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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Abstract
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°