Edinburgh Napier University

  The water vapour sorption properties of Sitka spruce (Picea abies)
have been investigated over a range of temperatures (14.2–43.8C) using a
dynamic vapour sorption apparatus. The sorption kinetics behaviour was evaluated
using the parallel exponential kinetics model which has been found to give very
accurate fits to the data in studies of foodstuffs or plant fibres, but has not been
previously applied to sorption studies with wood. Both the adsorption and
desorption kinetics curve can be deconvoluted into a fast and slow exponential
process. Under conditions of adsorption, the fast process appears to be associated
with the formation of monolayer water (determined using the Hailwood Horrobin
model) up to a relative humidity of 20%. Under desorption, there is no clear
differentiation between fast and slow processes. The area bounded by the sorption
hysteresis loop reduced as the temperature at which the isotherm was measured
increased, due to movement of the desorption curve only, with the adsorption
curve remaining the same at all temperatures. This behaviour is consistent with
sorption processes taking place on nanoporous glassy solids below the glass
transition temperature. The heat of wetting was determined from the temperature
dependence of the desorption isotherms by using the Clausius–Clapeyron equation,
yielding results that are consistent with literature values. However, doubts are raised in this paper as to the applicability of using the Clausius–Clapeyron
equation for analyses of this type.