Temperature-Dependent Solubility Transition of Na₂SO₄ in Water and the Effect of NaCl Therein: Solution Structures and Salt Water Dynamics

Dual, aqueous solubility behavior of Na₂SO₄ as a function of temperatures is still a natural enigma lying unresolved in the literature. The solubility of Na₂SO₄ increases up to 32.38 °C and decreases slightly thereafter at higher temperatures. We have thrown light on this phenomenon by analyzing the Na₂SO₄–water clusters (growth and stability) detected from temperature-dependent dynamic light scattering experiments, solution compressibility changes derived from the density and speed of sound measurements, and water structural changes/Na₂SO₄ (ion pair)–water interactions observed from the FT-IR and 2D DOSY ¹H NMR spectroscopic investigations. It has been observed that Na₂SO₄–water clusters grow with an increase in Na₂SO₄ concentration (until the solubility transition temperature) and then start decreasing afterward. An unusual decrease in cluster size and solution compressibility has been observed with the rise in temperature for the Na₂SO₄ saturated solutions below the solubility transition temperature, whereas an inverse pattern is followed thereafter. DOSY experiments have indicated different types of water cluster species in saturated solutions at different temperatures with varying self-diffusion coefficients. The effect of NaCl (5–15 wt %) on the solubility behavior of Na₂SO₄ at different temperatures has also been examined. The studies are important from both fundamental and industrial application points of view, for example, toward the clean separation of NaCl and Na₂SO₄ from the effluent streams of textile and tannery industries.