The sorption–desorption hysteresis observed in many nanoporous solids, at vapor pressures low enough for the liquid (capillary) phase of the adsorbate to be absent, has long been vaguely attributed to some sort of ‘pore collapse’. However, the pore collapse has never been documented experimentally and explained mathematically. The present work takes an analytical approach to account for discrete molecular forces in the nanopore fluid and proposes two related mechanisms that can explain the hysteresis at low vapor pressure without assuming any pore collapse nor partial damage to the nanopore structure. The first mechanism, presented in Part I, consists of a series of snap-through instabilities during the filling or emptying of non-uniform nanopores or nanoscale asperities. The instabilities are caused by non-uniqueness in the misfit disjoining pressures engendered by a difference between the nanopore width and an integer multiple of the thickness of a monomolecular adsorption layer. The wider the pore, the weaker the mechanism, and it ceases to operate for pores wider than about 3 nm. The second mechanism, presented in Part II, consists of molecular coalescence, or capillary condensation, within a partially filled surface, nanopore or nanopore network. This general thermodynamic instability is driven by attractive intermolecular forces within the adsorbate and forms the basis for developing a unified theory of both mechanisms. The ultimate goals of the theory are to predict the fluid transport in nanoporous solids from microscopic first principles, determine the pore size distribution and internal surface area from sorption tests, and provide a way to calculate the disjoining pressures in filled nanopores, which play an important role in the theory of creep and shrinkage.
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