New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy

Abstract

Surface functionalities and adsorption evolution of water molecules on silica gel samples were studied by second derivative near infrared spectroscopy. Four different silica gel samples with varying surface area were used in this experiment. Each of the dry samples was allowed to equilibrate with surrounding air and the near infrared spectra were accumulated at different time intervals using reflectance technique. The evolved spectra were analysed using second derivative technique to study the adsorption evolution of water molecules on the surface. The second derivative spectral profiles have revealed some interesting features about the surface functionalities and some events of adsorption on silica gel surface that have never been reported in the literature. The near infrared spectroscopic evidence is provided for the first time to show the presence of hydrogen bonded silanol groups on silica gel surface. The results and the events show that the adsorption of water molecules readily takes take place on the hydrogen bonded vicinal silanol groups and spread over the space and ends with the hydrogen bonding at free silanol groups. Water molecules then build over these molecules to form a network of hydrogen bonded water molecules. There is clear evidence that the presence of hydrogen bonded silanol groups is a necessary condition for the effective adsorption activities on silica gel surface. Furthermore, the study reveals that while mono and multi-layer water molecular adsorption takes place at certain active sites of the surface, a large portion of the surface is free from any adsorption activities.