Interactions competitive adsorption

Chen, J., Dickinson, E., and Iveson, G. 1993. Interfacial interactions, competitive adsorption and emulsion stability. Food Structure 12 135-146. 

One can further elaborate a model to have a concrete form of /(ft), depending on which aspect of the adsorption one wants to describe more precisely, e.g., a more rigorous treatment of intermolecular interactions between adsorbed species, the activity instead of the concentration of adsorbates, the competitive adsorption of multiple species, or the difference in the size of the molecule between the solvent and the adsorbate. An extension that may be particularly pertinent to liquid interfaces has been made by Markin and Volkov, who allowed for the replacement of solvent molecules and adsorbate molecules based on the surface solution model [33,34]. Their isotherm, the amphiphilic isotherm takes the form... 

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... 

We have also calculated the competitive adsorption with the introduction of a non-zero value of the surfactant-surfactant interaction parameter, i.e. xi2 calculated composition in... 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

The observation that no hydrogenolysis of the C-X bond takes place as long as either nitro compounds or reaction intermediates are present can be explained by the strong adsorption of these molecules, thereby preventing the interaction of the C-Cl bond with the catalyst. The mode of action of the modifiers is less clear. It could be due to a modification of the catalytic properties of the Raney nickel or also to a competitive adsorption between the effective modifiers and the... 

With 0CO > 1/3 (i.e., for coverages beyond the completion of the y/3 x y/3 R 30° structure) a Pd(lll) surface is no longer able to dissociatively adsorb oxygen. Since this is a necessary prerequisite for C02 formation, the reaction is inhibited by CO if its coverage is too high. At lower CO concentrations on the surface oxygen can be co-adsorbed. Both components then form separate domains on the surface [competitive adsorption (182)] as becomes evident from LEED observations (172). The mean domain diameter is at least of the order of 100 A i.e., the coherence width of the electrons used with this technique. This indicates the existence of repulsive interactions between Oad and COad. As can be seen from the schematic sketch of Fig. 32b, eventual product formation can then only occur along the boundaries of these islands. 

Chen J. and Dickinson E. 1995c. Protein/surfactant interfacial interactions. Part 3. Competitive adsorption of protein + surfactant in emulsions. Colloids Surf. A Physicochem. Engin. Aspects 101 77-85. 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

The transitory poisoning by scavengers is explained by competitive adsorption of a halogen-containing species on catalyst sites that are needed for the oxidation of CO and hydrocarbons. In the case of EDB it is thermodynamically probable that HBr 33), or Br2 is the actual adsorbed species (66). The possible interactions of EDB and EDC with TEL and the resulting loss in noble metal surface area on the one hand, and catalyst activity on the other, are very complex (66). 

The high molar mass species reside mostly in the aqueous phase with a number of peptide groups residing in the oil/water interface [293]. Although these latter surfactants are less effective at reducing interfacial tension, they can form a viscoelastic membrane-like film around oil droplets or air bubbles. These tend to be used in the preparation of, for example, O/W emulsions. These trends are by no means exclusive, mixtures are the norm and competitive adsorption is prevalent. Caseinate, one of the most commonly used surfactants in the food industry, is itself a mixture of interacting proteins of varying surface activity [814],... 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Nondiagonal [k] if the permeating components interact with each other within the membrane this will be the case for example if bulk diffusion and/or competitive adsorption effects are involved. 

Less-polar solvent molecules B (CHCI3, CH2CI2, benzene, etc.) that do not localize nevertheless interact with adsorption sites. This is illustrated in Fig. Id for the binary mixtures A/B (A is nonpolar), and is contrasted in Fig. le for adsorption of a mobile phase A/C, where C is localizing. When nonlocalizing molecules of a polar mobile phase M are adjacent to localizing molecules of solute X (Fig. Ic) or solvent C (Fig, If), these noncova-lent interactions of M with surface sites can interfere with or displace corresponding interactions between the localized molecule and its site. This effect is referred to as site-competition delocalization. 

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