In this study, the adsorption enthalpy and entropy of water in the nanoconfinement of periodic mesoporous organosilicas with fine-tuned surface chemistry were investigated by integrating thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a continuous-flow humidity generator, providing a direct method to better understand the water adsorption process at very low surface coverage. For comparison, the commonly used indirect isosteric method was employed, yielding generally comparable results to the simultaneous DSC-TGA measurements despite differing experimental conditions. Only the calorimetric method revealed that not only more hydrophilic materials exhibit high absolute values of adsorption enthalpy of water at the first surface contact but also hydrophobic materials, as they contain surface silanol groups that can strongly interact with water until these sites are saturated. Beyond this initial humidity region of water-surface contact, both methods yield largely consistent results at higher relative humidity, clearly distinguishing between hydrophilic and hydrophobic surface behavior. More hydrophobic materials have lower absolute values of adsorption enthalpy, reflecting weaker interaction strength and less structuring before reaching the condensation enthalpy of water (-44 kJ mol-1), while hydrophilic materials exhibit higher absolute values of adsorption enthalpy before water-water interactions dominate. Additionally, results from the isosteric method indicate that, at higher surface coverage, hydrophilic materials exert a long-range structural influence extending into distant water layers. Furthermore, the influence of surface functionality and temperature on water vapor sorption isotherms was analyzed, revealing the temperature independence of both the water aggregate size prior to, and the relative pressure at, the inflection point of capillary condensation. In contrast, the desorption branch primarily governs the temperature dependence of the adsorption-desorption hysteresis. This study presents the first detailed enthalpic analysis of water adsorption in PMOs using a continuous-flow calorimetric method, providing a foundation for future research on water-surface functionality interactions either at different loadings under isothermal conditions or across varying temperatures.

Spherulites are spherical crystals that are polycrystalline assemblies of radially organized crystallites. Despite their wide prevalence and relevance to fields ranging from geology to medicine, the dynamics of spherulitic crystallization and the conditions required for such growth remain ill-understood. Here, we reveal the conditions for controlled spherulitic growth of sodium sulfate crystals from evaporating aqueous solutions mixtures of sulfate salts at room temperature. We reveal that divalent metal ions in the salt solutions induce spherulitic growth of sodium sulfate through non-classical nucleation and self-assembly of (nearly)-oriented nanocrystals. A key result is the very high viscosity ( 111 Pa ⋅ s) of the highly supersaturated solutions at the onset of spherulite precipitation. This allows for slow dynamics that facilitates the formation of a large number of mesoscopic prenucleation clusters, that subsequently show diffusion-limited growth and assemble into the spherulitic shapes. The spherulites are found to be metastable structures that form in out-of-equilibrium conditions. As the supersaturation decreases during growth, Na2SO4 spherulites can also evolve into other shapes depending on the evaporation rate. These findings shed light on the conditions that govern spherulite formation and provide practical strategies for tuning their morphology. Spherulites, prevalent in fields from geology to medicine, have poorly understood crystallization dynamics and growth conditions. Here, the authors demonstrate that divalent metal ions in sulfate salt solutions induce spherulitic growth of sodium sulfate through non-classical nucleation, revealing high viscosity and slow dynamics as key factors.

Salt crystallisation pressure is one of the main sources of weathering in porous building materials. In our work we study the origin of this phenomenon using single crystals of KAl(SO4)2⋅12H2O (potassium aluminium sulphate dodecahydrate) as model system. Crystals were grown immersed in a solution of a specific supersaturation (σ=10−80%). Evidence of crystallization pressure was the vertical lift of weights on top of the growing crystal, which was measured by in-situ displacement sensors. A clear supersaturation threshold of σ≃30% for a 0.5 N weight is required to initiate lifting in our model system. Above this value, the maximum displacement increases nonlinearly with σ and depends on crystal orientation, interfacial wettability, and applied load. Hydrophobic interfaces suppress crystal growth by limiting mass transport, whereas hydrophilic confinement supports continuous growth until the local supersaturation is exhausted. In saturated solutions, on the other hand, a load leads to dissolution. Based on the observations, we propose a mechanism for crystallization pressure for which the growth of the unloaded faces adjacent to the confined face is key. These results refine our understanding of crystallization pressure in confined spaces and can help explain and mitigate salt-induced damage in porous materials.

