Fabian Glaab

Stabilization of amorphous calcium carbonate in inorganic silica-rich environments.

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.

Formation and Evolution of Chemical Gradients and Potential Differences Across Self‐Assembling Inorganic Membranes

Silica gardens are well-known examples for the self-assembly of inorganic material (see figure). The growth of hollow tubes results in the spontaneous formation of two compartments with highly dissimilar pH and ion concentrations, which cause electrochemical potential differences across the membrane. Initially generated gradients are relieved over time through dynamic diffusion and precipitation processes.

Experimental techniques for the growth and characterization of silica biomorphs and silica gardens.

Silica biomorphs and silica gardens are canonical examples of precipitation phenomena yielding self-assembled nanocrystalline composite materials with outstanding properties in terms of morphology and texture. Both types of structures form spontaneously in alkaline environments and rely on simple, and essentially similar, chemistry. However, the underlying growth processes are very sensitive to a range of experimental parameters, distinct preparation procedures, and external conditions. In this chapter, we report detailed protocols for the synthesis of these extraordinary biomimetic materials and identify critical aspects as well as advantages and disadvantages of different approaches. Furthermore, modifications of established standard procedures are reviewed and discussed with respect to their benefit for the control over morphogenesis and the reproducibility of the experiments in both cases. Finally, we describe currently used techniques for the characterization of these fascinating structures and devise promising ways to analyze their growth behavior and formation mechanisms in situ and as a function of time.

Precipitation and Crystallization Kinetics in Silica Gardens

Abstract Silica gardens are extraordinary plant‐like structures resulting from the complex interplay of relatively simple inorganic components. Recent work has highlighted that macroscopic self‐assembly is accompanied by the spontaneous formation of considerable chemical gradients, which induce a cascade of coupled dissolution, diffusion, and precipitation processes occurring over timescales as long as several days. In the present study, this dynamic behavior was investigated for silica gardens based on iron and cobalt chloride by means of two synchrotron‐based techniques, which allow the determination of concentration profiles and time‐resolved monitoring of diffraction patterns, thus giving direct insight into the progress of dissolution and crystallization phenomena in the system. On the basis of the collected data, a kinetic model is proposed to describe the relevant reactions on a fundamental physicochemical level. The results show that the choice of the metal cations (as well as their counterions) is crucial for the development of silica gardens in both the short and long term (i.e. during tube formation and upon subsequent slow equilibration), and provide important clues for understanding the properties of related structures in geochemical and industrial environments.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group VI Terminal Borylene Complexes

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.