in which w is for sorbed water and l is for liquid water at its
saturation pressure p0. The quantity p/p0 is also referred to as
the water activity (aw), which, in turn, when multiplied by 100
is defined as the RH. In a water sorption experiment, p/p0 or
aw is fixed and the water content of the solid is measured. The
p/p0 value is changed and water content is measured over a
range from p/p0 = 0 to typically 0.9. The isotherm establishes
the relation of p/p0 to water content, which in this analysis will
use water mole fraction Xw.