Confined Water: Structure, Dynamics, and Thermodynamics

Abstract

Understanding the properties of strongly confined water is important for a variety of applications such as fast flow and desalination devices, voltage generation, flow sensing, and nanofluidics. Confined water also plays an important role in many biological processes such as flow through ion channels. Water in the bulk exhibits many unusual properties that arise primarily from the presence of a network of hydrogen bonds. Strong confinement in structures such as carbon nanotubes (CNTs) substantially modifies the structural, thermodynamic, and dynamic (both translational and orientational) properties of water by changing the structure of the hydrogen bond network. In this Account, we provide an overview of the behavior of water molecules confined inside CNTs and slit pores between graphene and graphene oxide (GO) sheets. Water molecules confined in narrow CNTs are arranged in a single file and exhibit solidlike ordering at room temperature due to strong hydrogen bonding between nearest-neighbor molecules. Although molecules constrained to move along a line are expected to exhibit single-file diffusion in contrast to normal Fickian diffusion, we show, from a combination of molecular dynamics simulations and analytic calculations, that water molecules confined in short and narrow CNTs with open ends exhibit Fickian diffusion because of their collective motion as a single unit due to strong hydrogen bonding. Confinement leads to strong anisotropy in the orientational relaxation of water molecules. The time scale of relaxation of the dipolar correlations of water molecules arranged in a single file becomes ultraslow, of the order of several nanoseconds, compared with the value of 2.5 ps for bulk water. In contrast, the relaxation of the vector that joins the two hydrogens in a water molecule is much faster, with a time scale of about 150 fs, which is about 10 times shorter than the corresponding time scale for bulk water. This is a rare example of confinement leading to a speedup of orientational dynamics. The orientational relaxation of confined water molecules proceeds by angular jumps between two locally stable states, making the relaxation qualitatively different from that expected in the diffusive limit. The spontaneous entry of water inside the hydrophobic cavity of CNTs is primarily driven by an increase in the rotational entropy of water molecules inside the cavity, arising from a reduction in the average number of hydrogen bonds attached to a water molecule. From simulations using a variety of water models, we demonstrate that the relatively simple SPC/E water model yields results in close agreement with those obtained from polarizable water models. Finally, we provide an account of the structure and thermodynamics of water confined in the slit pore between two GO sheets with both oxidized and reduced parts. We show that the potential of mean force for the oxidized part of GO sheets in the presence of water exhibits two local minima, one corresponding to a dry cavity and the other corresponding to a fully hydr...

Electrocrystallization of supercooled water confined by graphene walls

“…Understanding the mechanisms of phase transitions in confined water is one of open problems in condensed matter physics, which stimulates performance of numerous experimental, numerical and theoretical studies [1][2][3][4][5][6][7][8] . So, investigations of the properties of water in a confinement attract the special attention of scientists because of its importance for understanding many biological and geological processes, such as the microscopic processes in membranes and cells; transport processes in pores; capillary phenomena; dynamic processes in the depths of satellites of some planets of the Solar system 9,10 .…”

Possible relations between supercooled and glassy confined water and amorphous bulk ice

“…The size and dimensionality of the cavity as well as the structure, hydrophobicity and roughness of its inner surface all affect the structure and dynamics of the confined water. [6][7][8][9][10][11][12][13][14][15][16] However, in the deeply supercooled regime, experimental results have shown that the dynamics of confined water is rather universal, irrespective of whether the water is located in a solid porous material, hydrating a biomolecule or in a solution below or close to the glass transition temperature, T g , of the entire system. 17 In this paper we will mainly discuss results obtained from differential scanning calorimetry (DSC), dielectric spectroscopy, quasielastic neutron scattering (QENS) and relaxation nuclear magnetic resonance (NMR) measurements.…”

Acyclic Janus‐AT Nucleoside Host Channels Precisely Lock Water into Single‐File Wires with Local Rotational Flexibility

“…The behavior of water molecules in nano‐sized confinement has drawn intensive research interest . Very little research concerning the dynamic behavior of the water wire within precise hydrophilic confinement has been reported.…”