Efficient, durable, and cost-effective thermochemical heat storage is essential for advancing renewable energy utilization and waste heat recovery. This study introduces millimeter-sized, semi-spherical granules of potassium carnallite (KMgCl3 & sdot; 6 H2O), an abundant and low-cost double salt of magnesium chloride (MgCl2), as a promising material for long-term thermal energy storage in large-scale fixed-bed systems. Unlike MgCl2, which suffers from hydrolysis, material degradation, and over-hydration at moderate humidity levels, carnallite demonstrates remarkable stability, resisting liquefaction and hydrolysis under operational conditions. Comprehensive analysis confirms that the granules maintain good structural integrity during cycling, although measurable inter-particle agglomeration is observed, with no phase segregation or loss of chemical homogeneity. In cyclic tests conducted under controlled charging and discharging conditions, carnallite achieves a gravimetric energy density exceeding 470 kJkg-1 and a volumetric power density of 95 kW m-3. Notably, the material retains over 90 % of its initial capacity after fifteen consecutive cycles, significantly outperforming MgCl2, which exhibits substantial degradation under similar conditions. These findings highlight carnallite's exceptional potential as a stable, high-performance candidate for scalable thermochemical heat storage systems. Further research into enhancing particle durability will unlock its full potential for large-scale energy storage applications.

Salt crystallization pressure is one of the main sources of weathering in porous building materials. In our work we study the origin of this phenomenon using single crystals of KAl(SO4)(2)center dot 12H(2)O (potassium aluminium sulphate dodecahydrate) as model system. Crystals were grown immersed in a solution of a specific supersaturation (sigma = 10-80 %). Evidence of crystallization pressure was the vertical lift of weights on top of the growing crystal, which was measured by in-situ displacement sensors. A clear supersaturation threshold of sigma similar or equal to 30 % for a 0.5 N weight is required to initiate lifting in our model system. Above this value, the maximum displacement increases nonlinearly with sigma and depends on crystal orientation, interfacial wettability, and applied load. Hydrophobic interfaces suppress crystal growth by limiting mass transport, whereas hydrophilic confinement supports continuous growth until the local supersaturation is exhausted. In saturated solutions, on the other hand, a load leads to dissolution. Based on the observations, we propose a mechanism for crystallization pressure for which the growth of the unloaded faces adjacent to the confined face is key. These results refine our understanding of crystallization pressure in confined spaces and can help explain and mitigate salt-induced damage in porous materials.

Available thermodynamic data for (NH4)2SO4(aq) have been critically evaluated and used to parameterize an extended ion interaction (Pitzer) model. The model satisfactorily represents the available activity and thermal data from below the eutectic temperature to 110 °C. At near ambient temperatures (5–40 °C) the model accurately predicts the water activities in supersaturated solutions to about 30 mol·kg–1. The model has been used to determine the solubility products of (NH4)2SO4(s) from the eutectic temperature to 110 °C. Solubilities in the ternary Tutton salt forming systems (NH4)2SO4–MgSO4–H2O, (NH4)2SO4–FeSO4–H2O, (NH4)2SO4–ZnSO4–H2O, and (NH4)2SO4–CuSO4–H2O have been used to determine the ternary interaction parameters in the Pitzer model and to calculate the solubilities and deliquescence humidities of the Tutton salts (NH4)2MIISO4·6H2O (with M = Mg, Fe, Zn, Cu). Using additional literature data of decomposition vapor pressures, the phase diagrams of the Tutton salts have been established and the suitability of the salts as thermochemical heat storage materials has been critically evaluated.

