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

作者：Chen Jiahao; Wang Zhengjia; Oyola Reynoso Stephanie; Gathiaka Symon M; Thuo Martin^*

来源：Langmuir, 2015, 31(25): 7047-7054.

DOI：10.1021/acs.langmuir.5b01662

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 similar to 0.2-0.5 nm and compared the wetting properties of n-alkanethiolate (C-10-C-10) SAMs fabricated on these surfaces. Static contact angles, theta(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 Delta theta(sin - (n+1)vertical bar) approximate to 1.7 degrees (n = even) and Delta theta(sin - (n+1)vertical bar) approximate to 3.1 degrees (n = odd). A gradual increase in theta(s), with increasing length of the molecule was observed, with values ranging from water 104.7-110.7 degrees(overall Delta theta(s) = 6.0 degrees while for the evens Delta theta(E)(s) = 4.4 degrees and odds Delta theta(O)(s) = 3.5 degrees) to diethylene glycol 72.9-80.4 degrees (overall Delta theta(s) = 7.5 degrees while for the evens Delta theta(E)(s) = 2.9 degrees and odds Delta theta(O)(s) = 2.4 degrees) and hexadecane 40.4-49.4 degrees (overall Delta theta(s) = 9.0 degrees while for the evens Delta theta(E)(s) = 3.7 degrees and odds Delta theta(O)(s) = 2.1 degrees). This article establishes that the gradual increase in theta(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 theta(s) on surface roughness we infer that (i) RMS roughness approximate to 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 theta(s) is similar to 3 degrees.

• 出版日期2015-6-30