Comparing the ice nucleation properties of the kaolin minerals kaolinite and halloysite

Abstract

Abstract. Heterogeneous ice nucleation on dust particles in the atmosphere is a key mechanism for ice formation in clouds. However, the conditions of a particle surface for efficient ice nucleation are poorly understood. In this study, we present results of immersion freezing experiments using differential scanning calorimetry on emulsified mineral dust suspensions, involving the two chemically identical, but morphologically different, kaolin minerals of kaolinite and halloysite. Kaolinite occurs in a platy morphology, while halloysites form predominantly tubular structures. We investigated six different halloysite and two different kaolinite samples. Our results show that, on average, the halloysite samples not only exhibit a higher ice nucleation (IN) activity than the kaolinite samples but also a higher diversity in terms of freezing onset temperatures and heterogeneously frozen fraction. Repeating the freezing experiments after shortly milling the samples led to a decrease in freezing onset temperatures and in the heterogeneously frozen fraction of the halloysite samples, bringing their IN activity closer to that of the kaolinites. To interpret these findings, the freezing experiments were complemented by dynamic vapor sorption (DVS), BET (Brunauer–Emmett–Teller) surface area measurements, pore ice melting experiments with slurries, and transmission electron microscopy (TEM) before and after milling. These measurements demonstrate an increase in surface area and the destruction of tubes by milling and provide evidence for the influence of the tubular structure of the halloysites on their IN activity. We identify the OH–Al–O–Si–OH functionalized edges as being the most likely site for ice nucleation, as the high geometric diversity of the edges best accounts for the high diversity in IN activity of halloysites. We hypothesize that the stacking of layers and the number of stacks in halloysite tubes and kaolinite platelets affect the freezing temperature, with thicker stacks having the potential to freeze water at higher temperatures. The notion that the edges constitute the IN-active part of kaolin minerals is further supported by comparing kaolin minerals with montmorillonites and feldspars, all of which exhibit enhanced IN activity in the presence of ammonia and ammonium-containing solutions. As OH–Al–O–Si–OH functionalized edge surfaces are the only surface type that kaolin particles have in common with montmorillonites and feldspars, the common feature of IN activity enhancement in ammoniated solutions can only be explained by ice nucleation occurring at the edges of kaolin minerals.

Ice nucleation by smectites: the role of the edges

“…Immersion freezing experiments with single particles revealed that not all kaolinite particles are IN active and that the fraction of IN inactive particles increases with decreasing particle size, in accordance with the assumption that ice nucleation is restricted to rare nucleation sites (Lüönd et al, 2010;Welti et al, 2012;Wex et al, 2014;Nagare et al, 2016). Interestingly, kaolinite and halloysite showed an increased IN activity in the presence of ammonia or ammonium solutes (Kumar et al, 2019b;Klumpp et al, 2023), pointing to a similar chemical makeup of nucleation sites as feldspars.…”

“…Illite NX and SE consist of 86 wt % and 77 wt % illite, respectively, with kaolinite as the major impurity (10 wt %) and only traces of feldspars and quartz. Moreover, the kaolin mineral halloysite exhibited IN activity up to 245 K in emulsion freezing experiments for samples without quartz or feldspar impurities (Klumpp et al, 2023). Thus, the special peaks of SAu-1 and MX-80 are within the freezing range observed for clay minerals in emulsion freezing experiments, and an origin from smectite INPs is likely.…”

“…In emulsion freezing experiments, KGa-1b and KGa-2 both exhibit onset freezing temperatures around 240-242 K (Pinti et al, 2012). On the other hand, freezing onset temperatures up to 245 K are reached by halloysites, which are chemically almost identical to kaolinites but typically occur in the form of cylindrical tubes (Churchman et al, 1995;Joussein et al, 2005;Pasbakhsh et al, 2013;Klumpp et al, 2023). Immersion freezing experiments with single particles revealed that not all kaolinite particles are IN active and that the fraction of IN inactive particles increases with decreasing particle size, in accordance with the assumption that ice nucleation is restricted to rare nucleation sites (Lüönd et al, 2010;Welti et al, 2012;Wex et al, 2014;Nagare et al, 2016).…”

Significance of the surface silica/alumina ratio and surface termination on the immersion freezing of ZSM-5 zeolites

“…22 Clay aluminosilicate minerals have also been extensively studied both theoretically [24][25][26][27][28] and experimentally. [29][30][31][32][33][34][35] Kaolinite is a common clay mineral and has a unique arrangement of a layer of alumina tetrahedra with hydroxyl groups and a layer of silica tetrahedra with bound oxygen groups. 36 The alumina basal plane is more ice active than the silica basal plane, and in fact, Soni and Patey showed in computational studies that removing the Si layer does not change the ice nucleation activity of the kaolinite by the alumina layer.…”

Molecular simulations reveal that heterogeneous ice nucleation occurs at higher temperatures in water under capillary tension

“…Meanwhile, less attention has been given to the pressure dependence of j het , which will be the focus of this study. Negative pressure in water is prevalent throughout nature, occurring in tree xylem (Jacobsen et al, 2007), in water capillary bridges in soil (Seiphoori et al, 2020), and within nano-sized pores on atmospheric ice nucleating particles (David et al, 2020;Marcolli, 2020;Klumpp et al, 2023). Negative Laplace pressure, given by ∆P ≈ σ lv /2r, where σ lv ≈ 0.7 J m −2 is the liquid-vapor surface tension of water, becomes significant for nucleation when the air-water interface radius of curvature r is on the order of nanometers, leading to negative pressures of order 100s of atmospheres.…”