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In recent years, we have come to appreciate the astounding intricacy of the formation process of minerals from ions in aqueous solutions. The original ‘textbook’ image o these phenomena, stemming from the adaptation of classical nucleation and growth theories, has increased in complexity due to the discovery of a variety of precursor and intermediate species [e.g. 1], including solute clusters (e.g. prenucleation clusters PNCs), liquid(-like) phases, as well as amorphous and nanocrystalline solids etc. In general, these precursor or intermediate species constitute different, often short-lived points along the pathway from dissolved ions to the final solids (typically crystals in this context). In this regard synchrotron-based scattering (SAXS/WAXS/HEXD appears to be the perfect tool to follow in situ and in a time-resolved manner the crystallization pathways because of the temporal and spatial length scales that can be directly accessed with these techniques.
Here, I show how we used scattering to probe the crystallization mechanisms o calcium sulfate. CaSO4 minerals (i.e. gypsum, anhydrite and bassanite) are widespread in natural and industrial environments. During the last several years, a number o studies have revealed indeed that nucleation in the CaSO4-H2O system is non-classical My SAXS data demonstrate that gypsum precipitation, involves formation and aggregation of sub-3 nm primary species. These species constitute building blocks o an amorphous precursor phase [2]. Further, I show how in situ high-energy X-ray diffraction experiments and molecular dynamics (MD) simulations can be combined to derive the atomic structure of the primary CaSO4 clusters seen at small-angles [3]. fitted several plausible structures to the derived pair distribution functions and explored their dynamic properties using unbiased MD simulations based on polarizable force fields. Finally, based on combined SAXS/WAXS, broad-q-range measurements, show that the process of formation of bassanite, a less hydrated form of CaSO4, is very similar to the formation of gypsum: it also involves the aggregation of small primary species into larger disordered aggregates [4].
Based on these recent insights I formulated a tentative general model for calcium sulfate precipitation from solution. This model involves primary species that are formed through the assembly of multiple Ca2+ and SO42- ions into nanoclusters. These nanoclusters assemble into poorly ordered (i.e. amorphous) hydrated aggregates which in turn undergo ordering into coherent crystalline units of either gypsum o bassanite (and possibly anhydrite). Determination of the structure and (meta)stability of the primary species is important from both a fundamental, e.g. establishing a general non-classical nucleation model, and applied perspective; e.g. allow for an improved design of additives for greater control of the nucleation pathway
In recent years, we have come to appreciate the astounding intricacy of the formation process of minerals from ions in aqueous solutions. In this context, a number of studies have already revealed that nucleation in the CaSO4-H2O system is non-classical, where the formation of the different crystalline phases involves several steps including a common amorphous precursor. In this contribution a holistic view of the formation mechanism of gypsum and bassanite from solution will be presented.
In short, our in situ and time-resolved scattering data demonstrate that calcium sulfate precipitation starts with the formation and aggregation of well-defined sub-3 nm primary species. These species constitute building “bricks'' of an amorphous precursor phase. We characterised the “bricks” by combining information obtained at different length-scales accessible at the mesoscale (from small-angle scattering) and at the atomic-length-scale (wide-angle scattering and high-energy diffraction). From these scattering data we derived pair distribution functions of the clusters and restricted their external shapes and dimensions. This allowed us to propose a structure of the primary species and to explore their dynamic properties with unbiased MD simulations using polarizable force fields. The formation of the amorphous phase involves the aggregation of these small primary species into larger disordered aggregates exhibiting “brick-in-the-wall” structure. The actual crystallisation occurs by the restructuring and coalescence of the “bricks” into a given calcium sulfate phase depending on the thermodynamic conditions of the solution. Importantly, these rearrangement processes by no means continue until a (nearly-)perfect homogeneous single crystal is obtained. Instead they come to a stop or at least significantly slow down. Such a process thus yields a final imperfect mesocrystal, composed of smaller domains rather than a continuous crystal structure, within which the domains are separated by an amorphous calcium sulfate phase.
