Ice Nucleation By Smectites: The Role of the Clay Mineral Edges

ANAND KUMAR, Kristian Klumpp, Chen Barak, Giora Rytwo, Michael Plötze, Thomas Peter, Claudia Marcolli, Eidgenössische Technische Hochschule Zürich (ETHZ)

     Abstract Number: 698
     Working Group: Aerosols, Clouds and Climate

Abstract
Smectite clay minerals have been shown to promote ice nucleation in the immersion freezing mode and likely contribute to the population of ice nucleating particles in the atmosphere. Smectites are layered aluminosilicates, forming platelets that might swell or even delaminate in water by intercalation of water molecules between their layers. We investigated the ice nucleation (IN) activity of a variety of natural and synthetic smectites with different exchangeable cations. The montmorillonites STx1b and SAz1, nontronite SWa1 and hectorite SHCa1 are rich in Ca²⁺ as the exchangeable cation. Synthetic Laponite is a pure Na⁺ smectite and synthetic mica-montmorillonite Barasym carries ammonium as the exchangeable cation. In emulsion freezing experiments, all samples except Laponite exhibited one or two heterogeneous freezing peaks with onsets between 239 K and 248 K and a large variation in IN activity, yet without clear correlation with exchangeable cation, type of smectite or mineralogical impurities in the samples. Furthermore, replacing with Ca²⁺ in Barasym reduced its IN activity to that of other Ca-rich montmorillonites. Stepwise exchange of the native cations in STx1b with Y³⁺ and Cu²⁺ showed no influence on IN activity. However, aging of smectite suspensions in pure water up to several months revealed a decrease in IN activity, which we attribute to the delamination of smectites in suspensions, which occurs over long timescales. The dependence of IN activity on platelet stacking and thickness can be explained if the hydroxylated chains forming at the edges are the location of IN, since the edges need to be thick enough to host a critical ice embryo. We hypothesize that at least three smectite layers need to be stacked together to host a critical ice embryo on clay mineral edges and that the larger the surface edge area is the higher the freezing temperature.