Thermodynamic Model of Charging the Gas/Water Interface

In this work, charging of the water layer at the gas/water interface is analyzed experimentally and theoretically. Experimental data on the surface tension and surface potential in the acidic range are presented. Literature electrokinetic data indicate that the isoelectric point of the gas/water interface is pH 3.8. In the acidic region, a plot of the surface potential versus pH has a slope of 55% of the Nernstian value. The surface tension exhibits a minimum at pH 3.8. A thermodynamic model based on the distribution of ions between the bulk and the interface is developed. The interpretation of the surface and electrokinetic potential data provided equilibrium constants for the distribution of ions between the bulk and the interface and the ionization equilibrium constant of interfacial water. A significantly higher ionization of interfacial water with respect to the bulk phase was observed. The affinity of hydrogen and hydroxide ions to accumulate at the interface is higher than the affinity to accumulate in the bulk of the solution, especially for hydroxide ions. The minimum of the surface tension at the isoelectric point is presented and analyzed. The thermodynamic concept, in accordance with the Gibbs isotherm, takes into account the variation of the composition at the interface. The effect of electrolytes on the surface tension, that is, the Jones–Ray effect, is also discussed.