  Hindered adsorbed layers completely filling the nanopores must cause
significant deviations from the classical BET isotherms for multimolecular
adsorption of vapor in porous solids. Since the point of transition from free
to hindered adsorption moves into narrower nanopores as the pore vapor pressure
(or humidity) decreases, the area of the surface of multimolecular adsorption
layer exposed to vapor gets modified by an area reduction factor that decreases
with increasing thickness of the layer, and thus also with increasing vapor
pressure. The area reduction factor does not affect the rates of direct
adsorption or condensation from the vapor, which represent a local process, but
imposes a lateral constraint on the total area and volume of the free portion
of the adsorption layer in direct contact with vapor. Assuming a power-law for
the dependence of the area reduction factor on the number of molecular layers,
one can express the modified isotherm in terms of logarithmic or
polylogarithmic (a.k.a Jonqui\`ere) functions. Comparisons with some published
experimentally obtained isotherms show that the present modification of the BET
theory for hindered adsorption goes in the right direction. Detailed
calibration of the theory and an extension for indirect communication of vapor
molecules with the molecules adsorbed in nanopores less than a few nm wide will
require further research.
