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.

Thermosensitive nanogels were prepared from photo-cross-linkable copolymers of N-isopropylacrylamide (NIPAAm) and 2-dimethylmaleinimido ethylacrylamide (DMIAAm). The colloidal nanogels were formed by UV-irradiated solutions of thermosensitive polymers in water at 45 °C. The size of nanogels could be controlled by the concentration of photopolymer solutions, the amount of chromophore DMIAAm in the photopolymer chains, and sodium dodecyl sulfate (SDS) concentrations. Analytical ultracentrifugation could reveal the degree of swelling of the different nanogel preparations, confirming the data from dynamic light scattering. In addition, the quantitative determination of un-cross-linked polymer species was possible. Even the determination of the degree of swelling distribution was reported for a nanogel for the first time. Hydrogel particles could be prepared with low SDS concentration or even in the absence of a surfactant. Therefore, the effects on swelling produced by the addition of alcohols to these nanogels could be investigated, excluding the influence of SDS on the swelling behavior of these nanogels.

The self-assembly of double-hydrophilic poly(ethylene oxide)–poly(2-methyl-2-oxazoline) diblock copolymers in water has been studied. Isothermal titration calorimetry, small-angle X-ray scattering, and analytical ultracentrifugation suggest that only single polymer chains are present in solution. In contrast, light scattering and transmission electron microscopy detect aggregates with radii of ca. 100 nm. Pulsed field gradient NMR spectroscopy confirms the presence of aggregates, although only 2% of the polymer chains undergo aggregation. Water uptake experiments indicate differences in the hydrophilicity of the two blocks, which is believed to be the origin of the unexpected aggregation behavior (in accordance with an earlier study by Ke et al. [Macromolecules 2009, 42, 5339–5344]). The data therefore suggest that even in double-hydrophilic block copolymers, differences in hydrophilicity are sufficient to drive polymer aggregation, a phenomenon that has largely been overlooked or ignored so far.

Investigation of the thermo-reversible properties of different poly(N-isopropyl acrylamide) samples, including microgels and block copolymers, with a combination of methods such as electron microscopy, dynamic light scattering, analytical ultracentrifugation, electrophoresis and ultrasound resonator technology allows comprehensive characterisation of the phase transition. By the combination of methods, it was possible to show that the precipitated polymer phase contains at 40°C between 40 and 50 vol.% of water. Besides free bulk water, there is also bound water that strongly adheres to the N-isopropyl acrylamide units (about 25 vol.%). Ultrasound resonator technology, which is a non-sizing characterisation method, revealed for the microgel particles two more temperatures (at about 35 and between 40°C and 50°C depending on the chemical nature) where characteristic changes in the ultrasound attenuation take place. Moreover, the experimental data suggest that the phase transition temperature is related to surface charge density of the precipitated particles.

No abstract