The Surface Properties of Glasses as Affected by Heavy Metal Ions

JSGT 1948 V32 T247-T259

The present picture of the atomic structure of glass, the network theory of W. A. Zachariasen and B. E. Warren, represents a first approximation which has been found very useful for glasses which contain only ions of the noble-gas type. Most glass-forming ions have eight outer electrons, a stable structure characteristic of the atoms of the rare or noble gases, neon, argon and krypton. The stability of this arrangement in the electric fields of neighbouring ions of opposite charge makes it possible to treat some ions as rigid spheres and to estimate their forces on the basis of their charge and size alone (ionic potential). Large ions such as BaH or ions having an electronic structure which differs from that of the noble-gas atoms (Zn²⁺, Cd²⁺, Pb²⁺) cannot be treated as rigid spheres, but their deformation has to be taken into consideration. The properties of lead oxide-containing glasses were first explained by K. Fajans & N. J. Kreidl, who called attention to the importance of the symmetry of forces in the constitution of glass. From the nature of these deformed ions it was to be expected that surface properties would be more strongly affected by their presence than bulk properties. Based on a chemical picture of the deformed lead ion a number of properties will be explained, for example, that some lead oxide-containing glasses have a positive temperature coefficient of surface tension. Furthermore, the presence of Pb²⁺, or similar ions (Tl+, Bi³⁺, Sn²⁺), provides a bridge between the structures of glasses and of noble metals. These ions make noble metals soluble in glass melts, thus making the gold and copper ruby glasses possible. The paper provides a common theoretical basis for subsequent publications on the hygroscopicity of glass, adhesion phenomena and on the catalytic action of glass in respect to the gas nucleation and thermal decomposition of metastable compounds. Four new methods are discussed which provide better insight into the structure of the glass surface at room as well as at higher temperatures.

W. A. Weyl