H2O-NH4Cl system: Thermodynamics and structure of supersaturated (8A•28m) solutions at 298.15 K

Thermodynamic characteristics are determined for supersaturated (8A center dot 28m) aqueous ammonium chloride solutions at 298.15 K: activity as well as partial molar enthalpy, volume, and excess entropy of water, osmotic coefficients, density, and salt hydration numbers. The relative humidity above the solution decreases from 75.6% (8m solution) to 44.7% (28m solution). A directly proportional dependence is found between the enthalpy and entropy characteristics. The negative values of the nonideal portion of the Gibbs energy are due to structural effects (excess entropy). The excess entropy of water has the same value 1.8 J/(mol center dot K) in supersaturated 12m NaCl and 28m NH4Cl. The ratio 12/28 = 3/7 is also valid for other "isoentropic" molalities of the said salts.