This study investigate the combined effect of surface roughness and solvent polarity on the behavior of an n-alkanethiolate self-assembled monlayers (SAM) by studying the hydrophobicity of the SAM. Previously our studies showed the origin and limit of the phenomenon called the odd-even effect, which is a zigzag oscillation in the contact angle of SAMs with increasing carbon chain length. However, although we predicted the roughness upper limit for odd-even effect to appear should be around Rrms = 1nm, we still observed several cases of odd-even effect with SAMs formed on substrate beyond 1nm limit, primarily with the non-polar solvent hexadecane. With a closer examination of the contact angle of the SAM, we noticed that an asymmetric behavior of the odd-even effect occuring from C10 to C16 SAMs, and the degree of asymmetry depends heavily on both the surface roughness and the polarity of solvent. We thus defined a new term, the asymmetric displacement, to describe the asymmetric behavior in wetting. By plotting asymmetric displacement versus surface roughness and solvent polarity, we concluded that the previously discovered Rrms = 1nm limit is the upper limit for SAM-dominant behavior, while beyond Rrms = 1.5 nm is the lower limit for surface-dominate behavior, with a transition zone in between. We also deducted that the dominating type of interaction (polar or dispersive) is determined by the ratio of surface tension.

Effect of solvent polarity on the roughness-dependent wetting behavior of SAMs

2019 Spring National Meeting

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