of FIG. 1.
FIG. 1.

Snapshots of –Si(OH)2(CH2)6OH monolayers at (a) near zero-load and zero-adhesion state (separation 49 Å) and (b) at ∼3.25 GPa (separation 38 Å), under sliding at a constant velocity of 10 m/s and 300 K. Top-down views at near zero-load and zero-adhesion (c) and at ∼3.25 GPa (d) are also shown. Light blue spheres represent carbon atoms, red oxygens, gray hydrogens, and yellow silicons. In (c) and (d) the hydrogen atoms of the CH2 groups are not shown for clarity.

Image of FIG. 2.
FIG. 2.

Distribution of tilt angles observed in –Si(OH)2(CH2)6OH monolayers at near zero-load state (left curve) and at ∼3.25 GPa (right curve) under sliding at a constant velocity of 10 m/s and 300 K.

Image of FIG. 3.
FIG. 3.

Distribution of hydrogen bonds for –Si(OH)2(CH2)6OH monolayers as a function of the oxygen-hydrogen separation at constant separations of 38 Å (solid line), and 42 Å (slashed line), and constant load of 3 GPa (dotted line).

Image of FIG. 4.
FIG. 4.

Normal load as a function of separation for –Si(OH)2(CH2)6OH (circles) and Si(OH)2(CH2)5CH3 (squares) monolayers simulated under constant separation (filled symbols) and constant normal load (open symbols) at a sliding velocity of 10 m/s and 300 K. Standard deviations of the data are smaller than the size of the symbols, except for the system with OH-terminated chains, using the constant load ensemble, which shows standard deviations of up to 0.25 Å at the average separation of 43.82 Å.

Image of FIG. 5.
FIG. 5.

Shear stress (a) and number of (b) interlayer hydrogen bonds, (c) interlayer non-matching hydrogen bonds, and (d) interlayer matching hydrogen bonds, as a function of time for –Si(OH)2(CH2)6OH monolayers simulated at 300 K and a constant separation of 42 Å and sliding velocity of 10 m/s.

Image of FIG. 6.
FIG. 6.

Shear stress as a function of normal load at a sliding velocity of 10 m/s and 300 K for silica coated with –Si(OH)2(CH2)6OH chains at constant separation (filled circles) and constant normal load (open circles). Results for the –Si(OH)2(CH2)5CH3 monolayers are also shown for constant separation (filled squares) and constant load (open squares) ensembles. Lines are provided as a guide to the eye.

Image of FIG. 7.
FIG. 7.

Shear stress as a function of normal load under sliding at a constant velocity of 10 m/s and 300 K for mixed –Si(OH)2(CH2)6OH and –Si(OH)2(CH2)5CH3 monolayers using the constant separation ensemble. Values for pure –Si(OH)2(CH2)6OH and –Si(OH)2(CH2)5CH3 monolayers are depicted with filled circles and filled squares, respectively. Open circles and open squares represent mixed systems with XOH = 0.75 and XOH = 0.25, respectively. Filled and open diamonds represent the two distributions of surface chains at XOH = 0.5. Lines are provided only as a guide to the eye. (Inset graph) Interpolations of the shear stress at constant load as a function of the surface concentration for OH-terminated chains at normal loads of 3 GPa (circles) and 0 GPa (zero-load) (squares).

Image of FIG. 8.
FIG. 8.

Sum of intra- and interlayer hydrogen bonds as a function of the surface concentration for mixed –Si(OH)2(CH2)6OH and –Si(OH)2(CH2)5CH3 monolayers sliding at a constant velocity of 10 m/s and 300 K in the constant separation ensemble. Filled and open symbols represent results at 0.15 and 3.25 GPa, respectively. The lines are provided as a guide to the eye and represent linear (0.15 GPa) and quadratic (3.25 GPa) fittings. Inset graph: Shear stresses as a function of the sum of intra- and inter-layer hydrogen bonds. Symbols represent the same states as the main figure.

Image of FIG. 9.
FIG. 9.

Snapshot from simulations of mixed –Si(OH)2(CH2)6OH (50%) and –Si(OH)2(CH2)11CH3 (50%) monolayers under sliding at constant velocity of 10 m/s and 300 K using the constant separation ensemble at a normal load of 0.15 GPa. Hydrogens linked to the carbon atoms are not shown to facilitate the localization of OH groups. Color scheme is the same as Fig. 1.

Image of FIG. 10.
FIG. 10.

Shear stress as a function of normal load under sliding at constant velocity of 10 m/s and 300 K for silica coated with mixed –(CH2)6OH-terminated (50%) and –(CH2)nCH3-terminated (50%) chains using the constant separation ensemble. Values for pure –(CH2)6OH-terminated chains are depicted with filled circles. Open squares represent the mixed systems with n = 5, filled squares n = 7, open circles n = 9, and filled triangles n = 17. Lines are provided only as a guide to the eye. (Inset graph) Interpolation of the shear stress at constant normal load as a function of the number of the methylene groups in the (CH2)nCH3-terminated chains at normal loads of 3 GPa (circles) and 0 GPa (zero-load) (squares).

Image of FIG. 11.
FIG. 11.

Distributions of oxygen atoms in –Si(OH)2(CH2)6OH chains along the normal direction for the mixed –Si(OH)2(CH2)6OH (50%) and –Si(OH)2(CH2)nCH3-t (50%) monolayers at 300 K and a constant separation of ∼38 Å (normal load ∼3 GPa) and a sliding velocity of 10 m/s. (a) n = 5, (b) n = 9, (c) n = 11, and (d) n = 17. z = 0 indicates the position at the center of the simulated system between the two monolayers.

Image of FIG. 12.
FIG. 12.

Shear stress as a function of simulation time for the mixed –Si(OH)2(CH2)6OH (50%) and –Si(OH)2(CH2)9CH3 (50%) monolayer under sliding at a constant velocity of 10 m/s and 300 K using the constant separation ensemble at a normal load ∼3 GPa.

Tables

Generic image for table
Table I.

Number of hydrogen bonds for –Si(OH)2(CH2)6OH monolayers under sliding at a constant velocity of 10 m/s and 300 K. A hydrogen bond is defined by oxygen – hydrogen and oxygen – oxygen separations of less than 2.6 Å and 3.6 Å, respectively, following the work of Kelkar et al. 60

Generic image for table
Table II.

Number of hydrogen bonds for mixed monolayers of –Si(OH)2(CH2)6OH and –Si(OH)2(CH2)5CH3 chains at different compositions under sliding at a constant velocity of 10 m/s and 300 K.

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/content/aip/journal/jcp/136/24/10.1063/1.4729312
2012-06-25
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Examining the frictional forces between mixed hydrophobic – hydrophilic alkylsilane monolayers
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/24/10.1063/1.4729312
10.1063/1.4729312
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