(425g) The Importance of Spreading Pressure on Adsorption Based Surface Energy Measurements; The Case of IGC

In his work on “The Spreading of Fluids on Glass”,¹ published in 1919, Sir William Bate Hardy argues that “Whether primary or secondary spreading does or does not occur on a fluid face depends mainly upon the relative value of the surface tensions, but on a clean solid face it must depend wholly upon the value of vapour tension”. This constitutes the first scientific statement highlighting the importance of spreading pressure in the formation of solid-fluid interfaces. Nonetheless, the concept of spreading pressure has been discussed, by Hardy and other prominent members of the then scientific community, well before 1919, in the context of its importance in the spreading of oils on water.^2-4

In 1937 Bangham and Razouk apply Gibb’s 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 70’s.^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 Carbamazepine’s 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.

References

1. Hardy WB. The spreading of fluids on glass. Philosophical Magazine. 1919, 38, 49-55.

2. Hardy WB. The tension of composite fluid surfaces and the mechanical stability of films of fluid. Proceedings of the Royal Society of London Series A, Mathematical and Physical Sciences. 1912, 86, 610-635.

3. Hardy WB. The influence of chemical constitution upon interfacial tension. Proceedings of the Royal Society of London Series A, Mathematical and Physical Sciences. 1913, 88, 303-313.

4. Hardy WB. The tension of composite fluid surfaces. No. II. Proceedings of the Royal Society of London Series A, Mathematical and Physical Sciences. 1913, 88, 313-333.

5. Bangham DH, Razouk RI. Adsorption and the wettability of solid surfaces. Transactions of the Faraday Society. 1937, 33, 1459-1463.

6. Bangham DH. The gibbs adsorption equation and adsorption on solids. Transactions of the Faraday Society. 1937, 33, 805-811.

7. Bangham DH, Razouk RI. The wetting of charcoal and the nature of the adsorbed phase formed from saturated vapours. Transactions of the Faraday Society. 1937, 33, 1463-1472.

8. Braun J, Guillet JE. Determination of crystallinity of olefin copolymers and polyolefin powders by inverse gas chromatography. Polymer Chemistry. 1975, 13, 1119-1131.

9. Ho R, Heng JYY. A review of inverse gas chromatography and its Development as a tool to Characterize Anisotropic surface properties of pharmaceutical solids. KONA Powder and Particle. 2013, 30, 164-180.

10. Jefferson EA, Williams DR, Heng JYY. Computing the surface energy distributions of heterogeneous crystalline powders. Journal of Adhesion Science and Technology. 2011, 25, 339-355.

11. Smith RR, Williams DR, Burnett DJ, Heng JYY. A new method to determine dispersive surface energy site distributions by inverse gas chromatography. Lagmuir. 2014, 30, 8029-8035.

12. Yao Z, Heng JYY, Lanceros-Méndez S, et al. Study on the surface properties of colored talc filler (CTF) and mechanical performance of CTF/acrylonitrile-butadiene-styrene composite. Journal of Alloys and Compounds. 2016, 676, 513-520.