Membranes - Introduction to Carbon Capture and Sequestration - page 341

(a)

(b)

(c)

(d)

Figure 7.6.5 Changing the chemistry of our model membrane
Chemical modifi cations of the membrane: (a) the reference material, (b) changes in the
chemistry in the cavities, (c) changes to the pore window, and (d) changes to both the
cavities and the windows. The red lines indicate that the interactions of an adsorbed
molecule have locally changed.

important; increasing the barrier interaction energy will decrease the dif-
fusion coeffi cient. So here we see that we can tune the permeation of our
material by controlling the diffusion coeffi cient.
In practical situations, we would most likely change the chemistry of
both the cavity and the window region at the same time. If this chemistry
could modify the energy by, say, adding a constant term to both the window
energy, U w →

U , we see that
this would not infl uence the diffusion coeffi cient but would change the
Henry coeffi cient. That is, by changing both the interactions in the window
region and the cavity, we can tune the Henry coeffi cient and hence the per-
meation (see Figure 7.6.6 ). Of course, this would require a very delicate
control of the chemistry. It is exactly this type of molecular control which is
the topic of modern synthetic chemistry applied to membrane separations.

U w + ∆ U , and the cavity energy, U c → U c + ∆