A classical view on nonclassical nucleation Smeets, Paul J. M.; Finney, Aaron R.; Habraken, Wouter J. E. M. ...
Proceedings of the National Academy of Sciences - PNAS,
09/2017, Letnik:
114, Številka:
38
Journal Article
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Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO₃) is often used as a model system to investigate nucleation mechanisms. Despite ...its great importance in geology, biology, and many industrial applications, CaCO₃ nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid–liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO₃ in dilute aqueous solutions. We propose that a dense liquid phase (containing 4–7 H₂O per CaCO₃ unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO₃ in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca2+ + z CO₃2− → z CaCO₃. The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.
Despite its importance in many industrial, geological and biological processes, the mechanism of crystallization from supersaturated solutions remains a matter of debate. Recent discoveries show that ...in many solution systems nanometre-sized structural units are already present before nucleation. Still little is known about the structure and role of these so-called pre-nucleation clusters. Here we present a combination of in situ investigations, which show that for the crystallization of calcium phosphate these nanometre-sized units are in fact calcium triphosphate complexes. Under conditions in which apatite forms from an amorphous calcium phosphate precursor, these complexes aggregate and take up an extra calcium ion to form amorphous calcium phosphate, which is a fractal of Ca(2)(HPO(4))(3)(2-) clusters. The calcium triphosphate complex also forms the basis of the crystal structure of octacalcium phosphate and apatite. Finally, we demonstrate how the existence of these complexes lowers the energy barrier to nucleation and unites classical and non-classical nucleation theories.
Although monodisperse amorphous silica nanoparticles have been widely investigated, their formation mechanism is still a topic of debate. Here, we demonstrate the formation of monodisperse ...nanoparticles from colloidally stabilized primary particles, which at a critical concentration undergo a concerted association process, concomitant with a morphological and structural collapse. The formed assemblies grow further by addition of primary particles onto their surface. The presented mechanism, consistent with previously reported observations, reconciles the different theories proposed to date.
The macroscopic friction behavior of water-swollen cross-linked poly(ethylene glycol)-based polyurethane coatings (PEG-based PU coatings) with varying PEG precursor mass is measured against a glass ...counter surface. Experimental data such as the water uptake and the indentation modulus are used to calculate an accurate value for the molar mass between cross-links
M
c
, which, in turn, is used for the estimation of the actual coating mesh size
ξ
. The friction, swelling and indentation data obtained are used to successfully deduce an empirical model for the quantitative description of the aqueous friction behavior of these coatings depending on the mesh size of the coatings and the sliding velocity only.
Liquid‐Phase (Scanning) Transmission Electron Microscopy (LP‐(S)TEM) has become an essential technique to monitor nanoscale materials processes in liquids in real‐time. Due to the pressure difference ...between the liquid and the microscope vacuum, bending of the silicon nitride (SiNx) membrane windows generally occurs. This causes a spatially varying liquid layer thickness that makes interpretation of LP‐(S)TEM results difficult due to a locally varying achievable resolution and diffusion limitations. To mediate these difficulties, it is shown: 1) how to quantitatively map liquid layer thickness for any liquid at less than 0.01 e− Å−2 total dose; 2) how to dynamically modulate the liquid thickness by tuning the internal pressure in the liquid cell, co‐determined by the Laplace pressure and the external pressure. It is demonstrated that reproducible inward bulging of the window membranes can be realized, leading to an ultra‐thin liquid layer in the central window area for high‐resolution imaging. Furthermore, it is shown that the liquid thickness can be dynamically altered in a programmed way, thereby potentially overcoming the diffusion limitations towards achieving bulk solution conditions. The presented approaches provide essential ways to measure and dynamically adjust liquid thickness in LP‐(S)TEM experiments, enabling new experiment designs and better control of solution chemistry.
Real‐time monitoring of nanoscale material processes in liquids by liquid‐phase (scanning) transmission electron microscopy is advanced by rapid dynamic control and quantification of the liquid layer thickness. These methods enable performing the largest part of the experiment using a thick liquid layer to avoid confinement effects and adjusting to a thin liquid layer whenever needed for imaging at higher resolution.
A kinetic Monte Carlo method to simulate photodegradation of a polymer coating is applied to the weathering process of a polyester-urethane clearcoat during artificial exposure under different ...conditions. Firstly, the optimised simulation parameters that yield the best match with experimentally measured results on the depth-resolved ester and urethane bond fractions are determined and compared for two different aerobic exposure experiments (one in a Weather-Ometer (WOM) and one in a Suntest equipment). Secondly, several other quantities that are obtained from the simulations, but cannot be determined experimentally, are compared, such as the fraction of newly formed crosslink bonds, absorptivity states, oxidised states, the fraction of radicals, the concentration of oxygen and the total amount of remaining material. Depth-inhomogeneity of the rate of photon absorption leads to the formation of distinct depth gradients in the WOM simulation, while a much more homogeneous evolution is obtained for the Suntest-air simulation. Photo-oxidative damage in the WOM simulation is more concentrated on the upper layer of the coating, resulting in the extensive evaporation of highly oxidised material, whereas degradation in the Suntest-air simulation is more spread out over the entire coating thickness, resulting in less material loss.
This paper describes the gelation of highly concentrated graphene/polymer dispersions triggered by mild heating. The gel formation is only dependent on the concentration of graphene with 3.25 mg mL−1 ...as the minimum value for graphene network formation. The graphene gel is then utilized for the preparation of colloidally stable and highly concentrated (52 mg mL−1) graphene pastes that demonstrate excellent performance in screen printing down to lines of 40 μm in width. Printed patterns dried at 100 °C for only 5 min exhibit sheet resistances of 30 Ω −1 at 25 μm thickness, thus, removing the need for long‐time high temperature annealing, doping, or other treatments. Such a low drying temperature, high printing definition, and compatibility with industrially relevant plastic and paper substrates brings high‐volume roll‐to‐roll application in printed flexible electronics within reach.
Graphene screen printing using a paste obtained by gelation of highly concentrated graphene dispersions is demonstrated. The graphene paste shows a high printing definition, enabling printing of 40 μm wide lines on PET foil and paper substrates. A sheet resistance of 30 Ω −1 at 25 μm is achieved after drying at 100 °C for 5 min, which brings high‐speed, high‐volume applications within reach.
A theory is presented for the onset of shear thickening in colloidal suspensions of particles, stabilized by an electrostatic repulsion. Based on an activation model, a critical shear stress can be ...derived for the onset of shear thickening in dense suspensions for a constant potential and a constant charge approach of the spheres. Unlike previous models, the total interaction potential is taken into account (sum of attraction and repulsion). The critical shear stress is related to the maximum of the total interaction potential scaled by the free volume per particle. A comparison with experimental investigations shows the applicability of the theory.
The electroviscous effects in very dilute aqueous dispersions of amorphous silica (Ludox) were investigated at various levels of salt, pH, and volume fraction (<0.01) of solids. Viscosities were much ...higher than predicted from existing theories, which is ascribed to the formation of a thick, gel-like surface layer on the particle. The volume of a particle adjusts itself almost reversibly to the salt and pH levels of the liquid and can grow up to four times the dry volume. This explains the apparent discrepancy between published dry and wet particle sizes and also the reported anomalously large number of bound water layers around a particle. The existence of a gel layer leads to an abnormally large amount of surface conductance; this may explain the anomalities found in electrophoresis. The validity of the model is also supported by published results of the amount of nonbulk water as found with NMR.
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