Donnan effect significance

Nanofiltration retains 40%-95% of organic compounds and multivalent ions and from —300% (negative retention by the Donnan effect) to - -50% chloride ions depending on the concentration of multivalent ions. Retention is greatly affected by electrostatic forces, as discussed in Section 35.3.2, and sulfate ions, in particular, significantly affect the retention of chloride ions [24]. 

If, however, a carrier-mediated transport membrane containing charged species — in the form of either mobile ions or fixed sites — were placed between two electrolytic mixtures, significant Donnan effects could be expected. For example, consider a membrane in which the carrier is a counterion to the permeant. The permeant would be expected to be preferentially included in the membrane phase. If significant inclusion were to occur, the use of simple first-kind boundary conditions would be inappropriate and could lead to underestimation of flux. On the other hand, if the permeant and carrier were coions, the permeant could be excluded and failure to account for exclusion could lead to overprediction of flux. Further complications would arise if the complex were charged or if other charged species were present, since the net charge density inside the membrane defines Donnan equilibrium conditions. 

For membrane processes involving liquids the mass transport mechanisms can be more involved. This is because the nature of liquid mixtures currently separated by membranes is also significantly more complex they include emulsions, suspensions of solid particles, proteins, and microorganisms, and multi-component solutions of polymers, salts, acids or bases. The interactions between the species present in such liquid mixtures and the membrane materials could include not only adsorption phenomena but also electric, electrostatic, polarization, and Donnan effects. When an aqueous solution/suspension phase is treated by a MF or UF process it is generally accepted, for example, that convection and particle sieving phenomena are coupled with one or more of the phenomena noted previously. In nanofiltration processes, which typically utilize microporous membranes, the interactions with the membrane surfaces are more prevalent, and the importance of electrostatic and other effects is more significant. The conventional models utilized until now to describe liquid phase filtration are based on irreversible thermodynamics good reviews about such models have been reported in the technical literature [1.1, 1.3, 1.4]. 

Since the Donnan effect is significant only when the fixed charge concentration in the polymer network c g is significantly greater than the mobile ion concentration in the gel c s, under these circumstances equation 12 may be simplified to the following ... 

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. 

Electrophoretic elution and "switch" monoclonal antibodies are combined in a new rapid recycle method an affinity-mediated membrane transport process reported by Dall-Bauman and Ivory (8). In this modeling paper, a "switch" monoclonal antibody incorporated into a supported liquid membrane is used to facilitate the transport of human growth hormone from a high-pH to a low-pH environment. Electrochemical effects, including Donnan equilibria between the membrane and external environments, and imposition of external electrical fields, significantly affected the flux of protein across the membrane. Experimental confirmation of the simulation results could introduce affinity-mediated transport as a powerful new biospecific separation method. 

The mathematical model described here has illustrated that electrochemical effects can significantly influence protein flux in an affinity-mediated transport system. The system considered consists of a supported liquid membrane containing a pH-sensitive monoclonal antibody as carrier and human growth hormone as permeant. On a microscopic scale, Donnan inclusion of the hormone can increase the flux of hormone into the membrane. This allows more complex to be formed and simultaneously generates a steep hormone concentration gradient which drives a greater flux of free hormone than would occur in the absence of inclusion. 

Another factor which may influence the liquid junction potential is termed the "suspension effect" in which the presence of colloids or suspended particles, e.g., red blood cells, produce an anomalous liquid junction potential. It has been suggested that this phenomenon is caused by the effect of colloidal particles on the relative rates of diffusion, i.e., transference numbers, of the salt bridge electrolyte. Another possibility is that colloids with ion-exchange properties give rise to a Donnan potential across the suspension/supernatant liquid interface. Whatever the cause, the effect may be significant and must be avoided in accurate studies with electrodes. 

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