Exploring the Role of Adsorption and Surface State on the Hydrophobicity of Rare Earth Oxides

Abstract

Rare earth oxides (REOs) are attracting attention for use as cost-effective, high-performance dropwise condensers because of their favorable thermal properties and robust nature. However, to engineer a suitable surface for industrial applications, the mechanism governing wetting must be first fully elucidated. Recent studies exploring the water-wetting state of REOs have suggested that these oxides are intrinsically hydrophobic owing to the unique electronic structure of the lanthanide series. These claims have been countered with evidence that they are inherently hydrophilic and that adsorption of contaminants from the environment is responsible for the apparent hydrophobic nature of these surfaces. Here, using X-ray photoelectron spectroscopy and dynamic water contact angle measurements, we provide further evidence to show that REOs are intrinsically hydrophilic, with ceria demonstrating advancing water contact angles of ≈6° in a clean surface state and similar surface energies to two transition metal oxides (≳72 mJ/m). Using two model volatile species, it is shown that an adsorption mechanism is responsible for the apparent hydrophobic property observed in REOs as well as in transition metal oxides and silica. This is correlated with the screening of the polar surface energy contribution of the underlying oxide with apparent surface energies reduced to <40 mJ/m for the case of nonane adsorption. Moreover, we show that the degree of surface hydroxylation plays an important role in the observed contact angle hysteresis with the receding contact angle of ceria increasing from ∼10° to 45° following thermal annealing in an inert atmosphere. Our findings suggest that high atomic number metal oxides capable of strongly adsorbing volatile species may represent a viable paradigm toward realizing robust surface coating for industrial condensers if certain challenges can be overcome.

Effect of Nd3+ Ion Concentration on the Corrosion Resistance of API X70 Steel in Chloride‐Rich Environments

“…e hydrophobic nature of rare earth oxides has been associated with its unique electronic configuration characterized by the presence of nonaccessible metallic orbitals that reduce its affinity for water, and only the oxygen atoms attached directly to the rare earth are able to form hydrogen bonds with water molecules [31,32]. Similar observations have been reported, indicating that, by increasing the atomic number of the lanthanides, their reactivity decreases due to the fact that the external electrons have more nuclear attraction [4].…”

Atmosphere-Mediated Superhydrophobicity of Rationally Designed Micro/Nanostructured Surfaces

“…Good water repellency was displayed on noble metals, [22][23][24] metal oxides, 25,26 rare earth oxides, [27][28][29][30] boron nitride, 31,32 and graphene, 33,34 which was initially attributed to the intrinsic low surface energy of the materials. 25,27,30 However, the intrinsic hydrophobicity has been debated, 25,[27][28][29][30]32 with many studies 22,28,29,35,36 showing adsorption of atmospheric volatile organic compounds (VOCs) playing an important role. The majority of these VOCs are composed of alkanes, olefins, aromatic hydrocarbons, halogenated hydrocarbons and oxygen-containing organic molecules, stemming from the biochemical, 37 industrial, 38 and human processes.…”

Dropwise condensation on solid hydrophilic surfaces

“…Maintaining a high intrinsic wettability (i.e.,  a ≪ 90°) with minimal contact angle hysteresis is difficult to achieve. Many studies have noted the formation of discrete "flat" water droplets on hydrophilic substrates during condensation due to atmospheric contamination of volatile organic compounds (VOCs) after cleaning (13,14); however, the corresponding receding contact angle is typically close to zero (15), resulting in filmwise condensation (16). Hence, for the past eight decades, dropwise condensation of water has been achieved mainly via the deposition of hydrophobic conformal coatings.…”

“…Smooth silicon wafers (P type, 〈1 0 0〉 orientation, 0 to 100 ohm·cm, single side polished) were cleaned with acetone, IPA, and DI water and then dried with nitrogen. Afterward, the wafers were treated with air plasma for 10 min before being left out uncovered in a lab environment in Urbana, IL (40°06′14″N, 88°12′44″W) to adsorb hydrocarbons and VOCs for 1 week (13)(14)(15).…”