Isothermal compressibility of supercooled water and evidence for a thermodynamic singularity at −45°C

Using a capillary technique for small samples, the isothermal compressibility κT of water has been measured to −26°C. Accelerating increases of κT at the lower temperatures can be described by an expression of the form κT=Aεγ where ε= (T−Ts)/Ts, which is known to describe anomalies encountered in the vicinity of a thermodynamic singularity located at Ts. The implication that the thermodynamic and certain other properties of water at lower temperatures may be decomposed into a normal component and an anomalous component which diverges at Ts=−45°C is supported by analysis of numerous other thermodynamic and relaxation data which extend into the supercooled regime. The anomalous characteristics are shown to originate primarily in the sensitivity of the volume to temperature changes, suggesting a geometrical basis for the cooperative behavior. The singularity at Ts=−45°C may be a lambda transition associated with the cooperative formation of an open hydrogen-bonded network, but the near coincidence of Ts with the experimental homogeneous nucleation temperature suggests, as an alternative, that Ts may correspond to the limit of mechanical stability for the supercooled liquid phase.