Evolution of Chemical Bonding and Crystalline Swelling-Shrinkage of Montmorillonite upon Temperature Changes Probed by in Situ Fourier Transform Infrared Spectroscopy and X-ray Diffraction

Clay minerals are distributed in Earth's crust and troposphere and in Martian crust where temperature varies. Understanding the changes of chemical bonding and crystalline swelling-shrinkage of montmorillonite (Mnt) upon temperature changes is fundamental for studying its surface reactivity and interaction in specific surroundings. However, such an issue remains poorly understood. Here, in situ high- and low-temperature Fourier transform infrared (HT- and LT-FTIR) spectroscopy and X-ray diffraction (HT- and LT-XRD) were performed to study the evolution of chemical bonding and crystalline swelling-shrinkage of sodium-montmorillonite (NaMnt) upon temperature changes. The FTIR results show that the hydroxyl content in NaMnt decreased when the temperature increased from 20 to 700 °C, while it is independent of temperature from 0 to -150 °C. The formation of hydroxyls at the "broken" layer edges of NaMnt is related to adsorbed water molecules on the surfaces, and its content increased when the particle size of the NaMnt decreased. The water molecules in the interlayer space of NaMnt could bond to the tetrahedral sheet of NaMnt through Si₂O-H₂O bonds. HT- and LT-XRD results reveal that all of those water molecules in NaMnt were removed after heating to 100 °C in the heating chamber. The NaMnt was transformed from a state of monolayer interlayer water molecules at 20 °C to a dehydrated state at 100 °C, and then to a dehydroxylated state at 700 °C. Accordingly, the basal spacings of NaMnt were changed from 1.27 to 0.97 nm and then to 0.96 nm, respectively. When NaMnt was cooled from 20 to -268 °C, a crystalline swelling of NaMnt occurred with an increase of 0.03 nm of basal spacing. This work demonstrates that high/low temperature has a remarkable effect on the hydroxyls and the water molecules in NaMnt, which in turn affects its swelling-shrinkage performance. These findings provide some in-depth understanding of the changes of chemical bonding and crystalline swelling-shrinkage of montmorillonite upon temperature changes and the reasons behind these, which might be helpful for the design of engineering Mnt in high-/low-temperature applications.

Enhanced Sorption of Supercritical CO2 and CH4 in the Hydrated Interlayer Pores of Smectite

Understanding the long-term confinement of supercritical fluids in the clay pores of subsurface rocks is important for many geo-energy technologies, including geological CO₂ storage. However, the adsorption properties of hydrated clay minerals remain largely uncertain because competitive adsorption experiments of supercritical fluids in the presence of water are difficult. Here, we report on the sorption properties of four source clay minerals-Ca-rich montmorillonite (STx-1b), Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-for water at 20 °C up to relative humidity of 0.9. The measurements unveil the unsuitability of physisorption analysis by N₂ (at 77 K) and Ar (at 87 K) gases to quantify the textural properties of clays because of their inability to probe the interlayers. We further measure the sorption of CO₂ and CH₄ on swelling STx-1b and nonswelling IMt-2, both in the absence (dehydrated at 200 °C) and the presence of sub-1W preadsorbed water (following dehydration) up to 170 bar at 50 °C. We observe enhanced sorption of CO₂ and CH₄ in STx-1b (50 and 65% increase at 30 bar relative to dry STx-1b, respectively), while their adsorption on IMt-2 remains unchanged, indicating the absence of competition with water. By describing the supercritical adsorption isotherms on hydrated STx-1b with the lattice density functional theory model, we estimate that the pore volume has expanded by approximately 6% through the formation of sub-nanometer pore space. By presenting a systematic approach of quantifying the smectite clay mineral's hydrated state, this study provides an explanation for the conflicting literature observations of gas uptake capacities in the presence of water.

Effects of Radiation Intensity, Mineral Matrix, and Pre-Irradiation on the Bacterial Resistance to Gamma Irradiation under Low Temperature Conditions

