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

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 (C₁₀–C₁₆) SAMs fabricated on these surfaces. Static contact angles, θ_s, 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^E and SAM^O are Δθ_{s|<i>n </i> – (<i>n</i>+1)|} ≈ 1.7° (<i>n</i> = even) and Δθ_{s|<i>n </i>– (<i>n</i>+1)|} ≈ 3.1° (<i>n</i> = odd). A gradual increase in θ_s with increasing length of the molecule was observed, with values ranging from water 104.7–110.7° (overall Δθ_s = 6.0° while for the evens Δθ_s^E = 4.4° and odds Δθ_s^O = 3.5°) to diethylene glycol 72.9–80.4° (overall Δθ_s = 7.5° while for the evens Δθ_s^E = 2.9° and odds Δθ_s^O = 2.4°) and hexadecane 40.4–49.4° (overall Δθ_s = 9.0° while for the evens Δθ_s^E = 3.7° and odds Δθ_s^O = 2.1°). This article establishes that the gradual increase in θ_s 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_s 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_s, 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

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_s) made by water droplets on <i>n</i>-alkanethiolate SAMs with an odd (SAM^O) or even (SAM^E) number of carbons (average θ_s range of 105.8–112.1°). When SAMs were fabricated on smooth “template-stripped” metal (M^TS) surfaces [root-mean-square (rms) roughness = 0.36 ± 0.01 nm for Au^TS and 0.60 ± 0.04 nm for Ag^TS], the odd–even effect, characterized by a zigzag oscillation in values of θ_s, was observed. We, however, did not observe the same effect with rougher “as-deposited” (M^AD) surfaces (rms roughness = 2.27 ± 0.16 nm for Au^AD and 5.13 ± 0.22 nm for Ag^AD). The odd–even effect in hydrophobicity inverts when the substrate changes from Au^TS (higher θ_s for SAM^E than SAM^O, with average Δθ_{s |<i>n</i> – (<i>n</i> + 1)|} ≈ 3°) to Ag^TS (higher θ_s for SAM^O than SAM^E, with average Δθ_{s |<i>n</i> – (<i>n</i> + 1)|} ≈ 2°). A comparison of hydrophobicity across Ag^TS and Au^TS showed a statistically significant difference (Student’s <i>t</i> test) between SAM^E (Δθ_{s |Ag evens – Au evens|} ≈ 5°; <i>p</i> < 0.01) but failed to show statistically significant differences on SAM^O (Δθ_{s |Ag odds – Au odds|} ≈ 1°; <i>p</i> > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of M^AD versus M^TS) and orientation of the terminal −CH₂CH₃ (by comparing SAM^E and SAM^O on Au^TS versus Ag^TS) play major roles in the hydrophobicity and, by extension, general wetting properties of <i>n</i>-alkanethiolate SAMs