Pore network interactions in ascending processes relative to capillary condensation

Abstract

It is currently accepted that domain interdependence in adsorption hysteresis (i.e. pore-blocking effects due to hindered liquid–vapour transitions in which the state of any domain depends on those adopted by its neighbours), occurs during the descending (desorption) processes associated with capillary evaporation. In contrast to this behaviour, network effects are thought to be absent during the ascending processes inherent in capillary condensation. However, we have considered the possibility of strong vapour–liquid transitions of an assisted kind taking place during capillary condensation. This situation seems to be the rule, rather than the exception, in a wide variety of porous materials. The interactive effect arises as a consequence of menisci coalescence at the meeting point of capillaries, and it becomes more important as the extent to which the network is filled with capillary condensate increases. When a critical proportion (which depends on the connectivity and geometry of the porous media) of filled elements in the network has been reached, the whole condensation occurs suddenly. This implies that the usual analysis of the ascending boundary curve does not lead to the true pore-size distribution. However, the ascending curve can be predicted from the size distribution and connectivity of the porous network.