  The sorption-desorption hysteresis observed in many nanoporous solids, at
vapor pressures low enough for the the liquid (capillary) phase of the
adsorbate to be absent, has long been vaguely attributed to changes in the
nanopore structure, but no mathematically consistent explanation has been
presented. 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 change in 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 second mechanism,
presented in Part II, consists of molecular coalescence 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 to develop 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, and to determine the pore size
distribution and internal surface area from sorption tests.
