Phase Diagram of Water Confined by Graphene

Zhenghan Gao,Nicolas Giovambattista,Ozgur Sahin,Nicolas Giovambattista,Ozgur Sahin

doi:10.1038/s41598-018-24358-3

Zhenghan Gao, Nicolas Giovambattista + Show 3 more

Open Access

https://doi.org/10.1038/s41598-018-24358-3

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Journal: Scientific Reports	Publication Date: Apr 18, 2018
Citations: 63	License type: open-access

Affiliation: Columbia University

Abstract

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene.

Highlights

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination
In order to improve our understanding of nanoconfined water, we study systematically the phase diagram of water confined by two parallel graphene sheets
We describe in detail the different phase transitions observed in water confined by graphene sheets

Summary

Introduction

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Common confining model surfaces include detailed realistic surfaces, such as carbon nanotubes (CNTs)[11,21,34,35,36], graphene sheets[23,31,32,37,38], SiO233,39, and MoS2 nanopores[40,41], which have potential applications in water desalination and purification[41,42], as well as model surfaces, such as unstructured smooth confining walls[32,43,44] These and other studies show that the unique properties of nanoconfined water depend strongly on the confining geometry and dimensions[36], and characteristics of the confining surfaces, such as chemistry[45], structure[33], and curvature[46]. Another recent study included the effects of varying D at constant temperature, though the range was narrow (0.65 nm < D < 0.75 nm) and lateral pressures were high (>500 MPa)[54]

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