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ChemPhysChem article available online "Entropy drives calcium carbonate ion association"
The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalized by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions. [ChemPhysChem (2016); DOI: 10.1002/cphc.201600653]
Cristina Ruiz Agudo has joined the team as a postdoctoral researcher. Welcome!
The Journal of Physical Chemistry Letters article available online "The molecular mechanism of iron(III) oxide nucleation"
A molecular understanding of the formation of solid phases from solution would be beneficial for various scientific fields. However, nucleation pathways are still not fully understood, whereby the case of iron (oxyhydr)oxides poses a prime example. We show that in the pre-nucleation regime, thermodynamically stable solute species up to a few nanometers in size are observed, which meet the definition of pre-nucleation clusters. Nucleation then is not governed by a critical size, but rather by the dynamics of the clusters that are forming at the distinct nucleation stages, based on the chemistry of the linkages within the clusters. This resolves a longstanding debate in the field of iron oxide nucleation, and the results may generally apply to oxides forming via hydrolysis and condensation. The (molecular) understanding of the chemical basis of phase separation is paramount for e.g. tailoring size, shape and structure of novel nanocrystalline materials. [J. Chem. Phys. Lett. (2016); DOI: 10.1021/acs.jpclett.6b01237]
Angewandte Chemie International Edition communication available online "Water as the key to proto-aragonite amorphous CaCO3"
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. [Angew. Chem. Int. Ed. (2016); DOI: 10.1002/anie.201603176]
ChemComm article available online "A solvothermal method for synthesizing monolayer protected amorphous calcium carbonate clusters"
A solvothermal method was developed for synthesizing organic monolayer protected amorphous calcium carbonate clusters using 10,12-pentacosadiynoic acid as ligand, ethanol as solvent and NaHCO3 decomposition as CO2 source, which can be extended to synthesize other monolayer protected mineral clusters. [ChemComm (2016); DOI: 10.1039/C6CC03010F]
Crystal Growth & Design article available online "pH-dependent schemes of calcium carbonate formation in presence of alginates"
From recent studies on bone and shell formation, the importance of polysaccharides in biomineralization processes is gradually being recognized. Through ion-complexation and self-assembly properties, such macromolecules have remarkable effects on mineralization. However their influences on the different regimes of crystallization including the interactions with precursor species are unclear. The present study therefore addresses calcium carbonate mineralization in presence of alginates, a class of linear co-polymeric saccharides composed of β-1,4 linked D-mannuronic and L-guluronic acid. During mineralization, this biopolymer is found to exert a pH dependent control over mineralization pathways in terms of the stability of pre-nucleation clusters, inhibitory effect towards nucleation and initially formed post-nucleation products. Remarkably in presence of this macromolecular additive, either amorphous or crystalline vaterite particles can be selectively nucleated in a pH dependent manner. This is validated by electron microscopy wherein vaterite particles are intimately associated with alginate assemblies after nucleation at pH 9.75. At lower pH, aggregates of amorphous particles are formed. Thus in addition to the general focus on biochemical properties of additives, solution pH, a physiologically fundamental parameter significantly alters the scheme of mineralization. [Cryst. Growth Des. (2016); DOI: 10.1021/acs.cgd.5b01488]
Minerals article available online "The Role of Chloride Ions during the Formation of Akaganéite Revisited"
Iron(III) hydrolysis in the presence of chloride ions yields akaganéite, an iron oxyhydroxide mineral with a tunnel structure stabilized by the inclusion of chloride. Yet, the interactions of this anion with the iron oxyhydroxide precursors occurring during the hydrolysis process, as well as its mechanistic role during the formation of a solid phase are debated. Using a potentiometric titration assay in combination with a chloride ion-selective electrode, we have monitored the binding of chloride ions to nascent iron oxyhydroxides. Our results are consistent with earlier studies reporting that chloride ions bind to early occurring iron complexes. In addition, the data suggests that they are displaced with the onset of oxolation. Chloride ions in the akaganéite structure must be considered as remnants from the early stages of precipitation, as they do not influence the basic mechanism, or the kinetics of the hydrolysis reactions. The structure-directing role of chloride is based upon the early stages of the reaction. The presence of chloride in the tunnel-structure of akagenéite is due to a relatively strong binding to the earliest iron oxyhydroxide precursors, whereas it plays a rather passive role during the later stages of precipitation. [Minerals 5, 778 (2015)]
"Prenucleation clusters and non-classical nucleation" is among Nano Today's most cited articles.