Ionizing radiation is one of the main factors limiting the survival of microorganisms in extraterrestrial conditions. The survivability of microorganisms under irradiation depends significantly on the conditions, in which the irradiation occurs. In particular, temperature, pressure, oxygen and water concentrations are of great influence. However, the influence of factors such as the radiation intensity (in low-temperature conditions) and the type of mineral matrix, in which microorganisms are located, has been practically unstudied. It has been shown that the radioresistance of bacteria can increase after their exposure to sublethal doses and subsequent repair of damage under favorable conditions, however, such studies are also few and the influence of other factors of extraterrestrial space (temperature, pressure) was not studied in them. The viability of bacteria Arthrobacter polychromogenes, Kocuria rosea and Xanthomonas sp. after irradiation with gamma radiation at a dose of 1 kGy under conditions of low pressure (1 Torr) and low temperature (-50 °C) at different radiation intensities (4 vs. 0.8 kGy/h) with immobilization of bacteria on various mineral matrices (montmorillonite vs. analogue of lunar dust) has been studied. Native, previously non-irradiated strains, and strains that were previously irradiated with gamma radiation and subjected to 10 passages of cultivation on solid media were irradiated. The number of survived cells was determined by culturing on a solid medium. It has been shown that the radioresistance of bacteria depends significantly on the type of mineral matrix, on which they are immobilized, wherein montmorillonite contributes to an increased survivability in comparison with a silicate matrix. Survivability of the studied bacteria was found to increase with decreasing radiation intensity, despite the impossibility of active reparation processes under experimental conditions. Considering the low intensity of radiation on various space objects in comparison with radiobiological experiments, this suggests a longer preservation of the viable microorganisms outside the Earth than is commonly believed. An increase in bacterial radioresistance was revealed even after one cycle of irradiation of the strains and their subsequent cultivation under favourable conditions. This indicates the possibility of hypothetical microorganisms on Mars increasing their radioresistance.

Role of Mono- and Divalent Surface Cations on the Structure and Adsorption Behavior of Water on Mica Surface

Understanding solid-water(vapor) interfacial systems is relevant for both industrial and academic scenarios for their presence in wide areas ranging from tribology to geochemistry. Using grand canonical Monte Carlo simulations, we have investigated the role of monovalent (lithium, Li⁺; sodium, Na⁺; and potassium, K⁺) and divalent (magnesium, Mg²⁺; calcium, Ca²⁺) cations on the structure and adsorption behavior of water on mica surface. The water density adjacent to the surface exhibits (a) oscillations due to hydration of surface cations (interfacial layer), (b) followed by a thick liquidlike layer. The thickness of the interfacial layer is strongly dependent on the hydration shell size and hydration energy of surface ions. Water molecules immediately next to the surface (contact layers) adsorb on ditrigonal (hexagonal) cavities of mica surface and form an ordered structure. The Li⁺, Na⁺, Mg²⁺, and Ca²⁺ surface ions are coadsorbed with water molecules on the ditrigonal cavities due to their smaller hydration shell. Majority of water molecules in the second contact layer hydrate the surface ions and, together with the rest of the water molecules, form hydrogen bonds among themselves. The structure of the water molecules in the third and subsequent layer is random and more bulk liquidlike, except those molecules that hydrate the surface ions. The adsorption isotherm of water on all ion-exposed mica surface exhibits three regimes: (a) an initial rapid increase in water loading for relative vapor pressure ( p/ p₀) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/ p₀ = 0.2 and 0.7, where the hydrogen bond formation between the water molecules of the interfacial layer occurs; and (c) exponential growth beyond p/ p₀ = 0.7 due to thickening of the liquidlike layer. The water loading on divalent-ion-exposed mica surface is higher compared to the monovalent ions case. Although the divalent ions have higher hydration energy, the fraction of water molecules hydrating the surface ions is less compared to nonhydrating water molecules. We found that ion hydration energy and size of hydration shell play a crucial role in their structure adjacent to mica surface. At lower water loadings, the surface ions form a hydration shell with surface bridging oxygens, whereas at higher water content, the hydration preference is shifted toward mobile water molecules. The detailed understanding obtained from this work will be useful in identifying the role of ions in cloud formation, nanotribological studies, and activities of biological molecules and catalysts.

Particle Size Controls on Water Adsorption and Condensation Regimes at Mineral Surfaces

Atmospheric water vapour interacting with hydrophilic mineral surfaces can produce water films of various thicknesses and structures. In this work we show that mineral particle size controls water loadings achieved by water vapour deposition on 21 contrasting mineral samples exposed to atmospheres of up to ~16 Torr water (70% relative humidity at 25 °C). Submicrometer-sized particles hosted up to ~5 monolayers of water, while micrometer-sized particles up to several thousand monolayers. All films exhibited vibrational spectroscopic signals akin to liquid water, yet with a disrupted network of hydrogen bonds. Water adsorption isotherms were predicted using models (1- or 2- term Freundlich and Do-Do models) describing an adsorption and a condensation regime, respectively pertaining to the binding of water onto mineral surfaces and water film growth by water-water interactions. The Hygroscopic Growth Theory could also account for the particle size dependence on condensable water loadings under the premise that larger particles have a greater propensity of exhibiting of surface regions and interparticle spacings facilitating water condensation reactions. Our work should impact our ability to predict water film formation at mineral surfaces of contrasting particle sizes, and should thus contribute to our understanding of water adsorption and condensation reactions occuring in nature.

