Surface potentials of aqueous electrolyte solutions

: The effect of added electrolytes on the surface potential of water was determined using the radioactive electrode technique. Changes in surface potential were found to vary from +64 mV for Na2SO4 at 1.8m to -180 mV for NaCNS at 7.5m. The Group IA chlorides in water gave surface potential differences that decreased in the order K(+) = NH4(+) Na(+) Li(+), and the surface potentials of the Group IIA cations decreased in the order Ba(++) Sr(++) Mg(++). At a constant anion concentration of 2m the surface potential differences due to the sodium salts were in the order SO4(--) CO3(--) CH3COO(-) Cl(-) NO3(-) Br(-) I(-) CNS(-). In general the anion with the smaller hydration energy gave the greatest decrease in surface potential. The magnitude of each surface potential change, however, does not appear to be a simple function of the hydration energy or of some related property. The surface potential changes must involve the orientation and structure of the water molecules at the water/air interface, which may be only partially dependent upon the ionic properties as determined in bulk solution.