Water is a surfactant in water

Textbooks and websites on the origin of the surface tension of water need to be rewritten, according to Maoyuan Liu, James K Beattie and Angus Anthony Gray-Weale. These sources describe a pristine, uncharged air/water interface. None recognise that the interface is in fact highly charged by the presence of adsorbed hydroxide ions, with hydronium ions providing charge balance in the double layer. Now Australian chemists at the Universities of Sydney and Melbourne have given an explanation for the millisecond relaxation time required for the air/water surface tension to reach its equilibrium value (72.7 mN/m at 293 K). Their results can be read in the article ‘The Surface Relaxation of Water’, published in The Journal of Physical Chemistry B.

Often-overlooked early work with water microjets, and its subsequent confirmation, indicates that a newly formed pristine air/water interface has a surface tension of 80-100 mN/m. This relaxes to the equilibrium value in a few milliseconds through a combination of autolysis of water and diffusion of hydroxide ions to the interface. The formation of the charged double layer lowers the surface tension, just like the adsorption of typical soap-like amphiphilic surfactants. Water itself acts as a surfactant. There are practical consequences for submillisecond processes such as ink-jet printing and spray coating.