The interaction of water vapor with soluble electrolytes has been comprehensively studied due to their ubiquity in nature and industry. Enormous experimental works have elucidated the mechanisms of water molecule adsorption, layer growth, and ion solvation in the water film on a NaCl crystal surface. In this study, we employ a new method to investigate the moisture uptake behavior on the NaCl crystal surface under varying humidity levels by using environmental scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (ESEM-EDX). Additionally, water vapor sorption, ESEM, and Raman microscopy are used to examine the role of water films in facilitating phase transitions and chemical reactions of salt mixtures. Our results indicate that the water uptake on a NaCl crystal surface initiates early and increases with rising humidity. A sudden increase in the condensation rate above 60% RH suggests ion solvation in the water film. In ternary mixtures, the water film occurs by water vapor adsorption or capillary condensation at the interface between two crystals in an early stage, where ions from both sides dissolve into the interfacial water film to form a nanoscale brine film. This process initiates the mutual deliquescence of a NaCl-KCl mixture at a relative humidity lower than the deliquescence humidity of either of the two single salts. For the solid-solid reaction NaNO3 + KCl → NaCl + KNO3, the process involves the dissolution of reactants and the precipitation of products, where the interfacial water film acts as a liquid bridge, promoting the ion diffusion and exchange between the two reactants.

Available thermodynamic data for (NH4)2SO4(aq) have been critically evaluated and used to parameterize an extended ion interaction (Pitzer) model. The model satisfactorily represents the available activity and thermal data from below the eutectic temperature to 110 degrees C. At near ambient temperatures (5-40 degrees C) the model accurately predicts the water activities in supersaturated solutions to about 30 molkg-1. The model has been used to determine the solubility products of (NH4)2SO4(s) from the eutectic temperature to 110 degrees C. Solubilities in the ternary Tutton salt forming systems (NH4)2SO4-MgSO4-H2O, (NH4)2SO4-FeSO4-H2O, (NH4)2SO4-ZnSO4-H2O, and (NH4)2SO4-CuSO4-H2O have been used to determine the ternary interaction parameters in the Pitzer model and to calculate the solubilities and deliquescence humidities of the Tutton salts (NH4)2MIISO46H2O (with M = Mg, Fe, Zn, Cu). Using additional literature data of decomposition vapor pressures, the phase diagrams of the Tutton salts have been established and the suitability of the salts as thermochemical heat storage materials has been critically evaluated.

Spherulites are complex polycrystalline structures that form through the self-assembly of small aggregated nanocrystals starting from a central point and growing radially outward. Despite their wide prevalence and relevance to fields ranging from geology to medicine, the dynamics of spherulitic crystallization and the conditions required for such growth remain ill-understood. Here, we report on the conditions to induce controlled spherulitic growth of sodium sulfate from evaporating aqueous solutions of sulfate salt mixtures at room temperature. We reveal that introducing divalent metal ions in the solution cause spherulitic growth of sodium sulfate. For the first time, we quantify the supersaturation at the onset of spherulitic growth from salt mixtures and determine the growth kinetics. Our results show that the nonclassical nucleation process induces the growth of sodium sulfate spherulites at high supersaturation in highly viscous solutions. The latter reaches approximately 111 Pa·s, triggered by the divalent ions, at the onset of spherulite precipitation leading to a diffusion limited growth. We also show that spherulites, which are metastable structures formed under out-of-equilibrium conditions, can evolve into other shapes when supersaturation decreases as growth continues at different evaporation rates. These findings shed light on the conditions under which spherulites form and offer practical strategies for tuning their morphology.

In this study, we investigated the crystallization of a salt mixture during drying in a porous medium. Specifically, we focused on the ternary system of NaCl-NaNO3-H2O, which is also encountered in situ. In order to study the crystallization, we use a specialized 4.7 T NMR setup that allows us to directly measure the NaCl and NaNO3 concentrations in a porous medium and track their ratio during drying, providing direct insight into the phase diagram. The measurements indicate that the equilibrium phase diagram alone is not sufficient to describe the physical processes that occur in porous media during drying experiments. In the case of forced drying in this study, where advection of the ions is dominant (Pe > 5), the measurements indicate that we need to take supersaturation into account and that crystallization is driven by transport. As a result, the ratio of a salt mixture will remain constant in the porous medium throughout the experiments, as was seen for this ternary system Na+, Cl-, NO3 - resulting in the formation of both NaCl and NaNO3. These results indicate that the rate of evaporation, in combination with the effect of supersaturation and solution transport in the pore system, allows the saturation degree given by the phase diagram to be surpassed. This phenomenon is critical when assessing mixed salt systems in porous media and should be considered when evaluating phase diagrams alone.

