Donnan salt exclusion

The Donnan salt exclusion parameter f expresses a property of polyelectrolyte solutions. It describes the bias when a salt is present in the bulk of the solution against the presence of mobile ions in the vicinity of the polyelectrolyte. In the case of the two phase system of a cross-linked ion exchange gel and an outer solution the exclusion is directly measurable experimentally. If the concentration of the salt in the external solution is CEex and that in the vicinity of the polyelectrolyte is CEnear, then the limiting value of Tfor a concentration Cpoiy of the polyelectrolyte tending to zero is formally defined by ... 

In addition to a sample peak, some anomalous peaks often appear in chromatograms. when multicomponent eluent is used [ref. 71, 72]. Such an induced peak can be observed when polyelectrolyte is chromatographed using a simple electrolyte solution as eluent the first peak corresponds to polyelectrolyte and the second to an induced peak which has the elution volume exactly the same as that of the eluent salt [ref. 33, 45-48, 73-76]. This effect of polyelectrolytes in exclusion chromatography has been explained in terms of Donnan salt exclusion established on the gel [ref. 33, 45-48, 73-76] a polyelectrolyte is barred from the gel interior and thus promote the diffusion of the penetrable eluent coion into the inner volume of the gel, the gel matrix acting as a semipermeable membrane. The eluent coion thus excluded from the polyelectrolyte zone produces an induced peak. It has been reported that the area of the induced peak allows to calculate the Donnan salt exclusion parameter [ref. 33, 74] or the osmotic coefficient [ref. 46-48, 76] of the polyelectrolyte. 

However, these interpretations in terms of the Donnan salt exclusion effect seem to be still open to discussion, because the difference in elution volume between small inorganic ions, such as chloride and nitrate ions, on tightly crosslinked hydrophilic gels cannot be attributed to their size differences, and furthermore, the eluent salt peak can also be produced by the injection of the sample ion which elutes slower than the eluent coion [ref. 16, 23, 26]. 

In sulfonated aromatic polymer membranes, therefore, Donnan ion exclusion appears to be an important contributor to performance. In contact with dilute feedwaters, these strongly anionic membranes exclude feedwater anions from entering the membranes by charge repulsion effects, thus inhibiting salt passage. In more concentrated salt solutions, the charge repulsion effects are shielded by the high ionic activity of the feedwater. 

Fig. 2. Hydrolyzed polyacrylamide gel swelling vs ionic strength. The salt increases the osmotic force in the water outside the gel. The gel s swelling is reduced because Donnan exclusion prevents small ions from entering the get (Reprinted with permission from [16], 1986 Pergamon Press)...

The adsorbance of NaBr, Ts, was large and negative. The Donnan exclusion factor rex = rs/rp was in fair agreement with that calculated from the wire model of Devore and Manning119 for the polyelectrolyte in salt solution. 

On account of the Donnan exclusion, the concentration of absorbed salt in the membrane — also called diffusible salt — is very low. Therefore it is explicable that if a salt solution is pressed through an ion exchange membrane, under certain conditions salt filtration occurs. 

If a salt solution is pressed through an ion-exchange membrane, salt filtration can take place caused by the Donnan exclusion. 

The rejection of salts by nanofiltration membranes is more complicated and depends on both molecular size and Donnan exclusion effects caused by the acid groups attached to the polymer backbone. The phenomenon of Donnan exclusion is described in more detail in Chapter 10. In brief, charged groups tend to exclude ions of the same charge, particularly multivalent ions while being freely permeable to ions of the opposite charge, particularly multivalent ions. 

Many nanofiltration membranes follow these rules, but oftentimes the behavior is more complex. Nanofiltration membranes frequently combine both size and Donnan exclusion effects to minimize the rejection of all salts and solutes. These so-called low-pressure reverse osmosis membranes have very high rejections and high permeances of salt at low salt concentrations, but lose their selectivity at salt concentrations above 1000 or 2000 ppm salt in the feed water. The membranes are therefore used to remove low levels of salt from already relatively clean water. The membranes are usually operated at very low pressures of 50-200 psig. 

Figure 5.14 Salt rejection with neutral, anionic and cationic nanofiltration membranes showing the effect of Donnan exclusion and solute size on relative rejections. Data of Peters et al. [36]...

Negative adsorption is a relatively import2mt phenomenon in concentrated disperse systems and in capillaries. It is responsible for the Donnan effect, for the exclusion of electrolytes from concentrated sols, dispersions and capillaries and the ensuing salt-sieving effect, already introduced in chapter I.l. It also plays a role in double layer relaxation as occurs in alternating fields or in particle-particle Interaction. As negative adsorption is a purely electrostatic feature and takes place far from the surface, in all these applications its computation from Polsson-Boltzmann statistics is reliable, especially at high ly l. 

Although for the present purpose the negative adsorption had to be suppressed, for others it is important Donnan exclusion, pH establishment in soils, and salt-sieving. Traditionally, these topics have greatly benefited from studies with clay colloids. 

Besides these phenomena, plug experiments can also be carried out to measure salt-sieving, the retention of electrolytes caused by the negative adsorption of co-lons from the (overlapping) double layers, edso known as the Donnan exclusion. The phenomenon was already met on p. 1 of Volume I and further analysed in sec. 3.5b. 

Ion implantation This study involves modifying the surface of nanofiltration membranes by ion implantation for increased salt rejection [55]. ions at two different intensities—lElO and 5E10 atoms/cm —were implanted on the surface of commercially available nanofiltration membranes to increase the negativity of the membrane surfaces. The objective was to increase the Donnan exclusion effect to improve salt rejection by the modified membranes. It was also noted that this modification did not significantly damage the semipermeable membrane surface. 

Hydrogels are crosslinked polymer networks with entrapped solvent. In the case of hydrogels containing polyectrolytes, in addition to solvent, ions and salt can be found in the gel as determined by the Donnan partition. This arises from the exclusion of ions of the same charge that sets a membrane potential at the gel/external electrolyte interface. 

When an ion exchange membrane is in contact with an electrolyte (salt solution), ions with the same charge (co-ions) as the fixed ions are excluded and cannot pass through the membrane, while the oppositely charged ions (counter-ions) can pass freely through the membrane. This effect is known as Donnan exclusion [112]. It reflects the abUity of the membrane to discriminate between ions of opposite charge. 

Charged UF membranes reject low concentrations of salts primarily by the Donnan exclusion mechanism. Because the fixed charged groups on the membrane skin reject ionic solutes via repulsion of coions, the rejection would be expected to depend on solute type and coion charge. Obviously, divalent and tri-valent ions are rejected better than monovalent ions. Highly hydrated ions are rejected better than poorly hydrated ions. 

In their paper concerning electrolyte effects in aqueous exclusion chromatography of inorganic salts, Neddermeyer and Rogers [ref. 12] rationalized the phenomena observed by invoking the existence of a Donnan diffusion [ref. 78], involving an internal volume of the gel penetrable to some, but not all, ionic solutes in solution. The peak of the eluent salt was attributed to the Donnan exclusion effect, on the assumption that the sample ion and the eluent coion were able to penetrate into the gel interior to different degrees. 

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