Frustration vs Prenucleation: Understanding the Surprising Stability of Supersaturated Sodium Thiosulfate Solutions

Gibbs classical nucleation theory predicts that a supersaturated solution will have transient nuclei that flitter in and out of existence. Only when one of these nuclei becomes larger than a critical size, will the solution crystalize. Recently, nonclassical nucleation theories have invoked the presence of prenuclei possibly associated with a liquid-liquid phase separation. However, there are few experimental observations of such prenuclei. Here, we use ultrafast optical Kerr-effect spectroscopy to measure the temperature-dependent low-frequency (sub-gigahertz to terahertz) anisotropic Raman spectra of supersaturated aqueous sodium thiosulfate solutions. Clear evidence of clusters is obtained in the spectra. However, on the basis of the inferred stability of these clusters, it appears that they frustrate rather than promote the formation of crystals. This would explain the surprising stability of supersaturated sodium thiosulfate and similar solutions.