Effects of Electrostatic Fields on the Surface Tension of Salt Solutions

Abstract

The surface tension of air/ solution interfaces with applied electrostatic fields of up to 6700 v/cm was measured using a surface energy balance. The concentration was 0–3.5%. It was found that fields of either polarity lower the surface tension of all investigated solutions. Magnitude of the surface tension changes and dependence on direction of the field lead to the conclusion that the potential drop at air/water interfaces is smaller than previously thought, that the water dipole orientation is only slight, and that the hydrogen in the oriented water molecule is facing the air. The experiments point out that the effect of the field is predominantly on the sodium ion. It is shown that the difference in the energy for adsorption at the interface between cations and anions cannot be neglected. Calculations on the basis of a modified Gibbs equation on the assumption of a diffuse double layer at the interface leads to unreasonably high surface deficiencies. Neglecting the diffuse double layer and reckoning on the basis of an inner double layer only leads to values of the same order of magnitude as those calculated from the Gibbs equation alone.