Alkanes PTMSP

The sorption of several penetrants in PTMSP has been studied as a function of temperature and pressure. For both solubility and diffiisivity isotherms, the experimental results show significant differences between n-alkanes and alcohols. A discussion of the experimental data is presented, considering the glassy matrix as a homogenous phase, and using thermodynamic arguments commonly applied to standard mixtures. It is thus possible to offer a unique description of the thermodynamic properties of the various mixtures examined, in spite of their rather different behaviour. Simple isotherms for the mobility coefficient are also calculated for all penetrants as a function of composition. Remarkably, they show very similar trends for both n-alkanes and alcohols. 

In this study an extensive set of experiments was conducted to determine solubility and diffiisivity of several vapours in PTMSP. Five diff ent penetrant species were considered, three alkanes of different molecular weight and two polar components. The effect of temperature on solubility and diffiisivity was also considered by performing several sorption experiments in the range between 285 and 330 K. 

Remarkably, upon comparing the solubility of n-pentane and n-hexane in PTMSP, it is observed that, at the same temperature and penetrant activity, the solute mass ratio dissolved in PTMSP is higher for the higher molecular weight component. The same trend is confirmed when the analysis is extended to the case of n-heptane, and it is clearly apparent from Figure 3, where the solubility isotherms for the three alkanes at 330 K are simultaneously reported. From the data shown in Figure 3, the solubility coefficient in the low pressure limit for n-heptane in PTMSP may be evaluated as So 27. 

In Figure 4 the solubility isotherms at three different temperatures are reported for ethanol in PTMSP. Unlike the case of n-alkanes, the solubility coefficient of ethanol in PTMSP is quite low in the range of low pressures it increases with penetrant content in the low concentration region and it decreases at higher penetrant fiigacity. The... 

The peculiar features shown by the sorption isotherms of ethanol in PTMSP are followed also by the solubility curves for methanol in PTMSP, which are presented in Figure 5. As in the case of ethanol, the methanol solubility coefBcient in the low pressure limit. So, is orders of magnitude lower than for alkanes, and the sorption isotherms are characterised by the previously mentioned S shape. 

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. 

From the experimental data presented herein, we are now able to estimate the mixing enthalpy for n-alkanes and alcohols in PTMSP and, thus, to evaluate the... 

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... 

In Figure 7, diffusivity isotherms at 300 K are presented for n-pentane, n-hexane, ethanol and methanol. The measured values of D are orders of magnitude higher than the typical values of diffusion coeffidents in glassy polymers. The diffusivity of the two alkanes in PTMSP show similar trends as a function of the penetrant content. For both n-pentane and n-hexane, the diffusion coefficient exhibits a maximum variation of about 40% and a rather flat maximum value at intermediate concentrations. 

The diffusivity isotherms obtained for ethanol and methanol are similar to each other and quite different from those of the alkanes. Very large diffusion coefficients have been measured in the low penetrant concentration range. Then, the diffusion coefficients decrease to a minimum value at approximately 7% penetrant weight fraction. Afterwards, the diffusivity increases again and shows a local maximum value at o ll%. The latter trend is also observed for diffusivity coefficients measured at temperatures of 285 and 330 K. However, for the sake of conciseness, these data are not presented explicitly. The present data also confirm the diffusivity behaviour of ethanol in PTMSP reported previously (6). 

A clear interpretation of the concentration and temperature dependence of the mass transport properties of both alkanes and alcohols in PTMSP can be obtained when it is recognised that the diffusion coefficient, A results from the product of the mobility, L, and a pure thermodynamic factor, a, defined as... 

Similar isotherms for the mobility coefficient are observed at aU t peratures and, moreover, the ratio between the mobility coefficients at different temperatures for the same component are essentially independent of concentration. This observation permits the evaluation of a unique activation energy, E., for the mobility coefficient of a given penetrant in PTMSP. The values of the activation energy of the mobility coefficient in PTMSP for n-alkanes and alcohols are reported in Table I. 

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. 

By considering the solid glassy mixture as a homogeneous phase, the apparent qualitative differences between the solubility of alkanes and alcohols in PTMSP may be easily interpreted as simple quantitative differences in energetic and entropic contributions to the free energy of mixing. Indeed, the chemical potentials of the penetrant species in the mixture show similar isotherms for every case considered. The difference in the value of the chemical potential for alkanes and alcohols in PTMSP is mainly due to differences in energetics of the sorption process, and not to entropic effects. 

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