Basic adsorption phenomena

Adsorption isotherms can be applied to any surface. In the following we focus our attention on surfaces covered with adsorption layers under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from soluble adsorption layers for example. Another phenomenon is the spread of surfactant molecules tangential to the surface that effect takes place if the adsorption layer is inhomogeneous (cf. Fig. 1.1). 

The effect of surface dilational elasticity was first formulated by Gibbs (1957) as 

In Chapter 3 we will show that the modulus of surface elasticity is really quite a complex quantity. Whereas the flow properties of liquid/air and liquid/liquid interfaces are determined strongly by elastic forces, the solid/liquid interface is mainly related to wetting processes. 

Other effects of significance for the application of surfactants include forced and spontaneous emulsification, stabilisation of emulsions and dispersions, the wetting of hydrophobic surfaces by surfactants solutions, and many more. 

We will now discuss surfactant adsorption at a liquid interface. When the surface of a surfactant solution has been formed the adsorption equilibrium takes time to be established. This process is determined by transport from the bulk to the surface, mainly by diffrision processes (undisturbed diffiision, laminar or convective diffusion). 

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Surface Chemistry of Surfactants and Basic Adsorption Phenomena... 

Table 6.10 gives a list of several isotherms, including the ones already discussed, and some of their main characteristics. The isotherms we have presented here are focused on particular aspects of the adsorption phenomenon. For example, the Langmuir isotherm focuses on the basic process of transferring a molecule from the bulk to the electrode the Temkin isotherm focuses on interpreting the adsorption process in terms of the heterogeneity of the adsorbing surface etc. 

Whatever quantity is used to determine the amount adsorbed (pressure, mass or gas flow), one has to choose between the two basic procedures, discontinuous and continuous. In the past there has been some misunderstanding about the significance of these terms. We prefer to focus attention on the adsorption phenomenon itself, as will be made clear in the following sections. 

In another vein, double layers play a role In the salt-sieving phenomenon, mentioned In the Introduction to Volume I, and already known to Aristotle. When seawater percolates over a compact sediment of slllcate-like particles, under some conditions the effluent Is potable. Basically the phenomenon is attributable to the negative adsorption of (in this case) anions, leading to the Donnan expulsion of electrolyte, see sec. 3.5b. Over-demand may lead to salt penetration the screening of the double layers around the silica particles (reduction of x ) makes the pores between them effectively wider. For this problem technical solutions had to be found. 

The general phenomenon of polymer adsorption/retention is discussed in some detail in Chapter 5. In that chapter, the various mechanisms of polymer retention in porous media were reviewed, including surface adsorption, retention/trapping mechanisms and hydrodynamic retention. This section is more concerned with the inclusion of the appropriate mathematical terms in the transport equation and their effects on dynamic displacement effluent profiles, rather than the details of the basic adsorption/retention mechanisms. However, important considerations such as whether the retention is reversible or irreversible, whether the adsorption isotherm is linear or non-linear and whether the process is taken to be at equilibrium or not are of more concern here. These considerations dictate how the transport equations are solved (either analytically or numerically) and how they should be applied to given experimental effluent profile data. 

Actually, this sample was completely separated into water and Ag agglomerates after about 3 h. On the contrary, silver colloids prepared in the presence of surfactants showed relatively good stability compared with the sample prepared in a simple aqueous solution without a surfactant. But depending on the surfactant species, quite different stabilities were observed. Especially in the case of Tween 20 and SDS, the colloids remained basically stable for weeks. The silver nanoparticles in the colloid solution were adsorbed on the silica wall of glass vials. This effect was observed for Tween 20 just after 3 min from the start of the reduction in our experimental conditions, but in the case of NP-9 and SDS, this phenomenon was observed after 1 day. Such adsorption phenomenon was also reported by Liz-Marzan and Lado-Tourino [18] after their experiment on silver reduction through... 

In the 2nd period ranging from the 1930s to the 1950s, basic research on flotation was conducted widely in order to understand the principles of the flotation process. Taggart and co-workers (1930, 1945) proposed a chemical reaction hypothesis, based on which the flotation of sulphide minerals was explained by the solubility product of the metal-collector salts involved. It was plausible at that time that the floatability of copper, lead, and zinc sulphide minerals using xanthate as a collector decreased in the order of increase of the solubility product of their metal xanthate (Karkovsky, 1957). Sutherland and Wark (1955) paid attention to the fact that this model was not always consistent with the established values of the solubility products of the species involved. They believed that the interaction of thio-collectors with sulphides should be considered as adsorption and proposed a mechanism of competitive adsorption between xanthate and hydroxide ions, which explained the Barsky empirical relationship between the upper pH limit of flotation and collector concentration. Gaudin (1957) concurred with Wark s explanation of this phenomenon. Du Rietz... 

