Properties of Self-Assembled Monolayers Revealed via Inverse Tensiometry

Abstract

Self-assembled monolayers (SAMs) have emerged as a simple platform technology and hence have been broadly studied. With advances in state-of-the-art fabrication and characterization methods, new insights into SAM structure and related properties have been delineated, albeit with some discrepancies and/or incoherencies. Some discrepancies, especially between experimental and theoretical work, are in part due to the misunderstanding of subtle structural features such as phase evolution and SAM quality. Recent work has, however, shown that simple techniques, such as the measurement of static contact angles, can be used to delineate otherwise complex properties of the SAM, especially when complemented by other more advanced techniques. In this article, we highlight the effect of nanoscale substrate asperities and molecular chain length on the SAM structure and associated properties. First, surfaces with tunable roughness are prepared on both Au and Ag, and their corresponding n-alkanethiolate SAMs are characterized through wetting and spectroscopy. From these data, chain-length- and substrate-morphology-dependent limits to the odd-even effect (structure and properties vary with the number of carbons in the molecules and the nature of the substrate), parametrization of gauche defect densities, and structural phase evolution (liquidlike, waxy, crystalline interfaces) are deduced. An evaluation of the correlation between the effect of roughness and the components of surface tension (polar-γ and dispersive-γ) reveals that wetting, at nanoscale rough surfaces, evolves proportionally with the ratio of the two components of surface tension. The evolution of conformational order is captured over a range of molecular lengths and parametrized through a dimensionless number, χ. By deploying a well-known tensiometry technique (herein the liquid is used to characterize the solid, hence the term inverse tensiometry) to characterize SAMs, we demonstrate that complex molecular-level phenomena in SAMs can be understood through simplicity.

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry, not Monotonically by Hydrophobicity

“…Through the analysis based on Bragg's law, it can be concluded that the long‐chain SAMs have stronger tendency to form ordered hexagonal structures with symmetry, as compared to the short‐chain ones . Moreover, the evidence from nuclear magnetic resonance (NMR) spectroscopy, sum frequency generation (SFG) studies and molecular dynamics (MD) simulations also suggest that the molecular mobility of long‐chain SAMs is significantly restricted because the strong intermolecular van der Waals attraction reduces the free volume and increases the packing density, and the great lag in the molecular correlation and orientation could also imply the crystalline‐like arrangement of the long‐chain hydrocarbon chains . In addition, surface nanoscale structure and molecular mobility can be tuned within the SAM by coadsorbing two kinds of thiols with different chain lengths ,,.…”

Substrate-Induced Liquid Layering: A New Insight into the Heterogeneous Nucleation of Liquid Metals

“…The existence of covalent bonds between at the S/L interfaces renders a strong stability of the liquid layers. The covalently bonded liquid layers, which form spontaneously when in contact with TiB 2 and sapphire substrates, are analogous to the layered water [6] near solid substrates and the self-assembled monolayers (SAMs) grown above the metal substrates [53,54]. Hence, the theories in these systems can be borrowed to analyze the liquid metal layering near the nucleant substrates (normally ceramics).…”

“…It is necessary to explore the factors that might contribute to these differences. The surface roughness of the substrate is a vital factor that greatly influences the behaviors of the nearsurface layering or the self-assembly of atoms (or molecules) [8,53,54]. In this work, we focus on the atomically smooth surfaces, and our results show that when liquid Al is in contact with the atomically smooth surfaces of (0001) sapphire and (0001) TiB2, stratified liquid structures develop spontaneously.…”

Stabilization of Undercooled Metals via Passivating Oxide Layers

“…We infer that, due to the nature of the carboxyl ligands,shorter chains would tend to pack better on the curved surface due to limited degree of freedom and thus form the most ideal and equally distributed passivating layer. [34] Formic acid (lowest pK a ), however might be too acidic, hence high etching rate that is detrimental towards the integrity of the native oxide and the metal itself,r esulting in significant dealloying (Supporting Information, Figure S14b). Longer chains would also lead to phase transformation into am ore independent system (selfassembled monolayer), [34] thus increasing the interfacial distance and reducing the intensity of the chemical potential gradient and mirror charge within the surface oxide.…”