Transmission, sorption and diffusivity: methods for determining the transport of water vapour through films


The transport of water molecules through solids is commonplace and of significance to diverse industries. Broadly
speaking the interest lies in preventing ingress of water vapour
and the associated product protection. Equally there are
applications dependent on the transport of water vapour such
as where a controlled release of water vapour is required. In
all such cases knowledge of the physical properties governing
the rate of transport in different climatic conditions is essential.
Molecular transport through solids is termed permeation or
transmission and can either be due to absorption where
molecules diffuse in the solid or due to adsorption where
molecules diffuse through the pore structure of the solid. The
permeation or transmission rate, F, will be a function of three
i) The solid dimensions (i.e. the ratio of area, A, to

thickness, t, for a film)
ii) Temperature, T: Molecular diffusion in solids is an
activated process which requires energy to overcome
a potential barrier and increasing temperature will
therefore assist diffusion.
iii) Concentration gradient, ΔC: Net transport requires a
difference in the concentration of molecules across
the solid so that there will be a corresponding net rate
of transfer from the region of high concentration
(analogous to the flow of heat from a region of high
temperature in the presence of a thermal gradient).