Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks.

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

Alkane-based eutectic phase change materials doped with carbon nanomaterials.

Cold thermal energy storage is an issue of increasing importance on a global scale particularly in the format of passive thermal protection. This study presents three eutectic Phase Change Materials (ePCMs) composed of n-alkanes, which provide passive temperature control (their operation is automatically induced by exceeding the limit temperature without the need for a control system) around 4 °C (277.2 ± 2 K) and are chemically neutral. The solid-liquid equilibrium (SLE) in the following binary systems was investigated: {n-tetradecane + n-heptadecane}, {n-tetradecane + n-nonadecane}, {n-tetradecane + n-heneicosane}, allowing the determination of two ePCMs with enthalpies close to 220 J g-1 and one significantly lower (155.5 J g-1). Moreover, two solid-liquid-liquid equilibrium (SLLE) phase diagrams were determined for systems: {n-tetradecane + 1,6-hexanediol} and {n-tetradecane + 1,12-dodecanediol}. In addition, the work provides a systematic analysis of the problem of designing ePCMs with specific properties and the aspects that need to be considered. The possibility of predicting the parameters of eutectic mixtures using the UNIFAC (Do) equation and the equation of ideal solubility was verified. A method for predicting the enthalpy of melting of eutectic was also proposed and confronted with the results of DSC analysis. Thermodynamic studies have been supplemented with measurements and correlation of experimental data of ePCMs density and dynamic viscosity as a function of temperature. The final issue is the improvement of the thermal conductivity of paraffins by the addition of nanomaterials such as Single Wall Carbon Nanotubes (SWCNTs), Expandable Graphite (GIC) or Expanded Graphite (EG). The possibility of forming a long-lasting composite material composed of ePCMs and 1 wt% of SWCNTs with a thermal conductivity significantly higher compared to pure ePCMs has been proven via stability testing under operating conditions.

High-Performance Ionanofluids from Subzipped Carbon Nanotube Networks.

Investments in the transfer and storage of thermal energy along with renewable energy sources strengthen health and economic infrastructure. These factors intensify energy diversification and the more rapid post-COVID recovery of economies. Ionanofluids (INFs) composed of long multiwalled carbon nanotubes (MWCNTs) rich in sp2-hybridized atoms and ionic liquids (ILs) display excellent thermal conductivity enhancement with respect to the pure IL, high thermal stability, and attractive rheology. However, the influence of the morphology, physicochemistry of nanoparticles and the IL-nanostructure interactions on the mechanism of heat transfer and rheological properties of INFs remain unidentified. Here, we show that intertube nanolayer coalescence, supported by 1D geometry assembly, leads to the subzipping of MWCNT bundles and formation of thermal bridges toward 3D networks in the whole INF volume. We identified stable networks of straight and bent MWCNTs separated by a layer of ions at the junctions. We found that the interactions between the ultrasonication-induced breaking nanotubes and the cations were covalent in nature. Furthermore, we found that the ionic layer imposed by close MWCNT surfaces favored enrichment of the cis conformer of the bis(trifluoromethylsulfonyl)imide anion. Our results demonstrate how the molecular perfection of the MWCNT structure with its supramolecular arrangement affects the extraordinary thermal conductivity enhancement of INFs. Thus, we gave the realistic description of the interactions at the IL-CNT interface with its (super)structure and chemistry as well as the molecular structure of the continuous phase. We anticipate our results to be a starting point for more complex studies on the supramolecular zipping mechanism. For example, ionically functionalized MWCNTs toward polyionic systems─of projected and controlled nanolayers─could enable the design of even more efficient heat-transfer fluids and miniaturization of flexible electronics.