title: 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°.
authors.affiliation: †Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011, United States.|‡Micro-Electronic Research Center, Iowa State University, 133 Applied Sciences Complex I, 1925 Scholl Road, Ames, Iowa 50011, United States.
authors.forename: Jiahao
authors.initials: J
authors.lastname: Chen
authors.affiliation: †Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011, United States.
authors.forename: Zhengjia
authors.initials: Z
authors.lastname: Wang
authors.affiliation: †Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011, United States.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Oyola-Reynoso
authors.affiliation: §Department of Chemistry, Auburn University, 179 Chemistry Building, Auburn, Alabama 36849, United States.
authors.forename: Symon M
authors.initials: SM
authors.lastname: Gathiaka
authors.affiliation: †Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011, United States.|‡Micro-Electronic Research Center, Iowa State University, 133 Applied Sciences Complex I, 1925 Scholl Road, Ames, Iowa 50011, United States.|∥Biopolymer and Biocomposites Research Team, Center for Bioplastics and Biocomposites, Iowa State University, 1041 Food Sciences Building, Ames, Iowa 50011, United States.
authors.forename: Martin
authors.initials: M
authors.lastname: Thuo
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D005026:Ethylene Glycols / Q000737:chemistry; D017155:Wettability
chemical_list: D000473:Alkanes; D005026:Ethylene Glycols; C013484:diethylene glycol; C007932:n-hexadecane