Salt damage to porous building materials, which becomes relevant in case of high pore fillings and cyclic crystallization events, affects numerous objects of our cultural heritage. For single salts, the critical threshold values for crystallization cycles are given by the deliquescence humidity and the crystallization humidity, the latter depending on the degree of supersaturation prior to crystallization. Contamination of built heritage with only one single salt is unlikely, so ion mixtures must be considered for a more realistic assessment of the damage potential. For salt mixtures, the boundary conditions, i.e., crystallization humidities and the relative humidity range relevant for crystallization and dissolution processes, are different from the conditions for single salts. This paper investigates whether the damage potential of a salt is also different when precipitating from a mixture rather than from its pure solution. For that purpose, the supersaturation by cooling and by evaporation is used as a measure for the damage potential and is determined for different systematically selected mixture compositions of the Na+-K+-Cl--NO3 --H2O system and respective subsystems. The results indicate that the supersaturation of some salts is indeed affected by accompanying ions, while for others, the impact is low.

This study investigates the behavior of water confined in cylindrical nanopores with hydrophilic, charged, and hydrophobic surfaces using water vapor sorption at 298.15 K. MCM-41 silica and periodic mesoporous organosilicas (PMOs) serve as host materials, with PMOs featuring a silica backbone linked by organic bridging units, allowing precise control over surface chemistry. Using a consistent divinylbenzene-based structure, the study assesses how functional groups influence confined water properties. A novel hydrophilicity index is introduced, incorporating contact angle data and adsorption layer thickness to quantify surface chemistry effects. Hydrophilic materials exhibit distinct adsorption profiles compared to hydrophobic PMOs, where surface chemistry and confinement influence water density, adsorption behavior, and t-layer proportions. In hydrophobic materials, such as divinylbenzene- and N,N-dimethyldivinylaniline-bridged PMOs, thicker water adsorption layers with lower average density are observed, which can be attributed to the localization of water near silanol groups and its absence near hydrophobic regions. Fluorinated PMOs fluctuate between hydrophobic and hydrophilic tendencies depending on water saturation. Introduction of surface charges enhances water adsorption, leading to a larger t-layer and higher adsorbed water density. These findings provide critical insights into how surface chemistry and pore size collectively govern water behavior, offering a framework for future studies on nanoconfined water.

The impact of surface hydrophilicity on the freezing, melting, and nonfreezable layer of nanoconfined water remains debated. Variability in material types, pore sizes, surface functionality organization, and experimental conditions complicate direct comparisons and conclusions. To address this, periodic mesoporous organosilicas (PMOs) with a uniform divinylbenzene bridging unit were used. By fine-tuning hydrophilicity and pore sizes, confined water's phase behavior was studied via differential scanning calorimetry (DSC). Comparing gas-phase adsorption and incipient wetness as filling methods revealed an increased t-layer density with the latter. Surface hydrophilicity has little effect on melting point depression in larger pores but becomes increasingly influential as pore size decreases. The nonfreezable layer thickness was evaluated using the Gibbs-Thomson equation and geometric enthalpy-based calculations. In hydrophobic PMOs, water exhibited larger melting point depression, lower specific enthalpies, and thicker interfacial layers than in hydrophilic ones. In contrast, charged PMOs behaved differently: despite higher hydrophilicity, confined water exhibited a larger melting temperature depression, lower specific enthalpy, larger critical pore radius, and comparatively thicker t-layers, likely due to higher disorder of the hydrogen-bonding network close to the surface. Moreover, the t-layer density did not follow a simple trend based solely on hydrophilicity. These results highlight the complex interplay between pore size, surface chemistry, and interfacial water behavior, offering valuable insights into confined water properties and phase transitions.

