title: Effect of Substrate Morphology on the Odd-Even Effect in Hydrophobicity of Self-Assembled Monolayers.
abstract: Surface roughness, often captured through root-mean-square roughness (Rrms), has been shown to impact the quality of self-assembled monolayers (SAMs) formed on coinage metals. Understanding the effect of roughness on hydrophobicity of SAMs, however, is complicated by the odd-even effect-a zigzag oscillation in contact angles with changes in molecular length. We recently showed that for surfaces with Rrms> 1 nm, the odd-even effect in hydrophobicity cannot be empirically observed. In this report, we compare wetting properties of SAMs on Ag and Au surfaces of different morphologies across the Rrms∼ 1 nm limit. We prepared surfaces with comparable properties (grain sizes and Rrms) and assessed the wetting properties of resultant SAMs. Substrates with Rrmseither below or above the odd-even limit were investigated. With smoother surfaces (lower Rrms), an inverted asymmetric odd-even zigzag oscillation in static contact angles (θs) was observed with change from Au to Ag. Asymmetry in odd-even oscillation in Au was attributed to a larger change in θsfrom odd to even number of carbons in the n-alkanethiol and vice versa for Ag. For rougher surfaces, no odd-even effect was observed; however, a gradual increase in the static contact angle was observed. Increase in the average grain sizes (>3 times larger) on rough surfaces did not lead to significant difference in the wetting properties, suggesting that surface roughness significantly dominated the nature of the SAMs. We therefore infer that the predicted roughness-dependent limit to the observation of the odd-even effect in wetting properties of n-alkanethiols cannot be overcome by creating surfaces with large grain sizes for surfaces with Rrms> 1 nm. We also observed that the differences between Au and Ag surfaces are dominated by differences in the even-numbered SAMs, but this difference vanishes with shorter molecular chain length (≤C3).
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.|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: Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego , Pharmaceutical Sciences Building, 9500 Gilman Drive, La Jolla, California 92093, 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.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Oyola-Reynoso
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