ADSORPTION Of WATER ON SILICA and SILICATE GLASSES

Restricted (Penn State Only)
Author:
Akkopru Akgun, Betul
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
January 28, 2013
Committee Members:
  • Carlo G Pantano, Thesis Advisor
Keywords:
  • water adsorption
  • silicate glasses
  • physisorption
  • chemisorption
Abstract:
The aqueous surface chemistry of glasses has a significant effect on material properties, including chemical durability, mechanical strength and electrical conductivity. Thus, a fundamental understanding of water adsorption phenomena on glass surfaces is enormously important. This study examined adsorption phenomena on multicomponent glass surfaces by DRIFTs (Diffuse reflectance Fourier transform infrared spectroscopy) and TG-MS (Thermogravimetric mass spectroscopy). In particular, the effect of non-bridging oxygen and alkali/alkaline earth ions on the variation and concentration of surface adsorption sites, and eventually the degree of hydration, were examined on dry- and wet-ground multicomponent silicate glasses. For this purpose, commercial Ba-silicate, Ba-, Ca-, Mg-boroaluminosilicate, float glass, fused silica and fused quartz glasses have been used. The adsorbed water on the glass surface was distinguished from bulk water, trapped in interstices of silicate network, by D2O exchange experiments. The variation in the glass surface composition as a function of the chemical composition and grinding media (wet or dry) was characterized by XPS (X-ray photoelectron spectroscopy). The effect of chemical structure on physically adsorbed water and chemisorbed water species on different multicomponent glasses were identified via adsorption/desorption experiments using in-situ DRIFTs. The degree of hydration was found to vary depending on the concentration of Si-OH groups which is strongly dependent on the presence of (1) modifier ions or non-bridging oxygen, (2) glass intermediates (Al2O3 or B2O3) and (3) water already trapped in the bulk structure of the glass. For Ba-silicate and float glass with many non-bridging oxygen sites, the number of silanol groups (SiOH) and subsequent hydration rate is higher compared to fused silica and fused quartz. Moreover, BaCO3 formation on the surface of Ba-silicate network also enhanced the degree of hydration. The number of water molecules on/in Ba-silicate glasses was found three times higher than pure silica counterparts. The presence of Al2O3 and B2O3 in modifier-containing silicate glasses decreases the number of NBO’s due to their charge compensation of [AlO4]- and [BO4]-groups in the glass network structure. This limits the hydroxylation and subsequent hydration process. For this reason, the number of water adsorption sites in/on Ba-boraluminosilicate glasses was found considerably smaller than Ba-silicate. Besides the effect of chemical composition, the mechanical milling of glass powders in aqueous environments also promotes the degree of hydration. The effect of milling media is more predominant in Ba-silicate glasses with the high number of non-bridging oxygen.