Adsorption charge reversal

R.O. James, T.W. Healy, Adsorption of hydrolyzable metal ions at the oxide — water interface. II. Charge reversal of Si02 and Ti02 colloids by adsorbed Co(II), La(III), and Th(IV) as model systems, J. Colloid Interface Sd. 40 (1972) 53-64. 

CEC, cation exchange capacity A measure of the amount of cations that will adsorb to the negatively charged surface of a clay mineral. It is usually measured in units of meq of charge per lOOg of clay mineral. This adsorption is reversible. 

Recent studies indicate that the adsorption of metal ions is controlled only in part by the concentration of the free (aquo) metal ion of considerable importance is the ability of hydroxo and other complex ions and molecules to adsorb. There have been two apparently divergent approaches to describe the role played by hydroxo metal complexes in adsorption at solid-aqueous electrolyte interfaces. Matijevic et al. (9) have proposed that specific hydrolysis products—e.g., Al8(OH)2o4+ in the A1(III)-H20 system, are responsible for extensive coagulation and charge reversal of hydrophobic colloids. It has also been demonstrated by Matijevic that the free (aquo) species of transition and other metal ions... 

Charge Reversal and Variation of the IEP(s) by Specific Adsorption (44). To achieve zero charge in the presence of a specifically adsorbed ionic species, the pH must be shifted away from the IEP(s) to increase or decrease hydrogen ion adsorption, whichever is appropriate. The new pH at which zero charge is observed is of course not an IEP(s). 

Because ionic species adsorbed in response to coulombic attraction alone obviously cannot adsorb in amounts larger than those equivalent to the original surface charge—i.e., they cannot reverse the sign of the surface charge—charge reversal must occur in response to specific adsorption alone. Similarly at the isoelectric point, the surface charge is zero hence, adsorption by coulombic attraction alone will not occur. Thus, adsorption which results in a shift in the isoelectric point must be specific adsorption. 

Because of the surface charge reversal as the electrolyte concentration increases, it is more realistic to also include the adsorption of anions among the adsorption equilibria, since they can have a relatively high concentration near a positively charged surface. 

FIGURE 4.31 Changes in potential with distance (a) charge reversal due to the adsorption of surface-active or polyvalent counterions (b) adsorption of surface-active co-ions. 

The adsorption of polyelectrolytes at charged surfaces is used frequently for both charge neutralization and charge reversal. Polyelectrolyte multilayers formed by polycation-polyanion pairs provide additional opportunities to control surface morphology and film thickness [24],... 

In summary, the fact that the VSC-VSP model predicts adsorption edge shift and the effect of charge reversal on the pH-dependence of adsorption, neither of which can be accounted for on the basis of the model of James and Healy ( ), lends support to its use in accurately describing heavy metal adsorption. The agreement between theory and experiment shown in Figure 7 additionally supports the use of the VSC-VSP model. 

James, R. 0. and Healy, T. W. Adsorption of hydrolyzable metal ions at the oxide-water interface. I. Cobalt (II) adsorption on silicon dioxide and titanium dioxide as model systems II. Charge reversal of silicon dioxide and titanium dioxide colloids by adsorbed cobalt (II), lanthanum (III) and thorium (IV) as model systems III. 

We are not aware of theoretical predictions for the technologically most important high-salt regime. Still these results confirm charge reversal upon polyelectrolyte binding which has been observed in the fabrication of polyelectrolyte multilayers. It is also obvious that the complexity of electrostatic conditions at the interface and their change upon adsorption will cause complicated adsorption kinetics, as we have discussed above. 

TTie initial addition of CTAB caused a reduction in the negative surface charge of the clay (by the adsorption of CTA on the negative sites of the clay), and this was accompanied by reduction in the negative mobihty of the clay. When complete neutralisation of the clay particles occurred (at the i.e.p.), maximum flocculation of the clay suspension occurred and this was accompanied by a maximum in On further increases in CTAB concentration, a further adsorption of CTA occurred, and this resulted in charge reversal and restabilisation of the clay suspension. This was accompanied by a reduction in cr. ... 

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