CrystEngComm article available online "Synergy of Mg2+ and poly(aspartic acid) in additive-controlled calcium carbonate precipitation"
Additive-controlled precipitation of calcium carbonate is central to various fields of research. Technically, scale formation is an important problem, where polycarboxylates are most commonly employed as inhibitors. Herein, we show that the combination of poly(aspartic acid) with magnesium ions leads to synergistic effects that bring about a dramatic increase in the efficiency towards inhibition of nucleation and growth of nanoscopic CaCO3 precursors. These effects can also be crucial in biomineralization processes, where polycarboxylates and magnesium ions are thought to play important roles. [CrystEngComm (2015); DOI: 10.1039/C5CE00452G]
Julian Gale visits our group as a Senior Fellow of the Zukunftskolleg, and will stay until July 3rd. Julian's research concerns the development and application of theoretical or computational methods to problems in chemistry, physics, geochemistry, mineralogy and materials science. Current foci include; polymorphism, crystallisation, mineral-water interfaces, fuel cells, lithium battery materials, solvent extraction, and desalination membranes. Within the framework of the theory colloquium of the Physics Department, Julian will give a talk in P603 on Monday June 8, 13:30.
CrystEngComm article available online "Disordered amorphous calcium carbonate from direct precipitation"
Amorphous calcium carbonate (ACC) is known to play a prominent role in biomineralization. Different studies on the structure of biogenic ACCs have illustrated that they can have distinct short-range orders. However, the origin of so-called proto-structures in synthetic and additive-free ACCs is not well understood. In the current work, ACC has been synthesised in iso-propanolic media by direct precipitation from ionic precursors, and analysed utilising a range of different techniques. The data suggest that this additive-free type of ACC does not resemble clear proto-structural motifs relating to any crystalline polymorph. This can be explained by the undefined pH value in iso-propanolic media, and the virtually instantaneous precipitation. Altogether, this work suggests that aqueous systems and pathways involving pre-nucleation clusters are required for the generation of clear proto-structural features in ACC. Experiments on the ACC- to-crystalline transformation in solution with and without ethanol highlight that polymorph selection is under kinetic control, while the presence of ethanol can control dissolution re-crystallisation pathways. [CrystEngComm (2015), DOI: 10.1039/c5ce00720h]
Faraday Discussions article available online "High-resolution insights into the early stages of silver nucleation and growth"
Nucleation and growth of silver nanoparticles has already been investigated with various experimental and computational tools, however, owing to inherent problems associated with the analytical characterization of nucleation processes, there is a generic lack of experimental data regarding the earliest precursors and smallest Ag(0) clusters. Here, we address this problem by the application of Synthetic Boundary Crystallization Ultracentrifugation, utilizing a multiwavelength detector for the first time, complemented by a specialized Titration Assay. These techniques shed new light on silver nanoparticle precursors existing in the pre-nucleation regime and the initially nucleated ensemble of nanoclusters. For the first time, we present experimental data of UV-vis spectra for fractionated silver clusters. These allow for unsurpassed insights into the sequence of nucleation and early growth species as well as their optical properties. [Faraday Discussions (2015), DOI: 10.1039/C4FD00269E]
Grazyna Durak has joined the team as a postdoctoral researcher. Welcome!
Grazyna's project is a collaboration with Thomas Böttcher.
Nanoscale article available online "New Insights into the Early Stages of Silica-Controlled Barium Carbonate Crystallisation"
Recent work has demonstrated that the dynamic interplay between silica and carbonate during co-precipitation can result in the self-assembly of unusual, highly complex crystal architectures with morphologies and textures resembling those typically displayed by biogenic minerals. These so-called biomorphs were shown to be composed of uniform elongated carbonate nanoparticles that are arranged according to a specific order over mesoscopic scales. In the present study, we have investigated the circumstances leading to the continuous formation and stabilisation of such well-defined nanometric building units in these inorganic systems. For this purpose, in-situ potentiometric titration measurements were carried out in order to monitor and quantify the influence of silica on both the nucleation and early growth stages of barium carbonate crystallisation in alkaline media at constant pH. Complementarily, the nature and composition of particles occurring at different times in samples under various conditions were characterised ex situ by means of high-resolution electron microscopy and elemental analysis. The collected data clearly evidence that added silica affects carbonate crystallisation from the very beginning on (i.e. already prior to, during, and shortly after nucleation), eventually arresting growth at the nanoscale by cementation of BaCO3 particles within a siliceous matrix. Our findings thus shed novel light on the fundamental processes driving bottom-up self-organisation in silica-carbonate materials and, for the first time, provide a direct experimental proof that silicate species are responsible for the miniaturisation of carbonate crystals during growth of biomorphs, hence confirming previously discussed theoretical models for their formation mechanism. [Nanoscale (2014), DOI: 10.1039/C4NR05436A]
Yu-Chieh Huang has joined the team as a PhD student. Welcome!
Bingqiang Lu has joined the team as a postdoctoral researcher. Welcome!
Denis Gebauer has been elected as a new member of the executive board of the Graduate School Chemistry.
Denis Gebauer has been promoted to a Research Fellow (5-year fellowship) of the Zukunftskolleg of the University of Konstanz.