Further comments on narrow pores

The lower pressure sub-region is characterized by a considerable enhancement of the interaction potential (Chapter 1) and therefore of the enthalpy of adsorption consequently the pore becomes completely full at very low relative pressure (sometimes 0 01 or less), so that the isotherm rises steeply from the origin. This behaviour is observed with molecular sieve zeolites, the enhancement of the adsorption energy and the steepness of the isotherm being dependent on the nature of the adsorbent-adsorbate interaction and the polarizability of the adsorbate. - 

In the higher pressure sub-region, which may be extended to relative pressure up to 01 to 0-2, the enhancement of the interaction energy and of the enthalpy of adsorption is relatively small, and the increased adsorption is now the result of a cooperative effect. The nature of this secondary process may be appreciated from the simplified model of a slit in Fig. 4.33. Once a monolayer has been formed on the walls, then if molecules (1) and (2) happen to condense opposite one another, the probability that (3) will condense is increased. The increased residence time of (1), (2) and (3) will promote the condensation of (4) and of still further molecules. Because of the cooperative nature of the mechanism, the separate stages occur in such rapid succession that in effect they constitute a single process. The model is necessarily very crude and the details for any particular pore will depend on the pore geometry. 

This second sub-region will give rise to a rounded knee to the isotherm. Thus a purely microporous solid which contains both categories of micropore will give rise to a Type 1 isotherm, having a very steep initial 

If mesopores are present in addition to micropores, the isotherm will be of Type IV, with the characteristic hysteresis loop but, as explained in 

Chapter 3, there is often a region immediately preceding the lower closure point, in which increased adsorption is brought about by reversible capillary condensation. The meniscus now tends to be somewhat ill defined owing to its small dimensions (p. 153), but the mechanism can still be thought of in Kelvin terms, where the driving force is the pressure difference across an interface. 

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