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Direct Raman evidence for a weak continuous phase transition in liquid water
Natalie K Alphonse1, Stephanie R Dillon, Ralph C Dougherty
1Department of Chemistry and Biochemistry and of Mathematics, Florida State University, Tallahassee, 32306-4390, USA.
The Journal of Physical Chemistry. A
|June 16, 2006
Summary
This study reveals a liquid-phase transition in water near 34.6°C, indicated by Raman depolarization ratios. This transition involves changes in water
Area of Science:
- Physical Chemistry
- Materials Science
- Thermodynamics
Background:
- Liquid water exhibits complex structural behavior.
- Previous studies suggest anomalies in water's properties near room temperature.
- Understanding water's phase transitions is crucial for various scientific disciplines.
Purpose of the Study:
- To investigate the presence and nature of a liquid-phase transition in degassed ultrapure water.
- To correlate Raman depolarization ratio data with known thermal and viscous properties of water.
- To identify the structural changes associated with the observed phase transition.
Main Methods:
- Measurement of Raman depolarization ratio of degassed ultrapure water as a function of temperature (303.4–314.4 K).
- Analysis of the data in conjunction with literature values for heat capacity and viscosity.
- Interpretation of results in the context of water's molecular structure and connectivity.
Main Results:
- A distinct phase transition in liquid water was identified at 307.7 K (34.6°C) and 5.8 kPa.
- The transition is characterized as weak-continuous, aligning with a shallow minimum in heat capacity.
- Changes in the pressure coefficient of viscosity and coordination shell measurements support a structural shift involving tetrahedrally coordinated water.
Conclusions:
- Liquid water undergoes a liquid-liquid phase transition near 34.6°C.
- Tetrahedrally coordinated water structures are implicated in this transition, with planar structures dominating above it.
- Water is a structured liquid with temperature-dependent local structures, exhibiting distinct phases.
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