Effect of added salt on the surface tension of aqueous solution of disodium monolauryl phosphate

Abstract

Disodium monolauryl phosphate was synthesized by the pyrophosphoric acid method. The surface tension of its solution was measured in aqueous sodium chloride and sodium hydroxide by changing its concentration. The surface excess was calculated by applying the Gibbs adsorption equation. Szyskowski equation was verified using the activity of disodium monolauryl phosphate, a, and it was confirmed that the values of surface excess agreed with those calculated from the Langmuir adsorption equation using the activity, a, instead of the concentration. The activity, a, was defined as a=γ±(2C+C')2 C where γ±is the mean activity coefficient calculated from the Debye-Huckel equation, C is the concentration of disodium monolauryl phosphate, and C' is the added salt concentration. On the basis of the discussion of micelle formation, it was also concluded that the micelle formation occured at the constant activity independent of the added salt concentration.