Cintia M. CORREA 1, Gauthier LEGRAND 2, Thibaut DIVOUX 2, Sébastien MANNEVILLE2, Frederik PHILIPPI 1, Agilio PÁDUA 1, Margarida COSTA GOMES1
1École Normale Supérieure de Lyon and CNRS, Laboratoire de Chimie, France
2École Normale Supérieure de Lyon and CNRS, Laboratoire de Physique, France
Porous ionic liquids can be prepared as stable suspensions of metal-organic frameworks (MOFs) in voluminous phosphonium salts.[1] Their applications are diverse and promising: gas absorption and separations, reaction media for gaseous solutes, and delivery of small molecules.[2]
Suspensions or dispersions of particles in a liquid are currently applied in several industrial processes as liquid abrasives, to prepare ceramics, medicines, foodstuffs or inks. One of the characteristics that need to be fulfilled to allow for real and practical applicability is their long-term stability.3 Understanding thermodynamically and kinetically the factors that cause phase separation, aggregation/sedimentation, and formation of microdomains is crucial to assign a viable application to these systems. Suspensions require to be stable and fluid all along the process of interest, respecting all the conditions involved (i.e. pressure, temperature, etc) or easily re-dispersed in case of phase separation.3
Our research group has prepared and characterized type III porous liquids using ionic liquids, often based on phosphonium cations, and MOFs such as ZIF-8.[2,4] In addition to acting as excellent gas absorbers, an attribute that stands out is the visual stability of the suspensions that remain stable for more than 22 months for the system ZIF-8 in [P6,6,6,14][NTf2], independently of the MOF concentration (5 or 10 % w/w).[2]
Rheology allows for the study of the flow of matter and the deformation behaviour of fluids correlating structural information with flow properties and thus is an interesting tool to elucidate stability, and molecular interactions in the mixture to assess the potential use of suspensions for different applications.[5,6] In this context, the main goal of this study was to measure the rheological properties, in a temperature range from 20 to 80°C, of suspensions prepared with ZIF-8 in two phosphonium-based ionic liquids with different cations, same anion and dissimilar viscosities ([P6,6,6,14][NTf2] and [P(3O1)4[NTF2]), in the concentrations of 0, 5, 10 and 20% w/w.
Both pure ionic liquids showed Newtonian behaviour, while suspensions containing ZIF-8 showed behaviour described by the Bingham model, most likely indicating an attractive inter-particle interaction.
[1] Avila J. et al. Angew. Chem. Int. Ed. 2021, 60, 12876
[2] Avila J. et al. Mater. Adv. 2022, 3, 8848
[3] Larsson M. et al. Annual Transactions - The Nordic Rheology Society. 2012, 20, 209.
[4] Costa Gomes M. et al. Angew. Chem. Int. Ed. 2018, 57, 11909.
[5] Elhamarnah Y. et al. J. Mol. Liq. 2019, 277, 932.
[6] Shakeel A. et al. ACS Sustainable Chem. Eng. 2019, 7, 133586.