The significance of partition coefficients

In order to determine the most important influences on the partition coming from the structures of the permeant molecules, 12 compounds have been selected from Table 9-7 and studied closer under different aspects. The selection contains compounds with different functional groups and molecular structures. An additional important criterion for the composition of this mixture is they can be easily separated into individual components by gas chromatography. By doing this, four to eight compounds could be measured simultaneously. Comparisons with measurements made using individual compounds showed no difference beyond measurement uncertainty, up to a concentration of 1 to 2 % (mass fraction) in methanol. 

It can be seen from the data in Table 9-8 that the partition coefficients of the selected compounds show no significant differences with respect to the liquid phase used. The values for the unpolar limonene are slightly higher in the polyolefin/methanol system than in the polyolefin/ethanol system because the methanol has a higher polarity than ethanol. For the polar cis-3-hexenol compound this difference disappears. 

Since the measurement of the partition coefficients for LDPE goes much more rapidly than for the other polyolefins, it is practical to apply the K values obtained for LDPE to other polyolefins. This can be done without any large errors. 

For an ideal solution y = 1, and the equilibrium concentration of the permeant varies proportionally to the mole fraction over the complete mole fraction range. A practically ideal system is for example a solution of heptanol in methanol (Fig. 9-6). 

Compound Eth.% LDPE 100 LDPE 80 LDPE 60 LDPE 40 LDPE 20 PVC 100 PVC 40 PVC 20 

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