Contamination and Wettability: Rare Earth Oxides

Many applications require materials that are both intrinsically hydrophobic and robust. The current technology in hydrophobic coating is limited to organic materials that are easily degraded and require lengthy processes to apply and maintain. A recent publication in Nature Materials, by Azimi et al., suggested that rare-earth oxides (REOs) are intrinsically hydrophobic (WCA of 105o) because their electronic structure prohibits their bonding with interfacial water. This is a potentially transformative discovery because metal oxides are much more robust than organic coatings, and therefore could be used in a much wider range of applications. However, the hydrophobicity of REOs is also quite unexpected because all other metal oxides are known to be super-hydrophilic in their pristine states. In addition, given that rare earth metal ions bind water strongly in aqueous solution and bulk REO surface strongly adsorbs water, it is puzzling why REOs surface would be hydrophobic. This work will show that REO’s are actually intrinsically super hydrophilic (WCA of 0o) and only exhibit hydrophobic properties upon adsorption of ambient air carbon based contaminants. Time evolution of hydrophobicity with carbon contamination is analyzed alongside different surface contamination cleaning techniques