The basic premise of the original kinetic description of inhibition was that, for a reaction to proceed on a surface, one or more of the reactants (A) must be adsorbed on that surface in reversible equilibrium with the external solution, having an equilibrium adsorption constant of KA, and the adsorbed species must undergo some transformation involving one or more adsorbed intermediates (n) in the rate-limiting step, which leads to product formation. The product must desorb for the reaction cycle to be complete. If other species in the reaction mixture (I) can compete for the same adsorption site, the concentration of the adsorbed reactant (Aad) on the surface will be lower than when only pure reactant A is present. Thus, the rate of conversion will depend on the fraction of the adsorption sites covered by the reactant (0A) rather than the actual concentration of the reactant in solution, and the observed rate coefficient (fcobs) will be different from the true rate coefficient (ktme). In its simplest form the kinetic expression for this phenomenon in a first-order reaction can be described as follows ... 

Stabilization of a radical anion of humic acid may be caused by an adsorption effect. Bijl (3) observed that solid barium hydroxide octahydrate turned blue when placed in a solution of quinhydrone the blue solid was highly paramagnetic. Under the conditions we used for preparing these salts, insoluble sodium humate (with a large surface area) could have stabilized the anion radical by adsorption from the basic solution. Weiss and McNeil (18) observed a similar phenomenon with base soluble xanthenes, and proposed that biradicals may be formed in such a system. His compounds, however, do not appear to have the structural requirements to satisfy such a stabilized system. The recent report by Weber (29) on the spin content increase associated with the basification of a naphthoquinone-naphthohydroquinone system seems to parallel our observations quite closely. 

This Volume deals with various aspects of surface tensions and interfacial tensions. Together with the phenomenon of adsorption (enrichment of molecules at interfaces), these tensions constitute the basic characteristics of interfaces. 

Chemical kinetics also plays a basic role in the study of the nature of catalytic activity. Studies of the catalyst and reactants in the absence of appreciable over-all reaction, such as studies of the electronic properties of catalytic solids or optical studies of adsorbed molecular species can provide valuable information about these materials. In most cases, however, kinetic data are ultimately needed to establish the relation and relevance of any information derived from such studies to the catalytic reaction itself. For example, a particular adsorbed species may be observed and studied by a spectral technique yet it need not play any essential role in the catalytic reaction since adsorption is a more general phenomenon than catalytic activity. On the other hand, kinetics studies can provide information about the variation, as a function of experimental conditions, of the relative number of adsorbed species that play a basic role in the reaction. Consequently, such information may make it possible to identify which, if any, of the adsorbed species studied by the use of a direct analytical technique are relevant to the reaction. As another example, when studies are made of the solid state properties of a given catalytic solid, the question as to which, if any, of these properties are related to catalytic activity must ultimately be answered in terms of consistency with the observed behavior of the reaction system. 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

In the narrow sense modelling is a simplified description of a physical phenomenon (substance or process) which serves for writing basic equations representing the properties of the system. In the broader sense the term model is used for theory and also includes basic equations, as well as the resulting relationships. Physical chemistry is concerned with both theoretical and experimental aspects. Figure 1 is a scheme that represents activity in a certain field. The example is given for the physical chemistry of ionic adsorption. 

It is a characteristic aspect of the MB method that it frequently overestimates the values of both surface area and CEC (Hahner et al., 1996). Typical explanations for this phenomenon are that MB adsorbs in multiple layers (Hang Brindley, 1970) or that it does not lie flat on the mineral surface (De et al., 1974 Shelden et al.,1993 Hahner et al.,1996 Bujdak Komadel, 1997). The exact reason remains unclear because the mechanisms of MB adsorption to clay minerals are largely unknown and expected to be rather complex (Bodenheimer Heller, 1968). Thus, the problem of MB adsorption is basically a structural one and of the kind for which MD simulations are particularly well suited. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

The stop-effect, a drastic increase of the reaction rate when the feed concentration of a reactant is switched to zero, was studied for the dehydration of ethanol to ethylene on 7-alumina at 180 and 200°C. Two basic models exist in the literature to describe this phenomenon. They were discriminated on the basis of transient and periodic experiments, coupled with FTIR data of the adsorbed species. The model that best describes these measurements postulates the adsorption of ethanol on two different sites, S and S2, with a free S2 site being necessary for ethylene formation. 

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