Donnan effects

The Donnan effect acts to exclude like-charged substrate ions from a charged surface region, and this exclusion, as well as the concentration of oppositely charged ions, can be expressed in terms of a Donnan potential pD. Thus for a film of positively charged surfactant ions S one can write... 

The contribution of the Donnan effect is that of the second term in Eq. XV-13, that is. 

The spontaneous depolarization of an excitable cell in response to a stimulus. Gibbs-Donnan effect... 

The equilibrium (also known as the Donnan effect) established across a semipermeable membrane or the equivalent of such a membrane (such as a solid ion-exchanger) across which one or more charged substances, often a protein, cannot diffuse. Diffusible anions and cations are distributed on the two sides of the membrane, such that the sum of concentrations (in dilute solutions) of diffusible and nondiffusible anions on either side of the membrane equals the sum of concentrations of diffusible and nondiffusible cations. Thus, the diffusible ions will be asymmetrically distributed across the membrane and a Donnan potential develops. 

For our purpose here, we are interested in the following question How do the membranes maintain the ionic environments in the cell over long periods of time while simultaneously allowing transmembrane passage of the needed ions The mechanism that makes this possible is the so-called Donnan equilibrium we discuss the details of this equilibrium in this chapter, but a qualitative picture of the Donnan effect can be obtained with the help of a simplified model of a cell shown in Figure 3.1 (See, also, Section 3.5a.). 

Clearly, the results of osmotic pressure measurements on solutions of charged colloidal particles, such as proteins, will be invalid unless precautions are taken either to eliminate or to correct for this Donnan effect. Working at the isoelectric pH of the protein will eliminate the Donnan effect but will probably introduce new errors due to coagulation of the protein. Working with a moderately large salt concentration and a small protein concentration will make the... 

Sometimes, thanks to the Donnan effect, by modifying the pH it is possible to retain in the reaction ambient molecules that otherwise would pass the membrane. In fact, some membranes can become electrically charged at acidic and alkaline pHs, then repulsive or attractive interactions between the substrate molecules and the membrane surface may occur if the charges are of the same or of different sign, respectively. Repulsive interactions increase rejection values, whereas attractive ones decrease them. 

Donnan distribution, leading to abnormally high tt, is a most serious source of error for Mn of polyanions measured by osmometry. The Donnan effect is allegedly overcome by very dilute concentrations that never exceed 25 g L 1 for the polyanion and i = 0.3 mol L-1 for the solvent (Wagner, 1949). A 25-g L-1 polysaccharide concentration is in the semidilute-to-concentrated domain, outside the theoretical dilution limit of osmometry where a power-law dependence of tt/c, on ct is expected. 

Bell and co-workers84 confirmed the lowering of osmotic pressure in the presence of salts as observed by Oakley and Young.32 These authors suggested that as disaggregation and the Donnan effect were unlikely, an explanation might be that the salt caused a decreased asymmetry in the molecule with a resultant entropy change. 

Donnan effects (Hindering of the passage of free ligand) (Mapleson 1987)... 

A Donnan effect that prevents self-repulsion of the similarly charged adsorbed IPR ions at higher ionic strength, the higher surface concentration of the IPR ( LH in Equation 3.5) partially compensates for the decreased magnitude of the electrostatic surface potential due to the increased Scoi in Equation 3.5. 

In experiments where the ligand and macromolecule are charged, the equilibrium may be affected by the Donnan potential of the charged macromolecule, which is confined to one compartment. Donnan effects can be minimised by carrying out the dialysis in sufficiently high ionic strength media, such as in the presence of 0.1 M NaCl or KC1. 

Donnan effect, i.e. the expulsion of electrolyte by charged particles. As which is positive, cannot be measured without simultaneously measuring (also positive), cannot be measured by titration, but changes of <7 are coupled to those in T° via the Esln-Markov coefficient, see (3.4.14-16). so that after all they can be assessed except for a constant. The negative adsorption of neutral electrolyte can be measured as an Increase in the bulk concentration caused by the addition of the particles. Negative adsorption studies are therefore useful to compute the ionic components of charge for a diffuse double layer. 

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. 

For Isolated spheres of water, the value to be taken for the equilibrium electrolyte concentration in a bulk phase, c( ), may be a problem. In the situation of fig. 3.16b, and c do not approach this value. When there is equilibrium with an external bulk phase, as with vesicles, this is no problem, because c (r = 0) and c.(r = 0) and c(oo) are simply related via the Boltzmann equation. When Boltzmann s law applies, the equilibrium concentration c(=o) in a (virtual) bulk phase can be written as c( o) = (c (r = 0) c (r = 0)). However, if there is no such equilibrium (say, for microdrops of water formed in a nonconducting oil. under highly dynamic conditions) c(o ) may differ between one drop and the other and nothing can be said in general. Alternatively, the negative adsorption of electrolyte can be computed if y is known (this is the Donnan effect). 

Figure 3.33. Changes in ion concentrations near a surface due to charging of this surface. Symmetrical electrolyte Donnan effect.

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