Concentration-driven surface transition in the wetting of mixed alkanethiol monolayers on gold

Abstract

The construction of mixed monolayers containing hydrophobic and hydrophilic components for which the contact angles for three different liquids vary as a highly nonlinear function of the monolayer composition is reported. It is suggested that a prewetting, crystalline-like layer of water, possibly formed from bulk vapor, is present near the hydrophilic surface, because of an enhanced surface chemical potential ("surface field"). As the concentration of the hydrophilic component is lowered, increasing "quenched randomness" in the distribution of surface fields destroys the surface condensed water phase, thus triggering the observed nonlinearity in the contact angles. The microscopic structure of the water molecules adsorbed on an OH surface is revealed by continuum Monte Carlo simulations, with realistic force fields, and the scenario is supported by mean-field calculations on a simplified lattice-gas model. The observed wetting behavior at 30% relative humidity was altered for a relative humidity 52%, as well as when the surface of the monolayer was molecularly roughened by the addition of two CH2 groups to the hydrophobic (CH,-terminated) component of the mixed monolayers. It is suggested that this transitional phenomenon is due to a possible (true or rounded) surface phase transition, due to the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. IntroductionWetting behavior of ordered and random surfaces has generated considerable interest recently,] in particular, the understanding of its relationship to the surface structure at the molecular level. Therefore, polymeric2 and monolayer surfaces, especially those of thiols on gold,) have been intensively investigated as model systems. Modern theories of wetting are based on phase diagrams representing competing ordering between bulk and surface

Comparative electrochemical study of self-assembled monolayers of 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and 2-mercaptobenzimidazole formed on polycrystalline gold electrode

“…The SAMs have been used in various areas such as molecular electronics [2], tribology [3], sensor fabrication [4], studies of electron transfer kinetics [5], and biomolecular investigations [6]. Formation of SAMs is based on spontaneous adsorption of the molecules with polar head groups designed to interact and bind specifically and chemically to the solid surfaces, especially metals, from a gas or more generally from a liquid phase.…”

Analysis of Self-Assembled Monolayers by Thermal Desorption Mass Spectrometry: Neighborhood Interaction and Hydrogen/Deuterium Exchange

“…18 Ulman proved this concentration-driven surface transition in the wetting of mixed alkanethiol monolayers on gold by a Monte-Carlo simulation and mean-field calculations on a simplified lattice-gas model. 19 For random mixing, the fraction of asymmetric dimer was mathematically as follows: …”

Gold nanoparticles chemisorbed by a terphenyldithiol self-assembled monolayer for fabrication of a protein biosensor

“…In the let two decades, self-assembled monolayers (SAMs) of organic molecules on metal surfaces, in particular those of thiol molecules on Au(111) have attracted much attention, because of their potential applications in molecular technology [37][38][39]. These films usually have an interfacial region with well-defined composition, structure, and thickness, which serves as an ideal model system for understanding a variety of interfacial phenomena, for example wetting [40,41], adhesion [42,43], and catalysis [44].…”