High adsorption affinity

Polymers typically exhibit a high-affinity adsorption isotherm as shown in Fig. XI-5 here the adsorbed amount increases very rapidly with bulk concentration and then becomes practically independent of concentration. 

FIGURE 16.2 Typical adsorption isotherms, relations between amount of adsorbate attached to the surface of adsorbent and its concentration in supernatant. It shows the high-affinity adsorption isotherm, which is typical for many polymers in contact with an active adsorbent in a weak solvent. Up to a certain extent of the surface saturation, the saturation threshold, all macromolecules are attached to the adsorbent surface. 

Type G is a high-affinity adsorption isotherm. The molecules bind so strongly that no rest can be detected in the solution or gas phase. The difference to the Langmuir type is quantitative not qualitative. It is often observed for polymer or proteins adsorbing from solution. 

L-type describes high-affinity adsorption between the adsorbate and adsorbent and usually indicates chemisorption (e.g., phosphate-soil interactions)... 

The pattern of the curves of quaternary deposition corresponds to a high-affinity adsorption. For many authors, who link the surface affinity for softeners to the ion exchange capacity, such a pattern reinforces the electrostatic attraction mechanism [23,24,30],... 

Thus, deviations from the ideal Langmuir isotherm can be caused both by intermolecular interactions, which result in an enthalpy of mixing, and by area differences between molecules, which produce a non-ideal entropy of mixing [18]. For a simple case where the interactions are of the Frumkin type and the partial molar areas of solvent and surfactant are constant the entropic effect of area differences results in typical features of macromolecular adsorption, e.g., a steep initial increase of adsorption ( high affinity adsorption) and a very slow rise once the surface is approximately half filled [18]. 

High-affinity adsorption is reflected in the shape of the adsorption isotherm F(Ceq) the initial part of the isotherm practically merges with the F-axis, because at low polymer supply aU of the polymer is adsorbed until the interface is saturated as shown in Figure 15.12. 

FIGURE 15.12 High-affinity adsorption isotherms, typical for polymer adsorption. 

Fig. 3. (a) High affinity adsorption isotherm (b) molecular weight dependence of surface... 

The adsorption of HDP on the surface of the original sepiolite gives a high-affinity adsorption isotherm. The amount of HDP adsorbed in the region of ion exchange (i.e., for the immeasurably low equilibrium concentrations) is 0.2 mmol/g, which is in... 

Fig. XI-5. Adsorption isotherm from Ref. 61 for polystyrene on chrome in cyclohexane at the polymer theta condition. The polymer molecular weights x 10 are (-0) 11, (O) 67, (( )) 242, (( )) 762, and (O) 1340. Note that all the isotherms have a high-affinity form except for the two lowest molecular weights.

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

A good example of a surface-modified lens is the Sola/Bames-Hind Hydrocurve Flite lens, introduced in 1986. The material for the commercial Hydrocurve lens, bufilcon A [56030-52-5] contains methacrylic acid and has a high affinity for protein and subsequent deposition. The surface of the Flite lens was chemically modified with the addition of diazomethane (190) to reduce the surface charge. In vitro testing demonstrated a decrease in protein adsorption (191). 

Tetrahydrofuran (THE) is known as a liquid that exhibits high affinity toward various adsorbents and therefore effectively suppresses the adsorption of many polymers. Still, THE could not fully suppress adsorption, e.g., of... 

Cavallaro and McBride (1984a) observed that the removal of Fe oxides from two clay soils reduced Zn adsorption. Shuman (1976) reported that the removal of Fe oxides resulted in an increase or decrease in Zn adsorption, but later in another similar study (1988) he found that the removal of either amorphous or crystalline Fe oxides increased Zn adsorption capacity and decreased Zn-bonding energy. The author explained that adsorption sites on the Fe oxide coatings were not as numerous as those released when the coatings were removed. Elliott et al. (1986) observed that DCB extraction of Fe oxides from two subsoils of the Atlantic Coastal Plain increased heavy metal adsorption. Wu et al. (1999) found that Cu adsorption on the fine clay fraction increased after dithionite treatment with possible exposure of much more high-affinity sites for Cu on the fine clay. 

Adsorption of the polymer molecule causes a reduction of its conformational entropy (Norde 2003b). Flence, adsorption takes place only if the loss in conformational entropy is compensated by sufficient favorable interactions between polymer segments and the interface. Because the polymer molecule attaches with many segments at the interface, it adsorbs tenaciously with a very high affinity, even if the interaction of the individual segments with the interface is rather weak. The high affinity manifests itself by the adsorption being irreversible with respect to variations of the polymer concentration in solution. 

The adsorption isotherms of PEG 6000 and AO,000 in fig. 1 show the typical high affinity character for precipitated and for pyrogenic silica with the following trends ... 

Adsorption Isotherm The adsorption Isotherm of PVA on polystyrene latex was of the high affinity type, as previously found (15). 

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