Nanoscale condensation of water on self-assembled monolayers

Abstract

We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10-8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry. 2011 The Royal Society of Chemistry. We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage ($2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures ($10 À8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 C under atmospheric conditions did lead to evaporation from the surface.We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry.

Optimizing atomic resolution of force microscopy in ambient conditions

“…In typical conditions the humidity is so large that several water layers form on any surface. 39,40 The effect of this is a drastic lowering of Q and the dramatic increase of E ts where the tip is close enough to the surface to resolve atoms. This can be seen by the large excitation required in Fig.…”

“…Laboratory air also has a significant amount of water vapor, where the partial pressure of water depends on temperature and relative humidity (RH). If RH is greater than zero, all surfaces (hydrophobic or hydrophilic) exposed to air 39,40 adsorb a water layer with a thickness dependent on the exposure time, temperature, RH, and the sample's hydrophilic or hydrophobic character. showed that by poking the surface, the tip could be used to create monoatomic terraces due to plastic deformation of the KBr(001) cleavage plane in UHV.…”

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…Similarly, the thickness of brine films on silica increased over 100 times with surface roughness [104]. Multilayer adsorption is observed on rather hydrophobic self-assembled monolayers, from d ~ 2 nm at low humidity up to 4 nm at 65% RH [105,106]. Steps in mica terraces lead to nucleation of water deposits of height of 7 nm at 65% RH [107].…”

Molecular Clusters, Skins, and Ultrathin Films

“…In addition, the volume of water molecules confined to 5.1 and 2.8 nm TiO 2 pores increases by 4 and 7.5 %, respectively, with respect to that in the bulk [55]. More interestingly, a monolayer film of water at room temperature manifests 'quasi-solid' and hydrophobic nature that prevents the monolayer water from being wetted by a water droplet [21,56].…”