Odd-even effect in the hydrophobicity of n-alkanethiolate self-assembled monolayers depends upon the roughness of the substrate and the orientation of the terminal moiety.

The origin of the odd–even effect in properties of self-assembled monolayers (SAMs) and/or technologies derived from them is poorly understood. We report that hydrophobicity and, hence, surface wetting of SAMs are dominated by the nature of the substrate (surface roughness and identity) and SAM tilt angle, which influences surface dipoles/orientation of the terminal moiety. We measured static contact angles (θs) made by water droplets on n-alkanethiolate SAMs with an odd (SAMO) or even (SAME) number of carbons (average θs range of 105.8–112.1°). When SAMs were fabricated on smooth “template-stripped” metal (MTS) surfaces [root-mean-square (rms) roughness = 0.36 ± 0.01 nm for AuTS and 0.60 ± 0.04 nm for AgTS], the odd–even effect, characterized by a zigzag oscillation in values of θs, was observed. We, however, did not observe the same effect with rougher “as-deposited” (MAD) surfaces (rms roughness = 2.27 ± 0.16 nm for AuAD and 5.13 ± 0.22 nm for AgAD). The odd–even effect in hydrophobicity inverts when t...