Adsorption layers

Short-Chain Organics. Adsorption of an organic dispersant can reduce polarizabiHty attraction between particles, ie, provide semisteric stabilization, if A < A.p < A or A < A.p < A (T = dispersant) and the adsorption layer is thick. Adsorption in aqueous systems generally does not foUow the simple Langmuir profile because the organic tails on adsorbed molecules at adjacent sites attract each other strongly. 

The drawback of the described adsorbents is the leakage of the bonded phase that may occur after the change of eluent or temperature of operation when the equilibrium of the polymer adsorption is disturbed. In order to prepare a more stable support Dulout et al. [31] introduced the treatment of porous silica with PEO, poly-lV-vinylpyrrolidone or polyvinylalcohol solution followed by a second treatment with an aqueous solution of a protein whose molecular weight was lower than that of the proteins to be separated. Possibly, displacement of the weakly adsorbed coils by the stronger interacting proteins produce an additional shrouding of the polymer-coated supports. After the weakly adsorbed portion was replaced, the stability of the mixed adsorption layer was higher. 

According to the concepts, given in the paper [7], a significant difference between the values of yield stress of equiconcentrated dispersions of mono- and polydisperse polymers and the effect of molecular weight of monodisperse polymers on the value of yield stress is connected with the specific adsorption on the surface of filler particles of shorter molecules, so that for polydisperse polymers (irrespective of their average molecular weight) this is the layer of the same molecules. At the same time, upon a transition to a number of monodisperse polymers, properties of the adsorption layer become different. 

The situation becomes most complicated in multicomponent systems, for example, if we speak about filling of plasticized polymers and solutions. The viscosity of a dispersion medium may vary here due to different reasons, namely a change in the nature of the solvent, concentration of the solution, molecular weight of the polymer. Naturally, here the interaction between the liquid and the filler changes, for one, a distinct adsorption layer, which modifies the surface and hence the activity (net-formation ability) of the filler, arises. Therefore in such multicomponent systems in the general case we can hardly expect universal values of yield stress, depending only on the concentration of the filler. Experimental data also confirm this conclusion [13],... 

This formula may be useful as a rheological method for determining the thickness of adsorption layer, which is formed as a result of interaction between a dispersion medium and filler, by the results of measuring the t] versus q> dependence. Especially curious phenomena, connected with surface effects, arise when a mixture of fillers of different nature is used according to concentration of an active filler the introduction of the second (inert) filler can either increase or decrease the viscosity of a multicomponent system [35],... 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

The concepts of interface rheology are derived from the rheology of three-dimensional phases. Characteristic for the interface rheology is the coupling of the motions of an interface with the flow processes in the bulk close to the interface. Thus, in interface rheology the shear and dilatational stresses of the interface are in equilibrium with the corresponding shear stress in the bulk. An important feature is the compressibility of the adsorption layer of an interface in contrast, the flow elements of the bulk are incompressible. As a result, compression or dilatation of the adsorption layer of a soluble surfactant is associated with desorption and adsorption processes by which the interface tends to reinstate the adsorption equilibrium with the bulk phase. 

The change in surface wettability (measured by the contact angle) with concentration for the three surfactants is plotted in Fig. 2.54 (Zhang and Manglik 2005). The contact angle reaches a lower plateau around the CMC where bilayers start to form on the surface. Wettability of non-ionic surfactants in aqueous solutions shows that the contact angle data attains a constant value much below CMC. Direct interactions of their polar chain are generally weak in non-ionics, and it is possible for them to build and rebuild adsorption layers below CMC. The reduced contact an-... 

FIG. 3 Plots of half the range of attraction (see Fig. 2) and the apparent thickness of the ethanol adsorption layer vs. the ethanol concentration. 

FIG. 6 Plausible structure of the adsorption layer composed of ethanol clnsters. 

The equilibrium time required for adsorption of metals on biomass was studied for various initial metal concentrations and the results were shown in Figure 2. The adsorption increases rapidly with time in the initial period of adsorption and approaches an equilibrium at about 120min for all the concentrations studied (10-100 mg/L). The slow but gradual increase of metal biosorption after 120min indicates that the adsorption occurs through a continuous formation of adsorption layer in the final period of adsorption. 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Snyder and Soczewinski created and published, at the same time, another model called the S-S model describing the adsorption chromatographic process [19,61]. This model takes into account the role of the mobile phase in the chromatographic separation of the mixture. It assumes that in the chromatographic system the whole surface of the adsorbent is covered by a monolayer of adsorbed molecules of the mobile phase and of the solute and that the molecules of the mobile phase components occupy sites of identical size. It is supposed that under chromatographic process conditions the solute concentrations are very low, and the adsorption layer consists mainly of molecules of the mobile phase solvents. According to the S-S model, intermolecular interactions are reduced in the mobile phase but only for the... 

Using the sensor technique for studying photolysis of adsorbed layers of cetene on metal oxides we observed tlie decay of adsorption layers under the influence of ultraviolet light. The reaction yields methylene radicals in the surface layer... 

The specified decrease of the radical concentration in the gas phase near the film surface and in tiie layer adsorbed on the film is caused by the fact that interaction of these prides with cetene molecules becomes stronger as concentration of the latter increases. Another reason for the decrease of the radical concentration is the decrease of the diffusion coefficients of active particles in the gas and on the siu-face. This results in a growth of the time it takes for active particles from a gas phase to reach the film surface. Furthermore, it leads to an increase in the time it takes for active particles in the adsorption layer to reach the centers of chemisorption. 

Another competitive process is associated with the cell effect in the adsorption layer... 

Figure 4.29 demonstrates validity of equation (4.26) in the case of butane at various pressures. It is seen that the dots representing two experimental series fit the same straight line. This means (according to the equation) that K2 values are equal and independent of the pressure of butane. In liquefied butane, the value increases significantly. This indicates that adsorption involves not only hydrogen atoms produced in the adsorption layer, but those produced in distant layers of the liquid adjacent to the surface. 

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