Preferential adsorption phenomena

Polymers in Binary Solvents. Cosolvency Effect Preferential Adsorption Phenomena... 

In general, to explain the observed cosolvent effects, the preferential adsorption phenomena have been invoked. Flowever few topics in the physical chemistry of polymers have evoked so many theories but so little consensus as preferential adsorption. When a polymer is dissolved in a binary solvent mixture, usually one of the solvents preferentially solvates the polymer. This solvent will then be found in a greater proportion in the proximities of the macromolecule with respect to the bulk solution composition. This variation of the solvent composition can cause interesting phenomena such as cosolvency as was discussed before, [11, 91, 92] non - cosolvency [93, 94], and some times variation of the unperturbed polymer dimensions [95,96]... 

Preferential Adsorption Phenomena Thermodynamical Description. Association Equilibria Theory... 

There are some important structural aspects of the polymer which are necessary to take into account in the analysis of the polymer behavior in mixture solvents, such as its polarity, chemical structure, microtacticity, molecular weight. The analysis of these properties shows that they are determinant factors in preferential adsorption phenomena involved. It has been pointed out that the effect of tacticity, and particularly the molecular weight, is a complex problem. In the case of poly(2-vinylpyridine), when the polar solvent is preferentially adsorbed, preferential solvation is independent of molecular weight but when the non-polar solvent is adsorbed, there is a dependence on the molecular weight. ... 

The goal of this review was to present aspects of the preferential adsorption phenomena of solvents and polymers with a focus on their thermodynamic aspects. The idea behind this was to attract the attention of polymeric physico-chemists to this area, which is sufficiently related to a lot of different effects. In fact, preferential adsorption occupies a special place in the solubility of polymers in mixed solvents. 

Surface charge at mineral surfaces can be generated by one or more mechanisms. The most commonly encountered charging mechanisms include dissociation of surface acid groups [12-14], lattice substitution [15-17], preferential hydration of surface lattice ions, and preferential adsorption phenomena [18-20]. In some cases, a surface may be charged by the orientation of dipoles or by surface polarization. Of particular concern in the case of alkali halides is the preferential hydration of... 

HON Hong, P.-D. and Huang, H.-T., Effect of co-solvent complex on preferential adsorption phenomenon in polyvinyl alcohol ternary solutions. Polymer, 41, 6195, 2000. 

In the case of poly(methacrylates) mentioned above there is a decreasing in the X values when the aromatic ring has alkyl groups as substituents. This behavior was explained taken into account at least two factors. If the model accepted to explain the preferential adsorption is that which consider that this phenomenon occurs along the polymer chain, the rigidity of the macromolecule must influence the amount of the adsorbed solvent. 

It was observed that lyophobic sols are stable due to their electric charge. As most of the lyophobic sols are prepared by preferential adsorption of ions, the mutual repulsion of particles is responsible for their stability. The sols become unstable, as soon as they are robbed off their charge. However, in case of lyophilic sols, the stability is due to electric charges as well as solvation—a phenomenon in which the colloid particle is surrounded by a thin film of the solvent. The layer of the solvent forms an envelope around each colloidal particle and thus pre-forms an envelope around each colloidal particle and thus prevents the particles from coming together. Groups like... 

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

B. In this case there is a preferential adsorption of relatively straight adsorbates, compared to more angular or nonlinear isomers. The phenomenon appears to be the result of a linear spacing of sites on the alumina surface, and these sites are better matched to the atoms of linear adsorbates. A final example of the matching of adsorbate atoms and adsorbent sites has been claimed to exist in the preferential adsorption of aromatics on charcoal or graphite see Section 7-3. Here it has been observed that the... 

The interesting phenomenon where a mixture of two poor solvents or nonsolvents for a polymer provides a medium that acts as a good solvent for die polymers has been the objective of many studies, by light scattering, " viscometry, " sorption equilibrium, and fluorescence. From these techniques, it has been possible to appreciate how the second virial coefficient Aj and the intrinsic viscosity [11] preferential adsorption coefficient A, and excimer and monomer emission ratio Ie/Im are involved by changing solvent composition. They present ([T)], A2) a maximum or a variation at a certain solvent composition where the polymer behaves as through it were dissolved in a good solvent. 

Polymers dissolved in mixed solvents show the phenomenon of Preferential Adsorption. Experimentally, the preferential adsorption eoeffieient, X, is determined. Xis the volume of one of the liquids sorbed in exeess by the polymer (per unit mass of polymer). In general, the Flory-Huggins model of polymer solutions is used to deseribe the Preferential Adsorption. More reeently, equation of state theories have been applied. ... 

In the present work, the concentration effect has been correlated with the preferential solvation phenomenon. Eirst, the observed non-exclusion mechanism has been quantified by means of the distribution coefficient, Kp, representing the concentration ratio of macromolecules inside the pores (i.e., in the quasistationary phase) to those in the mobile phase. Second, the shifts in the elution volume owing to either adsorption or, on the contrary, incompatibility of the solute with the gel packing, have been related to the preferential or selective sorption phenomenon. From a thermodynamic viewpoint, the... 

The preferential solvation phenomenon in a ternary polymer system indicates which component, (1) or (2), is preferentially adsorbed by component (3) it is quantified by means of the preferential solvation coefficient, A. From a chromatographic point of view, adsorption or, conversely, incompatibility between solute (2) and gel matrix (3) can be detected through the coefficient K, as seen before. Therefore, both parameters, A and Kp, have a similar physical meaning in the sense that an equilibrium between a binary and a ternary phase is estabhshed and, consequently, one could try to relate them. Accordingly, it is expected that when ATp > 1 [meaning compatibility between polymer (2) and gel matrix (3)], values of A < 0 will be obtained, denoting a preferential solvation of... 

The phenomenon of catalyst poisoning arises from the preferential adsorption of substances on the catalyst s active site, or the formation of a strong chemical bond or a new compound. These changes will affect the performance of the catalj t, and will not free the catalyst to participate in the adsorption or reaction of the reactants. The catalyst s activity will decline, or even more seriously, completely lose its activity. Because the poison can selectively adhere to different active sites, poisoning may also cause a decline in the catalyst s selectivity. ... 

Fig. 6.16 Schematic representation of the immersion experiment (explanation of the symbols in the text). The adsorption phenomenon is assumed not to affect the enthalpy of the bulk of solid, ms hs changes in the mass of the solvent are to be neglected when immersion is carried out in a sufficient excess of dilute solution containing a preferentially adsorbed solute (i.e., M = A/°)...

It has been repeatedly pointed out that one of the most significant results of the physical phenomenon of surface activity is the preferential adsorption of amphiphilic molecules at interfaces, resulting in potentially dramatic changes in the characteristics of those interfaces. The ability to reproducibly control such adsorption and interfacial modifications is of immeasurable technological importance, not to mention the fact that our very existence as living organisms would be impossible had such a phenomenon not been a direct consequence of natural laws as we understand them. This chapter is concerned with one of the most important overall areas of impact of surfactants on our technological existence emulsion formation and stabilization. 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

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],... 

In the presence of NAPL, the concentration of contaminants in the soil moisture (Cw) can be calculated simply from the solubility of the compounds (equation 3 in Table 14.3). Adsorption of contaminants to the soil particles is a much more complex phenomenon, which depends both on contaminant properties and on soil characteristics. The simplest model for describing adsorption is based on the observation that organic compounds are preferentially bound to the organic matter of soil, and the following linear equation is proposed for calculating the adsorbed concentration (Cs) ... 

Gibbs adsorption equation is an expression of the natural phenomenon that surface forces can give rise to concentration gradients at Interfaces. Such concentration gradient at a membrane-solution Interface constitutes preferential sorption of one of the constituents of the solution at the interface. By letting the preferentially sorbed Interfacial fluid under the Influence of surface forces, flow out under pressure through suitably created pores in an appropriate membrane material, a new and versatile physicochemical separation process unfolds itself. That was how "reverse osmosis" was conceived in 1956. 

As it was noted before, Preferential or Selective Adsorption is a very common phenomenon in ternary systems composed of a polymer and a binary solvent mixture. There is a great variety of ternary systems that have been studied, mainly those containing at least one polar component [97], In many cases, specific interactions between polar groups are important, and the formation of hydrogen bonds have to be taken into account. This is the case, especially, when alcohols are components of the systems. 

The electrostatic repulsion between the colloids can also be strengthened by adsorption of polyelectrolytes with the same net charge as the colloids. Such adsorption has been observed experimentally by several groups [55,56]. Another example is adsorption of polyelectrolytes on clay particles and in Fig. 13 it is shown that more salt must be added to coagulate the clay particles when the polyelectrolyte concentration has been increased (except for very low concentrations of polyelectrolytes, which has been described above). The polyelectrolytes only adsorb on equally charged clay particles in the presence of salt [51]. There are many explanations to this phenomenon and one theory is that the adsorption preferentially takes place at edges of the clay particles and it has been found that the probability for adsorption is higher for short polymers [56]. 

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