Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part 3 – Aluminosilicates

Abstract

<p><strong>Abstract.</strong> Aluminosilicates (such as feldspars, clay minerals and micas) and quartz (a crystalline form of silica), constitute the majority of airborne mineral dust. Despite similarities in structures and surfaces they differ greatly in terms of their ice nucleation (IN) efficiency. Here, we show that determining factors for their IN activity include surface ion exchange, NH<sub>3</sub> or NH<sub>4</sub><sup>+</sup> adsorption, as well as surface degradation due to the slow dissolution of the minerals. We performed immersion freezing experiments with the (Na-Ca)-feldspar andesine, the K-feldspar sanidine, the clay mineral kaolinite, the micas muscovite and biotite, and gibbsite (Al(OH)<sub>3</sub>, a mineral form of aluminum hydroxide) and compare their IN efficiencies with those of the previously characterized K-feldspar microcline and quartz. Samples were suspended in pure water as well as in aqueous solutions of NH<sub>3</sub>, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>Cl and Na<sub>2</sub>SO<sub>4</sub>, with solute concentrations corresponding to water activities <i>a<sub>w</sub></i> = 0.88–1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous (<i>T</i><sub>het</sub>) and homogeneous (<i>T</i><sub>hom</sub>) freezing as well as heterogeneously frozen water volume fractions (<i>F</i><sub>het</sub>). Suspensions in pure water of andesine, sanidine and kaolinite yield <i>T</i><sub>het</sub> = 242.8 K, 241.2 K and 240.3 K, respectively, while no discernable heterogeneous freezing signal is present in case of the micas or gibbsite (i.e. <i>T</i><sub>het</sub> ≈ <i>T</i><sub>hom</sub> ≈ 237.0 K). The presence of NH<sub>3</sub> and/or NH<sub>4</sub><sup>+</sup>-salts as solutes has distinct effects on the IN efficiency of most of the investigated minerals. When feldspars and kaolinite are suspended in very dilute solutions of NH<sub>3</sub> or NH<sub>4</sub><sup>+</sup>-salts (< 0.1 molal, corresponding to <i>a<sub>w</sub></i> > ~ 0.99), Thet shifts to higher temperatures (by 2.6–7.0 K compared to the pure water suspension). Even micas and gibbsite develop weak heterogeneous freezing activities in ammonia solutions. Conversely, suspensions containing Na<sub>2</sub>SO<sub>4</sub> cause <i>T</i><sub>het</sub> of feldspars to clearly fall below the water-activity-based immersion freezing description (Δ<i>a<sub>w</sub></i> = const) even in very dilute Na<sub>2</sub>SO<sub>4</sub> solutions, while <i>T</i><sub>het</sub> of kaolinite follows the Δ<i>a<sub>w</sub></i> = const curve. The water activity determines how the freezing temperature is affected by solute concentration alone, i.e. if the surface properties of the ice nucleating particles are not affected by the solute. Therefore, the complex behavior of the IN activities can only be explained in terms of solute-surface-specific processes. We suggest that the immediate exchange of the native cations (K<sup>+</sup>/Na<sup>+</sup>/Ca<sup>2+</sup>) with protons, when feldspars are immersed in water, is a prerequisite for their high IN efficiency. On the other hand, excess cations from dissolved alkali salts prevent surface protonation, thus explaining the decreased IN activity in such solutions. In kaolinite, the lack of exchangeable cations in the crystal lattice explains why the IN activity is insensitive to the presence of alkali salts (Δ<i>a<sub>w</sub></i> = const). We hypothesize that adsorption of NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> on the feldspar surface rather than ion exchange is the main reason for the anomalous increased Thet in dilute solutions of NH<sub>3</sub> or NH<sub>4</sub><sup>+</sup>-salts. This is supported by the response of kaolinite to NH<sub>3</sub> or NH<sub>4</sub><sup>+</sup>, despite lacking exchangeable ions. Finally, on longer timescales (hours to days) the dissolution of the feldspars in water or solutions becomes an important process leading to depletion of Al and formation of an amorphous layer enriched in Si within less than an hour. This hampers the IN activity of andesine the most, followed by sanidine, then eventually microcline, the least soluble feldspar.</p>

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part 2 – Quartz and amorphous silica

“…Furthermore, in Part 3 (Kumar et al, 2018b) we show that micas (muscovite and biotite) and gibbsite, which reveal no IN activity in pure water emulsion freezing experiments, develop a heterogeneous freezing signal in the presence of NH3 and (NH4)2SO4. We therefore tested the amorphous silica samples for a similar effect.…”

“…In Part I of this series, we showed that heterogeneous freezing onsets of microcline showed strong deviations from the wateractivity-based description to higher freezing temperatures from predicted values when suspended in dilute NH3/NH 4 + containing solutions and substantial decrease in IN efficiency in more concentrated solutions of inorganic salts including NH 4 + containing salts and NH3 (Kumar et al, 2018a). In Kumar et al (2018b) we extended this investigation to other aluminosilicates and found that an increase of Thet in the presence of NH 4 + containing solutes is a general feature of feldspars, clay minerals and micas, while the decrease in IN efficiency at higher solute concentration is a more specific characteristic of K-feldspars and most pronounced for microcline. Conversely, SA quartz follows quite well the water-activity-based prediction of Thet in case of pH neutral suspensions.…”

“…Analogous to the parameterization for ∆ w hom ( ) based on the thermodynamic homogeneous IN description of Koop et al (2000), het ∆ het ( ) assumes that the water activity dependence of Thet is determined by solute-driven changes in the structure of the water alone, while interactions of the solute with the INP surface are excluded. From Kumar et al (2018a;2018b) we know that the assumption of ∆ w het fails when there are specific interactions between the solute and the mineral surface. As can be seen from This decrease in Thet is less pronounced in the presence of NH3 than for suspensions in NaOH.…”

“…Besides surface functionalization, surface charge is a factor that has been shown to influence IN activity (Marcolli et al, 545 2016;Abdelmonem et al, 2017;Kumar et al, 2018b). Abdelmonem et al (2017) found that the freezing temperature of water at the α-Al2O3 (0001) surface is highest when the surface is close to the PZC and the water molecules at the surface are least ordered.…”

“…In this paper and Part 3 (Kumar et al, 2018b) of the companion papers, we relate IN activities of mineral surfaces more closely with the mineral surface properties by investigating the differences in IN activity of structurally similar minerals in pure water and aqueous solutions. In Part 3 we investigate the differences in IN activities of aluminosilicates and whether the enhancement observed for microcline in dilute NH3/NH 4 + containing solutions (shown in Part 1) is a more general property of aluminosilicates.…”