The effect of boron oxide on the composition, structure, and adsorptivity of glass surfaces

Boron oxide has been added to commercial silicate glasses for many years to aid in lowering melting temperatures, lowering thermal expansion, and controlling chemical durability. The fact that simple borate glasses have rather high thermal expansion and low chemical durability attests to the unique influence of boron oxide additions upon the properties of silicate glasses. However, the impact of boron oxide additions upon surface properties of multicomponent borosilicates such as adsorption and reactivity is not yet well understood. In particular, the presence of multiple coordination states for boron is expected to introduce adsorption sites with different acidic or basic behavior, but their existence is yet unproven. To investigate these effects, multicomponent sodium aluminosilicate glasses have been prepared with varying sodium and boron concentrations and drawn into moderately high-surface-area continuous filament fibers.;A relatively new technique, boron K-edge Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is applied to study the local boron coordination at fracture and melt-derived fiber surfaces of these glasses. This structural information is combined with surface compositional information by X-ray Photoelectron Spectroscopy (XPS) to characterize the local atomic structure of boron at the as-formed glass surface. Finally, this information is used to interpret the adsorptivity of these as-formed and leached surfaces toward short-chain alcohol molecules through a new Inverse Gas Chromatography---Temperature Programmed Desorption (IGC-TPD) experiment. The results clearly show that boron additions to alkali-free glass surfaces introduce a unique adsorption site which is not present on boron-free glass surfaces and is easily removed by leaching in acidic solutions.