The presence of salts and related salt-induced damage represent one of the major threats to the preservation of our built heritage. Identifying critical relative humidity values that facilitate crystallization cycles is essential for understanding damage risks and extents. This knowledge helps in developing recommendations for favorable, damage-avoiding climates, particularly in controllable indoor environments. While for single salts their deliquescence humidity is known, for multi-ion mixtures relevant for the built heritage multiple transitions happen over a range of relative humidity. Modeling of equilibrium crystallization pathways is possible, e.g. using the Pitzer formalism. However, for complex mixtures, only predictions can be given, which need to be validated through experimental results. This work focuses on the use of dynamic water vapor sorption measurements to investigate phase transitions in salt mixtures, demonstrating its applicability, scrutinizing different influencing factors and an appropriate interpretation of results. Additionally, presenting an experimental design that delivers reliable results for the conservation of cultural heritage is crucial. In addition to single salts, mixtures from the common hygroscopic system Na+–K+–Mg2+–Ca2+–Cl––NO3––H2O are investigated, including their behavior in a stone material. The identified transitions are compared to the calculated behavior using the ECOS–Runsalt model. The presented results are accurate and reproducible. They show the ability to determine the critical relative humidity ranges (in bulk and in porous materials) and validate thermodynamic models.

Salt weathering significantly degrades building materials, necessitating a thorough understanding of influencing factors. While prior research has focused on relative humidity (RH), temperature effects on salt crystallisation and dissolution remain less explored. This study examines selected single salts and mixtures, using the ECOS/RUSALT thermodynamic model to assess equilibrium behaviour across temperatures from 1 °C to 50 °C and RH values from 15% to 98%. Results show that crystallisation and dissolution RH generally decrease with rising temperature. Single salts exhibit monotonic changes, whereas mixtures behave variably. Calcium-rich mixtures have lower mutual crystallisation and dissolution RH than sulfate-rich ones, with further reductions in magnesium-containing mixtures. Lower temperatures promote the formation of more output salts. Model limitations are acknowledged to explain discrepancies between predictions and real-world observations. These findings enhance understanding of salt behaviour under climatic variations, aiding strategies to mitigate salt damage in building materials.

Salt weathering is one of the most threatening issues for the preservation of our built cultural heritage comprised of porous materials. After salt accumulation in the pores, cyclic crystallization and dissolution processes can lead to material damage when the crystallization pressure, which is directly influenced by the supersaturation of the pore solution, acts on the material. Following the presentation of the damage potential and achievable supersaturation of potassium nitrate in a previous paper, this study reports results for the nitrates of sodium, magnesium and calcium for the first time. Two different types of damage tests with impregnated sandstone specimens were performed to assess their actual damage potential, also including the well investigated sodium chloride for comparison. Significant deterioration was revealed for all investigated salts as a result of a high supersaturation of the pore solution at the time when crystals grow against the pore wall. Surface roughness measurements were used to compare the extent of damage and to rank the damage potentials of the salts. Investigations on the supersaturation, which is directly linked to the crystallization pressure and is suggested to be used as a quantitative and comparable measure for the damage potential, revealed high values at the onset of crystallization for all salts in cooling (also in porous host materials) and evaporation experiments. The results of this comprehensive study clearly reveal a high damage potential of nitrate salts, formerly often neglected in discussions on damage-relevant salts.

The hydration of salt hydrates is often described as a solution mediated nucleation and growth mechanism, occurring between a reagent and a product in thermodynamic equilibrium with each other. If a system possesses more than one hydrate phase, the kinetic pathway may involve additional mechanisms due to the formation of intermediate hydrate species. We elected CuSO4 as our model system and analyzed the pathway leading from CuSO4·H2O (C1H) to CuSO4·5H2O (C5H), while CuSO4·3H2O (C3H) being a possible intermediate. We found that C1H hydration is mediated by the formation of C3H and that C5H does not nucleate directly from C1H, at the studied conditions. The hydration pathway therefore is characterized by the same mechanism occurring twice, nucleation and growth of C3H and nucleation and growth of C5H. Analysis of the hydration kinetics of C1H revealed that C5H nucleates rapidly from C3H, as if the metastability of C3H was reduced when starting from C1H. Therefore, we concluded that the hydration kinetics of C1H are probably controlled by the growth process of C5H. Despite being controlled by a single reaction process, we show that a single front 1D diffusion model is insufficient to describe the reaction kinetics at the tablet level. Understanding of these complex transformations is necessary to evaluate the suitability of these reactions for application, in particular with respect to the achieved power output.

