Point of zero charge , shift

In work related to flotational studies Chibowski has measured the effect on the surface charge of n-alcohol and n-alkane adsorption on quartz particles from aqueous KCl solution. Maximum effect was found with Cf,—Cg n-alcohols, where the pH at the point of zero charge shifted from 5.7 to 8—9 and for Cg—C13 n-alkanes, with a corresponding shift to pH 7. 

Because of the adsorption equilibrium for H+ and OFT ions between the surface of semiconductors and an aqueous (aq) solution, the semiconductor surface attains the point of zero charge (PZC). The flat-band potential U[h of most semiconductors including all oxides and also other compounds such as n- and p-type GaAs, p-type GaP, and n- and p-type InP in an aqueous solution is determined solely by pH and shifts proportionately with pH with a slope of -59 mV/decade, that is, pH, for example,... 

The particular enhancement of a radical dimerisation reaction, Eqs. (45a) and (45b), is observed on hydrophobic metals like Hg, Pb, and Sn at potentials close to their respective point of zero charge where electrosorption is strongest and diminishes at more negative and positive potential. If the reduction potential of the oxo-compound, which in accordance with the equilibrium reaction (45b) is pH dependent (60 mV per pH unit), is shifted by more than 100 mV away from the point of zero charge, then the ketyl radicals are no longer adsorbed strongly enough and they are displaced from the... 

Another behavior is observed in solutions with KOH, KCNS, and KI (Fig. 5.4). An increase in the concentration leads to a shift of the maximum to negative potentials. Explanation The anions bind specifically to mercury and shift the point of zero charge. Iodide adsorbs more strongly than cyanide, and cyanide more strongly than hydroxide. A negative potential must be applied in order to drive the anions away from the surface. 

Figure 12. Shift in point-of-zero charge of andalusite through acid-base cycling in a streaming potential apparatus. Key O, cycle 1 , cycle 2 0, cycle 3 A, cycle 4 and V, cycle 5. (Reproduced, with permission, from Ref. 42. Copyright 1966, A1ME.)...

A new feature is that a common intersection point is no longer found. The point of zero charge moves to the left with increasing specific adsorption of cations. This shift Is almost Independent of the inner layer capacitance. Close to the point of zero charge the specific adsorption is superequivalent, as in fig. 3.20c. Making the surface potential more negative, a point is reached where... 

Hence, the binding enthalpy of the proton can be obtained from the shift in the point of zero charge with temperature. The more basic an oxide the higher ads H+ stronger the p.z.c. decreases with temperature. More... 

Other tests Include the requirement of realistic values for the numerous parameters (Ng, pK, pK, Ag gG j. Cj, and, depending on the model, C ) and realistic dependencies. Sometimes for these parameters independent information is available after drying, Ng is obtainable from functional gas adsorption studies (sec. 1.3f) or spectroscopically, the enthalpic part of pK by calorimetiy, Cj and Cj from shifts in the point of zero charge (last paragraph of sec. 3.8b). Regarding dependencies, pK, pKj, and should be independent of pH and... 

Going one step further, the interpretation of the temperature shift of the point of zero charge requires a model for the adsorption equilibrium. From the data and the model, A G° is obtainable. If measured as a function of temperature. 

Figure 3.34. Salt effect on the surface charge near the point of zero charge. Top, a common Intersection point Is found bottom, with Increasing electrolyte concentration, the intersection points shift to lower pH and to more positive surface charge.

Table 3.4. Direction of the shift of point of zero charge and isoelectric point caused by specific adsorption. "Down" and "up" refer to change in the negative logarithm of the concentration of the charge-determining ions.

The tendency of water to direct the negative sides away from the liquid has also been observed for water adjacent to silver iodide. In this case the argument stems from the shift of the points of zero charge upon displacing the water by a monolayer of a number of aliphatic alcohols . Anticipating a conclusion from sec. 3.12, tdl these alcohols adsorb with their hydrocarbon moieties to the solid. 

Cattania, M.G. et al.. An experimental correlation between points of zero charge and X-ray photoelectron spectroscopy chemical shifts of oxides. Colloids Surf. A. 76, 233, 1993. 

Fig. Ill-17. The shift in point of zero charge position due to adsorbed ionic surfactants...

Fairhurst et al (1995) found that adsorption of HA at 5 mgL (as organic carbon) on 50 mgL" hematite decreased with increasing pH from 65% at pH 2 to 10% at pH 10. The apparent point of zero charge (pHpZc) of the colloids was shifted from pH 8 (no HA) to a pH <2. Such charge inversion of colloids... 

During the sol-gel conversion, the typical blue shift occurs as the solvent molecules become more ordered at the developing surface where the ruthenium complex is most likely to reside in gels > pH 2, which is usually quoted as being the point of zero charge for the surface (10). 

Its indirect influence may manifest itself through a shift of the point of zero charge and a related change in the double-layer structure, as was mentioned above in connection with data on electroreduction of anions. However, in the above-mentioned hydrogen overpotential correlations, the effect did not play any significant role. 

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