Yaroslav Grosu

Evolution of the energetic characteristics of {silicalite-1 + water} repulsive clathrates in a wide temperature range

Recently {lyophobic porous powders + liquid} systems were proposed to be used for nontraditional energy storage and conversion purposes. This article reports the experimental study of the mechanical behavior, within the pressure-volume (PV) diagram, of the {hydrophobic silicalite-1 + water} system in the temperature range 10-80 °C. Repeated recordings of PV-isotherms and thermal effects of the repulsive clathrate during successive compression-decompression runs were performed using scanning transitiometry. An unexpected steady decline in the intrusion-extrusion pressure and volume of embedded water was found during the forced (repeated) intrusion of water into the pores of silicalite-1 and its spontaneous extrusion at constant temperature. A discussion of possible reasons of unconventional behavior of these heterogeneous systems as well as a thermodynamic analysis is presented.

Stability of zeolitic imidazolate frameworks: effect of forced water intrusion and framework flexibility dynamics

Stability of metal–organic frameworks is one of the central issues for their successful usage in an increasingly wide range of applications. Particularly Zeolitic Imidazolate Frameworks (ZIFs) are known for their high stability. Herein we use the two most stable representatives ZIF-8 and ZIF-67 to show that the concomitant effect of pressure and temperature upon water intrusion/extrusion cycles is strikingly higher compared to the separate effects of either pressure or temperature and leads to previously unobserved irreversible structural changes. We also explore the effect of compression–decompression speed on the pronounced breathing effect of indicated ZIFs as part of high-pressure operation and show that framework relaxation time may be very long and should be taken into account for potential applications.

Intrusion and extrusion of water in hydrophobic nanopores

Significance Molecular springs, constituted by nanoporous materials immersed in a nonwetting liquid, are compact, economical, and efficient means of storing energy, owing to their enormous surface area. Surface energy is accumulated during liquid intrusion inside the pores and released by decreasing liquid pressure and thus triggering confined cavitation. State-of-the-art atomistic simulations shed light on the intrusion and extrusion of water in hydrophobic nanopores, revealing conspicuous deviations from macroscopic theories, which include accelerated cavitation, increased intrusion pressure, and reversible intrusion and extrusion processes. Understanding these nanoscale phenomena is the key to a better design of molecular springs as it allows relating the characteristics of the materials to the overall properties of the devices, e.g., their operational pressure and efficiency. Heterogeneous systems composed of hydrophobic nanoporous materials and water are capable, depending on their characteristics, of efficiently dissipating (dampers) or storing (“molecular springs”) energy. However, it is difficult to predict their properties based on macroscopic theories—classical capillarity for intrusion and classical nucleation theory (CNT) for extrusion—because of the peculiar behavior of water in extreme confinement. Here we use advanced molecular dynamics techniques to shed light on these nonclassical effects, which are often difficult to investigate directly via experiments, owing to the reduced dimensions of the pores. The string method in collective variables is used to simulate, without artifacts, the microscopic mechanism of water intrusion and extrusion in the pores, which are thermally activated, rare events. Simulations reveal three important nonclassical effects: the nucleation free-energy barriers are reduced eightfold compared with CNT, the intrusion pressure is increased due to nanoscale confinement, and the intrusion/extrusion hysteresis is practically suppressed for pores with diameters below 1.2 nm. The frequency and size dependence of hysteresis exposed by the present simulations explains several experimental results on nanoporous materials. Understanding physical phenomena peculiar to nanoconfined water paves the way for a better design of nanoporous materials for energy applications; for instance, by decreasing the size of the nanopores alone, it is possible to change their behavior from dampers to molecular springs.