Surface potential-pH characteristics

Surface Potential-pH Characteristics in the Theory of the Oxide-Electrolyte Interface... 

The measurement of the surface potential asa function of pH for an oxide provides valuable information for the determination of the parameters which describe the surface reactions. Ionizable surface site theories of the formation of surface charge and potential at an oxide surface in contact with a liquid electrolyte involve many more parameters than can be directly experimentally determined. Additional assumptions are required to evaluate these parameters, which explains why there is often no agreement in the literature about their value. A mathematical treatment of the amphoteric surface site model is given which exhibits the characteristic quantities which can be experimentally measured. It is shown that the measurement of both the surface potential i/>o and the surface charge 00 are required to completely determine these characteristic quantities. This approach is applied to Si02 and AI2O3, two surfaces for which both charge and potential measurements are available. 

One possible solution is to obtain new experimental data, which is independent of co/pH curves. The zeta potential is of course a possibility, but it suffers from the intrinsic indeterminacy of the exact location in the double layer where it occurs. Another possibility is the surface potential, Vo, which will be defined below. Variations of Vo can be measured by using electrolyte/insulator/semiconductor structures. It has been shown by Bousse et al. (14) that the Vo/pH characteristics are determined mainly by the number of charged but uncomplexed surface sites, and are insensitive to complex-ation. This means that combined consideration of tro/pH and Vo/pH characteristics should lead to a more complete and reliable determination of model parameters. 

Tphe force-area (tt-A), surface potential-area (AV-A), and surface vis-cosity-area (-q-A) characteristics of long-chain fatty acid monolayers at the air-water interface have been extensively investigated over acid and neutral aqueous solutions (17). Studies of fatty acid monolayer isotherms at high pH and the specific cation effect on the isotherms are less numerous. The difficulties inherent in working at high pH are ... 

Based on the above, a RI-SS relationship representing a particular colloidal system would be constant, assuming that pH is constant, because the RI-SS relationship is expected to be pH dependent. Furthermore, at a certain pH value, a colloidal system that exhibits both negative and positive surface potential would be expected to coflocculate when the net electrical potential is zero or the system is at its PZC (Schofield and Samson, 1953 Quirk and Schofield, 1955 Evangelou and Garyotis, 1985). Under these conditions, the settling characteristics of the suspended solids would be independent of RI (Fig. 9.14a and b). 

Since the pioneering work of Knox et al. on CEC [9,10], porous silica particles have been used as the column packing material in the majority of research studies and applications. Porous silica has a number of characteristics that make it suitable for use in CEC. These are a large surface area, a high surface potential at moderate pH values, which allows the generation of a high EOF, and the commercial availability of materials with various surface chemistries. However, other support materials, such as polymeric phases [11] and alternative inorganic base materials [12], are also applicable in CEC. 

Fig. 10.1 (a) Basic design of ISFET. (b) characteristics of the device, (c) pH responsibility of the device, (d) Change in the surface structure and surface potential of silicon nitride layer in each pH solution... 

The acid-hase characteristics of the surface groups (relative speciation of surface groups as a function of pH in upper figure) determine the pH of zero potential (point of zero proton condition MeOHt = =MeO ). The Nernst equation—a surface potential dependence on pH of (RT/Fj In 10 (= 59 mvolt at 25°C)—is not fulfilled. The lines in the lower figure were calculated from alkalimetric and acidimetric titration curves using... 

In order to correlate chemieal analysis of aetivated membrane surface with other characteristic parameters, tangential streaming potential measurements at a constant NaCl concentration (5x10 " M) but different pHs were carried out with the DT200 sample and analysed using the local dissociation model [68-69], which allows the determination of the pKa and the munber of acid sites accessible on the membrane surface (N ), and their comparison with the same parameter for PAO polyamide/polysulfone composite membrane (the un-modified base membrane). The obtained values and surface roughness are ... 

The discussion of potential-pH diagrams in Sec. 1.2.2 showed that metals may be protected by an oxide or a hydroxide film, preferentially in neutral and weakly alkaline solutions when they are in dissolution equilibrium with the electrolyte. As a consequence, the metal is protected against corrosion by a surface layer due to its thermodynamic characteristics. Typical examples are copper and aluminum, which are passive in... 

Despite the difficulties encountered in the interpretation of the stationary polarization curves, due to the interference of different processes occurring with rates that depend in an intricate way on time, potential, pH in the bulk of the solution, anions, and metal surface characteristics, and... 

Rowell and co-workers [62-64] have developed an electrophoretic fingerprint to uniquely characterize the properties of charged colloidal particles. They present contour diagrams of the electrophoretic mobility as a function of the suspension pH and specific conductance, pX. These fingerprints illustrate anomalies and specific characteristics of the charged colloidal surface. A more sophisticated electroacoustic measurement provides the particle size distribution and potential in a polydisperse suspension. Not limited to dilute suspensions, in this experiment, one characterizes the sonic waves generated by the motion of particles in an alternating electric field. O Brien and co-workers have an excellent review of this technique [65]. 

Optimum conditions for the formation of CdS by the acidic method on metallic A1 substrate at 25 °C have been reported as follows pH 2.3, potential -1 V vs. SCE, and electrolysis time > 2 h [44]. Thermal treatment improved the characteristics of the films and their photovoltaic properties, which were evaluated by evaporating a CU2S layer on the CdS/Al film, to form a heterojunction cell. The influence of the deposition substrate on the formation and morphology of CdS was found to be important. The aluminum substrates gave the best results among Pt, Mo, and Al. In the case of molybdenum, surface blocking by adsorbed sulfur was considered. 

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