Abstract
This review discusses the Porter-Whitesides discrepancy in wetting properties of n-alkanethiolate self-assembled monolayers (SAMs). About 25 years ago, Whitesides and coworker failed to observe any odd-even effect in wetting, however, Porter and his coworker did, albeit in select cases. Most previous studies agreed with Whitesides’ results, suggesting the absence of the odd-even effect in hydrophobicity of n-alkanethiolate SAMs. Recent reports have, however, found the odd-even effect in hydrophobicity of n-alkanethiolate SAMs on smooth substrates, indicating that hydrophobicity, and analogous interfacial properties, of n-alkanethiolate SAMs significantly depends on the properties of substrate. Unfortunately, the Whitesides and Porter papers do not report on the quality of the surfaces used. Based on recent work, we inferred that the original discrepancy between Whitesides and Porter can be attributed to the quality of the surface. Odd-even effect of SAMs in charge transport, capacitance, friction, and SAM structure are also discussed in this review to inform the general discussion. The discrepancy between Porter's group and Whitesides’ group could be due to surface roughness, morphology, oxidation, and adventitious contaminants.
Highlights
The study of organic thin films began about 200 years ago, when Franklin discovered, in 1774, the calming effect of oil on water surface [1]
The odd-even effect of wetting properties was observed in CH3(CH2)nS–Au monolayers with methylene iodide, nitrobenzene or DMF, but, no odd-even effect when the probe liquid was water [28]. All these results demonstrate that structures and properties, such as packing density and wettability of self-assembled monolayers (SAMs), may be altered by substituting the terminal group of the self-assembled molecule, and, the interfacial wetting property between liquid and organic monolayer will be different when the probe liquid is changed
This study suggests that the odd-even effect is a combination of interface effect and variance of inherent properties of the SAMs
Summary
The study of organic thin films began about 200 years ago, when Franklin discovered, in 1774, the calming effect of oil on water surface [1]. When n-alkanethiolate SAMs are fabricated on gold, the arrangement of the sulfur atoms on the surface defines the free space (footprint) available for the molecules, which organize to optimize intermolecular interactions within geometric constraints. These interactions are governed by the substrate topology, head-group hybridization, and, molecular constituents [15]. (i) well-ordered, crystalline alkanethiolate SAM; (ii) defective SAM with domain boundary, a by-product of the assembly process; and, (iii) poorly formed SAM whose hydrocarbon chains are not fully extended under growth conditions and/or different substrates quality This type of poor assembly can be due to use of very dilute solution or not allowing the assembly process to equilibrate, i.e., go to completion. SAMs provide a convenient, flexible, and simple system with which to tailor the interfacial properties of metals, metal oxides, and semiconductors
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
