(720f) Heat Capacity Measurements of Water At Negative Pressures

Liquid water exhibits many anomalous properties.  Despite extensive study, the origin of these anomalies remains unclear.  Among the most intriguing of these properties are the measured divergences in thermodynamic and dynamic parameters of liquid water in the supercooled state[1].  Several observations motivate the pursuit of analogous measurements in the stretched, superheated state of liquid water:  1) there is a dearth of experimental data of any type in this regime[2], 2) theoretical[3]  and computational[4] studies point to the possibility of unusual features in the phase diagram at negative pressures, and 3) controversy remains about the locations and shapes of the kinetic stability limit and the spinodal that bound this metastable regime[5]. In this presentation, we will report on our measurements of the heat capacity of water in this stretched regime. 

Our method exploits the metastable equilibrium between liquid water and sub-saturated vapors[6]:  macroscopic voids within an organic hydrogel are filled with liquid water and allowed to equilibrate through the gel with vapors of controlled activity.  As water leaves the voids, the liquid is stretched and its pressure decreases until the liquid has reached the chemical potential defined by the vapor.  This technique allows for macroscopic volumes of liquid water to be put into a stretched state at well-defined temperature and chemical potential.   We will describe the synthesis of such samples and the experimental methods that allow for the extraction of the heat capacity of water from the composite (gel/water) material with a commercial calorimeter.   We will present heat capacity as a function of temperature and chemical potential and compare with predictions based on extrapolations of an empirical equation of state.  Finally, we will conclude with a discussion of the relevance of these measurements to the global understanding of water’s thermodynamic properties. 

[1]  C. A.  Angell, Ann. Rev. Phys. Chem 34 593 (1983)

[2]  K. Davitt et al,  J. Chem. Phys 133 174507 (2010)

[3]  P. H. Poole et al, Phys.Rev.Lett 73 12 (1994)

[4]  I. Brovchenko et al, J. Chem. Phys 123 044515 (2005)

[5]  F. Caupin and E. Herbert, C.R. Physique 7 1000 (2006)

[6]  T. D. Wheeler and A. D. Stroock, Nature 455 208 (2008)