Temperature Invariance of NaCl Solubility in Water: Inferences from Salt–Water Cluster Behavior of NaCl, KCl, and NH₄Cl

The growth and stability of salt–water clusters have been experimentally studied in aqueous solutions of NaCl, KCl, and NH₄Cl from dilute to near-saturation conditions employing dynamic light scattering and zeta potential measurements. In order to examine cluster stability, the changes in the cluster sizes were monitored as a function of temperature. Compared to the other cases, the average size of NaCl–water clusters remained almost constant over the studied temperature range of 20–70 °C. Information obtained from the temperature-dependent solution compressibility (determined from speed of sound and density measurements), multinuclear NMR (¹H, ¹⁷O, ³⁵Cl NMR), and FTIR were utilized to explain the cluster behavior. Comparison of NMR chemical shifts of saturated salt solutions with solid-state NMR data of pure salts, and evaluation of spectral modifications in the OH stretch region of saturated salt solutions as compared to that of pure water, provided important clues on ion pair–water interactions and water structure in the clusters. The high stability and temperature independence of the cluster sizes in aqueous NaCl shed light on the temperature invariance of its solubility.