Rare-Earth Oxides Are Naturally Hydrophobic

Hydrophobic coatings find use in industries such as energy, water, transportation, construction, and medicine. But common polymer coatings degrade in response to high temperatures or abrasion. Rare-earth oxides could provide more robust coating materials because their electron configuration makes them intrinsically hydrophobic, report researchers at Massachusetts Institute of Technology (Nat. Mater., DOI: 10.1038/nmat3545). Previous work demonstrated that ceria is hydrophobic, but the property was unexplained. A team led by Gisele Azimi and Kripa K. Varanasi speculated that the full octet of electrons in lanthanides’ outer 5s and 5p orbitals would shield their unfilled 4f orbitals, leaving the metal atoms less likely to form hydrogen bonds with water. The group synthesized oxides of the entire series from cerium to lutetium, except for radioactive promethium, to test the hypothesis. They found that all of the tested oxide materials are hydrophobic. The property persists even when samples are exposed to conditions that would degrade polymer coatings. Hydrophobic rare-earth oxide coatings could be easily applied at large scales by standard ceramic processing techniques, the researchers say.