Mechanism of Water Freezing in Solutions: Solutes Affect the Formation of Critical Ice Nuclei

The microscopic mechanisms by which solutes modulate water freezing are fundamental for controlling the freezing of various environmental and cryobiotic systems. Although our understanding of the initiation mechanisms of pure water freezing is becoming clearer, the microscopic pictures regarding ice nucleation in complex systems such as solutions still rely on theory assumption and empirical formulation. Here, we experimentally demonstrate that solutes modulate water freezing through affecting critical ice nucleus formation. Upon addition of a solute, which can be quantified by the single parameter of water activity (a_w), critical ice nuclei are more difficult to form because the solute increases the critical ice nucleus radius (r*) by decreasing a_w. The value of r* that can be applied to solution systems depends firmly on not only the nucleation temperature but also a_w. Furthermore, using molecular dynamics simulations, we give a microscopic picture of ice nucleus formation in solution and explain the underlying reasons for solute-induced changes in r*.