An FTIR study of water thin films on magnesium oxide

Transmission Fourier transform infrared (FTIR) spectroscopy is used to investigate the adsorption of water on dry nitrogen, cleaved MgO(100) surfaces under ambient conditions. The majority of the water physically adsorbs to the substrate in a reversible manner. The water thin film exhibits liquid-like hydrogen bonding even at the lowest coverages detected and at temperatures as low as -10 °C. A plot of the coverage of water on MgO as a function of the water vapor pressure resembles a typical BET adsorption isotherm, indicating that the layer grows in via three-dimensional island formation. This growth mechanism is suggested not only by the isotherm but also by the fact that IR absorption due to the OH stretch of the adsorbed water does not change as a function of coverage. Water does not wet the MgO surface; at relative humidities as high as 90% the water thin film consists of just five layers of water. At no time during this series of experiments were carbon-containing features seen in the FTIR spectra.