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Published on-line at 04:39:03 PM on Wednesday, September 18th, 2013
Evidence of two distinct local structures of water from ambient to supercooled conditions
Article Published 13 Sep 2013 Nature Communications A.Taschin, P. Bartolini, R. Eramo, R. Righini & R. Torre
Water can remain liquid below its melting point and stays in its metastable phase known as supercooled water. Several thermodynamic and dynamic observables of liquid water show an anomalous behaviour. The water anomalies are connected with the formation of various structural aggregates whose features are modified by the temperature/pressure conditions. A crucial point is the physical origin of these structural aggregations.
One of the possible explanation, hypothesize the existence of two liquid waters:the low-density (LD) and the high-density (HD) water forms. The LD phase is characterized by tetrahedral intermolecular coordination, whereas the HD one shows more packed structures with distorted networks. A hidden first-order transition between these two water forms would be the origin of the measured anomalous behaviour and critical phenomena.
The liquid water dynamics takes place on an extremely fast time scale, less than picoseconds (10-12 sec). The fast and continuous redefinition of the water local structures makes the experimental capture of the LD and HD species very difficult and there are no experimental compelling evidences of the two water forms to date.
We performed a time-resolved optical Kerr effect based on femto-second laser source, that enable to measure the very fast dynamics of water molecules in an extended temperature range, and with unprecedented data quality. We measured the inter-molecular vibrations and the rearrangement processes in the liquid and supercooled phase of water. The experiment measures the water intermolecular vibrations and the structural relaxation process. The analysis of the experimental data enables to characterize the intermolecular vibrational modes and their interplay with the structural relaxation process. The results bring evidence of the coexistence of two local configurations in water, which are interpreted as high-density and low-density forms.
Correspondence and requests for materials should be addressed to R.T. (email:lens.unifi.it>torrelens.unifi.it).
NATURE COMMUNICATIONS|4:2401|DOI:10.1038/ncomms3401| www.nature.com/naturecommunications
Source: Nature Communication
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