Thermally Activated Condensation and Evaporation in Cylindrical Pores

Abstract

In a confined geometry, thermal activation gives rise to specific condensation or evaporation mechanisms. Accounting for this phenomenon is therefore important for characterizing porous materials from their sorption isotherms. However, most phenomenological models discussing thermal activation in strong confinement are only qualitative. Here, we use a semimacroscopic approach to compute the temperature and diameter dependences of the condensation and evaporation for a fluid confined in a cylindrical nanopore in the presence of thermal activation. In particular, our model predicts the effect of confinement on cavitation. Our results show how thermal activation controls the existence and shape of the hysteresis loop in porous materials with noninterconnected cylindrical pores, and shed a new light on the applicability of the classical Barrett−Joyner−Halenda method for determining pore diameter distributions in porous materials. This work paves the way to a phenomenological multiscale approach able to describe condensation and evaporation in materials with connected pores.

On Condensation and Evaporation Mechanisms in Disordered Porous Materials

“…To modelize the effect of connections between pores, we combine a realistic calculation of the condensation and evaporation processes within a single cylindrical pore with a lattice description of the pore network. This approach compromises between those used by Guyer and McCall, to discuss evaporation by percolation in three-dimensional (3D) disordered porous networks, and by Puibasset to model condensation and evaporation in a single corrugated cylindrical pore.…”

Nucleation of Capillary Bridges and Bubbles in Nanoconfined CO2

“…Capillary processes are at the center of a wide range of phenomena, including the onset of phase transitions in confined geometries, − the control of friction between sliding surfaces, − or the formation of capillary liquid bridges during inflammatory processes of the lungs. , These fluid contacts can actually take place over vastly different length scales and have been reported for colloidal systems − and in nanotribology and nanolithography. − Recent advances on the theoretical front have led to the extension of capillary theory (CT), whose tenets are rooted in a macroscopic approach, to nanoscopic systems by connecting the free energy barrier of nucleation for the fluid contacts to the contributions arising from the surface tension …”

Direct observation of homogeneous cavitation in nanopores

“…Moreover, a similar discrepancy has been found for the cavitation of dibromomethane in Vycor [19]. These results cast a doubt on the very principle of using extensions of homogeneous CNT [2,9] to predict the cavitation threshold in nanoporous materials.…”

“…In this paper, we elucidate this paradoxical situation by comparing evaporation of hexane from silicon and alumina membranes with pores transverse size in the range 20-60 nm, large enough to allow a direct comparison to bulk CNT [9]. For both membranes, we directly evidence cavitation and accurately check its activated nature.…”

“…Fundamental understanding of which of these processes is effective is crucial for many applications, ranging from characterizing porous materials [2,3] to controlling the shrinkage of concrete [4]. In particular, this requires establishing whether cavitation occurs in the bulk of pores or on their surface, and, in the first case, whether pores are large enough for homogeneous classical cavitation theory [5] (CNT) to hold or whether confinement has to be considered [6][7][8][9].…”