Heterogeneous ice nucleation and water-activity-based nucleation theory

Background: homogeneous ice nucleation and water activity

In a recent study, Koop et al. (2000) described a new thermodynamical model for homogeneous ice nucleation of supercooled aqueous solutions. Considering an extensive set of experimental data, the authors concluded that homogeneous ice nucleation depends not on the nature of a solute, but solely on the water activity, a_w, of the solution. a_w(T) is defined as the quotient of the vapor pressure of the solution and the water pressure of pure water at the same temperature. At the ice melting point, the aqueous solution and the ice are in equilibrium and hence their vapor pressures are equal. Thus, the water activity at its ice melting point, a_w(T_m), of any given aqueous solutions can be calculated exactly. At the ice freezing point, on the other hand, the above mentioned equilibrium conditions are no longer fulfilled. Furthermore no measurements of a_w are to my knowledge available in the supercooled regime. However, Koop et al. (2000) found that a_w for a series of solutions show just a slight temperature dependence above the ice melting point. If this trend is extrapolated below the ice melting point, a_w at the ice freezing point (T_f,hom) can be equated to a_w(T_m).

Heterogeneous ice nucleation

Heterogeneous ice freezing experiments of samples containing various ice nuclei (IN) have been investigated, whereas all measurements represents the immersion mode. The results are published in a recent manuscript:

Zobrist et al., Heterogeneous ice nucleation in aqueous solutions: the role of water activity, J. Phys. Chem. A, 112, 3965-3975, 2008.article online

One (nonadecanol) and three inorganic IN (silver iodide, synthesized silica particles ans mineral dust) have been used, which exhibit different chemical and physical surface properties. The aqueous solutions exist of various inorganic (e.g., ammonium sulfate, sodium chloride), organic (e.g., malonic acid) or of mixtures of organic/inorganic solutes (malonic acid/sodium chloride), respectively. The results for the different IN show a consistent picture, indicating that the heterogeneous ice freezing temperatures are generally decreasing with decreasing water activity of the solution, which is similar to homogeneous ice nucleation. The study also highlights that a constant offset with respect to the ice melting curve, Δa_w,het, describes the data series for each ice nucleus very well, whereas Δaw,het exhibits values of 0.101, 0.173, 0.181 and 0.195 for nonadecanol, silica, silver iodide and Arizona test dust, respectively. The results of this study confirm the conclusion that the kinetically driven heterogeneous freezing process can be properly described by thermodynamic quantities for ice nucleation in the immersion mode.