The water molecules, when they are in the form of moisture in a solid, have a different enthalpy from the enthalpy of pure water. If the water is bound to the solid, this enthalpy is less than the enthalpy of pure water by the energy of this binding.
Consequently the moisture in a solid has a higher heat of evaporation than for pure water. The difference between this actual heat of evaporation for the moisture in the solid and the latent heat of evaporation of purewater is called the net isosteric heat of sorption or the enthalpy of wetting. By knowing the net isosteric heat of sorption, the actual energy required for drying a product to a specific moisture content can be calculated.