On the behavior of water confined in hydrophobic nanopores
Dr. Alberto Giacomello works at the University of Rome "La Sapienza" and is the recipient of a starting ERC grant:
Water in extreme hydrophobic confinement can vaporize at ambient conditions; this behavior is of utmost relevance for a number of biological and technological issues, including hydrophobic gating of ion channels , superhydrophobic surfaces capable of self-recovery [2,3], and porous materials for energy applications . The mechanism and the kinetics of nucleation of the vapor phase sensitively depends on the level of hydrophobicity, on the geometry, and on the size of confinement [3,5]. In this context, nanoporous materials immersed in water represent a powerful means to characterize the phase behavior of water in extreme confinement by relatively simple macroscopic measurements, to which atomistic simulations or theory lend a microscopic interpretation.
In this contribution we explore by means of experiments, theory, and rare-event molecular dynamics the effect of pore morphology on the spontaneous extrusion of liquids from hydrophobic nanopores . High-pressure water intrusion and extrusion experiments performed on two porous materials with similar nominal diameter and hydrophobicity showed qualitatively different extrusion behaviors: the first material with cylindrical, independent pores displayed irreversible liquid intrusion, while the second one, characterized by interconnected pores, exhibited extrusion (vapor nucleation) at pressures as large as few megapascals. Leveraging macroscopic capillary models and extensive molecular dynamics simulations we propose an explanation of this peculiar behavior based on the internal morphology of the pores and, in particular, on the presence of small-scale roughness or pore interconnections in the second materials.
Additional experiments with mercury confirmed that this mechanism is generic for nonwetting liquids and is indeed connected to the pore topology. Altogether the present results suggest a rational way to design heterogeneous systems for energy and nanofluidic applications in which the extrusion behavior can be controlled via the pore morphology and the liquid characteristics.
If time allows, the relevance of results [5,6] for biological phenomena, and in particular for hydrophobic gating of ion channels, will be discussed. This is the subject of the newly started ERC project HyGate which exploits multiscale simulation techniques to investigate the mechanisms through which biological ion channels switch on and off ionic currents, with the final goal of designing bioinspired nanosensors.