Macroscopic adsorption data

In this work we present five methodologies, drawn from the interface science, which should be used jointly in order to achieve the aforementioned targets. These methodologies are based on potentiometric titrations, microelectrophoresis and macroscopic adsorption data. Experimental results concerning various TMIS/support combinations have been used to illustrate the application of the aforementioned methodologies. 

The aforementioned general approach is schematized in Figure 2.6. It involves the application of several methodologies based on macroscopic adsorption data and potentiometric titrations as well as microelectrophoretic mobility or streaming potential measurements, the appKcation of spectroscopic techniques as well as the application of electrochemical (equilibrium) modeling, quantum-mechanical calculations and dynamic simulations. 

Methodologies Based on Macroscopic Adsorption Data and Potentiometric Titrations as well as on Microelectrophoretic Mobility or Streaming Potential Measurements... 

A successful modeling must describe the macroscopic adsorption data (adsorption isotherms, adsorption edges, the aforementioned plots amount of the H+ ions released (adsorbed) vs. amount of cationic (anionic) TMIS adsorbed , potentiometric titrations, microelectrophoretic mobility or steaming potential data over a wide range of pH, ionic strength and concentrations of the TMIS in the solution, using the minimum number of adjustable parameters. 

Adsorption isotherms obtained from the model have been shown to agree very closely with the predictions of recently published statistical theories (9,13). While there can be no doubt that the more sophisticated, statistical models provide more information on the nature of the adsorption process and the structure of the adsorbed film, because of its simple form, the macroscopic model can offer a powerful tool for the analysis, interpretation and utilization of adsorption data. 

Microscopic Subreactions and Macroscopic Proton Coefficients. The macroscopic proton coefficient may be used as a semi-empirical modeling variable when calibrated against major system parameters. However, x has also been used to evaluate the fundamental nature of metal/adsorbent interactions (e.g., 5). In this section, macroscopic proton coefficients (Xj and v) calculated from adsorption data are compared with the microscopic subreactions of the Triple-Layer Model ( 1 ) and their inter-relationships are discussed. 

One should realize that adsorption isotherms are purely descriptions of macroscopic data and do not definitively prove a reaction mechanism. Mechanisms must be gleaned from molecular investigations (e.g., the use of spectroscopic techniques). Thus the conformity of experimental adsorption data to a particular isotherm does not indicate that this is a unique description of the experimental data, and that only adsorption is in operation. 

In the field of adsorption from solution, many discussions and reviews were published about the measurement of the adsorbed amount and the presentation of the corresponding data [14, 45—47]. Adsorption isotherms are the first step of any adsorption study. They are generally determined from the variation of macroscopic quantities which are rigorously measurable far away from the surface (e.g., the concentration of one species, the pressure, and the molar fraction). It is then only possible to compare two states with or without adsorption. The adsorption data are derived from the difference between these two states, which means that only excess quantities are measurable. Adsorption results in the formation of a concentration profile near an interface. Simple representations are often used for this profile, but the real profile is an oscillating function of the distance from the surface [15, 16]. Without adsorption, the concentration should be constant up to the soHd surface. Adsorption modifies the concentration profile of each component as well as the total concentration profile. It must be noted also that when the liquid is a pure component its concentration profile, i.e., its density, is also modified. Experimentally, the concentration can be measured at a large distance from the surface. The surface excess of component i is the... 

During the early development of these surface complexation models, the selection of a particular reaction was based on its ability to lit adsorption data collected in macroscopic experiments in which the extent of sorption was expressed by quantifying the mass of solute lost from solution as a function of pH. Model refinements were based on attempts to more accurately describe surface charge be-... 

Some of the experimental data modeled in this chapter were obtained from the literature and the experimental methods are reported therein. In all cases, sodium nitrate was used as the background electrolyte. The Co(II) adsorption data were collected by Katz and Hayes (1995a,b). Macroscopic and XAS sorption data forCd(II) were collected previously by Honcymanf 1984) ami Papeliset al. (1995), respectively. The spectroscopic data from Papelis et al. (1995) were collected on... 

