The interactions between additives and mineral precursors and intermediates are at the heart of additive-controlled crystallisation, which is of high importance for various fields. In this ...commentary, we reflect on potential modes of additive control according to classical nucleation theory on one hand, and from the viewpoint of the so-called pre-nucleation cluster pathway on the other. This includes a brief review of the corresponding literature. While the roles of additives are discussed generally, i.e., without specific chemical or structural details, corresponding properties are outlined where possible. Altogether, our discussion illustrates that “non-classical” nucleation pathways promise an improved understanding of additive-controlled scenarios, which could be utilised in targeted applications in various fields, ranging from scale inhibition to materials chemistry.
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► We review the recent literature on prenucleation clusters. ► The prenucleation cluster pathway is a non-classical concept of nucleation. ► Prenucleation clusters are solutes with ...“molecular” character in solution. ► The stable clusters may have been concealed by the ion pair concept and activity effects. ► Non-classical nucleation is a valuable concept for understanding biomineralization.
In this contribution, we review the recent literature on an alternative crystallization pathway involving stable clusters prior to nucleation and show that the prenucleation cluster pathway is a truly non-classical concept of nucleation. Prenucleation clusters are solutes with “molecular” character in aqueous solution. It becomes evident that the stable clusters may have been concealed by the so-called ion pair concept and activity effects. We show that non-classical nucleation via stable prenucleation clusters is a highly valuable concept for a novel understanding of phenomena observed in bio- and biomimetic mineralization, which however may hardly be rationalized by means of classical nucleation theory. The importance to combine experimental and theoretical studies is emphasized.
Understanding the role of polymers rich in aspartic acid (Asp) and glutamic acid (Glu) is the key to gaining precise control over mineralization processes. Despite their chemical similarity, ...experiments revealed a surprisingly different influence of Asp and Glu sequences. We conducted molecular dynamics simulations of Asp and Glu peptides in the presence of calcium and chloride ions to elucidate the underlying phenomena. In line with experimental differences, in our simulations, we indeed find strong differences in the way the peptides interact with ions in solution. The investigated Asp pentapeptide tends to pull a lot of ions into its vicinity, and many structures with clusters of calcium and chloride ions on the surface of the peptide can be observed. Under the same conditions, comparatively fewer ions can be found in proximity of the investigated Glu pentapeptide, and the structures are characterized by single calcium ions bound to multiple carboxylate groups. Based on our simulation data, we identified three reasons contributing to these differences, leading to a new level of understanding additive–ion interactions.
The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to ...monitor the early stages of the crystallization of ibuprofen as a model system for small organic molecules. Ibuprofen undergoes liquid–liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid–liquid miscibility gap were analyzed and confirmed. An increase in viscosity sustains the kinetic stability of the dense liquid intermediate. Since the distances between ibuprofen molecules within the dense liquid phase are similar to those in the crystal forms, this dense liquid phase is identified as a precursor phase in the nucleation of ibuprofen, in which densification is followed by generation of structural order. This discovery may make it possible to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments.
The nucleation mechanism of crystals of small organic molecules was studied for the crystallization of ibuprofen as a model system. Ibuprofen undergoes liquid–liquid phase separation prior to nucleation and forms a dense liquid phase that is identified as a precursor phase in the nucleation of ibuprofen. These results may provide insight for the development of new pharmaceutical formulations.
Despite numerous studies on the nucleation and crystallization of iron (oxyhydr)oxides, the roles of species developing during the early stages, especially primary clusters and intermediate amorphous ...particles, are still poorly understood. Herein, both ligand‐free and ligand‐protected amorphous iron oxide (AIO) clusters (<2 nm) were synthesized as precursors for magnetite formation. Thermal annealing can crystallize the clusters into magnetite particles, and AIO bulk phases with domains of pre‐aligned clusters are found to be direct precursors to crystals, suggesting a non‐classical aggregation‐based pathway that differs from the reported oriented attachment or particle accretion mechanisms.
Magnetic lines: Magnetite formation via thermal annealing of amorphous iron oxide clusters proceeds from domains of pre‐aligned clusters that can be direct precursors to crystals. This phenomenon cannot be understood as oriented attachment, and seems to represent a key step in this non‐classical, aggregation‐based pathway.
