Matthias Kellermeier

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.

Alkali Metal Oligoether Carboxylates—A New Class of Ionic Liquids

On the way to greener ILs: The combination of a short oligoether carboxylate (CH3O-(CH2CH2O)3-CH2COO-) with small alkali metal cations leads to the formation of a new class of ionic liqs. that exhibit high thermal and electrochem. stability as well as low cytotoxicity.

The multiple effects of amino acids on the early stages of calcium carbonate crystallization

Abstract Proteins have found their way into many of Nature’s structures due to their structural stability, diversity in function and composition, and ability to be regulated as well as be regulators themselves. In this study, we investigate the constitutive amino acids that make up some of these proteins which are involved in CaCO3 mineralization – either in nucleation, crystal growth, or inhibition processes. By assaying all 20 amino acids with vapor diffusion and in situ potentiometric titration, we have found specific amino acids having multiple effects on the early stages of CaCO3 crystallization. These same amino acids have been independently implicated as constituents in liquid-like precursors that form mineralized tissues, processes believed to be key effects of biomineralization proteins in several biological model systems.

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.

Oligoether Carboxylates: Task-Specific Room-Temperature Ionic Liquids

Recently, a new family of ionic liquids based on oligoether carboxylates was introduced. 2,5,8,11-Tetraoxatridecan-13-oate (TOTO) was shown to form room-temperature ionic liquids (RTILs) even with small alkali ions such as lithium and sodium. However, the alkali TOTO salts suffer from their extremely high viscosities and relatively low conductivities. Therefore, we replaced the alkali cations by tetraalkylammonium (TAA) ions and studied the TOTO salts of tetraethyl- (TEA), tetrapropyl- (TPA), and tetrabutylammonium (TBA). In addition, the environmentally benign quaternary ammonium ion choline (Ch) was included in the series. All salts were found to be ionic liquids at ambient temperatures with a glass transition typically at around -60 °C. Viscosities, conductivities, solvent polarities, and Kamlet-Taft parameters were determined as a function of temperature. When using quaternary ammonium ions, the viscosities of the resulting TOTO ionic liquids are >600 times lower, whereas conductivities increase by a factor of up to 1000 compared with their alkali counterparts. Solvent polarities further reveal that choline and TAA cations yield TOTO ionic liquids that are more polar than those obtained with the, per se, highly polar sodium ion. Results are discussed in terms of ion-pairing and structure-breaking concepts with regard to a possible complexation ability of the TOTO anion.

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 rationalised 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.

Biodegradability and cytotoxicity of choline soaps on human cell lines: effects of chain length and the cation

Using choline as a counterion in fatty acid surfactants substantially increases their water solubility as compared to classical sodium and potassium soaps, and thereby enables the application of desirable longer-chain derivatives at ambient temperature. Since choline can be decomposed both physiologically and environmentally, corresponding fatty acid soaps are considered to be highly biocompatible. Recent toxicity and biodegradability studies of choline ionic liquids, including anions such as short- and middle-chain alkanoates, have verified the expected low toxic impact. However, according to the European Cosmetic Directive 76/768/EEC, all salts of choline are forbidden in cosmetic products, mainly just due to its classification as a quaternary ammonium ion. In order to facilitate their application in the future, we have investigated the biodegradability of choline soaps (ChCm) with alkyl chain lengths of m = 12–18 according to the OCDE 301F standard. Further, the cytotoxicity of ChCm surfactants with m = 8–16 was determined, both for odd- and even-numbered fatty acids. Studies were carried out using two different human cell lines, namely cervix carcinoma cells (HeLa) and keratinocytes (SK-Mel-28). For a better comparability to common soaps and to shed light on the influence of the cation, sodium and potassium homologues were also investigated. Results reveal an unexpected non-linear relationship between the hydrophobic chain length and the IC50 value. Most importantly, the presented data show that IC50 values of ChCm surfactants coincide with those of the widely applied sodium and potassium soaps. This demonstrates that choline carboxylate surfactants are harmless and thus strongly supports their applicability in customer end products.

Amino acids form prenucleation clusters: ESI-MS as a fast detection method in comparison to analytical ultracentrifugation

Electrospray ionisation mass spectrometry (ESI-MS) is a fast method which is able to provide molecular mass information with high precision. In this contribution, we show that prenucleation clusters—species recently found to play a pivotal role in crystallisation processes—are detected in addition to monomers by analytical ultracentrifugation (AUC) for the whole range of DL-amino acids, while higher oligomers are simultaneously observed in ESI-MS spectra. This suggests ESI-MS is a fast method to identify systems, which form prenucleation clusters. The occurrence of these clusters as relevant precursors in non-classical nucleation scenarios thus appears to be a more common phenomenon than so far assumed.

Choline alkylsulfates - New promising green surfactants

In this work we show how a new promising green and highly water-soluble surfactant can be designed based on recent progress in the knowledge of counterion-headgroup binding and crystallization behavior. The result is the combination of a most classical surfactant anion, dodecylsulfate (DS), with choline (Ch), a natural green cation. The advantage of the physiological metabolite choline is its bulky structure that prevents ChDS from easy crystallization and thus leads to a considerable lowering of the Krafft point down to 0°C. The counterion-headgroup binding is reflected by the aqueous phase behavior of ChDS. Conductivity, surface tension, and cryo-TEM measurements allow the characterization of the dilute micellar region, while the penetration scan technique enables the establishment of a preliminary aqueous phase diagram. In addition, the influence of different mono- and divalent salts on the solubility of ChDS is investigated. The results are compared to the alkali sulfate and alkylcarboxylate homologs, and reveal that ChDS is less sensitive towards addition of salts than, for instance, choline carboxylates due to an increased counterion-headgroup association. Further, cytotoxicity tests on HeLa and SK-Mel 28 cells are presented and compared to other surfactants, showing that ChDS is no more harmful than its sodium counterpart SDS. Taken together, our findings highlight that the harmless green cation choline is of great potential for the design of new surfactants.

Investigating the Early Stages of Mineral Precipitation by Potentiometric Titration and Analytical Ultracentrifugation

Despite the importance of crystallization for various areas of research, our understanding of the early stages of the mineral precipitation from solution and of the actual mechanism of nucleation is still rather limited. Indeed, detailed insights into the processes underlying nucleation may enable a systematic development of novel strategies for controlling mineralization, which is highly relevant for fields ranging from materials chemistry to medicine. In this work, we describe experimental aspects of a quantitative assay, which relies on pH titrations combined with in situ metal ion potentiometry and conductivity measurements. The assay has originally been designed to study the crystallization of calcium carbonate, one of the most abundant biominerals. However, the developed procedures can also be readily applied to any compound containing cations for which ion-selective electrodes are available. Besides the possibility to quantitatively assess ion association prior to nucleation and to directly determine thermodynamic solubility products of precipitated phases, the main advantage of the crystallization assay is the unambiguous identification of the different stages of precipitation (i.e., prenucleation, nucleation, and early postnucleation) and the characterization of the multiple effects of additives. Furthermore, the experiments permit targeted access to distinct precursor species and intermediate stages, which thus can be analyzed by additional methods such as cryo-electron microscopy or analytical ultracentrifugation (AUC). Regarding ion association in solution, AUC detects entities significantly larger than simple ion pairs, so-called prenucleation clusters. Sedimentation coefficient values and distributions obtained for the calcium carbonate system are discussed in light of recent insights into the structural nature of prenucleation clusters.