Lower Closure Point for Nitrogen or Argon Adsorption in Mesoporous Solids: Window-Induced Evaporation or Surface-Induced Cavitation?

Isotherms for nitrogen adsorption at 77 K and argon at 87 K in mesoporous adsorbents exhibit a hysteresis loop with a lower closure point (LCP) at a reduced pressure P/P-0 of approximately 0.4. This observation has been attributed to the homogeneous nucleation of vapor bubbles in the condensate within a cavity when it has been stretched beyond its tensile strength limit; on this basis, it is argued that the LCP is a property of the adsorbate fluid. However, our extensive computer simulations of homogeneous cavitation invariably give an LCP of 0.25, rather than 0.4. In our quest for the microscopic origin of the observed LCP, we have explored the possible role of two properties of a solid adsorbent: surface heterogeneity and the nature of the conduits connecting a cavity to the surrounding gas. In the first case, cavitation originates from the birth of nanobubbles formed at weakly adsorbing patches on the pore walls, which we refer to as surface-induced cavitation; in the second case, referred to as window-induced evaporation, the condensate in the cavity evaporates because of the snapping of the adsorbate in the short conduits (or windows) connecting the cavity to the surrounding gas. Our simulations show that the evaporation of the condensate from a cavity occurs at a reduced pressure of around 0.4 only when the holding potential of the patches in the surface-induced model is so weak that an open adsorbent with a similar holding potential would be nonwetting. The hypothesis that weakly adsorbing patches are the reason for loop closure at a relative pressure of 0.4 must therefore be rejected because this condition is unlikely to be found in real adsorbent solids. On the other hand, window-induced evaporation of the condensate from a cavity at 0.4 occurs when the windows are of nanodimensions. We argue that windows of nanodimensions are very probable in adsorbents consisting of agglomerations of microparticles.

Automated summary

Through simulations, two potential causes of adsorption loop closure have been identified: surface heterogeneity and window connections. For conventional adsorbents, surface-induced cavitation and nanodimension window-induced evaporation are likely scenarios.