Potential-dependent adsorption equilibrium electrolyte solutions

The equilibrium properties of foam films formed from aqueous solutions of decylmethyl sulfoxide have been studied in the presence of sodium chloride and potassium thiocyanate. Stable films were formed whose thicknesses depended on the electrolyte concentration. As the electrolyte concentration was increased, a sudden increase in film thickness occurred but gradually decreased with further electrolyte addition. Examination of the electrophoretic mobility of dodecane droplets stabilized by decylmethyl sulfoxide showed an increase in mobility at about the same concentration. These data indicated that the thicker foam films were charge stabilized owing to the adsorption of the anions. The surface pressures and surface potentials of monolayers of octadecyl sulfoxide were also investigated. 

Rosene and Manes studied the effect of pH on the total adsorption from aqueous solutions of sodium benzoate + benzoic acid by activated charcoal. They interpreted their data in terms of the Polanyi potential theory applied to bisolute adsorption (see later p. 117), in which the concentrations of neutral benzoic acid and benzoate anions depend on the pH of the solution (activity coefficient corrections were ignored). They confirmed that, at constant total equilibrium concentration, the adsorption dropped from a relatively high plateau for pH <2 down to a small adsorption at pH >10. The analysis of results is somewhat more complex than with essentially non-electrolyte adsorption, and in this case there were additional effects involving chemisorption of benzoate ion by residual ash in the carbon which had, therefore, to be eliminated. Even with ash-extracted carbon there was evidence of some residual chemisorption. The theoretical analysis correlated satisfactorily with the experimental data on the basis that at pH >10 sodium benzoate is not physically adsorbed and that the effect of pH is completely accounted for by its effect on the concentration of free acid. In addition the theory explains successfully the increase in pH (called by the authors hydrolytic adsorption ) when solutions of sodium benzoate are treated with neutral carbon. However, no account is taken in this paper of the effect of pH on the surface charge of the carbon. 

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