Measurement of partition and diffusion coefficients

The dimensionless relative solubility constant Sr = K in Eq. (9-32) is a measure of the partitioning of a substance between two phases, as is the absolute solubility constant coming from Henry s law in Eq. (9-3). The absolute solubility constants can be directly determined using one of the methods for gases, but this is limited to only relatively volatile substances. Large experimental errors like adsorption on the polymer surface and cell walls cannot be avoided in these methods for substances with low volatilities. 

A measurement system where the substance studied goes from a gas phase through the film and into a second gas phase is shown in Fig. 9-3 (Franz, 1992). 

Even when swelling processes are avoided, particularly for less volatile substances and certain structures, the differences between permeation measurements made over a gas phase and those made in direct contact with a liquid can be anticipated. The 

In the quasi steady state, the very slight concentration changes occuring over time in the two solvents can be taken into account through a simplified permeation calcula- 

This method for measuring the change in sorption over time cannot be used to measure polymers that absorb permeants very quickly like LDPE, since it produces large errors. In any case the sorption method is suitable for determining of the equilibrium concentration ce of permeant in the polymer and consequently for the measurement of K. 

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