The second step is to find good estimates of the rate parameters. The rate parameters can be obtained from collision theory, transition state theory, as well as first principles calculations such as DFT Calorimetric measurements of heats of adsorption is possible for the surface intermediates with a gas phase precursor. Otherwise, the surface energetic must be estimated. As we have mentioned earlier, the computational cost of DFT is overriding its utility and accuracy in the present-day capabilities. Eventually, the parameter space must be constructed with two major constraints. The first constraint requires the consistency with the thermodynamics and the second constraint requires that the macroscopic rate data can be reproduced. Unity bond index-quadratic exponential potential (UBI-QEP) method of Shustorovich (1986, 1998) offers a relatively accurate and affordable estimation of the surface energetics. 

In this survey, it is not attempted to show how successful surface complexation models are. Rather, it is attempted to show what can be done with them, what will be done with them in the future and what should not be done with them. The experimental aspects (e.g., the input data to the models) are discussed whenever judged important (certainly without full coverage but rather with focus on those aspects, which have not yet been addressed or details which have been of interest to the present author). In particular, the importance of combining methods and data is stressed. In recent years, there has been an increased interest in linking surface complexation models, which are traditionally based on macroscopic (adsorption, titration, and other) data, with structural information obtained with modem spectroscopic methods such as x-ray absorption spectroscopy (XAS). It is expected that the closer the agreement of the thermodynamic formulation of a surface chemical reaction with the actual structure of a surface complex is, the more reliable a prediction of the system behavior under more or less strongly varied conditions will be. 

The above-cited example on Cd/hematite indicates that some groups perform titrations in the presence of solutes different from innocent electrolytes. Such titrations may yield important macroscopic information on the proton balance of the suspension in the presence of such a solute (Table 2). However, the exact proton stoichiometry of some surface complex can rarely be inferred, because this would require that only one complex exists and that the protonation states of the surface groups, which are not contributing to that particular surface complex, are not affected by the adsorption process. This can, at best, be assumed in a quaUtative interpretation but can be quantitatively handled with the mean field approximation and the corresponding assumptions inherent to the respective computer programs. In fitting some models to adsorption data, proton data will constitute an independent and very valuable dataset representative of the system however, they may be restricted to sufficiently high solute to sorbent ratios. 

In the next sections, several selected aspects of macroscopic and microscopic data are discussed. Because of the importance of the acid-base properties of the soUds for the modeling concept, the discussed macroscopic data are limited to surface-charge density versus pH curves. A survey of actual adsorption data is beyond the scope of this chapter. 

Experimental observations of these empirical correlations clearly prove the postulated proportionality. These correlations suggest a similarity between the bond (with lower coordination) of the adsorbed particles to the modified surface and the bond to the surface of the pure macroscopic phase of the compound, which is relevant for the desublimation process. The adsorption behavior of atoms and compounds for most of the experiments used in the described correlations were evaluated using differently dehned standard adsorption entropies [65-70]. Adsorption data from more recent experimental results were evaluated applying the model of mobile adsorption [4]. Hence, data from previous experiments were re-evaluated using the latter model. These correlations based on estimated standard sublimation enthalpies allow predictions of adsorption enthalpies for selected compounds for the case of zero surface coverage. These results are only valid for experimental conditions using the same reactive gases, and thus, similarly modified stationary surfaces. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation suffer from the fundamental weakness of being macroscopic approaches that do not involve a direct examination of the solid phase. Information about the composition of an aqueous solution phase is not sufficient to permit a clear inference of a sorption mechanism because the aqueous solution phase does not determine uniquely the nature of its contiguous solid phases, even at equilibrium (49). Perhaps more important is the fact that adsorption and surface precipitation are essentially molecular concepts on which strictly macroscopic approaches can provide no unambiguous data (12, 21). Molecular concepts can be studied only by molecular methods. 

---