13398
J. Phys. Chem. B 2007, 111, 13398-13403
Using Heat Capacity and Compressibility To Choose among Two-State Models of Liquid Water Terry S. Carlton* Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074 ReceiVed: May 29, 2007; In Final Form: September 17, 2007
We extend to heat capacity Cp the model of Vedamuthu, Singh, and Robinson (J. Phys. Chem. 1994, 98, 2222). This model and that of Bartell (J. Phys. Chem. B 1997, 101, 7573) fit successfully, even in the supercooled region, the temperature dependence of Cp, volume, and isothermal compressibility κT. The Robinson model is superior for κT. Tanaka’s model (J. Chem. Phys. 2000, 112, 799) fails for Cp even after correction of a derivational error. All three models assume that the liquid consists of low-density component 1 and high-density component 2. We conclude that Robinson’s tactics, ignoring the intercomponent equilibrium constant and determining compositions solely from volumes, yield the most reliable compositions and individual-component properties. Our fits of the Robinson model to Cp yield at 0 °C H2 - H1 of (135 ( 35) J/g, H1 - Hice of 0.8∆Hfus, and C2 - C1 of (0.1 ( 0.7) J/K‚g. The enthalpy difference between the components is largely responsible for the rapid change of Cp at the lowest supercooled temperatures. We propose an adjustment to Speedy and Angell’s (J. Chem. Phys. 1976, 65, 851) experimental values of κT for supercooled water.
Introduction Water is a very unusual liquid. It has the highest heat capacity per gram of any room-temperature liquid except D2O, and its volume increases on freezing. Its volume V, isobaric heat capacity Cp, and isothermal compressibility κT have minima at 4, 35, and 46 °C, respectively. In supercooled (
