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New properties of supercooled confined water discovered
12 May 2011
Universidad de Barcelona
Study of a layer of water only one nanometre high confined between two hydrophobic plates. Hydrophobic nanoparticles were then added to the water layer in random positions to generate nanochannels or variable size.
A study led by the UB researcher Giancarlo Franzese and published in the journal Physical Review Letters suggests that hydrophobic nanoconfinement can alter the thermodynamics of water at supercool temperatures. These findings may have important applications in fields related to conservation at cryogenic temperatures (around -100 ºC), for example in the preservation of stem cells, blood and food products. The team behind the study, led by Giancarlo Franzese from the UB’s Department of Fundamental Physics, included researchers from Boston University and TU Berlin.
Water exhibits atypical fluid behaviour. One of its unique characteristics is the increase in heat capacity as water cools, an anomaly that enables us to regulate our body temperature. When water is supercooled – that is, when it is in liquid state at a temperature below its melting point – the range of anomalies expands. This irregular behaviour has generated fierce scientific debate over the last twenty years and could hold the key to understanding why water is so different to other liquids and why it is so important for biological organisms.
From a technical perspective, it is difficult to observe supercooled water directly and many researchers opt to use nanoconfinement. In this study, the team used Monte Carlo simulations to study a layer of water only one nanometre high – approximately equivalent to the diameter of three water molecules – confined between two hydrophobic plates. Hydrophobic nanoparticles were then added to the water layer in random positions to generate nanochannels or variable size.
This process led to a strong decrease in thermodynamic fluctuations, reflected in compressibility, thermal expansion coefficient and specific heat. The observed decrease occurred at all pressures tested, and at pressures in the region of 180 MPa fluctuations dropped by almost 99% for a concentration in nanoparticles of 25% by volume. The reduction was found to be as high as 90% even at a particle concentration ten times lower.
According to Giancarlo Franzese, the results show that the thermodynamic behaviour of water confined in hydrophobic nanochannels is very different to that of unconfined water, even allowing for the possible presence of more than one liquid phase within the range of temperatures and pressures tested.