Quantifying Gauche Defects and Phase Evolution in Self-Assembled Monolayers through Sessile Drops

Abstract

Self-assembled monolayers (SAMs) are widely used in surface modifications, specifically in tuning the surface chemistry of materials. The structure and properties of SAMs have been extensively studied often with sophisticated tools, even for the simplest n -alkanethiolate SAMs. In SAMs, especially in linear n -alkanethiolates, the properties are dependent on the chain length, which is best manifested in the so-called odd–even effect, a simple yet not fully understood phenomenon. One main challenge is fully delineating the origin of length-dependent properties, which can be due to the structure (ideal SAMs), defect evolution, or substrate-molecule effects. This study demonstrates that utilizing the wetting behavior of polar (water) and nonpolar (hexadecane (HD)) solvents on n -alkanethiolate SAMs formed on ultraflat gold and silver surfaces, the evolution of chain-length-dependent gauche defects can be revealed and parameterized through a newly defined dimensionless number (χ). The observation of the odd–even effect in hydrophobicity, however, depends on the thiol chain length, and it was only observed on longer-chain (>C 8 ) molecules. The trend in this odd–even effect demonstrates that there are three main transitions in the nature of wetting, hence structure, across n -alkanethiols. From wetting with HD, the role of dispersive components in wetting reveal that the SAMs are dynamic, which we attribute to rotations associated with previously reported evolution in gauche defects and changes in packing density. Therefore, from re-expression of the Young–Dupre equation, we define a new dimensionless number associated with molecular conformations, whose periodicity mirrors the energetics of Goodman’s conformations of n -alkanes in unbound states and associated four- or two-twist turns. Therefore, we infer that the evolution in surface energy is largely due to molecular conformations and associated relaxations of the bound thiolates.

Ambient synthesis of nanomaterials by in situ heterogeneous metal/ligand reactions

“…Analogous to self-assembled monolayers (SAM), longer chain lengths improve secondary interactions between chains and promote crystal growth. 35 The PXRD pattern shows that the dominant diffraction peak shifts to lower angles with increasing chain length (Fig. 6c), which signals an increasing interplanar distance.…”

Odd–even alternations in helical propensity of a homologous series of hydrocarbons

“…6 However, recently, odd-even effects have been linked to the liquid-like characteristics of self-assembled monolayers, specifically, the level of deviation of chains from the all-trans conformer to so called 'gauche defect' conformers, dynamic behaviour that appears to alternate in a zig-zag fashion with increasing chain length. 8,9,10 Furthermore, molecular dynamics simulations predict odd-even effects in the stretch-induced coil-to-helix transition of isotactic polypropylenes. 11 Here we show a rare manifestation of the odd-even effect in the conformational behaviour of all-syn contiguously methyl-substituted hydrocarbons, which are useful systems for the study and exploitation of helical conformers of hydrocarbons and display marked odd-even effects in their propensity to form regular helical conformers.…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…The wettability of nonpolar surfaces by nonpolar liquids arises from attractive dispersive interactions, which can reflect the chain packing density and surface morphology of the monolayers [ 25 , 95 ]. When nonpolar liquids were used (hexadecane and perfluorodecalin), the contact angles of the F8H10-C10 and F8H10-C12 SAMs were comparable, but lower than those obtained on the F8H10SH SAM, with the F8H10-C18 SAM exhibiting the lowest values.…”