Adsorption from electrolyte solutions surface charge

In the absence of specific adsorption of electrolyte ions, surface charge is considered to originate from acid-base dissociation of ionizable groups. In terms of acid groups (AH) and basic groups (B), the respective pH-dependent equilibrium between surface sites and solution at the interface can be represented as... 

As I see, an essential problem in this field is that the complex aspect of adsorption from electrolyte solutions shown in Fig. 1 has not been widely accepted. I have hardly found a systematic analysis of all probable simultaneous equilibria in a given adsorption system in the relevant literature. In most cases the solution condition (e.g., pH)-dependent dissolution of the solid phase, the surface precipitation, and the speciation in the aqueous phase are omitted in the evaluations either without mentioning them or with reference to some reasoning. To demonsteate some outcomes, it is worth inspecting a simple case of the surface-charge titration of a common aluminum oxide in detail. 

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. 

The charge density, Volta potential, etc., are calculated for the diffuse double layer formed by adsorption of a strong 1 1 electrolyte from aqueous solution onto solid particles. The experimental isotherm can be resolved into individual isotherms without the common monolayer assumption. That for the electrolyte permits relating Guggenheim-Adam surface excess, double layer properties, and equilibrium concentrations. The ratio u0/T2N declines from two at zero potential toward unity with rising potential. Unity is closely reached near kT/e = 10 for spheres of 1000 A. radius but is still about 1.3 for plates. In dispersions of Sterling FTG in aqueous sodium ff-naphthalene sulfonate a maximum potential of kT/e = 7 (170 mv.) is reached at 4 X 10 3M electrolyte. The results are useful in interpretation of the stability of the dispersions. 

A similar argument [7] was presented by Muller et al. [227] in their incisive analysis of adsorption of weak electrolytes from aqueous solution on ACs The solid surface charges in response to solution pH and ionic strength the resulting (smeared) surface electrostatic potential influences the adsorption affinity of the ionized solute. ... 

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