Adsorption phenomena

FIGURE 2.32 Adsorption-desorption isotherms of (a) water and (b) water or nitrogen on nanooxides. (Adapted from Powder Technol., 195, Gun ko, V.M., Zarko, V.I., Turov, V.V. et al.. Morphological and structural features of individual and composite nanooxides with alumina, silica, and titania in powders and aqueous suspensions, 245-258, 2009g, Copyright 2009, with permission from Elsevier.) 

However, the adsorption of water at ambient conditions resnlts in the formation of clusters at solid surfaces, especially in pores and at the surface containing hydroxyl groups (Brennan et al. 2001, Gun ko et al. 2005d, 2007d,f,i, 2009a, Chaplin 2011). 

The morphology of primary nanoparticles of nanooxides, as well as particles of higher hierarchic levels such as aggregates of primary particles and agglomerates of aggregates, is typical and nearly the same for all individual and complex nanooxides because of the flame synthesis conditions. Therefore, the nitrogen adsorption-desorption isotherms are of the same type for different oxides (Rgure 2.32). 

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

In some cases, hidden adsorption is responsible for the differences between electrode and purely chemical redox reactions. The aromatic derivatives of divalent sulfur on reduction at the mercury-dropping electrode do not show any adsorption waves within the corresponding polarogam. Thus, the reduction 

The addition of mercury in reaction mixtures of nitroarylsulfenates with in THF did not 

The dissimilarity found between the homogeneous and heterogeneous reduction processes can be attributed to a specific interaction of the sulfur-containing groups with the material of the electrode. 

the reduction of arenesulfenates at the mercury electrode can proceed by the formation of intermediate arenethiomercuric derivatives. Such derivatives are reduced just after their formation and more easily than the initial arenesulfenates. In line with this argument, it logically follows that the limiting currents of the polarographic waves would depend solely on the diffusion of substances to the electrode. In fact, diffusion currents have been observed experimentally. Experiments of electrolysis on mercury (a preparative scale) confirmed the general conclusion (Todres 1988). 

Generally, for chemical adsorption a significant overlap between substrate and electrode orbitals is needed, that is, a weak chemical bond has to be established. This implies that there must be an orientation effect, depending on the symmetry of orbitals involved, and the substrate molecule must be fairly close to the electrode surface. 

The combination of foreign ions with the surfaces of colloidal particles is, in general, called adsorption. It goes without saying that this behaviour is not limited to small particles and that a large crystal of silver chloride can equally well adsorb positive or negative particles. 

Chemical forces of various kinds can lead to such adsorption. Thus, the bond between ions and the AgGl molecule is due primarily to coulomb forces, but adsorption can equally well be caused by polarization forces alone, as for example when iodine is strongly adsorbed on the surface of GaF2. The calcium ions create dipoles in the iodine molecules, and the attraction of these dipoles by the charges of the calcium ions gives rise to the bond. In the same manner, alkali atoms, particularly those of Cs, are adsorbed by metal oxides. Oxides with adsorbed alkali atoms are of the very 

Calcium fluoride does not form a hydrate, yet the surface easily acquires a layer of water this, however, is not formed on crystals which form hydrates. This at first sight appears to be somewhat paradoxical, but the explanation is quite simple. The water molecules are held very strongly by the calcium fluoride, due to the strong field of the ions which also holds the crystal together and at the same time prevents the penetration of water. Water can only penetrate those compounds which have weaker ionic fields, but it will then no longer be held so strongly and certainly not on the surfaces of the crystal. 

It is a known fact that finely divided carbon can adsorb many gases, a property which is utilized in gas masks. Here the adsorption depends on the van der Waals-London forces, from which it follows that all strongly polarizable gases will be adsorbed, and, further, that a carbon mask is not suitable for the adsorption of CO. In practice, the efficacy of carbon for the adsorption of gases can be increased by adding other, usually ionic materials. 

Many electrochemical phenomena and processes are to a great extent influenced by different adsorption processes. Of prime importance is the adsorption on the electrode s surface of components of the electrolyte solution, as well of those participating in the electrode reaction, as those inert components that do not participate. 

The amount of species of the adsorbed substance j (adsorbate) per unit area of the trae surface area of the electrode or of any other adsorbent will be labeled Aj and will be called real adsorption (in contrast to the notion of Gibbs adsorption T see Section 10.4.1). In the limiting case, all adsorbed particles are packed right again the adsorbent s surface. This limiting case is called monolayer adsorption. In other cases, several layers of the adsorbate can form on the adsorbent s surface multilayer adsorption). 