Thin Ice Films at Mineral Surfaces

Ice films formed at mineral surfaces are of widespread occurrence in nature and are involved in numerous atmospheric and terrestrial processes. In this study, we studied thin ice films at surfaces of 19 synthetic and natural mineral samples of varied structure and composition. These thin films were formed by sublimation of thicker hexagonal ice overlayers mostly produced by freezing wet pastes of mineral particles at -10 and -50 °C. Vibration spectroscopy revealed that thin ice films contained smaller populations of strongly hydrogen-bonded water molecules than in hexagonal ice and liquid water. Thin ice films at the surfaces of the majority of minerals considered in this work [i.e., metal (oxy)(hydr)oxides, phyllosilicates, silicates, volcanic ash, Arizona Test Dust] produced intense O-H stretching bands at ∼3400 cm(-1), attenuated bands at ∼3200 cm(-1), and liquid-water-like bending band at ∼1640 cm(-1) irrespective of structure and composition. Illite, a nonexpandable phyllosilicate, is the only mineral that stabilized a form of ice that was strongly resilient to sublimation in temperatures as low as -50 °C. As mineral-bound thin ice films are the substrates upon which ice grows from water vapor or aqueous solutions, this study provides new constraints from which their natural occurrences can be understood.

Molecular simulation study of hydrated Na-rectorite

The swelling behavior of clay minerals is an important issue in industrial processes and environmental applications. Mixed-layer clay minerals containing a smectite fraction, such as rectorite, are neglected even though they could swell and exist in nature widely. The hydration of rectorite has not been well comprehended even though they are meaningful to mineralogy and industry. This study combines molecular dynamics (MD) and Monte Carlo (MC) simulations to disclose the swelling behavior of rectorite and compare with montmorillonite. From grand canonical Monte Carlo (GCMC) and MD simulations, we obtain swelling curves and swelling free-energy curves of rectorite with a relative humidity of 100%. With the comparisons of swelling free-energy minima, we find that the bilayer hydrate of Na-rectorite is more thermodynamically stable than the monolayer hydrate, which is similar to Na-montmorillonite. However, the interlayer sodium ions in rectorite show an asymmetrical distribution quite different from the symmetrical distribution in montmorillonite. Because of unequal layer charges between the smectite part and illite part of retorite, sodium ions prefer to distribute close to the illite part surface.

First principles calculation on the adsorption of water on lithium-montmorillonite (Li-MMT)

The interaction of water molecules and lithium-montmorillonite (Li-MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li-MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li-MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li-MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H(2)O/Li-MMT.

Water adsorption on clay minerals as a function of relative humidity: application of BET and Freundlich adsorption models

Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temperature (298 K) using horizontal attenuated total reflectance (HATR) Fourier transform infrared (FTIR) spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption analysis of water uptake on the clays. A detailed analysis of model fit integrity is reported. From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML) water coverage was determined and found to agree with previously reported gravimetric data. However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, respectively. The Freundlich adsorption model was found to fit the data well over the entire range of RH values studied and revealed two distinct water adsorption regimes. Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values greater than 19% (i.e., above ML water adsorption) relative to the kaolinite and illite clays. The difference in the observed water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites. It is suggested that different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms.

10th Anniversary review: applications of analytical techniques in laboratory studies of the chemical and climatic impacts of mineral dust aerosol in the Earth's atmosphere

It is clear that mineral dust particles can impact a number of global processes including the Earth's climate through direct and indirect climate forcing, the chemical composition of the atmosphere through heterogeneous reactions, and the biogeochemistry of the oceans through dust deposition. Thus, mineral dust aerosol links land, air, and oceans in unique ways unlike any other type of atmospheric aerosol. Quantitative knowledge of how mineral dust aerosol impacts the Earth's climate, the chemical balance of the atmosphere, and the biogeochemistry of the oceans will provide a better understanding of these links and connections and the overall impact on the Earth system. Advances in the applications of analytical laboratory techniques have been critical for providing valuable information regarding these global processes. In this mini review article, we discuss examples of current and emerging techniques used in laboratory studies of mineral dust chemistry and climate and potential future directions.