Formation of Precipitation Ellipsoidal Disks and Spheres in the Wake of a Planar Diffusion Front.

Pattern formation is one of the examples of self-organization. In the generation of patterns, the coupling between the mass transport of the chemical species and their chemical reactions plays an important role. Periodic precipitation (Liesegang phenomenon) is a type of pattern formation in which layered precipitation structures form in the wake of the diffusion front. Here, we show a new type of precipitation pattern formation in zeolitic imidazolate framework-67 in a solid hydrogel column in a test tube manifested in the generation of precipitation ellipsoidal disks and spheres in the wake of the planar diffusion front of the outer electrolyte (2-methylimidazole). To increase the probability of the emergence of ellipsoidal disks and spheres, the surfaces of the borosilicate test tubes were chemically treated and functionalized. To support the experimental findings, we developed a reaction-diffusion model that qualitatively describes the formation of precipitate ellipsoidal disks and spheres in a test tube.

Synthesis and composition modification of precipitate tubes in a confined flow reactor.

Precipitation reactions coupled to various transport phenomena, such as flow or diffusion, lead to the formation of different spatial gradients which can be influenced by tuning the experimental parameters (e.g., reactant concentration, flow rate, reactor geometry, etc.). Thereby it gives us the opportunity to change the micro and macrostructure of the products. Herein, we investigate the precipitate tube formation in a flow-driven system applying a horizontal confined geometry for individual and composite alkaline earth metal (Mg(II), Ca(II), Sr(II), and Ba(II))-carbonate systems. First we attempted to achieve tube-like structures in each reactive system by increasing the reactant concentrations. It is found that the precipitate tubes are not present in the magnesium-carbonate system even at extremely high concentrations. Therefore, we doped the magnesium solution with other alkaline earth metal ions, which resulted in the desired precipitate patterns. Besides the macroscopic characteristics, the microstructure of the crystals (morphology, crystal phase, size, and composition) could also be modified by combining the ions and varying their concentration ratio. In addition, by varying the relative concentration of the alkaline earth metal cations, separated and composite crystals could also be produced as different extrema. These were spatially isolated due to the reactor geometry, and thus the products, which contain the metal ions either homogeneously or individually, can be easily separated from each other.

Synthesis of Zeolitic Imidazolate Framework-8 Using Glycerol Carbonate.

In this study, we show that glycerol carbonate (GlyC), a bio-based derivative of glycerol, can be used as a suitable green solvent for the synthesis of metal-organic frameworks (MOFs). In particular, a zinc-based zeolitic imidazolate framework-8 (ZIF-8) was synthesized by exploring several different experimental conditions (in terms of temperature, reaction time, and reactants' concentrations) to find that the yield of the reaction and the quality of the products, measured in terms of crystallinity, surface area, and porosity, were in line with those obtained in the most commonly (non-green) used solvents. GlyC was also found to be reusable for several cycles, maintaining the same original quality as a solvent for the synthesis. Finally, some indicators for the assessment of the greenness of a process (E-factor and PMI) revealed a milder environmental impact of GlyC with respect to other solvents.

Non-autonomous zinc-methylimidazole oscillator and the formation of layered precipitation structures in a hydrogel.

Oscillations are one of the intrinsic features of many animate and inanimate systems. The oscillations manifest in the temporal periodic change of one or several physical quantities describing the systems. In chemistry and biology, this physical quantity is the concentration of the chemical species. In most chemical oscillatory systems operating in batch or open reactors, the oscillations persist because of the sophisticated chemical reaction networks incorporating autocatalysis and negative feedback. However, similar oscillations can be generated by periodically changing the environment providing non-autonomous oscillatory systems. Here we present a new strategy for designing a non-autonomous chemical oscillatory system for the zinc-methylimidazole. The oscillations manifested in the periodic change of the turbidity utilizing the precipitation reaction between the zinc ions and 2-methylimidazole (2-met) followed by a partial dissolution of the formed precipitate due to a synergetic effect governed by the ratio of the 2-met in the system. Extending our idea spatiotemporally, we also show that these precipitation and dissolution phenomena can be utilized to create layered precipitation structures in a solid agarose hydrogel.

Application of a chemical clock in material design: chemically programmed synthesis of zeolitic imidazole framework-8.

Here we show a time-programmed and autonomous synthesis of zeolitic imidazole framework-8 (ZIF-8) using a methylene glycol-sulfite clock reaction. The induction period of the driving clock reaction, thus, the appearance of the ZIF-8 can be adjusted by the initial concentration of one reagent of the chemical clock. The autonomously synthesized ZIF-8 showed excellent morphology and crystallinity.

Synthesis of zeolitic imidazolate framework-8 and gold nanoparticles in a sustained out-of-equilibrium state.

The design and synthesis of crystalline materials are challenging due to the proper control over the size and polydispersity of the samples, which determine their physical and chemical properties and thus applicability. Metal - organic frameworks (MOFs) are promising materials in many applications due to their unique structure. MOFs have been predominantly synthesized by bulk methods, where the concentration of the reagents gradually decreased, which affected the further nucleation and crystal growth. Here we show an out-of-equilibrium method for the generation of zeolitic imidazolate framework-8 (ZIF-8) crystals, where the non-equilibrium crystal growth is maintained by a continuous two-side feed of the reagents in a hydrogel matrix. The size and the polydispersity of the crystals are controlled by the fixed and antagonistic constant mass fluxes of the reagents and by the reaction time. We also present that our approach can be extended to synthesize gold nanoparticles in a redox process.

