Separate determinations of the surface excesses of surface-active ions and of gegenions at the air-solution interface

Abstract

The physical meaning of the slope of surface tension vs. the logarithm of the concentration under various conditions has been analyzed thermodynamically. The surface excesses (concentrations) of surface-active ions and of gegenions at the air-solution interface have been determined from the graphs of ( 1) the surface tension vs. the concentration of surface-active ions under the condition of a definite concentration of gegenions, and of ( 2) the surface tension vs. the concentration of the gegenions under the condition of a definite concentration of surface-active ions. The surface concentrations at the air-solution interface of aqueous solutions of perfluoro-octanoic acid, potassium perfluoro-octanoate, and barium perfluoro-octanoate were Γ C 7F 15COO- = 4.0, Γ H+ = 1.8; Γ C 7F 15COO- = 3.9, Γ K+ = 2.0; Γ C 7F 15COO- = 3.3-2.8, Γ Ba++ = 1.0-0.9. in 10 −10 mole/cm. 2 below, but near, the CMC. The surface excesses determined from the graphs of the ordinary surface tension vs. the concentration of surface-active agent were 5.8, 5.9, and 4.3-3.7 in 10 −10 mole/cm. 2. These values were of course equal to the respective summation of the surface excesses of surface-active ions and of gegenions within experimental error. The fact that the surface concentration of gegenions is about one half that of surface-active ions is noteworthy.