Exclusion Donnan

Donna Karan Donnan exclusion Donnan principle Donnazyme Donohue correlation Doors... 

In ion-exchange resins, diffusion is further complicated by electrical coupling effec ts. In a system with M counterions, diffusion rates are described by the Nernst-Planck equations (Helfferich, gen. refs.). Assuming complete Donnan exclusion, these equations canbe written... 

Nanofiltration membranes are negatively charged and reject multivalent anions at a much higher level than monovalent anions, an effect described as Donnan exclusion. Nanofiltration membranes have MgS04 retention and water permeability claims. 

There are two main factors that influence the selectivity of a sensor limits in discrimination of an interfering ion and upper limits in stability constant of an analyte-ionophore complex. While an ideal ionophore does not form complexes with interfering ions, too strong complexation with the primary ion leads to a massive extraction of analyte into membrane phase coupled with a coextraction of sample counter-ions, known as Donnan exclusion failure. In such cases, at high activities and lipophilicities of sample electrolytes, fli(org) increases and a breakdown of membrane permselectivity prevents the Nemst equation to hold. 

Unlike the pellicular packings used for ion exchange, the packings used in ion exclusion are derived from totally sulphonated polymeric materials. Separation is dependent upon three different mechanisms Donnan exclusion, steric exclusion and adsorption/partitioning. 

Donnan exclusion causes strong acids to elute in the void volumes of the column. Weak acids which are partially ionized in the eluent are not subject... 

On account of the relatively low water regain of cellulose acetate, the molal concentration of ionic groups in the swollen material exceeds Smmolal. This is comparable to the concentration of 300 ppm sodium chloride, a typical reverse osmosis product solution. Our homogeneous membranes are believed to be very similar to the active layer of an asymmetric membrane as developed by Loeb and Sourirajan. It is evident therefore that the concentration of fixed charges in the membrane is sufficient to exercise a significant Donnan exclusion of co-ions on the downstream side of the membranes in a reverse osmosis plant. 

With some stationary phases at low pH values (<4) benzyl amine as benzyl ammonium ion can be excluded by a Donnan potential from the pores, when positive charges are present at the surface. These could have stemmed from the manufacturing process or could have been introduced on purpose to shield amines from interacting with silanols. With an increasing pH, the Donnan exclusion decreases and at pH >5 benzyl amine is retarded increasingly. An example of this effect with a modern RP with low silanophilic properties is demonstrated in Figure 2.22, where the elution peaks of benzyl amine are presented as a function of pH. With these stationary phases, basic analytes cannot be separated at low pH values. 

Reverse osmosis models can be divided into three types irreversible thermodynamics models, such as Kedem-Katchalsky and Spiegler-Kedem models nonporous or homogeneous membrane models, such as the solution—diffusion (SD), solution—diffusion—imperfection, and extended solution—diffusion models and pore models, such as the finely porous, preferential sorption—capillary flow, and surface force—pore flow models. Charged RO membrane theories can be used to describe nanofiltration membranes, which are often negatively charged. Models such as Donnan exclusion and the... 

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)...

Separation is dependent on three different mechanims Donnan exclusion, steric exclusion, and adsorption/partitioning. Donnan exclusion causes strong acids to elute in the void volume of the column. Weak acids that are partially ionized in the eluent are not subject to the Donnan exclusion and can penetrate into the pores of the packing. Separation is achieved through differences in acid strength, size, and hydrophobicity. The major advantage of ion exclusion is the ability to handle samples that contain both strong and weak acids. 

The ion exclusion process is based on Donnan exclusion. In order for Donnan exclusion to operate in this application, the resin must be in the acid form. It is usually necessary the wash the ion exclusion column with acid between injections in order to remove alkali, alkaline earth and transition metal... 

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. 

As non-electrolytes are not submitted to the Donnan exclusion, the diffusion of non-electrolytes proceeds much faster than that of electrolytes. Therefore in principle it is possible to separate non-electrolytes from electrolytes with the aid of ion-exchange membranes. 

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

Nanofiltration Filtration for the treatment of water that removes suspended particles that are larger than about 0.001 pm (compare with Donnan exclusion, filtration, microfiltration, reverse osmosis, and ultrafiltration). 

Whatever the IEM, counterions carry most of the electric current, the concentration of co-ions being relatively low within the membrane itself. Moreover, the fixed ions are in electroneutrality with mobile ions in the interstices of the membrane and repel the co-ions, this type of action being usually called 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]...

Experiments with ion exchange membranes were described as early as 1890 by Ostwald [1], Work by Donnan [2] a few years later led to development of the concept of membrane potential and the phenomenon of Donnan exclusion. These early charged membranes were made from natural materials or chemically... 

Because the Donnan exclusion effect is much stronger for multivalent ions than for univalent ions, the polyelectrolyte layer rejects multivalent ions but allows the univalent ions to pass relatively unhindered. 

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