Membranes - Introduction to Carbon Capture and Sequestration - page 340

Figure 7.6.4 Effects of changes in the pore structure on the transport and thermody-
namic properties
In our model membrane, we can change the volume of our cavity and the diameter of
the window separating the cavities. Changing the window diameter has little effect on
the adsorption but a large effect on the diffusion coeffi cient. Changing the cavity
changes both the diffusion and the Henry coeffi cients.
Let us now use this model to study the effect of changes in the
chemistry. In our model, the chemistry is manifested in the energy terms
U c and U w . Let us assume that we can modify the interactions of our
adsorbed molecules with the wall of our pores (see Figure 7.6.5 ). These
changes will affect the permeation of the material.
For example, suppose we modify the chemistry such that the pores
are more attractive for our molecules. This would correspond to making
U c more negative, which in turn would increase the Henry coeffi cient.
However, if the energy in the cavity is lower, the free energy barrier for
hopping from one cage to another increases. Hence, the diffusion coef-
fi cient will decrease and, as the formula for the permeation shows, these
two effects will cancel. The net result is that the permeation will not
change signifi cantly if we modify the chemistry of the cages.
We can also try to modify the chemistry of the window. As the prob-
ability of fi nding a molecule on top of a free energy barrier is small, the
expression for the Henry coeffi cient shows that these window regions do
not contribute signifi cantly to the Henry coeffi cient. So changes in the
chemistry of the window region will not change the Henry coeffi cient. For
the diffusion coeffi cient, on the other hand, changes to the barrier are