Changes in the wettability of the surface of soda-lime glass and of sol-gel-derived lithium silicate thin films during storage at room temperature

Maintaining the water wettability for extended period is generally tough to achieve on solid surface. We stored a soda-lime glass plate in a container equipped with an air purifier with a fan filter, where the water contact angle increased from 3° to 33° in 2 days. X-ray photoelectron spectroscopic (XPS) spectra obtained on the glass surface showed that hydrocarbons increased in amount during storage, indicating their adsorption as one of the causes of the increased contact angle during storing. Next we prepared lithium silicate thin films with nominal Li/Si mole ratios of 0.12–0.86 on Si(100) wafers by sol-gel method from lithium nitrate-tetraethyl orthosilicate solutions. The films were stored in the container with the air purifier, and the water contact angle was measured as a function of storage time. The films of Li/Si ≥ 0.24 kept contact angles lower than 15° over 10 days although hydrocarbons increased in amount on the surface during storage, which was examined by XPS analysis on the film of Li/Si = 0.24. The film surface turned cloudy during storage, which we attributed to lithium carbonate precipitates by field emission scanning electron microscopy (FE-SEM) and infrared (IR) absorption spectroscopy. We thought that hydrocarbons adsorbed on lithium carbonate precipitates may be washed out when a water droplet is placed on the film surface in contact angle measurements, and that such hydrocarbon removal may lead to the long-term duration of the low contact angle. Elemental depth profile analysis by XPS showed that Li⁺ ions leach out from the film during soaking in water, resulting in the formation of a silica film. We expected the resulting silica film to be porous and hence to exhibit persistent low water contact angle due to capillary effect. However, the film showed higher initial contact angles, which further increased during storing. The silica film was unexpectedly relatively dense, which was revealed by refractive index measurements, not having porous nature with persistent hydrophilicity.