Mechanical energy storage within lyophobic heterogeneous systems (HLS), or “molecular springs,” relies on complex thermodynamic and structural couplings at the nanoscale. A multiscale study is necessary to link macroscopic mechanical behavior to the local molecular organization of the confined fluid (H_2O, D_2O). Experimental measurements are performed by combining high-pressure calorimetry and neutron scattering methods.
In this study, nanoporous matrices are characterized to isolate geometric, elastic and surface contributions. Their periodicity and hydrophobicity are validated by SAXS, physisorption, FTIR, and thermogravimetric analysis (TGA). Two families of materials are compared: rigid amorphous mesoporous silicas (MCM-41 and SBA-15) and metal-organic crystalline lattices (MOFs) such as ZIF-8 and MOF-74.
The thermodynamics of intrusion-extrusion cycles reveals a wide range of behaviors. High-pressure calorimetry shows that the forced intrusion of water into the lyophobic porosity is an endothermic process. Conversely, extrusion is driven by a thermally activated cavitation mechanism, generating mechanical hysteresis that defines the system’s function. These phenomena are systematically explored under several compression modes (gas and liquid) and applied to materials with distinct surface properties, both hydrophilic and hydrophobic.
The joint evolution of matrix deformations and intrusion profiles is established for the ZIF-8 lattice. Neutron diffraction coupled with DISCUS simulations demonstrates that intrusion is accompanied by a structural phase transition of the material. Flexibility and surface chemistry thus dictate the overall mechanical response.
Furthermore, the study of the fluid within the hydrophilic pores shows that isobaric measurements lead to the same ice structures under pressure as in the hydrophobic pores. While the Gibbs-Thomson effect is well known at atmospheric pressure (P_{atm}) and confirmed by our values, its formulation differs significantly under high pressure due to the thermodynamic compensation induced by the internal pressure.