Wettability, a Function of the Polarizability of the Surface Ions

Abstract

The wettability of crystals, glasses, and even of water itself can be temporarily decreased by bringing ions of high polarizability into their surfaces. Base exchange experiments are described where the hydrogen ions present in the surface layers of bentonite (a clay mineral of high exchange capacity) and of a soda‐lime glass are replaced by different cations. This substitution seems to have no particular effect on the hydrophilic and rheological properties of the carriers as long as their surfaces are kept in contact with water. After drying, however, the surfaces which contain ions of high polarizability become hydrophobic, at least temporarily. A porous clay film will no longer absorb water instantaneously after having been treated with Ni²⁺, Mn²⁺, Hg²⁺, or similar ions with incomplete outer electron shells. Contact angles with water up to 70° could be observed for a short period for Hg²⁺ and Pb²⁺ clays. Glass capillary tubes which have been treated with non‐noble gas‐type ions show a capillary rise which is much smaller than that observed with the tubes which have been treated with HCl and water only. This depression of the capillary rise, too, is temporary and can be observed only if the glass wall has been thoroughly dried previous to the experiment and if the capillary rise is measured in the upward direction. An explanation is presented for these and allied phenomena on the basis of the polarization of ions in the strongly asymmetrical forcefields of interfaces. The experiments are correlated with the hysteresis of the contact angle and with observations concerning adhesion phenomena and catalytic activities of heavy metal ions at interfaces.