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

• Published in: Langmuir 2014-10, Vol.30 (40), p.11985-11992
• Main Authors: Newcomb, Lucas B, Tevis, Ian D, Atkinson, Manza B. J, Gathiaka, Symon M, Luna, Rafael E, Thuo, Martin
• Format: Article
• Language: English
• Subjects: Alkanes - chemistry; Gold - chemistry; Hydrophobic and Hydrophilic Interactions; Silver - chemistry; Sulfhydryl Compounds - chemistry; Water - chemistry; Wettability
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la5032569

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 the substrate changes from AuTS (higher θs for SAME than SAMO, with average Δθs |n – (n + 1)| ≈ 3°) to AgTS (higher θs for SAMO than SAME, with average Δθs |n – (n + 1)| ≈ 2°). A comparison of hydrophobicity across AgTS and AuTS showed a statistically significant difference (Student’s t test) between SAME (Δθs |Ag evens – Au evens| ≈ 5°; p < 0.01) but failed to show statistically significant differences on SAMO (Δθs |Ag odds – Au odds| ≈ 1°; p > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of MAD versus MTS) and orientation of the terminal −CH2CH3 (by comparing SAME and SAMO on AuTS versus AgTS) play major roles in the hydrophobicity and, by extension, general wetting properties of n-alkanethiolate SAMs.