  Motivated by the puzzle of sorption hysteresis in Portland cement concrete or
cement paste, we develop in Part II of this study a general theory of vapor
sorption and desorption from nanoporous solids, which attributes hysteresis to
hindered molecular condensation with attractive lateral interactions. The
classical mean-field theory of van der Waals is applied to predict the
dependence of hysteresis on temperature and pore size, using the regular
solution model and gradient energy of Cahn and Hilliard. A simple "hierarchical
wetting" model for thin nanopores is developed to describe the case of strong
wetting by the first monolayer, followed by condensation of nanodroplets and
nanobubbles in the bulk. The model predicts a larger hysteresis critical
temperature and enhanced hysteresis for molecular condensation across nanopores
at high vapor pressure than within monolayers at low vapor pressure. For
heterogeneous pores, the theory predicts sorption/desorption sequences similar
to those seen in molecular dynamics simulations, where the interfacial energy
(or gradient penalty) at nanopore junctions acts as a free energy barrier for
snap-through instabilities. The model helps to quantitatively understand recent
experimental data for concrete or cement paste wetting and drying cycles and
suggests new experiments at different temperatures and humidity sweep rates.
