S—shaped adsorption isotherms of surface active electrolytes from aqueous solutions

Abstract

The modified Langmuir equation which uses the activity instead of the concentration gives S-shaped adsorption isotherms of electrolyte from aqueous solutions. The equation together with the Gibbs adsorption equation yields the modified Szyszkowski equation. According to these equations, the adsorption isotherms of disodium monoalkyl phosphates on the solution surface are found to be S-shaped, where the area occupied by one molecule at the saturated adsorption is 53 Å 2 and the energy of adsorption per one CH 2 group is 682 cal mole −1. Starting from the Hobbs theory based on the Gouy-Chapman equation of the diffuse double layer, it has been concluded that the effect of the counter ions on the critical micelle concentration can be explained according to the fact that the activity at the critical micelle concentration is constant irrespectively of the counter-ion concentration. The binding isotherms of hydrochloride and sodium salt of l-tryptophan to polyvinylpyrrolidone are also S-shaped, and the curves are changed to the Langmuir type if the activity is used instead of the concentration.