Hysteresis and scanning curves in linear arrays of mesopores with two cavities and three necks. classification of the scanning curves

摘要

Adsorption of argon at 87K in a linear array of slit mesopores composed of two cavities and three necks has been investigated using Grand Canonical Monte Carlo simulation. Hysteresis and scanning was found to depend on the relative size of the necks and cavities and on whether the necks are wider or narrower than the critical width that demarcates cavitation from pore blocking. There are 26 possible combinations for the linear array. By considering the behaviour of hysteresis scanning curves, we are able to identify four distinct groups: (I) Group 1: The descending scanning spans the boundary curve of the hysteresis loop due to stretching of the condensate in the small cavity. (ii) Group 2: The descending curve partially spans the loop and returns to the adsorption boundary. This occurs either because the condensate stretches in the small cavity, followed by evaporation via a pore blocking mechanism; or because the condensate evaporates as the meniscus recedes in the large neck that joins the two cavities. (iii) Group 3: The descending curve spans the loop as in Group 1 but there is a small sub-loop associated with emptying and filling of the large neck connecting the large cavity to the surrounding gas. (iv) Group 4: The descending scanning curve is similar to that in Group 2; but when the large cavity of the array is filled with adsorbate, and the small cavity is empty (except for an adsorbed film) the ascending curve partially spans the loop. This happens when molecular layers build-up in the small cavity (c.f. stretching of condensate in a descending scan) is followed by condensation, which results in the scanning curve returning to the desorption boundary (c.f. evaporation of the condensate and return to the adsorption boundary). There is also a sub-loop which has similar characteristics to those in Group 3.