The Surface Relaxationof Water.

We examine three possible explanations for the millisecondrelaxationtime of the dynamic surface tension of water: the diffusion of surfactantcontaminants from the aqueous phase to the surface, the reorientationof surface water molecules’ dipole moments, and the buildupof a charged surface layer of hydroxide ions. The relaxation timeexpected for hydroxide is by far the closest to the measured time.Our model for the surface layer agrees with static equilibrium experimentsand, as we show here, predicts the relaxation time. The results stronglyimply that the equilibrium surface of water is highly charged by aflow of hydroxide to the surface. The model predicts that neitherdiffusion nor autolysis dominates and shows that both processes areneeded to describe the relaxation of the surface tension. We finda salt and pH dependence of the relaxation time and propose furtherexperiments. [ABSTRACT FROM AUTHOR]