It's a strange behaviour indeed! One possible explanation could be the formation of a condensate layer at the outside surface at high pressure. This condensate may hinder the entrance of more molecules in the pores. This hypothesis may be prooved by a sudden reduction of pressure: in such a case you may observed a negative temperature step at the surface due to the endothermic "vaporisation" of the external CO2 film. This may be detected by IR emission reduction. You could find information on  how to measure and calculate such an effect in the following papers:

http://scitation.aip.org/content/aip/journal/rsi/69/5/10.1063/1.1148911

http://www.sciencedirect.com/science/article/pii/0009250995002804

Good luck to you

Vincent Bourdin

It's a strange behaviour indeed! One possible explanation could be the formation of a condensate layer at the outside surface at high pressure. This condensate may hinder the entrance of more molecules in the pores. This hypothesis may be prooved by a sudden reduction of pressure: in such a case you may observed a negative temperature step at the surface due to the endothermic "vaporisation" of the external CO2 film. This may be detected by IR emission reduction. You could find information on  how to measure and calculate such an effect in the following papers:

http://scitation.aip.org/content/aip/journal/rsi/69/5/10.1063/1.1148911

http://www.sciencedirect.com/science/article/pii/0009250995002804

Good luck to you

Vincent Bourdin

Oh this is the pressure on gas molecules which distorts/compress its structure and hence co2 enters, even at very low temperature this entered co2 becomes solidified also,

Change in steucture of of your porous material and co2 can be studied by raman spectroscopy. Research has been done on zif and co2, niyrogen and other gasses entrapment and release. Contact the renishaw guys, sone one have demonstrated it using raman instrument. Good luck