pH and the surface tension of water

• The surface tension of water is almost independent of pH between pH 1 and 13.
• The Gibbs isotherm and electroneutrality allow equal H+ and OH− in double layer.
• The pH independence arises because [H+] and [OH−] are coupled by Kw.
• The surface tension increases from pH 9 down to 4 in 10 mM NaCl or KBr.
• This is only consistent with the Gibbs isotherm with strong adsorption of OH−.

Despite the strong adsorption of hydroxide ions, the surface tension of water is almost independent of pH between pH 1 and 13 when the pH is adjusted by addition of HCl or NaOH. This is consistent with the Gibbs adsorption isotherm which measures the surface excess of all species in the double layer, if hydronium ions and hydroxide ions are adsorbed and sodium and chloride ions are not. The surface tension becomes pH dependent around pH 7 in millimolar NaCl or KCl solutions, for now sodium ions can replace hydronium ions as counterions to the adsorbed hydroxide ions.

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