Specific cation effects at the hydroxide-charged air/water interface

The zeta potentials of individual gas bubbles in water have been measured with a spinning tube zetameter as a function of added electrolyte concentrations. The bubbles have a negative charge at pH > 3 due to the preferential adsorption of hydroxide ions. Addition of alkali halide electrolytes from 10(-5) to 10(-2) M reduces the zeta potential due to double layer compression. The effect is independent of the anion among the sodium chloride, bromide and iodide salts, indicating that these do not compete with hydroxide ion at the surface. Lithium chloride is more effective than cesium chloride at reducing the zeta potential, indicating that electrostatic effects are more important than dispersion forces at these low ionic strengths.