(425g) The Importance of Spreading Pressure on Adsorption Based Surface Energy Measurements; The Case of IGC
AIChE Annual Meeting
Tuesday, October 31, 2017 - 4:45pm to 5:00pm
In 1937 Bangham and Razouk apply Gibbâs isotherm in gas-solid adsorption, in a work where they address the importance of spreading pressure.5-7 However, in adsorption based techniques developed for surface energy measurements, in the years followed the publication of this work, the importance of spreading pressure was omitted, on the basis that its value can be considered negligible. One such technique is Inverse Gas Chromatography (IGC), a widespread technique used since 70âs.8,9
This work shows that for low surface energy materials, such as organic solids and polymers, the influence of spreading pressure is sufficiently high, that it can lead to peculiar results if it is not taken into consideration. The example of two of Carbamazepineâs polymorphs, the P-Monoclinic and the Triclinic, is employed to show that in the absence of the spreading pressure correction, the results would suggest an increase in the surface energy of the material, with an increasing temperature; a peculiar result for the specific material.
Using tailored experiments a roadmap is developed for the spreading pressure correction of the data. Upon correction the trend of the surface energy becomes the one expected from relevant theory, showing a decrease of about 7 mJ/m2 for an increase in temperature from 25 oC to 60 oC. These findings reveal that appreciating the importance of spreading pressure is paramount for accurate surface energy measurements. Furthermore, the experiments suggest that a careful selection of the probes used in the measurements is important when the measurements are performed at temperatures well beyond ambient.
Computational models are employed to showcase that the surface energy distributions obtained from the spreading pressure adjusted results are much more realistic and in better agreement with the surface energy measurements obtained via wettability experiments on the individual facets of crystals of the P-Monoclinic polymorph.10-12 This does not only validate the importance of computational models for the elucidation of adsorption measurements, but it also stretches the importance of cross verification of the results with complimentary experimental techniques.
This work exposes the importance of spreading pressure on adsorption based surface energy measurements of organic materials and polymers. It highlights the need for complimentary experimental techniques, as also of computational modelling, for the in-depth understanding of interfacial phenomena at the low surface energy limit. It, finally, provides a framework, for researchers, to revisit some of their old data and examine them experimentally and/or computationally in the light of the new findings.
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