Preprints
https://doi.org/10.5194/egusphere-2022-526
https://doi.org/10.5194/egusphere-2022-526
 
17 Aug 2022
17 Aug 2022
Status: this preprint is open for discussion.

Ice nucleation by smectites: The role of the edges

Anand Kumar1,2, Kristian Klumpp2, Chen Barak3, Giora Rytwo3,4, Michael Plötze5, Thomas Peter2, and Claudia Marcolli2 Anand Kumar et al.
  • 1Department of Mechanical Engineering, The University of British Columbia, Vancouver V6T1Z1, Canada
  • 2Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, 8092, Switzerland
  • 3Environmental Physical Chemistry Laboratory, MIGAL Galilee Research Center, Kiryat Shmona 1101600, Israel
  • 4Environmental & Water Sciences Departments, Tel Hai College, Upper Galilee 1220800, Israel
  • 5Institute for Geotechnical Engineering, ETH Zurich, Zurich 8093, Switzerland

Abstract. Smectites, like other clay minerals, have been shown to promote ice nucleation in immersion freezing mode and likely contribute to the population of ice nucleating particles (INPs) in the atmosphere. Smectites are layered aluminosilicates, which form platelets that depending on composition might swell or even delaminate in water by intercalation of water molecules between their layers. They comprise among others montmorillonites, hectorites, beidellites, and nontronites. In this study, we investigate the ice nucleation (IN) activity of a variety of natural and synthetic smectite samples with different exchangeable cations. The montmorillonites STx-1b and SAz-1, the nontronite SWa-1, and the hectorite SHCa-1 are all rich in Ca2+ as the exchangeable cation, the bentonite MX-80 is rich in Na+ with a minor contribution of Ca2+, the synthetic Laponite is a pure Na+ smectite. The bentonite SAu-1 is rich in Mg2+ with a minor contribution of Na+, and the synthetic interstratified mica-montmorillonite Barasym carries NH4+ 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 quite large variation in IN activity, yet without clear correlation with the exchangeable cation, with the type of smectite, or with mineralogical impurities in the samples. To further investigate the role of the exchangeable cation, we performed ion exchange experiments. Replacing NH4+ with Ca2+ in Barasym reduced its IN activity to the one of other Ca-rich montmorillonites. In contrast, stepwise exchange of the native cations in STx-1b once with Y3+ and once with 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 with time, which we attribute to the delamination of smectites in aqueous suspensions, which may proceed 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 ice nucleation in smectites, 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. Comparison with reported platelet thicknesses of the investigated smectite samples suggests that the observed freezing temperatures are indeed limited by the surface area provided by the mostly very thin platelets. Specifically, Laponite, which did not show any IN activity, is known to delaminate into single layers of about 1 nm thickness, which would be too thin to host a critical ice embryo.

Anand Kumar et al.

Status: open (until 28 Sep 2022)

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Ice nucleation by smectites: The role of the edges Anand Kumar, Kristian Klumpp, Chen Barak, Giora Rytwo, Michael Plötze, Thomas Peter, Claudia Marcolli https://doi.org/10.3929/ethz-b-000554119

Anand Kumar et al.

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Short summary
Smectites are a major class of clay minerals that are ice nucleation (IN) active. They form platelets that swell or even delaminate in water by intercalation of water between their layers. 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. Edge sites on such clay particles play a crucial role in imparting IN ability to such particles.