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

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• 作者：Jiahao Chen ; Zhengjia Wang ; Stephanie Oyola-Reynoso ; Symon M. Gathiaka ; Martin Thuo
• 刊名：Langmuir
• 出版年：2015
• 出版时间：June 30, 2015
• 年：2015
• 卷：31
• 期：25
• 页码：7047-7054
• 全文大小：558K
• ISSN：1520-5827

文摘

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 鈭?.2鈥?.5 nm and compared the wetting properties of n-alkanethiolate (C₁₀鈥揅₁₆) SAMs fabricated on these surfaces. Static contact angles, 胃_s, formed between the SAM and water, diethylene glycol, and hexadecane showed an odd鈥揺ven effect irrespective of the solvent properties. The average differences in subsequent SAM^E and SAM^O are 螖胃_{s|n聽鈥€撯€?n+1)|} 鈮?1.7掳 (n = even) and 螖胃_{s|n鈥?/i>鈥撯€?n+1)|} 鈮?3.1掳 (n = odd). A gradual increase in 胃_s with increasing length of the molecule was observed, with values ranging from water 104.7鈥?10.7掳 (overall 螖胃_s = 6.0掳 while for the evens 螖胃_s^E = 4.4掳 and odds 螖胃_s^O = 3.5掳) to diethylene glycol 72.9鈥?0.4掳 (overall 螖胃_s = 7.5掳 while for the evens 螖胃_s^E = 2.9掳 and odds 螖胃_s^O = 2.4掳) and hexadecane 40.4鈥?9.4掳 (overall 螖胃_s = 9.0掳 while for the evens 螖胃_s^E = 3.7掳 and odds 螖胃_s^O = 2.1掳). This article establishes that the gradual increase in 胃_s with increasing molecular size in SAMs is due to asymmetry in the zigzag oscillation in the odd鈥揺ven 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 胃_s on surface roughness we infer that (i) RMS roughness 鈮?1 nm is a theoretical limit beyond which the odd鈥揺ven effect cannot be observed and (ii) on a hypothetically flat surface the maximum difference in hydrophobicity, as expressed in 胃_s, is 鈭?掳.