Summary of carrier mediated transport

Dialysis is a process where solutes diffuse from one side of Ihe membrane (the feed side) to the other side (the dialysate or permeate side) according to their concentration gradients. Separation between the solutes is obtained as a result of differences in diffusion rates across the membrane arising from differences in molecular size and difference in solubility. A typical concentration profile is shown in figure VI - 43a. Often boundary layer effects contribute as well and then the concentration profile as given in figure VI - 43b apply. 

Dialysis, or ordinary dialysis as discussed in this section, is referred to as the diffusion of neutral molecules. If electrolytes are separated with neutral membranes or with charged membranes, then Donnan effects arising from the unequal distribution of ions, interfere with the normal dialysis process. This type of dialysis is called Donnan dialysis or diffusion dialysis and these processes will be described in section VI.4.6. 

Dialysis is a diffusion process and at steady-state transport can be described by Pick s law which gives after integration across the,membrane (see figure VI - 43a) the following equadon 

Also in dialysis the transport resistance is not determined only by the membranebut frequendy bound layer iesisthnees have to be taken into account. This is drawn schematically in jf re VI - 43b. The overall mass transfer coefficient k( is obtained by the sum of the threje resistances according to 

Dialysis is mainly used to separate low molecular weight components from those of high molecular weight. Such a separation mechanism is based on differences in molecular weight as expressed by the Stokes-Einstein equation. Although dialysis is mainly employed wiA aqueous solutions, the process itself is not limited solely to such solutions. To achieve sufficient permeation rates the membrane must be highly swollen, which in turn implies that the membrane selectivity will decrease. An optimum must therefore be found between the diffusion rate and swelling. In addition, the membrane should be as thin as possible. 

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