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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Abstract
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