(488d) The Importance of the Amorphous Material Interface in the Measurement of Thermodynamic Parameters, Using Inverse Gas Chromatography | AIChE

(488d) The Importance of the Amorphous Material Interface in the Measurement of Thermodynamic Parameters, Using Inverse Gas Chromatography


Hadjittofis, E. - Presenter, Imperial College London
Zhang, G. G. Z., AbbVie Inc.
Heng, J., Imperial College London
Hansen Solubility Parameters (HSP) are thermodynamic quantities of crucial importance for the understanding of the behavior of polymeric materials with respect to different solvent systems.1 Traditionally, the measurement of HSP was based on lengthy solubility/swelling experiments, followed by extensive statistical analysis of the data. Inverse Gas Chromatography (IGC), a technique employed in polymer characterization since mid-70’s,2,3 was introduced as a tool for the determination of HSP about 25 years ago.4,5 Since then, it has gained a lot of momentum in the field, as it provides faster measurements, it is less labor intensive and requires much less amount of materials, such as polymer and solvents.

IGC enables the determination of the infamous Flory-Huggins χ solubility parameter5,6, for the interaction of the material with different solvents.7,8 The values of χ obtained can be then used to determine quantities such as Hansen Solubility Parameters (HSP) and diffusion coeffcients.9 Considering the chromatographic nature of the technique and the amorphous character of much of the materials of interest, it is evident that the flow rate of the carrier gas in the chromatographic column is of high importance, even for amorphous materials below their glass transition temperature (Tg). The interface between the solvent and the solute gives rise to capillary effects.10-12 Thus, for accurate measurements, enough time should be given for the solvent to overcome these limitations. Up to now, the influence of this interesting interfacial phenomenon has been omitted for such types of measurements. In the majority of the published literature the authors do not provide any rational for the selection of the carrier gas flow rate in their measurements and they seem to ignore its importance.

This work addresses the importance of interfaces in the measurement of HSP using a series of simple experiments on some model compounds. The HSP of crystalline materials (P-Monoclinic Carbamazepine and Monoclinic Acetaminophen) were measured at two different flow rates. The results are identical for all the flow rates suggesting that in crystalline materials the influence of diffusion is limited. Following that, Copovidone, a common polymeric pharmaceutical excipient, was employed for more advanced studies. Its Tg was determined both with DSC and IGC and it was found to be around 100 oC. Then its solubility parameters were measured at 30, 50, 70, 90 and 100 oC for different carrier gas flow rates. For measurements conducted well below the Tg, the value of HSP increases with decreasing flow rate whereas for the measurements on the Tg the HSP value appears to be independent of the flow rate. These findings highlight that at high temperatures the limitations induced by capillarity diminish, as expected from the analysis of these systems in terms of Fickian diffusion. Analysis of the χ parameters obtained, for different solvents and at different temperatures, reveal that the influence of entropic phenomena scales with decreasing flow rate and decreasing temperature. The findings of these work couple well with the theory proposed by de Gennes, which, in fact, encompasses the influence of local capillary phenomena; the importance of which have been predicted by J.W. Gibbs in his groundbreaking work “On the equilibrium of heterogeneous substances”, published in 1874.

Overall, this work addresses the influence of interfacial phenomena in the measurement of the bulk properties of amorphous materials; revealing the overwhelming importance of entropic interactions. It highlights the importance of experimental conditions, proposing a road-map of good experimental practice. This would provide the investigators with a magnifying glass enabling them to look the very fine details of the literature data.


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