Heat capacities of 0.0150 mol·kg −1 NaCl(aq) from 604 to 718 K at 32 MPa

Abstract

The difference in heat capacity between water and 0.0150 mol·kg −1 NaCl(aq) was measured from 604 to 718 K at 32 MPa with a modified flow calorimeter. The apparent molar heat capacities of NaCl(aq) calculated from these results drop quickly to a minimum. C p, φ = −18000 J·K −1·mol −1 at 668 K, rise to a maximum C p, φ = 12000 J·K −1·mol −1 at 686 K, and then decrease. These very large and rapidly varying effects are due to the proximity to the critical point of the solvent. The qualitative behavior of C p, φ is predicted by theories of dilute solutions near the critical point of the solvent. The magnitude of the effects are not predicted by these theories. These are the first measurements of heat capacities of aqueous solutions near the critical point of water.