Computer Simulation of Static and Dynamic Properties During Transient Sorption of Fluids in Mesoporous Materials

The hysteresis observed frequently in the adsorption isotherm of gas in porous material is currently subject to intense research. The nature of the hysteresis itself and in particular recent observations on the uptake dynamics inside the hysteresis loop are described as poorly understood puzzles even in recent literature (Wallacher, D.; Kunzer, N.; Kovalev, D.; Knorr, N.; Knorr, K. Phys. Rev. Lett. 2004, 92, 195704-1; Valiullin, R.; Naumov, S.; Galvosas, P.; Karger, J.; Woo, H.-J.; Porcheron, F.; Monson, P. Nature 2006, 443, 965-968). Detailed experiments combined with theoretical efforts have led to the identification of new problems rather than answers, e.g., the relaxation dynamics in pressure jump experiments is found to be "dramatically" slowed down inside the hysteresis loop (Valiullin et al. 2006). This has motivated the authors to postulate "a fundamental difference in the nature of the relaxation dynamics for states within the hysteresis region compared with those outside of this region". Here we present a computer simulation study on cylindrical pores. We determine the adsorption isotherm, i.e., a steady state property, and the uptake dynamics inside and outside of the hysteresis loop. Further, we study the behavior during incomplete passage through the hysteresis loop, a situation which has been characterized recently by PFG NMR (pulsed field gradient nuclear magnetic resonance) and again has motivated elaborate explanations (Naumov et al. 2008). It is noteworthy that all experimental observations are reproduced when describing diffusion by the Onsager ansatz which employs the gradient of the chemical potential as driving force. No ad hoc assumptions about a new transport mechanism inside the hysteresis region are needed.