Titanium dioxide has made a name for itself as a catalytic coating that renders glass self-cleaning. Despite the popularity, the molecular basis of some of TiO2’s useful properties puzzles scientists. For example, researchers know that shining ultraviolet light on TiO2 exposed to air and water makes the material’s surface hydrophilic by oxidizing ambient organic molecules, which produces hydroxylated species. It’s unclear though, why when left in the dark, the surface gradually switches to hydrophobic, and why it develops a so-called 2 x 1 overlayer, which suggests there are molecules binding to every other Ti atom. A team led by Jan Balajka and Ulrike Diebold of Technical University of Vienna and Melissa A. Hines of Cornell University has an answer. By controlling the purity of TiO2’s environment, the team shows that trace levels of acetic and formic acids in air bind to TiO2 and form hydrophobic layers that repel other air contaminants that are far more abundant. The acids, which were observed in Vienna and Ithaca, bind in a bidentate fashion, giving rise to the characteristic surface pattern (Science 2018, DOI: 10.1126/science.aat6752). Curiously, the layers are hydrophobic yet highly water soluble, which may play a role in TiO2’s self-cleaning properties, the team says.