where m is the water moisture (dry basis), mo is the mass (in grams)
of water in the monolayer per gram of dry membrane, aw is the
water activity, c = exp[(E1 − EL)/RT], as in the BET isotherm, is related
to the energy difference between the sorption energy of the first
water layer (E1) and the heat of liquifaction of pure water (EL), while
f = exp[(E2 − EL)/RT], is related to the energy difference between the
subsequent water layers (E2) and the heat of liquifaction of pure
water.