Limits to the Effect of Substrate Roughness or Smoothness on the Odd–Even Effect in Wetting Properties of n-Alkanethiolate Monolayers

Abstract

This study investigates the effect of roughness on interfacial properties of an n-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 n-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|n – (n+1)|) ≈ 1.7° (n = even) and Δθ(s|n – (n+1)|) ≈ 3.1° (n = 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°.

The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

“…Porter's data [21] showed odd-even effect; however, the oscillation in the values of contact angle was inverted compared to what we later observed [25,26,54].…”

“…The lower hydrophobicities of monolayers prepared from shorter alkanethiols (n < 9) is thought to be due to disorder in the shorter chain molecules and, potentially, interaction of the probe liquid with methylene (alkyl chain) instead of methylalthough coulombic interactions between the dipole of the probe liquid and the now proximal oxidized metal surface (due to bond formation) can also be envisioned. The value of the contact angle tend to plateau off with longer n-alkyl chains, and this was attributed to increased crystallinity of the SAMs [25,26].…”

“…[53] Template stripping can generate surfaces with roughness <1 nm [25], in which technique a glass slide or other solid support is glued to the exposed surface of a metal film deposited on silicon wafer, and then the metal film is peeled from the wafer to expose the surface that had been in direct contact with the wafer. Chen et al (2015) reported a new method, referred to as "remote mechanical annealing (ReMA)" to fabricate ultra-flat Au surface with RMS roughness as low as 0.2 nm [26].…”

Conformation-driven quantum interference effects mediated by through-space conjugation in self-assembled monolayers

“…Measuring well characterized, high-quality SAMs is paramount as, unlike top-down, single-molecule techniques (break junctions and so on) or few molecules techniques (conducting-probe AFM and so on. ), EGaIn is a bottom-up, large-area technique7 and is therefore sensitive to the detailed structure of the SAM because it defines the physical shape of the junction and the EGaIn//SAM interface212223.…”

Engineering tunable bio-inspired polymeric coatings for amphiphobic fibrous materials

“…This discourse, however, fails to capture the effect of surface chemicals and associated sub-nanometer roughness as captured through selfassembled monolayers. [48][49][50][51] Other ways to modify the surface of materials is by the dynamic process of selfassembly. The evolution of the concept of self-assembly, has eased engineering materials surface and interfaces.…”