Ice nucleation by smectites: the role of the edges
<p>Smectites, like other clay minerals, have been shown to promote ice nucleation in the 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 composit...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/4881/2023/acp-23-4881-2023.pdf |
| Summary: | <p>Smectites, like other clay minerals, have been shown to promote ice
nucleation in the 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 Ca<span class="inline-formula"><sup>2+</sup></span> as the exchangeable cation; the bentonite
MX-80 is rich in Na<span class="inline-formula"><sup>+</sup></span> with a minor contribution of Ca<span class="inline-formula"><sup>2+</sup></span>, and the
synthetic Laponite is a pure Na<span class="inline-formula"><sup>+</sup></span> smectite. The bentonite SAu-1 is rich
in Mg<span class="inline-formula"><sup>2+</sup></span> with a minor contribution of Na<span class="inline-formula"><sup>+</sup></span>, and the synthetic
interstratified mica-montmorillonite Barasym carries
NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a896672a5a2c6d19cc4695ffc843bfde"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-4881-2023-ie00001.svg" width="8pt" height="15pt" src="acp-23-4881-2023-ie00001.png"/></svg:svg></span></span> as the exchangeable cation. In
emulsion freezing experiments, all samples except Laponite exhibited one or
two heterogeneous freezing peaks with onsets between 239 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
NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a9b2fdba183dceff94210c316afa95ef"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-4881-2023-ie00002.svg" width="8pt" height="15pt" src="acp-23-4881-2023-ie00002.png"/></svg:svg></span></span> with Ca<span class="inline-formula"><sup>2+</sup></span> in Barasym reduced
its IN activity to that of other Ca-rich montmorillonites. In contrast,
stepwise exchange of the native cations in STx-1b once with Y<span class="inline-formula"><sup>3+</sup></span> and
once with Cu<span class="inline-formula"><sup>2+</sup></span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |