Surface Tension of Alkaline‐Earth Borates Containing 3 Mole % Potassium Oxide

The surface tensions of a series of barium, strontium, and calcium borates, each containing 3 mole % of K 2 O to counteract immiscibility, were measured by a ring detachment method. The order Ca < Sr < Ba was established for surface tensions of melts containing up to 40 mole % MO (alkaline‐earth oxide). Positive temperature coefficients of surface tension were observed for melts of low MO content. An interpretation of the observed inversions of surface tension from the order of ionic potential of the cations for these systems, as well as for other systems previously studied, is presented in terms of the Gibbs adsorption equation. A connection is indicated between inversions in the order of surface‐tension values and the presence of positive temperature coefficients.