where a and c are constants, R and T are gas constant and temperature, respectively. This approach is expected to work well for gas pairs such as H 2 /CH 4 or He/N 2 , with a significant difference in molecular size between the two species. However, a very important case of CO 2 and N 2 separation for carbon capture applications cannot be described by this theory. A compilation of data for CO 2 /N 2 separation for glassy polymers on a Robeson plot is shown in Fig. . On the same figure, we also show two empirical upper bounds (red and blue lines) drawn based on the experimental results , as well as a theoretical prediction based on Freeman's work (yellow line) . From this, we can say that it is difficult to discern a well defined boundary similar to other pairs of gases. In fact, as shown in Fig. , several possible boundaries could be drawn, possibly controlled by very different mechanisms.