Experimental Study of the Influence of the Adsorbate Layer Composition on the Wetting of Different Substrates with Water

Abstract

Wetting is strongly influenced by adsorbate layers, which are omnipresent on surfaces. The influence of the composition and thickness of adsorbate layers on the water contact angle of sessile drops on different substrates was systematically investigated in the present work. Measurements were carried out for gold-sputtered substrates. These new results are compared to results from a previous study, in which corresponding measurements were carried out for technical steel and titanium substrates. In all experiments, different pretreatments of the samples were used to obtain variations of the adsorbate layer. The samples were either exposed to an oil bath or not, and different cleaning agents were used. The analysis of the adsorbate layer was carried out with X-ray photoelectron spectroscopy (XPS). The results for the different substrates reveal that the water contact angle depends mainly on the composition of the adsorbate layer. The substrate has only an indirect influence, as it influences the composition of the adsorbate layer. The thickness of the adsorbate layers was between 1.4 and 14 nm and was large enough to prevent a direct influence of the substrate on the water contact angle. It is shown that using the information on the adsorbate layer composition from XPS and the results for the water contact angle obtained for the gold samples alone, the water contact angles on the steel and titanium samples can be predicted.

Molecular Dynamics Study of Wetting and Adsorption of Binary Mixtures of the Lennard-Jones Truncated and Shifted Fluid on a Planar Wall

“…These findings are in good agreement with experimental data, which do not show any influence of the substrate on the wetting for adsorbate layers thicker than 1 – 2 nm; cf. ref .…”

“…4 In most situations, the underlying substrate has no direct influence on the sessile drop when the adsorbate layer is thicker than about 1−2 nm. 5,6 In recent experimental studies on surfaces with adsorbate layer, that is, gold, steel, and titanium, Heier et al 2 observed that the contact angle depends only on the adsorbate layer composition for an adsorbate layer thicker than 1.4 nm. 1,2 In contrast, the wetting is influenced by the underlying substrate directly when the adsorbate layer is thinner than about 1 nm.…”

“…5,6 In recent experimental studies on surfaces with adsorbate layer, that is, gold, steel, and titanium, Heier et al 2 observed that the contact angle depends only on the adsorbate layer composition for an adsorbate layer thicker than 1.4 nm. 1,2 In contrast, the wetting is influenced by the underlying substrate directly when the adsorbate layer is thinner than about 1 nm. 7 Molecular simulations help gaining a detailed understanding of wetting phenomena and have been carried out by many authors, see for example, refs 8−10 Molecular simulation studies of surface wetting by a pure fluid obviously describe the wetting of atomically clean surfaces.…”

“…Technical surfaces are always contaminated by adsorbed residues, which form an adsorbate layer. The composition and thickness of that adsorbate layer depend on the pretreatment of the surface, the surrounding fluid (e.g., air), and the underlying substrate and can, for example, be studied by X-ray photoelectron spectroscopy (XPS). , Surfaces without adsorbate layers (i.e., atomically clean surfaces) can only be obtained by special treatments such as plasma cleaning and storage in ultrahigh vacuum. The adsorbate layer leads to a change in the wetting behavior compared to the atomically clean surface and it has long been known that the adsorbate layer strongly influences the contact angle of a sessile drop and must not be neglected in studies of wetting of surfaces .…”

“…The adsorbate layer leads to a change in the wetting behavior compared to the atomically clean surface and it has long been known that the adsorbate layer strongly influences the contact angle of a sessile drop and must not be neglected in studies of wetting of surfaces . In most situations, the underlying substrate has no direct influence on the sessile drop when the adsorbate layer is thicker than about 1–2 nm. , In recent experimental studies on surfaces with adsorbate layer, that is, gold, steel, and titanium, Heier et al observed that the contact angle depends only on the adsorbate layer composition for an adsorbate layer thicker than 1.4 nm. , In contrast, the wetting is influenced by the underlying substrate directly when the adsorbate layer is thinner than about 1 nm …”