Enthalpy, excess PTMSP

The quantitative differences observed between the two families of penetrants are due both to enthalpy and to entropy contributions. Based on a qualitative consideration of the interaction energies between the polymer matrix and the aliphatic or the alcoholic groups of the different penetrants, the enthalpic term in Equation 1 is mostly responsible for the quantitative differences observed in the isotherms. This was also the conclusion drawn in a previous study (75) where the solubility of ethanol and n-pentane in PTMSP were qualitatively compared with each other. Based on the solubility isotherms at different temperatures for n-alkanes and alcohols, we can now directly evaluate the excess enthalpy of mbdng in Eq.(l). In a previous discusdon of the comparison of solubility data for ethanol and n-pentane in PTMSP (6), it was assumed that the difference in chemical potential for the two penetrants was essentially due to the enthalpic term. TMs hypothesis is reasonable since the interaction energy of penetrant molecules with PTMSP segments should be significantly different, relative to interaction energies of pure penetrant molecules, between polar and non polar penetrants. 

Similarly, the excess enthalpy of n-hexane in PTMSP is evaluated from the data... 

In summary, the enthalpy of mixing is always negative and thus represents an important contributions in favour of the solubility of penetrants in PTMSP. For alcohols in PTMSP, the excess enthalpy is significantly smaller than for the n-alkanes and even shows negligible values in the low concentration range where the solubility... 

The solubility of several penetrants in PTMSP indicate relevant differences between polar and non-polar components. In particular, for the alcohols, unusual S shaped solubility isotherms are observed which cannot be explained by the dual mode model. Variations of solubility with temperature were also considered in this work, and the mixing enthalpy for n-pentane, n-hexane, ethanol and methanol in PTMSP was calculated. Significant negative mixing enthalpies were measured in all cases, and absolute values of the excess enthalpy of the sorption is much higher for the alkanes than for the alcohols. 

---