When the component j can exist in both phases (e.g., the electrolyte and the electrode) it will undergo redistribution after the phases have come into contact, and in particular, some of it will be transferred into the interior of the phase, where none of it had existed previously. In this case the term absorption (or bulk uptake) is used for the component. 

When the two phases in contact are condensed phases and the entire volume is taken up by incompressible substances, positive adsorption of one component must be attended by negative adsorption (desorption) of other components. This phenomenon is called adsorptive displacement. 

In the case of monolayer adsorption, a limiting adsorption value exists that is attained when the surface is covered completely by particles of a given substance (i.e., at full monolayer coverage). The limiting adsorption value depends on the effective surface area Sj taken up by one particle 1/5. This parameter characterizes the number of sites that can be occupied by adsorbed particles on a given surface. 

Whenever the concentration of a species at the interface is greater than can be accounted for by electrostatic interactions, we speak of specific adsorption. It is usually caused by chemical interactions between the adsorbate and the electrode, and is then denoted as chemisorption. In some cases adsorption is caused by weaker interactions such as van der Waals forces we then speak of physisorption. Of course, the solvent is always present at the interface so the interaction of a species with the electrode has to be greater than that of the solvent if it is to be adsorbed on the electrode surface. Adsorption involves a partial desolvation. Cations tend to have a firmer solvation sheath than anions, and are therefore less likely to be adsorbed. 

The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams. 

Molecules of fluid phases (f), i. e. gases, vapors, and liquids, can stick to the surface of solids (s) or other liquid phases (1). This phenomenon is called adsorption. It occurs in principle at any temperature and pressure and for all chemical species known so far [1.1-1.3]. The adsorbed molecules may have their place on the surface of the sohd and return to the gaseous phase. This phenomenon is called desorption. Often one can observe dynamic equilibrium between the number of molecules adsorbed and those desorbed in a certain time interval. Such a situation is called adsorption equilibrium. If these molecular flows to and from the surface do not match, we have either an adsorption process or a desorption process [1.4-1.6]. 

Additionally, in highly porous solids like zeolites and activated carbons there may be internal diffusion processes of the adsorbed molecules (admolecules). These can occur without external exchange of mass, i. e. at constant mass adsorbed, cp. Sects. 4, 5. An example for such a phenomenon is presented in Chap. 6, Fig. 6.29, [1.4, 1.7-1.9]. 

Adsorptive Gas or liquid whose molecules are interacting with the surface atoms of a solid phase. 

Adsorbent Solid phase with external and internal surfaces exposed to the molecules of a gas or liquid phase. 

Adsorbate Set of molecules being adsorbed on the surface of an (often porous) sohd material and forming a separate phase in the sense of thermodynamics, cp. Sect. 5. 

The adsorbed layer is the scene of various interesting phenomena, like re-orientation of the adsorbate molecules, co-adsorption, polylayer formation, surface aggregation, adsorption of oligomers and, finally, surface phase transformations. All these phenomena can be treated within the frames of either the STE model or the models based on the LBS approach in precisely the same way We express the equilibrium equations first in terms of chemical potentials and next we introduce into these equations the expressions of the chemical potentials given by Eqs. (13) and (14). In some cases certain modifications are needed, as discussed below. 

Ammonia synthesis Nj -I-H2 Iron oxide (FejO ) promoted with AtjOj and K2O NH3 

Oxidation of sulfur dioxide SO2 + O2 V2O5 or Pt SO3 used in the manufacture of oleum and sulfuric acid 

Hydrogenation of fats and edible oils Unsaturated oil + H2 Ni Saturated oil or fat 

Cracking Various petroleum fractions Combinations of sUica, alumina, and molecular sieves Wide range of compounds 

Polymerization Ethylene Aluminum alkyls and riCl4, M0O3, or C1O3 on alumina Polyethylene 

---

Adsorption is of technical importance in processes such as the purification of materials, drying of gases, control of factory effluents, production of high vacua, etc. Adsorption phenomena are the basis of heterogeneous catalysis and colloidal and emulsification behaviour. 

J. H. de Boer, Electron Emission and Adsorption Phenomena, Macmillan, New York, 1935. 

It turns out that many surfaces (and many line patterns such as shown in Fig. XV-7) conform empirically to Eq. VII-20 (or Eq. VII-21) over a significant range of r (or a). Fractal surfaces thus constitute an extreme departure from ideal plane surfaces yet are amenable to mathematical analysis. There is a considerable literature on the subject, but Refs. 104-109 are representative. The fractal approach to adsorption phenomena is discussed in Section XVI-13. 

