High-Pressure Infiltration–Expulsion of Aqueous NaCl in Planar Hydrophobic Nanopores

Abstract

Pressure-driven permeation of water in a poorly wettable material results in a conversion of mechanical work into surface free energy representing a new form of energy storage or absorption. When water is replaced by a concentrated electrolyte solution, the storage capacity of a nanoporous medium becomes comparable to high-end supercapacitors. The addition of salt can also reduce the hysteresis of the infiltration–expulsion cycle. Our molecular simulations provide a theoretical perspective into the mechanisms involved in the process and underlying structures and interactions in compressed nanoconfined solutions. Specifically, we consider aqueous NaCl in planar confinements of widths of 1.0 and 1.64 nm and pressures of up to 3 kbar. Open ensemble Monte Carlo simulations utilizing fractional exchanges of molecules for efficient addition–removal of ions have been utilized in conjunction with pressure-dependent chemical potentials to model bulk phases under pressure. Confinements open to these pressurized bulk, aqueous electrolyte phases show reversibility at narrow pore sizes and irreversibility in wider ones, consistent with experiment, as well as strong hysteresis at both pore size. The addition of salt results in significant increases in the solid–liquid interfacial tension in narrower pores and associated infiltration and expulsion pressures. These changes are consistent with strong desalination effects at the lower pore size observed irrespective of external pressure and initial concentration.

Predictive Thermodynamic Model for Intrusion of Electrolyte Aqueous Solutions in Nanoporous Materials

“…Then, ions penetrate at a much higher pressure of respectively 260 MPa and 140-240 MPa. This is supported by previous MC simulations performed by Zamfir and co-workers 54 where NaCl electrolyte penetrates a two-dimensional planar hydrophobic nanopores once water has been intruded inside it. The initial step consisting of intrusion of only water molecules is crucial for energy storage applications.…”

“…Then, we calculated the chemical potential in bulk electrolyte solutions containing 150 particles (composition given in Table S5), as in our previous study but for pressure ranging from 0.006 to 1000 MPa. Because concentration depends explicitly on the volume of the solution, it is more convenient to use molality as the relevant metric . This quantity will be required as input to set up the osmostat in zeosils filled with water molecules at different pressure.…”

“…Several hypotheses have been proposed such as ion desolvation, 20 osmotic effect, 41 confinement effect 51,52 or increase of solid−liquid interfacial tension. 53,54 Recently, it has been shown that intrusion of NaCl electrolyte solution in chabazite and LTA-type zeosils occurs in two steps. 21,30 First, only water molecules penetrate into the nanocages at 44 and 36 MPa in chabazite and LTA-type zeosils, respectively.…”

“…Although the mechanism of water intrusion in hydrophobic microporous materials has been highlighted by grand canonical Monte Carlo (GCMC) simulations, ,, the mechanism of electrolyte intrusion is still under debate; it is characterized by an increase of intrusion pressure that depends both on electrolyte nature and concentration. Several hypotheses have been proposed such as ion desolvation, osmotic effect, confinement effect , or increase of solid–liquid interfacial tension. , Recently, it has been shown that intrusion of NaCl electrolyte solution in chabazite and LTA-type zeosils occurs in two steps. , First, only water molecules penetrate into the nanocages at 44 and 36 MPa in chabazite and LTA-type zeosils, respectively. Then, the ions penetrate at a much higher pressure of 260 and 140–240 MPa, respectively.…”

Predictive Thermodynamic Model for Intrusion of Electrolyte Aqueous Solutions in Nanoporous Materials

“…Several hypotheses have been proposed such as ion desolvation, 20 osmotic effect, 41 confinement effect 51,52 or increase of solid-liquid interfacial tension. 53,54 Recently, it has been shown that intrusion of NaCl electrolyte solution in chabazite and LTA-type zeosils occurs in two steps. 21,30 First, only water molecules penetrate into the nanocages at respectively 44 and 36 MPa in chabazite and LTA-type zeosils.…”