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 (theta(s)) made by water droplets on n-alkanethiolate SAMs with an odd (SAM(O)) or even (SAM(E)) number of carbons (average theta(s) range of 105.8-112.1 degrees). When SAMs were fabricated on smooth template-stripped metal (M-TS) surfaces [root-mean-square (rms) roughness = 0.36 +/- 0.01 nm for Au-TS and 0.60 +/- 0.04 nm for Ag-TS], the odd-even effect, characterized by a zigzag oscillation in values of theta(s), was observed. We, however, did not observe the same effect with rougher as-deposited (M-AD) surfaces (rms roughness = 2.27 +/- 0.16 nm for Au-AD and 5.13 +/- 0.22 nm for Ag-AD). The odd-even effect in hydrophobicity inverts when the substrate changes from Au-TS (higher theta(s) for SAM(E) than SAM(O), with average Delta theta(s) (vertical bar n - (n + 1)vertical bar) approximate to 3 degrees) to Ag-TS (higher theta(s) for SAM(O) than SAM(E), with average Delta theta(s) (vertical bar n - (n + 1)vertical bar) approximate to 2 degrees). A comparison of hydrophobicity across Ag-TS and Au-TS showed a statistically significant difference (Student's t test) between SAM(E) (Delta theta(s) (vertical bar Ag evens - Au evens vertical bar) approximate to 5 degrees; P < 0.01) but failed to show statistically significant differences on SAM(O) (Delta theta(s) (vertical bar Ag odds) (- Au odds vertical bar) approximate to 1 degrees; p > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of M-AD versus M-TS) and orientation of the terminal -CH2CH3 (by comparing SAM(E) and SAM(O) on Au-TS versus Ag-TS) play major roles in the hydrophobicity and, by extension, general wetting properties of n-alkanethiolate SAMs.