An interesting question that arises is what happens when a thick adsorbed film (such as reported at for various liquids on glass [144] and for water on pyrolytic carbon [135]) is layered over with bulk liquid. That is, if the solid is immersed in the liquid adsorbate, is the same distinct and relatively thick interfacial film still present, forming some kind of discontinuity or interface with bulk liquid, or is there now a smooth gradation in properties from the surface to the bulk region This type of question seems not to have been studied, although the answer should be of importance in fluid flow problems and in formulating better models for adsorption phenomena from solution (see Section XI-1). 

This chapter on adsorption from solution is intended to develop the more straightforward and important aspects of adsorption phenomena that prevail when a solvent is present. The general subject has a vast literature, and it is necessary to limit e presentation to the essential features and theory. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

In Sec. II we briefly review the experimental situation in surface adsorption phenomena with particular emphasis on quantum effects. In Section III models for the computation of interaction potentials and examples are considered. In Section IV we summarize the basic formulae for path integral Monte Carlo and finite size scahng for critical phenomena. In Section V we consider in detail examples for phase transitions and quantum effects in adsorbed layers. In Section VI we summarize. 

Theoretieal studies aiming at the ineorporation of heterogeneity effeets into the formalism deseribing adsorption phenomena gained a new impetus... 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

Discussion. The Mohr method cannot be applied to the titration of iodides (or of thiocyanates), because of adsorption phenomena and the difficulty of distinguishing the colour change of the potassium chromate. Eosin is a suitable... 

A possible approach to interpretation of a low-frequency region of the G ( ) dependence of filled polymers is to compare it with a specific relaxation mechanism, which appears due to the presence of a filler in the melt. We have already spoken about two possible mechanisms — the first, associated with adsorption phenomena on a filler s surface and the second, determined by the possibility of rotational diffusion of anisodiametrical particles with characteristic time D 1. But even if these effects are not taken into account, the presence of a filler can be related with the appearance of a new characteristic time, Xf, common for any systems. It is expressed in the following way... 

If n = 1, the adsorbed amount would be proportional to the pressure in adsorption phenomena n > 1. 

Plonski, I.-H. Effects of Surface Structure and Adsorption Phenomena on the Active Dissolution of Iron in Acid Media 29... 

Adsorption phenomena from solutions onto sohd surfaces have been one of the important subjects in colloid and surface chemistry. Sophisticated application of adsorption has been demonstrated recently in the formation of self-assembhng monolayers and multilayers on various substrates [4,7], However, only a limited number of researchers have been devoted to the study of adsorption in binary hquid systems. The adsorption isotherm and colloidal stabihty measmement have been the main tools for these studies. The molecular level of characterization is needed to elucidate the phenomenon. We have employed the combination of smface forces measmement and Fomier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR) to study the preferential (selective) adsorption of alcohol (methanol, ethanol, and propanol) onto glass surfaces from their binary mixtures with cyclohexane. Om studies have demonstrated the cluster formation of alcohol adsorbed on the surfaces and the long-range attraction associated with such adsorption. We may call these clusters macroclusters, because the thickness of the adsorbed alcohol layer is about 15 mn, which is quite large compared to the size of the alcohol. The following describes the results for the ethanol-cycohexane mixtures [10],... 

Adsorption phenomena at electrode-electrolyte interfaces have a nnmber of characteristic special features. 

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

Parameter estimates should not differ by orders of magnitude from those evaluated using well established methods of thermodynamics or known from the literature several rules concerning adsorption phenomena have been worked out by Boudart et al. (1967) the optimal parameter estimates should not differ very much from the initial guesses if the latter were determined in well designed separate dedicated experiments. 

Frumkin AN. 1961. Hydrogen overvoltage and adsorption phenomena. Part I. In Delahay P, Tobias CW. editors. Advances in Electrochemistry and Electrochemical Engineering. Volume 1. New York Interscience. 

The appreciation of the importance of adsorption phenomena at liquid interfaces is probably as old as human history, since it is easily recognized in many facets of everyday life. It is not surprising that liquid interfaces have been a favorite subject of scientific interest since as early as the eighteenth century [3,4], From an experimental point of view, one obvious virtue of the liquid interfaces for studying adsorption phenomena is that we can use surface tension or interfacial tension for thermodynamic analysis of the surface properties. The interfacial tension is related to the adsorbed amount of surface active substances through the Gibbs adsorption equation. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

The described adsorption phenomena are characteristic for electrodes of sp metals. Transition metal electrodes are usually connected with irreversible chemisorption phenomena, discussed in Section 5.7. 

---