Enhancing the yield of calcium carbonate precipitation by obstacles in laminar flow in a confined geometry.

Flow-driven precipitation experiments are performed in model porous media shaped within the confinement of a Hele-Shaw cell. Precipitation pattern formation and the yield of the reaction are investigated when borosilicate glass beads of different sizes are used in a mono-layer arrangement. The trend of the amount of precipitate produced in various porous media is estimated via visual observation. In addition, a new method is elaborated to complement such image analysis based results by titration experiments performed on gel-embedded precipitate patterns. The yield of confined porous systems is compared to experiments carried out in unsegmented reactors. It is found that the obstacles increase the amount of product and preserve its radial spatial distribution. The precipitate pattern is successfully conserved in a slightly cross-linked hydrogel matrix and its microstructure is examined using SEM. The spatial distribution of the precipitate across the cell gap is revealed using X-ray microtomography.

Effects of radial injection and solution thickness on the dynamics of confined A + B → C chemical fronts.

The spatio-temporal dynamics of an A + B → C front subjected to radial advection is investigated experimentally in a thin solution layer confined between two horizontal plates by radially injecting a solution of potassium thiocyanate (A) into a solution of iron(iii) nitrate (B). The total amount and spatial distribution of the product FeSCN2+ (C) are measured for various flow rates Q and solution thicknesses h. The long-time evolution of the total amount of product, nC, is compared to a scaling obtained theoretically from a one-dimensional reaction-diffusion-advection model with passive advection along the radial coordinate r. We show that, in the experiments, nC is significantly affected when varying either Q or h but scales as nC∼Q-1/2V where V is the volume of injected reactant A provided the solution thickness h between the two confining plates is sufficiently small, in agreement with the theoretical prediction. Our experimental results also evidence that the temporal evolution of the width of the product zone, WC, follows a power law, the exponent of which varies with both Q and h, in disagreement with the one-dimensional model that predicts WC∼t1/2. We show that this experimental observation can be rationalized by taking into account the non-uniform profile of the velocity field of the injected reactant within the cell gap.

The impact of reaction rate on the formation of flow-driven confined precipitate patterns.

The production of solid materials via chemical reactions is abundant both in nature and in industrial processes. Precipitation reactions coupled with transport phenomena lead to enhanced product properties not observed in the traditional well-stirred systems. Herein, we present a flow-driven pattern formation upon radial injection in a confined geometry for various chemical systems to show how reaction kinetics modifies the emerging precipitation patterns. It is found that chemically similar elements, such as alkaline earth or transition metals react on very different time scales under the same experimental conditions. The patterns are quantified and compared both with literature results obtained in unconfined solution layers and with hydrodynamic simulations.

Magnetic-Field-Manipulated Growth of Flow-Driven Precipitate Membrane Tubes.

Chemobrionics is an emerging scientific field focusing on the coupling of chemical reactions and different forms of motion, that is, transport processes. Numerous phenomena appearing in various gradient fields, for example, pH, concentration, temperature, and so on, are thoroughly investigated to mimic living systems in which spatial separation plays a major role in proper functioning. In this context, chemical garden experiments have received increased attention because they inherently involve membrane formation and various transport processes. In this work, a noninvasive external magnetic field was applied to gain control over the directionality of membrane structures obtained by injecting one reactant solution into the other in a three-dimensional domain. The geometry of the resulted patterns was quantitatively characterized as a function of the injection rate and the magnitude of magnetic induction. The magnetic field was proven to influence the microstructure of precipitate tubes by diminishing spatial defects.

High-speed tracking of fast chemical precipitations.

Heterogeneous reactions taking place in the aqueous phase bear significant importance both in applied and fundamental research. Among others, producing solid catalysts, crystallizing biomorphs or pharmaceutically relevant polymorphs, and yielding bottom-up synthesised precipitate structures are prominent examples. To achieve a better control on product properties, reaction kinetics and mechanisms must be taken into account especially in dynamic systems where transport processes are coupled to chemistry. Since the characteristic time scale of numerous precipitation reactions falls below 1 s within the relevant concentration range, unique experimental protocols are needed. Herein we present a high-speed camera supported experimental procedure capable of determining such characteristic time scales in the range of 10 ms to 1 s. The method is validated both experimentally and by performing 3D hydrodynamic simulations.

Influence of microscopic precipitate structures on macroscopic pattern formation in reactive flows in a confined geometry.

