Liquid Membrane Transport Phenomenon

Types of Transport

I am not going to walk you through basic transport phenomenon now, but it is good to know that there are four basic types of transport systems, each of which has its own mechanisms and carrier types. In each of these systems, the one big item to notice is that regardless of mechanism, the complexes formed are that charge-neutrality must be maintained. Now then, the four systems are

Cation Transport

Cation Transport can occur in either of two ways, either symport or antiport, as shown in Figures 8 and 9. In the symport configuration, a neutral carrier moves the guest and co-transported anion together across the membrane. This occurs in four stages. If the outer side of the membrane is in contact with Aqueous Phase I, and the inner side of the membrane is in contact with Aqueous Phase II, then
    1. At the Phase I interface of the membrane, the guest salt is complexed with the carrier.
    2. That complex diffuses across the membrane.
    3. The release of that guest salt occurs at the Phase II interface of the membrane.
    4. The carrier diffuses back across the membrane, ready to continue.


Figure 8. Cationic Symport.

Sounds easy, right? It is, pretty much.

For the antiport transport, though, you utilize an anionic carrier, so the four stages are a slight bit different.

    1. At the Phase I interphase, the carrier tries to form a neutral complex with the guest cation.
    2. The ion-pair diffuses across the membrane.
    3. Cation-exchange reaction releases the guest cation to Phase II.
    4. The carrier complex with the counter-transported ion diffuses back across the membrane.


Figure 9. Cationic Antiport.

Since the mechanism of transport in symport is effectively M+ and X- being transported simultaneously, the symport transport has a concentration gradient of

Now, the concentration gradient for the antiport is slightly different; the mechanism is distinctly pH-dependant. This relationship shows itself in the extraction constant, and this effects the concentration gradient equation, which is now

Anion Transport

As you can see in Figure 10, anionic transport is similar in mechanism to the cationic transport, save for the use of a cationic carrier instead of an anionic one in the antiport configuration.


Figure 10. Anionic Antiport and Symport Transport.

Neutral Guest Transport

Neutral species are transported symport, using various carriers. Oxygen and CO have been transported as gases through the liquid membrane, but they use a mechanism different that that shown in Figure 11, or so researchers theorize.


Figure 11. Transport of Neutral Guest.

Switchable Transport

The use of photo- and electrochemistry has recently been investigated into increasing the rates at which the carrier complexes dissociate, which would, therefore, increase the transport rate. A sample of this is shown in Figure 12, but you can use your imagination to come up with exactly what is being down. Essentially, the switchable transport system works in addition to the regular transport system, and only the second step of the AC - e- --> AC+ --> A + C- reaction is accelerated.


Figure 12. Switchable Transport.