In recent years, we have come to appreciate the astounding intricacy of the formation process of minerals from ions in aqueous solutions. The original ‘textbook’ image of these phenomena, stemming from the adaptation of classical nucleation and growth theories, has increased in complexity due to the discovery of a variety of precursor and intermediate species [e.g. 1], including solute clusters (e.g. prenucleation clusters, PNCs), liquid(-like) phases, as well as amorphous and nanocrystalline solids etc. In general, these precursor or intermediate species constitute different, often short-lived, points along the pathway from dissolved ions to the final solids (typically crystals in this context). In this regard synchrotron-based scattering (SAXS/WAXS/HEXD) appears to be the perfect tool to follow in situ and in a time-resolved manner the crystallization pathways because of the temporal and spatial length scales that can be directly accessed with these techniques.
Here, we show how we used scattering to probe the crystallization mechanisms of calcium sulfate. CaSO4 minerals (i.e. gypsum, anhydrite and bassanite) are widespread in natural and industrial environments. During the last several years, a number of studies have revealed indeed that nucleation in the CaSO4-H2O system is non-classical. Our SAXS data demonstrate that gypsum precipitation, involves formation and aggregation of sub-3 nm primary species. These species constitute building blocks of an amorphous precursor phase [2]. Further, we show how in situ high-energy X-ray diffraction experiments and molecular dynamics (MD) simulations can be combined to derive the atomic structure of the primary CaSO4 clusters seen at small-angles [3]. We fitted several plausible structures to the derived pair distribution functions and explored their dynamic properties using unbiased MD simulations based on polarizable force fields. Finally, based on combined SAXS/WAXS, broad-q-range measurements, we show that the process of formation of bassanite, a less hydrated form of CaSO4, is very similar to the formation of gypsum: it also involves the aggregation of small primary species into larger disordered aggregates [4].
Based on these recent insights we formulated a tentative general model for calcium sulfate precipitation from solution. This model involves primary species that are formed through the assembly of multiple Ca2+ and SO42- ions into nanoclusters. These nanoclusters assemble into poorly ordered (i.e. amorphous) hydrated aggregates, which in turn undergo ordering into coherent crystalline units of either gypsum or bassanite (and possibly anhydrite). Determination of the structure and (meta)stability of the primary species is important from both a fundamental, e.g. establishing a general non-classical nucleation model, and applied perspective; e.g. allow for an improved design of additives for greater control of the nucleation pathway.
Dataset and Jupyter worksheet interpreting the (results from) small- and wide-angle scattering data from a series of boehmite/epoxy nanocomposites. Accompanies the publication "Competition of nanoparticle-induced mobilization and immobilization effects on segmental dynamics of an epoxy-based nanocomposite", by Paulina Szymoniak, Brian R. Pauw, Xintong Qu, and Andreas Schönhals.
Datasets are in three-column ascii (processed and azimuthally averaged data) from a Xenocs NanoInXider SW instrument. Monte-Carlo analyses were performed using McSAS 1.3.1, other analyses are in the Python 3.7 worksheet. Graphics and result tables are output by the worksheet.
In recent years, we have come to appreciate the astounding intricacy of the processes leading to the formation of minerals from ions in aqueous solutions. The original, and rather naive, ‘textbook’ image of these phenomena, stemming from the adaptation of classical nucleation and growth theories, has increased in complexity due to the discovery of a variety of precursor and intermediate species. These include solute clusters (e.g. prenucleation clusters, PNCs), liquid(-like) phases, as well as amorphous and nanocrystalline solids etc.. Does it, however, mean that all the minerals grow through intermediate phases, following a non-classical pathway?
In general, the precursor or intermediate species constitute different, often short-lived, points along the pathway from dissolved ions to the final solids (typically crystals in this context). In this regard synchrotron-based scattering (SAXS/WAXS/total scattering) appears to be the perfect tool to follow in situ and in a time-resolved manner the crystallization pathway because of the temporal and spatial length scales that can be directly accessed with these techniques. In this presentation we show how we used scattering to probe the crystallisation mechanisms of calcium sulfate, This system contains minerals that are widespread in diverse natural environments, but they are also important in various industrial settings. Our data demonstrate that calcium sulfate precipitation involves formation and aggregation of sub-3 nm anisotropic primary species. The actual crystallisation and formation of imperfect single crystals of calcium sulfate phases, takes place from the inside of the in itial aggregates. Hence, calcium sulfate follows a non-classical pathway.