Thanks to the coupling between chemical precipitation reactions and hydrodynamics, new dynamic phenomena may be obtained and new types of materials can be synthesized. Here we experimentally investigate how the characteristic microscopic crystal properties affect the macroscopic pattern obtained. To shed light on such interactions, different reactant solutions are radially injected into a calcium chloride solution at different volumetric flow rates in a confined geometry. Depending on the reactants used and the flow conditions, deformed precipitate membranes have been observed due to reaction-driven viscous fingering. In such cases we show that upon injection a large number of small particles is produced in situ by the reaction at the miscible interface between the two reactant solutions. Therefore, a colloidal gel composed of those tiny particles is pushed forward by the injected aqueous solution giving rise to a viscosity gradient-driven hydrodynamic instability.

Macroscale precipitation kinetics: towards complex precipitate structure design.

Producing self-assembled inorganic precipitate micro- and macro-structures with tailored properties may pave the way for new possibilities in, e.g., materials science and the pharmaceutical industry. One set of important parameters to maintain appropriate control over the yield falls in the frame of reaction kinetics, which affects the possible coupling between hydrodynamics and chemical reactions under flow conditions. In this study, we present a spectrophotometric method to experimentally determine the characteristic timescales of precipitation reactions. It is also shown that the nickel-oxalate model system - despite the fast chemical complexation equilibria taking place - can be kinetically described by either Classical Nucleation Theory or the classical homogeneous kinetics approach. The applicability of our results is illustrated via injection experiments intrinsically exhibiting coupling between chemistry and hydrodynamics. Therefore, we suggest that easy-to-handle power law functions may be applied to characterize the precipitation kinetics in flow systems.

Osmotic contribution to the flow-driven tube formation of copper-phosphate and copper-silicate chemical gardens.

We have produced hollow copper-containing precipitate tubes using a flow-injection technique, and characterized their linear and volume growth. It is shown that the ratio of the volume increase rate to that of pumping is constant independent of the chemical composition. It is also found that osmosis significantly contributes to the tube growth, since the inward flux of chemical species dominates during the precipitate pattern formation. The asymmetric hydrodynamic field coupled with the inherent concentration and pH gradients results in different particle morphology on the two sides of the precipitate membrane. While the tubes have a smooth outer surface, the inner walls are covered with nanoflowers for copper phosphate and with nanoballs for copper silicate.

Determination of the diffusion coefficient of hydrogen ion in hydrogels.

The role of diffusion in chemical pattern formation has been widely studied due to the great diversity of patterns emerging in reaction-diffusion systems, particularly in H+-autocatalytic reactions where hydrogels are applied to avoid convection. A custom-made conductometric cell is designed to measure the effective diffusion coefficient of a pair of strong electrolytes containing sodium ions or hydrogen ions with a common anion. This together with the individual diffusion coefficient for sodium ions, obtained from PFGSE-NMR spectroscopy, allows the determination of the diffusion coefficient of hydrogen ions in hydrogels. Numerical calculations are also performed to study the behavior of a diffusion-migration model describing ionic diffusion in our system. The method we present for one particular case may be extended for various hydrogels and diffusing ions (such as hydroxide) which are relevant e.g. for the development of pH-regulated self-healing mechanisms and hydrogels used for drug delivery.

Flow-driven control of calcium carbonate precipitation patterns in a confined geometry.

Upon injection of an aqueous solution of carbonate into a solution of calcium ions in the confined geometry of a Hele-Shaw cell, various calcium carbonate precipitation patterns are observed. We discuss here the properties of these precipitation structures as a function of the injection flow rate and concentrations of the reactants. We show that such flow-controlled conditions can be used to influence the total amount and the spatial distribution of the solid phase produced as well as the reaction efficiency defined here as the amount of product formed for a given initial concentration of the injected solution.

Self-Assembly of Charged Nanoparticles by an Autocatalytic Reaction Front.

In this work we present that aggregation of charged and pH sensitive nanoparticles can be spatiotemporally controlled by an autonomous way using the chlorite-tetrathionate autocatalytic front, where the front regulates the electrostatic interaction between nanoparticles due to protonation of the capping (carboxylate-terminated) ligand. We found that the aggregation and sedimentation of nanoparticles in liquid phase with the effect of reversible binding of the autocatalyst (H(+)) play important roles in changing the front stability (mixing length) and the velocity of the front in both cases when the fronts propagate upward and downward. Calculation of interparticle interactions (electrostatic and van der Waals) with the measurement of front velocity revealed that the aggregation process occurs fast (within a few seconds) at the front position.

Three-dimensional convection-driven fronts of the exothermic chlorite-tetrathionate reaction.

Horizontally propagating autocatalytic reaction fronts in fluids are often accompanied by convective motion in the presence of gravity. We experimentally and numerically investigate the stable complex three-dimensional pattern arising in the exothermic chlorite-tetrathionate reaction as a result of the antagonistic thermal and solutal contribution to the density change. By particle image velocimetry measurements, we construct the flow field that stabilizes the front structure. The calculations applied for incompressible fluids using the empirical rate-law model reproduce the experimental observations with good agreement.

Self-organization of calcium oxalate by flow-driven precipitation.

We have analyzed the emerging precipitate pattern of calcium-oxalate in a flow system. The circular symmetry is broken because of the hydrodynamic instability at the tip of the underlying gravity current. The presence of a concentration gradient maintained by the flow leads to the enrichment of the thermodynamically unstable calcium oxalate dihydrate form.