Structural features of concentrated aqueous NaCl solution in the sub- and supercritical state at different densities

The structural properties of 4.37 m NaCl(aq) at sub- and supercritical conditions (1. p = 25 MPa, Т = 450-750 K, ρ = 1.054-0.188 g/cm3; 2. p = 250 MPa, Т = 823 K, ρ = 0.867 g/cm3) are studied by the extended RISM integral equation method. The behavior of pair correlation functions and partial coordination numbers is analyzed. The structural features of the investigated system are discussed in the framework of a clustering hypothesis. According to the obtained results the solution structure transforms from a state characterized by hydrated ions and contact ion pairs (in the subcritical region) to a structure where non-H-bonded water molecules and low-molecular weight cluster conformations predominate (in the low density supercritical region). The structural features of the highly dense supercritical NaCl solution can be defined by free H2O molecules and larger clusters.