Thermodynamic and structural anomalies of water nanodroplets

Shahrazad M A Malek,Ivan Saika-Voivod,Peter H Poole

doi:10.1038/s41467-018-04816-2

Shahrazad M A Malek, Ivan Saika-Voivod + Show 1 more

Open Access

https://doi.org/10.1038/s41467-018-04816-2

Copy DOI

Abstract

Liquid water nanodroplets are important in earth’s climate, and are valuable for studying supercooled water because they resist crystallisation well below the bulk freezing temperature. Bulk liquid water has well-known thermodynamic anomalies, such as a density maximum, and when supercooled is hypothesised to exhibit a liquid–liquid phase transition (LLPT) at elevated pressure. However, it is not known how these bulk anomalies might manifest themselves in nanodroplets. Here we show, using simulations of the TIP4P/2005 water model, that bulk anomalies occur in nanodroplets as small as 360 molecules. We also show that the Laplace pressure inside small droplets reaches 220 MPa at 180 K, conditions close to the LLPT of TIP4P/2005. While the density and pressure inside nanodroplets coincide with bulk values at moderate supercooling, we show that deviations emerge at lower temperature, as well as significant radial density gradients, which arise from and signal the approach to the LLPT.

Highlights

Liquid water nanodroplets are important in earth’s climate, and are valuable for studying supercooled water because they resist crystallisation well below the bulk freezing temperature
Bulk samples of liquid water crystallise at a homogeneous nucleation temperature TH, which to date has prevented the direct observation of the liquid–liquid phase transition (LLPT) predicted to occur at lower T
From the Young–Laplace equation PL = 2γ/R, where R is the droplet radius and γ is the surface tension, PL inside a 1 nm droplet should exceed 102 MPa19, 20. This is high enough to approach the range of P in which the critical point of the proposed LLPT is estimated to occur in bulk water[14, 21]

Summary

Introduction

Liquid water nanodroplets are important in earth’s climate, and are valuable for studying supercooled water because they resist crystallisation well below the bulk freezing temperature. The crystallisation of pure water nanodroplets has attracted particular interest because the temperature at which freezing is observed, relative to bulk water, decreases dramatically with size, reaching 202 K for nanodroplets of radius 3.2 nm[7]. Both the specific heat and the isothermal compressibility of the liquid increase strongly as T decreases To account for these anomalies, several thermodynamic scenarios have been proposed, including the hypothesis that a liquid-liquid phase transition (LLPT) occurs in deeply supercooled water[13, 14]. Despite the importance of liquid water nanodroplets, and their potential to help clarify the behaviour of bulk water, relatively little is known of their fundamental thermophysical properties This is due to the significant experimental challenges associated with studying liquid nanodroplets that are not in contact with a supporting or confining surface.

Methods

Results

Conclusion