Adsorption measurements, indirect

Another property of importance is the pore volume. It can be measured indirectly from the adsorption and/or desorption isotherms of equilibrium quantities of gas absorbed or desorbed over a range of relative pressures. Pore volume can also be measured by mercury intrusion techniques, whereby a hydrostatic pressure is used to force mercury into the pores to generate a plot of penetration volume versus pres- sure. Since the size of the pore openings is related to the pressure, mercury intrusion techniques provide information on the pore size distribution and the total pore volume. 

Future important contributions of heats of immersion will be made in the field of solution adsorption despite the necessity for more exacting experimentation. The common problem in solution adsorption has been to define the nature and extent of the interface between solid particles and mixed liquids. Specifically, more information is needed concerning the orientation and solvation of adsorbed molecules as well as the composition and practical boundary of the adsorbed phase. Direct adsorption measurements yield only net changes in concentration and indirect approaches must be taken (66). Much can be learned, however, by measuring the heats of immersion of powders into two component solutions of varying composition where the adsorption of one component is predominant. This technique, also, is the only available method for measuring the heat of adsorption of... 

Ai and Suzuki [5,9] investigated the combination V2Os—P2Os. The acidity was measured indirectly by the activity for dehydration of isopropanol and was shown to decrease with increasing P2Os content. The activity for the oxidation of butene-1 and butadiene to maleic acid anhydride decreased accordingly. It was shown that the adsorption equilibrium constant of the olefin on the catalyst also decreased in the same way. 

There are two principal ways to obtain experimental test results from the statistical polymer method the comparison of the predicted weight distribution and other parameters of polymeric systems with these measured for classical polymers (direct test) and the comparison of predicted structural characteristics with these found from adsorption/desorption and other measurements (indirect test). For gels, the indirect test has many advantages, while the interpretation of experimental data is not always easy, e.g., chemisorption is sometimes mixed to physisorption. 

In writing this book, we have endeavoured to give an introductory survey of the principles, methodology and applications of the adsorption of gases and liquids by powders and porous solids. In particular, we hope that this book will meet the needs of all those students and non-specialists who wish to undertake adsorption measurements. In addition, we believe that certain sections of the book will be of interest to those scientists, engineers and other technologists who are already concerned, either directly or indirectly, with the characterization of finely divided or porous solids. 

Leenheer and Huffman (1976) designed a hydrophobic classification of dissolved organic carbon using resin adsorption as a means of fractionation. The procedure is based on adsorption chromatography onto XAD resins. Those organic substances that adsorb onto the resins with pH adjustment (low pH for acids and neutral pH for bases) are termed hydrophobic those organic substances that do not absorb are hydrophilic. Fulvic and humic acids are classified as hydrophobic substances. This classification procedure makes it possible to measure indirectly the amount of humic substances in water. A more detailed explanation of this procedure is presented by Leenheer (1981). 

The accurate information about the surface tension-surface coverage relationship which for soluble monolayers is contained in the e0—tt curves, can be used, for example, to interpret rate of adsorption measurements. For fluid-fluid interfaces, adsorption onto an initially clean surface can be assessed only indirectly by measuring the changing interfacial tension. An example is shown in Figure 9, giving the change in surface pressure... 

As already mentioned for other surface systems, an indirect but efficient way to monitor changes in the electronic structure of a supported metal particle with respect to the free species is to analyze variations of properties of an adsorbed probe molecule. This is similar to the experimental approach to examine the acidity or basicity of a given adsorption site by measuring indirectly the flow of charge between the adsorbate and the substrate [201] (see Section 4). Comparison of spectral signatures of adsorbed CO molecules (e.g., the frequency of the C-0 vibration) offers valuable information on the electronic state of the substrate. We analyzed computationally the properties of CO molecules adsorbed on free and supported model Pd4 and Pds clusters [176]. 

In summary, indirect "adsorption measurements" indicate some surprizing results. First a rather large effect of small changes in acid concentration is observed for the butyne-diol. It would be rather difficult to explain this in terms of adsorption theory because the adsorption isotherm in 4N hydrochloric acid crosses the one obtained in 6N hydrochloric acid. 

The Measuring methods of adsorption amount of flotation reagent on mineral surface are divided into direct method and indirect method. Direct method refers to that the adsorption amount of flotation reagent is direcdy measured. Indirect method refers to that the adsorption amount of flotation reagent is indirectly measured by computing the difference between initial concentration and residual concentration of reagent solution. 

It is well known, that the vast majority of metal ions hydrolyze in aqueous solutions, yielding a variety of solutes, the complexity of which increases with the charge of the cation. It has been established with numerous adsorbents in aqueous solutions of metal salts, that the adsorptivity of cations increases dramatically, as a result of their hydrolysis [11]. Indeed, the ions which do not specifically interact with solid surfaces, do so once they form hydrolyzed complexes. The enhanced uptake has been documented by direct adsorption measurements and indirectly by determining electrokinetic mobilities (electrokinetic potentials) as a function of the pH. The latter experiments invariably show the formation of charged sites on neutral surfaces or charge reversal on negatively charged surfaces, due to the chemisorption of hydrolyzed cationic solutes. 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

Note that in all the examples discussed so far, infrared spectroscopy gives its information on the catalyst in an indirect way, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult because of absorption by the support. Infrared emission and Raman spectroscopy, discussed later on in this chapter, offer better opportunities in this respect. 

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. 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

The main goal of the molecular dynamics computer simulation of ionic solvation and adsorption on a metal surface has been to test the above model and to provide more quantitative information about the different factors that influence the structure of hydrated ions at the interface. Unfortunately, most of the experimental information about these issues has been obtained from indirect measurements such as capacity and current-potential plots, although in recent years in situ experimental techniques have begun to provide an accurate test of the above model. For a recent review of experimental techniques and the theory of ionic adsorption at the water/metal interface, see the excellent paper by Philpott. ... 

---