Proton surface charge measurement

The titratable surface charge measurements can be interpreted to give a quantitative assessment of the interactions between the solid and the supporting electrolyte ions. Potentiometric titrations measure the adsorption or release of protons and the model developed by Yates ad. (29) and Davis et al. (17) proposes reactions between oxide surface groups and... 

The combination of shifts in EM with surface charge measurements and titration data can be interpreted to provide important information on the surface speciation, which is complimentary to spectroscopic data. Titration data provide information on the net proton or hydroxyl release from the surface. Among the techniques that can be utilized is suspension of the oxide in the background electrolyte, adjustment of the pH to the value of interest, adjustment of the background electrolyte with the adsorbing ion, then mixing the two and measuring the proton or hydroxyl mass necessary to return to the specified pH, in conjunction with determination of the concentration of anion adsorbed. These data are then reported in mol H or OH per mmol anion adsorbed. 

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

The double-layer model of the membrane consists of many particles (assume a diameter of 0.1 gm) that are impenetrable for solution and carry a surface charge. Electrical double layers exist around each particle, and because the dimensions of the membrane pores are of the same order as the double layers around the particles, double layers exist throughout the membrane pores. The potential measured by the underlying ISFET, with respect to the bulk potential, is on one hand determined by the mean pore potential, which is the net result of the contribution of all surface potentials of the charged particles, and on the other hand by the pH at the membrane-1 SFET interface. The measured ISFET response in equilibrium is therefore the same as that of an ISFET without a membrane, because the distribution of the protons between membrane and solution results in a pH difference, which compensates the mean membrane potential (this is the same mechanism as in the Donnan model). The relation between the surface charge on the particles a (C/cm2) and the surface potential jx of each particle is given by... 

First, assume that the surface charge on the membrane particles does not interact with the mobile protons (no proton release or uptake). An ion step will result in an increase in the double-layer capacitances of the particles and consequently in a decrease of the surface potentials fr, because the charge densities remain constant. The ISFET will measure a transient change in the mean pore potential. As a result of the potential changes, an ion redistribution will take place and the equilibrium situation is re-established. The theoretical maximum ion step response is the change in the mean pore potential. This is comparable with the Donnan model where the theoretical maximum is determined by the change in the Donnan potential at the membrane solution interface. 

The interaction of a cation with a neutral oxide group results in the release of a proton, while the association of an anion results in the adsorption of a proton. Accordingly, the formation of a negative site from a neutral site involves the release of a proton and the formation of a positive site involves the adsorption of a proton. Therefore, the titratable surface charge determined by potentiometric titration is a measure of both the formation of surface-ion complexes and the ionization of surface functional groups, and... 

Proton penetration into RUO2 has also been detected by other techniques, such as potential step (368, 377), current step (378, 379), membrane doublecell (366), and spectroelectrochemistry (380), and it has been attributed to bulk diffusion (permeation) in some cases (368,377) and regarded as unimportant for surface area measurement in other cases (372). In spite of the above, it is not clear why proton diffusion alone is assumed to be the reason for the effect observed it seems that a state of surface reaction involving increase or decrease of surface oxidation could account for the results equally well. Also, the connection between a solid state redox reaction (367,381) involving several Ru valence states in the oxide and the requirement (146,382) of proton intercalation to achieve charge balance at and OH sites as oxidation or reduction takes place was not noted. 

These results indicate, that the mechanism of surface charging of mineral oxides and related materials in polar organic solvents is similar to that in water, and the main difference is due to technical and theoretical difficulties related to measurement and interpretation of activities of proton and other inorganic ions in such media. 

The approach to proton adsorption from aqueous solution must be different from the approach to adsorption of other solutes, because water molecules can provide or absorb a practically unlimited number of protons (higher by several orders of magnitude than the concentration of any other species in solution and the concentration of surface sites) to balance the changes induced by adsorption. Thus, adsorption isotherms based on the concept of a distribution of a limited amount of adsorbate molecules between solution and surface are not applicable. Most authors accept this obvious fact, but a few others have used the same formalism for proton adsorption as is used for other solutes. For example, in [205], the surface charging of alumina is discussed in terms of adsorption isotherms (amount adsorbed vs. equilibrium concentration). Positive adsorption of protons is equivalent to negative adsorption of OH , and vice versa. In adsorption experiments, uptake of protons and release of OH cannot be distinguished. Only the net result of uptake/releasc of H and OH can be obtained, and independent curves of 11 and OH adsorption reported in the literature [206,207] must be based on measurements of other quantities. 

The micellar charge was also corroborated by the Guoy—Chapman diffused, double-layer model. At equilibrium, the surface charge of the micelle alters the ionic composition of the interface with respect to the bulk concentration. The difference between the actual proton concentration on the interface and the one measured at the bulk by pH electrode is observed as a pK shift of the indicator and is related with the Gouy-Chapman potential (Goldstein, 1972). 

---