The thermodynamical response functions and the origin of the anomalous behavior of liquid water

Abstract

The density maximum of water dominates the thermodynamics of the system under ambient conditions, is strongly P-dependent, and disappears at a crossover pressure P_cross ∼ 1.8 kbar. We study this variable across a wide area of the T–P phase diagram. We consider old and new data of both the isothermal compressibility K_T(T, P), the pressure constant specific heat C_P(T) and the coefficient of thermal expansion α_P (T, P). We observe that K_T(T) shows a minimum at T* ∼ 315 ± 5 K for all of the studied pressures, whereas, at the same temperature, C_P(T) has the minimal variation as a function of P in the interval 1 bar–4 kbar. We find the behavior of α_P also to be surprising: all the α_P(T) curves measured at different P cross at T*. The experimental data show a “singular and universal expansivity point” at T* ∼ 315 K and α_P(T*) ≃ 0.44 10⁻³ K⁻¹. Unlike other water singularities, we find this temperature to be thermodynamically consistent in the relationship connecting the three response functions. By considering also the P–T behavior of the self-diffusion coefficient D_S and of the NMR proton chemical shift δ we have the information that at T* the water local order points out, with decreasing T, the crossover from a normal fluid to the anomalous and complex liquid characterized by the many anomalies.