Direct visualization of solute locations in laboratory ice samples

Abstract

Abstract. Many important chemical reactions occur in polar snow, where solutes may be present in several reservoirs, including at the air–ice interface and in liquid-like regions within the ice matrix. Some recent laboratory studies suggest chemical reaction rates may differ in these two reservoirs. While investigations have examined where solutes are found in natural snow and ice, few studies have examined either solute locations in laboratory samples or the possible factors controlling solute segregation. To address this, we used micro-computed tomography (microCT) to examine solute locations in ice samples prepared from either aqueous cesium chloride (CsCl) or rose bengal solutions that were frozen using several different methods. Samples frozen in a laboratory freezer had the largest liquid-like inclusions and air bubbles, while samples frozen in a custom freeze chamber had somewhat smaller air bubbles and inclusions; in contrast, samples frozen in liquid nitrogen showed much smaller concentrated inclusions and air bubbles, only slightly larger than the resolution limit of our images (∼ 2 µm). Freezing solutions in plastic vs. glass vials had significant impacts on the sample structure, perhaps because the poor heat conductivity of plastic vials changes how heat is removed from the sample as it cools. Similarly, the choice of solute had a significant impact on sample structure, with rose bengal solutions yielding smaller inclusions and air bubbles compared to CsCl solutions frozen using the same method. Additional experiments using higher-resolution imaging of an ice sample show that CsCl moves in a thermal gradient, supporting the idea that the solutes in ice are present in mobile liquid-like regions. Our work shows that the structure of laboratory ice samples, including the location of solutes, is sensitive to the freezing method, sample container, and solute characteristics, requiring careful experimental design and interpretation of results.

The morphology of ice and liquid brine in an environmental scanning electron microscope: a study of the freezing methods

“…ESEMs are therefore becoming increasingly popular within research into pure ice (Nair et al, 2018;Magee et al, 2014;Chen et al, 2017) and ice-impurity interactions under static and dynamically changing conditions (Krausko et al, 2014;Yang et al, 2017). While electron microscopy visualizes the sample surface in detail, microcomputed tomography has been recently applied to investigate solute locations in the frozen bulk (Hullar and Anastasio, 2016). Using this technique, the authors were able to visualize the locations of solutes (CsCl or rose bengal), gas, and ice in 3-D.…”

“…The impurities' locations were most often deduced from the sample preparation methods; for example, freezing an aqueous solution involved the precondition of placing the impurities inside the ice matrix. Subsequent breaking of the ice into small pieces was proposed to bring the impurities to the ice surface (Kahan et al, 2010); this assumption was recently questioned (Hullar et al, 2018). Other studies suggested that freezing an aqueous solution places the impurities prevalently inside the ice interior, whereas deposition of the organics from the vapor phase accommodates them on the ice surface (Ondrušková et al, 2018;Vetráková et al, 2017;Krausko et al, 2015a, b;Kania et al, 2014;Heger et al, 2011;Hullar et al, 2018;Bartels-Rausch et al, 2013).…”

“…The location and time evolution of impurities within a frozen sample are crucial in multiple respects: they determine the mechanical and optical properties of the material and thus must be considered in relation to snowpack stability, avalanches, sea ice mechanics, and climate change research (Blackford et al, 2007;Hobbs, 2010;Dash et al, 2006;Wåhlin et al, 2014). The location of impurities most probably influences the compounds' reactivity because their availability to incoming light radiation and/or gaseous reactants substantially differs depending on whether the impurities are located on the ice surface or buried in the frozen bulk (Hullar and Anastasio, 2016;Bartels-Rausch et al, 2014). However, the information combining the reactivity of compounds with their locations is essentially missing.…”

The Morphology of Ice and Liquid Brine in the Environmental SEM: A Study of the Freezing Methods

“…These brine pools occasionally appear at the locations of caverns formed by bubbles, thus 445 delineating the grain boundary, which would otherwise be difficult to observe. 446 The fact that ice is a very dynamic medium allowing reconstitution at sub-zero temperatures was 454 previously evidenced using, for example, time-lapse X-ray tomography; up to 60 % of the total ice mass 455 recrystallized during 12 hours at -15 °C (Pinzer and Schneebeli, 2009;Hullar and Anastasio, 2016). Our 456 research team is not the first one to have observed disjoint behavior of the ice surface in relation to 457 the underlying ice crystals; fresh ice growing in steps of 100 -4,000 nm to traverse the ice grains 458 regardless of the boundaries was already noticed for planar ice growth (Ketcham and Hobbs, 1968).…”

“…The results have 28 straightforward implications for artificially prepared and naturally occurring salty ices. probably influences the compounds' reactivity as their availability to incoming light radiation and/or 39 gaseous reactants substantially differs depending on whether the impurities are located on the ice 40 surface or buried in the frozen bulk (Hullar and Anastasio, 2016). However, the information combining 41 the reactivity of compounds with their locations is essentially missing.…”