Adsorption of cations

Wliether the potentials are derived from quantum mechanical calculations or classical image forces, it is quite generally found that there is a stronger barrier to the adsorption of cations at the surface than anions, in agreement with that generally. ... 

It is not always necessary for the resin to be in the hydrogen form for adsorption of cations, especiaHy if a change in the pH of the Hquid phase is to be avoided (see also Hydrogen-ION activity). Eor example, softening of water, both in homes and at industrial sites, is practiced by using the resin in the form. 

The compatibihty value is mainly related to the affinity of the dye for the particular fiber because for basic dyes on modified acryhc fibers there is htde possibihty for migration and therefore this does not play a significant part in determining compatibihty. The rate of dyeing of a specific mixture of dyes of the same compatibihty value is not determined by the value itself. The adsorption of cationic dyes is induenced by the presence of others in the dyebath the presence of cationic retarding agents and electrolytes also induences the rate of exhaustion. It is therefore possible to have a combination of dyes with a compatibihty value 5 that under specific dyebath conditions exhausts more rapidly than a combination based on dyes of compatibihty value 3. 

SG sols were synthesized by hydrolysis of tetraethyloxysilane in the presence of polyelectrolyte and surfactant. Poly (vinylsulfonic acid) (PVSA) or poly (styrenesulfonic acid) (PSSA) were used as cation exchangers, Tween-20 or Triton X-100 were used as non- ionic surfactants. Obtained sol was dropped onto the surface of glass slide and dried over night. Template extraction from the composite film was performed in water- ethanol medium. The ion-exchange properties of the films were studied spectrophotometrically using adsorption of cationic dye Rhodamine 6G or Fe(Phen) and potentiometrically by sorption of protons. 

Serizawa and Akashi [95] analyzed the monolayer adsorption of polystyrene latex particles with cationic polyvinylamine grafted on their surface, while Serizawa et al. [96,97] used commercial anionic latex particles. Both types of particles were adsorbed on polyelectrolyte-coated substrates previously prepared by alternating adsorption of cationic and anionic polyelectrolytes such as polyallylamine hydrochloride (PAH) and polystyrene sulfonate sodium salt (PSS) according to the method described by Decher [164]. Using... 

Partial Charge Transfer Specific adsorption of ions is often attended by a partial transfer of their charge to the metal surface for instance, in the specific adsorption of cations M + on platinum... 

Specific adsorption occurs, i.e. ions enter the compact layer, in a considerable majority of cases. The most obvious result of specific adsorption is a decrease and shift in the maximum of the electrocapillary curve to negative values because of adsorption of anions (see Fig. 4.2) and to positive values for the adsorption of cations. A layer of ions is formed at the interface only when specific adsorption occurs. 

Figure 10.56 Adsorption of cationic softeners by cotton and acrylic fabrics at 30 °C [483]...

Figure 6.8 The generalised effect of molecular weight and cationicity on adsorption of cationic polyelectrolytes on non-porous surfaces.

But factors other than the surface charge can become important such as the effects of specific adsorption of cations and anions on the degree of surface protonation (see Example 5.1). 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

The efficacy of CE separation depends considerably on the type of capillary. Fused-silica capillaries without pretreatment are used most frequently. Its outside is coated with a polymer layer to make it flexible and to lessen the occurrence of breakage. The polymer coating has to be dissolved with acid or burned away at the detection point. Capillaries with an optically transparent outer coating have also found application in CE. The objectives of the development of chemically modified capillary walls were the elimination of electro-osmotic flow and the prevention of adsorption on the inner wall of the capillary. Another method to prevent the adsorption of cationic analyses and proteins is the use of mobile phase additives. The modification of the pH of the buffer, the addition of salts, amines and polymers have all been successfully employed for the improvement of separation. 

In addition to anion adsorption, there exists the possibility of adsorption of cations at negative potentials along with coadsorption phenomena. For example, mixed layers of alkali cations with iodine on Au(llO) [291] or cyanide on Pt(lll) [292] have been reported. In fact, coadsorption has proven to be quite common among numerous underpotential metal deposition reactions as described below. 

Adsorption of cations on iron oxides (Table 11.3) may be specific or non specific. With non specific adsorption, there is at least one water molecule between the adsorbing species and the surface functional group. Specific adsorption involves interaction with deprotonated surface hydroxyl groups to form mono- and bi-nuclear, inner sphere complexes, i.e. 

Colloidal suspensions and emulsions acquire their charge owing to preferential adsorption of cations or anions, thus a negative suspension or emulsion will be precipitated on the addition of an electrolyte containing a readily adsorbable cation, the added electrolyte however contains an anion as well, and tends to counteract the effect of the cation, hence the amounts of various electrolytes containing a common cation necessary to produce... 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

Dissolved polymer molecules can be adsorbed by polymer particles via electrostatic attractive force or hydrophobic interaction. When polyelectrolyte is adsorbed on an opposite-charge particle, the polymer molecules usually have a loop-and-tail conformation and, as a result, inversion of charge occurs. For example, sulfatecarrying particles behave as cationic ones after they adsorb poly(lysine). Then poly(-styrene sulfonate) can be adsorbed on such cationic particles and reinvert the charge of particles to anionic (14). Okubo et al. pointed out that the alternate adsorption of cationic and anionic polymers formed a piled layer of polyelectrolytes on the particle, but the increment of adsorbed layer thickness was much less than expected. This was attributed to synchronized piling of two oppositely charged polyelectrolytes (15). 

A. M. Grancaric, T. Pusic, I. Soljacic, and V. Ribitsch, Influence of Electrokinetic Potential on Adsorption of Cationic Surfactants," Textile Chemist Colorist 29(12) 33 (1997). 

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