2 research outputs found

    Limits to the Effect of Substrate Roughness or Smoothness on the Odd–Even Effect in Wetting Properties of <i>n</i>‑Alkanethiolate Monolayers

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    This study investigates the effect of roughness on interfacial properties of an <i>n</i>-alkanethiolate self-assembled monolayer (SAM) and uses hydrophobicity to demonstrate the existence of upper and lower limits. This article also sheds light on the origin of the previously unexplained gradual increase in contact angles with increases in the size of the molecule making the SAM. We prepared Au surfaces with a root-mean-square (RMS) roughness of ∼0.2–0.5 nm and compared the wetting properties of <i>n</i>-alkanethiolate (C<sub>10</sub>–C<sub>16</sub>) SAMs fabricated on these surfaces. Static contact angles, θ<sub>s</sub>, formed between the SAM and water, diethylene glycol, and hexadecane showed an odd–even effect irrespective of the solvent properties. The average differences in subsequent SAM<sup>E</sup> and SAM<sup>O</sup> are Δθ<sub>s|<i>n </i> – (<i>n</i>+1)|</sub> ≈ 1.7° (<i>n</i> = even) and Δθ<sub>s|<i>n </i>– (<i>n</i>+1)|</sub> ≈ 3.1° (<i>n</i> = odd). A gradual increase in θ<sub>s</sub> with increasing length of the molecule was observed, with values ranging from water 104.7–110.7° (overall Δθ<sub>s</sub> = 6.0° while for the evens Δθ<sub>s</sub><sup>E</sup> = 4.4° and odds Δθ<sub>s</sub><sup>O</sup> = 3.5°) to diethylene glycol 72.9–80.4° (overall Δθ<sub>s</sub> = 7.5° while for the evens Δθ<sub>s</sub><sup>E</sup> = 2.9° and odds Δθ<sub>s</sub><sup>O</sup> = 2.4°) and hexadecane 40.4–49.4° (overall Δθ<sub>s</sub> = 9.0° while for the evens Δθ<sub>s</sub><sup>E</sup> = 3.7° and odds Δθ<sub>s</sub><sup>O</sup> = 2.1°). This article establishes that the gradual increase in θ<sub>s</sub> with increasing molecular size in SAMs is due to asymmetry in the zigzag oscillation in the odd–even effect. Comparison of the magnitude and proportion differences in this asymmetry allows us to establish the reduction in interfacial dispersive forces, due to increasing SAM crystallinity with increasing molecular size, as the origin of this asymmetry. By comparing the dependence of θ<sub>s</sub> on surface roughness we infer that (i) RMS roughness ≈ 1 nm is a theoretical limit beyond which the odd–even effect cannot be observed and (ii) on a hypothetically flat surface the maximum difference in hydrophobicity, as expressed in θ<sub>s</sub>, is ∼3°

    Odd–Even Effect in the Hydrophobicity of <i>n</i>‑Alkanethiolate Self-Assembled Monolayers Depends upon the Roughness of the Substrate and the Orientation of the Terminal Moiety

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    The origin of the odd–even effect in properties of self-assembled monolayers (SAMs) and/or technologies derived from them is poorly understood. We report that hydrophobicity and, hence, surface wetting of SAMs are dominated by the nature of the substrate (surface roughness and identity) and SAM tilt angle, which influences surface dipoles/orientation of the terminal moiety. We measured static contact angles (θ<sub>s</sub>) made by water droplets on <i>n</i>-alkanethiolate SAMs with an odd (SAM<sup>O</sup>) or even (SAM<sup>E</sup>) number of carbons (average θ<sub>s</sub> range of 105.8–112.1°). When SAMs were fabricated on smooth “template-stripped” metal (M<sup>TS</sup>) surfaces [root-mean-square (rms) roughness = 0.36 ± 0.01 nm for Au<sup>TS</sup> and 0.60 ± 0.04 nm for Ag<sup>TS</sup>], the odd–even effect, characterized by a zigzag oscillation in values of θ<sub>s</sub>, was observed. We, however, did not observe the same effect with rougher “as-deposited” (M<sup>AD</sup>) surfaces (rms roughness = 2.27 ± 0.16 nm for Au<sup>AD</sup> and 5.13 ± 0.22 nm for Ag<sup>AD</sup>). The odd–even effect in hydrophobicity inverts when the substrate changes from Au<sup>TS</sup> (higher θ<sub>s</sub> for SAM<sup>E</sup> than SAM<sup>O</sup>, with average Δθ<sub>s |<i>n</i> – (<i>n</i> + 1)|</sub> ≈ 3°) to Ag<sup>TS</sup> (higher θ<sub>s</sub> for SAM<sup>O</sup> than SAM<sup>E</sup>, with average Δθ<sub>s |<i>n</i> – (<i>n</i> + 1)|</sub> ≈ 2°). A comparison of hydrophobicity across Ag<sup>TS</sup> and Au<sup>TS</sup> showed a statistically significant difference (Student’s <i>t</i> test) between SAM<sup>E</sup> (Δθ<sub>s |Ag evens – Au evens|</sub> ≈ 5°; <i>p</i> < 0.01) but failed to show statistically significant differences on SAM<sup>O</sup> (Δθ<sub>s |Ag odds – Au odds|</sub> ≈ 1°; <i>p</i> > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of M<sup>AD</sup> versus M<sup>TS</sup>) and orientation of the terminal −CH<sub>2</sub>CH<sub>3</sub> (by comparing SAM<sup>E</sup> and SAM<sup>O</sup> on Au<sup>TS</sup> versus Ag<sup>TS</sup>) play major roles in the hydrophobicity and, by extension, general wetting properties of <i>n</i>-alkanethiolate SAMs