Calcium carbonate is an abundant substance that can be created in several mineral forms by the reaction of dissolved carbon dioxide in water with calcium ions. Through biomineralization, organisms ...can harness and control this process to form various functional materials that can act as anything from shells through to lenses. The early stages of calcium carbonate formation have recently attracted attention as stable prenucleation clusters have been observed, contrary to classical models. Here we show, using computer simulations combined with the analysis of experimental data, that these mineral clusters are made of an ionic polymer, composed of alternating calcium and carbonate ions, with a dynamic topology consisting of chains, branches and rings. The existence of a disordered, flexible and strongly hydrated precursor provides a basis for explaining the formation of other liquid-like amorphous states of calcium carbonate, in addition to the non-classical behaviour during growth of amorphous calcium carbonate.
Stable Prenucleation Calcium Carbonate Clusters Gebauer, Denis; V?lkel, Antje; C?lfen, Helmut
Science (American Association for the Advancement of Science),
12/2008, Letnik:
322, Številka:
5909
Journal Article
Recenzirano
Odprti dostop
Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. Huge amounts of carbon dioxide are retained as carbonate ions, and calcium ions represent a major contribution ...to water hardness. Despite its relevance, little is known about the precipitation mechanism of calcium carbonate, and specified complex crystal structures challenge the classical view on nucleation considering the formation of metastable ion clusters. We demonstrate that dissolved calcium carbonate in fact contains stable prenucleation ion clusters forming even in undersaturated solution. The cluster formation can be characterized by means of equilibrium thermodynamics, applying a multiple-binding model, which allows for structural preformation. Stable clusters are the relevant species in calcium carbonate nucleation. Such mechanisms may also be important for the crystallization of other minerals.
The suggested role of amorphous calcium carbonate in pathological mineralization highlights that future studies of such stones should not be based on analytical techniques that are sensitive only to ...crystalline CaCO3. Self-assembly processes of alkaline earth carbonates in the presence of silica, on the other hand, produce a unique class of composite materials with complex morphologies, as studied by Opel et al. Ochiai and Utsunomiya 21 investigate the crystal chemical properties of hydrous rare-earth phosphates, forming at an ambient temperature with fractions of an amorphous component with an increasing ionic radius. Ochiai, A.; Utsunomiya, S. Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature.
Calcium orthophosphates (CaPs), as hydroxyapatite (HAP) in bones and teeth are the most important biomineral for humankind. While clusters in CaP nucleation have long been known, their speciation and ...mechanistic pathways to HAP remain debated. Evidently, mineral nucleation begins with two ions interacting in solution, fundamentally underlying solute clustering. Here, we explore CaP ion association using potentiometric methods and computer simulations. Our results agree with literature association constants for Ca
and H
PO
, and Ca
and HPO
, but not for Ca
and PO
ions, which previously has been strongly overestimated by two orders of magnitude. Our data suggests that the discrepancy is due to a subtle, premature phase separation that can occur at low ion activity products, especially at higher pH. We provide an important revision of long used literature constants, where association of Ca
and PO
actually becomes negligible below pH 9.0, in contrast to previous values. Instead, CaHPO
dominates the aqueous CaP speciation between pH ~6-10. Consequently, calcium hydrogen phosphate association is critical in cluster-based precipitation in the near-neutral pH regime, e.g., in biomineralization. The revised thermodynamics reveal significant and thus far unexplored multi-anion association in computer simulations, constituting a kinetic trap that further complicates aqueous calcium phosphate speciation.
Water as the Key to Proto-Aragonite Amorphous CaCO3 Farhadi-Khouzani, Masoud; Chevrier, Daniel M.; Zhang, Peng ...
Angewandte Chemie (International ed.),
July 4, 2016, Letnik:
55, Številka:
28
Journal Article
Recenzirano
Temperature and pH value can affect the short‐range order of proto‐structured and additive‐free amorphous calcium carbonates (ACCs). Whereas a distinct change occurs in proto‐vaterite (pv) ACC above ...45 °C at pH 9.80, proto‐calcite (pc) ACC (pH 8.75) is unaffected within the investigated range of temperatures (7–65 °C). IR and NMR spectroscopic studies together with EXAFS analysis showed that the temperature‐induced change is related to the formation of proto‐aragonite (pa) ACC. The data strongly suggest that the binding of water molecules induces dipole moments across the carbonate ions in pa‐ACC as in aragonite, where the dipole moments are due to the symmetry of the crystal structure. Altogether, a (pseudo‐)phase diagram of the CaCO3 polyamorphism in which water plays a key role can be formulated based on variables of state, such as the temperature, and solution parameters, such as the pH value.
The binding of water molecules above 45 °C at pH 9.80 induces dipole moments across carbonate ions in proto‐aragonite amorphous CaCO3 as in the anhydrous crystalline polymorph aragonite, where the dipole moments are due to the crystal structure. This key role of water was inferred from IR (see picture) as well as NMR spectroscopy in combination with Ca K‐edge EXAFS analysis.
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