ON THE PREDOMINANT ELECTRON-DONICITY OF POLAR SOLID-SURFACES

The reasons for the predominant electron-donicity of almost all solid polar surfaces and its implications are discussed in this paper. By co ntact angle or interfacial tension measurements, the electron-acceptin g as well as the electron-donating properties of polar liquids can be ascertained, through the interplay between their energies of adhesion and cohesion. For the solid-liquid interface, direct interfacial tensi on measurements are not possible, but indirectly, solid/liquid interfa cial tensions of polar systems can be obtained by contact angle measur ement. However, as the energy of cohesion of a solid does not influenc e the contact angle formed by a liquid drop placed upon its surface, o ne can only measure the solid surface's residual polar property, manif ested by the energy of adhesion between solid and liquid. This residua l polar property is of necessity the dominant component; in most cases this turns out to be its electron donicity. When, by means of contact angle measurements with polar liquids, both electron-accepting and el ectron-donating potentials are found on a polar solid, it is most like ly still partly covered with a polar liquid: usually water. The amount of residual water of hydration of a polar solid follows from its pola r (Lewis acid-base) surface tension component (gamma(AB)). The degree of orientation of the residual water of hydration on a polar solid can be expressed by the ratio of the electron-donating to electron-accept ing potentials (gamma(+)/gamma(-)), measured on the hydrated surface.