Capillary condensation of nitrogen in MCM-41 and SBA-15

Abstract

We performed measurements of a series of adsorption–desorption isotherms of nitrogen into MCM-41 and SBA-15 mesoporous molecular sieves with different pore sizes in a wide temperature range between a bulk triple point (Tt) and a bulk critical point (Tc). The hysteresis loop shrinks with increasing temperature and eventually disappears at the hysteresis critical temperature (Tch), in accord with the appearance of metastable states in a single pore. We estimated the exact pore sizes of our samples from comparing the experimental equilibrium pressure at 77 K with the NLDFT isotherms, where the condensation pressures at 77 K were determined by extrapolating a plot of the equilibrium pressure versus temperature measured above Tch. The pore critical temperature (Tcp) was also determined from the inflection point in a plot of the inverse slope of the adsorption step against temperature. Tch is always lower than Tcp. Both plots of (Tc−Tch)/Tc versus d/rp and (Tc−Tcp)/Tc versus d/rp form straight lines passing through the origin, where d is the molecular diameter, although the former plot deviate from the linear relationship at large pores; Tch approaches Tcp in large pores. This strongly suggests that in large pores the energy barrier from the metastable state to the stable state is too large to be overcome within the period of observations even at the high temperatures just below Tcp.

Continuous adsorption in highly ordered porous matrices made by nanolithography

“…Mean density functional theory 15 , molecular dynamics simulations of adsorption and desorption by diffusive mass transfer into model pores 16 and grand canonical Monte Carlo simulations 17 confirm this classical picture of adsorption in a single pore. In contrast to theoretical predictions, experiments with mesoporous matrices characterized by a trivial porous network topology consisting of straight and not interconnected pores show that adsorption is always characterized by pronounced hysteresis loops independent of whether the pores are open at one or at both ends 7,12,[18][19][20][21] . In other words, in these materials the pores appear not to be independent during an adsorption isotherm.…”

Simple Phenomenological Model for Phase Transitions in Confined Geometry. 2. Capillary Condensation/Evaporation in Cylindrical Mesopores

“…Interestingly, noting  L t=the amount of adsorbed liquid per surface area and  V the vapor specific chemical potential, from (13) we recover the celebrated Gibbs' adsorption equation, namely:…”

“…1 Such a hysteresis loop shrinks and disappears as the temperature increases. 11,12,13,14,15 This hysteretic behavior, which is common to both nanoporous materials with independent, unconnected pores or connected pores, is often explained using one of the following model. 16 In the case of connected pores, the hysteresis loop is ascribed to the existence of constrictions or bottlenecks between pores that cause a delay in the desorption process compared to the adsorption 17,18,19 .…”

Water filling of hydrophilic nanopores

“…19 These conclusions, however, seem to be at odds with the experimental data presented by Morishige and Ito, who probed nitrogen in several MCM-41 samples to show that neither capillary condensation nor evaporation must necessarily take place at the thermodynamic equilibrium in irreversible isotherms. 21 The condensation of fluids in confined environments can produce a rich phase diagram with several phase transitions. 22,23 Computer simulations offer insight into the microscopic structure and processes associated with the phase transformations in nanopores.…”