In this study, nanocomposites of Calcium hydroxide (Ca(OH)2) and Silica (SiO2) were developed using tetraethylorthosilicate precursor, and the impact of ammonia concentration on the reaction kinetics of Ca(OH)2 to CaO conversion were studied. The Ca(OH)2 nanoparticles were synthesized by the sol-gel method. The formation of Ca(OH)2 particles was confirmed through X-ray diffraction. Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) provided evidence for the presence of a SiO2 layer on Ca(OH)2. Thermogravimetric analyses were carried out under N2 atmosphere to study the dehydration efficiency of the Ca(OH)2 system in the presence of SiO2. The results were found to be consistent with the findings from XRD and Raman analysis. Differential Scanning Calorimetry (DSC) was done to determine the impact of ammonia and silica on the endothermic peak of Ca(OH)2 and CaCO3. TG results were used to determine the weight loss, activation energy (Ea), entropy (, enthalpy (, and Gibbs free energy (.

The potential use of carnallite (KMgCl 3 . 6H 2 O) for low temperature thermochemical heat storage has been evaluated. Carnallite is an incongruently soluble double salt and its possible decomposition and dehydration reactions have been carefully evaluated from available thermodynamic data and the phase diagram has been constructed. The dehydration and rehydration reactions have been experimentally studied and the reaction products have been characterized using thermogravimetric mass spectrometry, water vapor sorption, X-ray powder diffraction, scanning electron microscopy, Raman microscopy and calorimetry. The results of the experimental investigation of these reactions confirm the model predictions and clearly show that a thermochemical cycle including carnallite and its dihydrate (KMgCl 3 . 2H 2 O) is very promising for low temperature domestic thermochemical storage. In comparison to both pure MgCl 2 . 6H 2 O and also other salts mentioned in the literature, the double salt carnallite offers significant advantages. It has a higher deliquescence humidity and, thus, is much less sensitive to over-hydration and liquefaction. It can be easily dehydrated at only 100 degrees C and is much less prone to hydrolysis (HCl release). Carnallite has also a good reversibility between dehydration and hydration and the re -formation of carnallite by rehydration is easily achieved at low water vapor pressure and faster reaction rate than the hydration of MgCl 2 . 2H 2 O to MgCl 2 . 6H 2 O. The enthalpy of hydration is slightly higher for the hydration of KMgCl 3 . 2H 2 O than for the hydration of MgCl 2 . 2H 2 O. The only price to be paid is a slightly reduced storage density of the double salt due to its larger molar volume. However, the theoretical storage density of 1.52 kJ cm - 3 is still excellent. Also, carnallite is a low-cost material as it is available as industrial waste material or can be easily synthesized.

A large number of exquisite sandstone grottoes remaining along the Silk Road in Northwest China suffer from severe salt damage due to the vulnerability of the sandstone materials used and long-term continuous fluctuation of environmental conditions. Exploring the mechanism of salt weathering is crucial for the conservation of these grottoes. This study aims to quantitatively analyze the damage potential and the main impact factors of salt weathering in 14 grottoes along the Silk Road that were selected and investigated by using visual evaluation, salt analysis including thermodynamic modeling, and a comprehensive evaluation based on the entropy weight method. The results indicate that salt weathering of the sandstone grottoes is primarily characterized by efflorescences, peeling, and powdering. The overall salt content within the weathering layer typically exceeds a threshold of 1 %. Within the semi-arid area, the humidity fluctuations critical for significant volume changes in crystalline salts fall within the ranges 46–76 % and 16–30 %. In the semi-humid area, these ranges are 58–90 %, 42–50 %, and 16–30 %. A total of 14 salts can precipitate in the study area with NaCl, NaNO3, KNO3, Na2SO4·xH2O and several double salts occuring most frequently. Generally, the damage degree of sandstones grottoes in the semi-humid area is more serious than that in the semi-arid area. Through a comparison of visual inspection and more comprehensive quantitative evaluation, it has been confirmed that the salt damage is primarily influenced by the factors salt content, salt type, and environmental conditions. Furthermore, the order of influence for each factor on salt weathering can be ranked as follows: salt content > salt type > environmental conditions. This study quantitatively reveals the impact of various factors on salt weathering and proposes a method to evaluate the salt damage potential, which is of great significance for the preventive conservation of sandstone grottoes.