The Surface Relaxation of Water

We examine three possible explanations for the millisecond relaxation time of the dynamic surface tension of water: the diffusion of surfactant contaminants from the aqueous phase to the surface, the reorientation of surface water molecules’ dipole moments, and the buildup of a charged surface layer of hydroxide ions. The relaxation time expected for hydroxide is by far the closest to the measured time. Our model for the surface layer agrees with static equilibrium experiments and, as we show here, predicts the relaxation time. The results strongly imply that the equilibrium surface of water is highly charged by a flow of hydroxide to the surface. The model predicts that neither diffusion nor autolysis dominates and shows that both processes are needed to describe the relaxation of the surface tension. We find a salt and pH dependence of the relaxation time and propose further experiments.