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The sorption-desorption hysteresis observed in many nanoporous solids, atvapor pressures low enough for the the liquid (capillary) phase of theadsorbate to be absent, has long been vaguely attributed to changes in thenanopore structure, but no mathematically consistent explanation has beenpresented. The present work takes an analytical approach to account fordiscrete molecular forces in the nanopore fluid and proposes two relatedmechanisms that can explain the hysteresis at low vapor pressure withoutassuming any change in the nanopore structure. The first mechanism, presentedin Part I, consists of a series of snap-through instabilities during thefilling or emptying of non-uniform nanopores or nanoscale asperities. Theinstabilities are caused by non-uniqueness in the misfit disjoining pressuresengendered by a difference between the nanopore width and an integer multipleof the thickness of a monomolecular adsorption layer. The second mechanism,presented in Part II, consists of molecular coalescence within a partiallyfilled surface, nanopore or nanopore network. This general thermodynamicinstability is driven by attractive intermolecular forces within the adsorbateand forms the basis to develop a unified theory of both mechanisms. Theultimate goals of the theory are to predict the fluid transport in nanoporoussolids from microscopic first principles, and to determine the pore sizedistribution and internal surface area from sorption tests.

Theory of Sorption Hysteresis in Nanoporous Solids: I. Snap-Through Instabilities