Rapid Assessment of Crystal Nucleation and Growth Kinetics: Comparison of Seeded and Unseeded Experiments.

In this work, we outlined an experimental workflow enabling the rapid assessment of primary and secondary nucleation and crystal growth kinetics. We used small-scale experiments in agitated vials with in situ imaging for crystal counting and sizing to quantify nucleation and growth kinetics of α-glycine in aqueous solutions as a function of supersaturation at isothermal conditions. Seeded experiments were required to assess crystallization kinetics when primary nucleation is too slow, especially at lower supersaturations often encountered in continuous crystallization processes. At higher supersaturations, we compared results from seeded and unseeded experiments and carefully analyzed interdependencies of primary and secondary nucleation and growth kinetics. This approach allows for the rapid estimation of absolute values of primary and secondary nucleation and growth rates without relying on any specific assumptions about functional forms of corresponding rate expressions used for estimation approaches based on fitting population balance models. Quantitative relationships between nucleation and growth rates at given conditions provide useful insights into crystallization behavior and can be explored to rationally manipulate crystallization conditions for achieving desirable outcomes in batch or continuous crystallization processes.

The unexpected dominance of secondary over primary nucleation.

It is still a challenge to control the formation of particles in industrial crystallization processes. In such processes, new crystals can be generated either by primary or secondary nucleation. While in continuous stirred tank crystallization processes, secondary nucleation is thought to occur due to the shear or attrition of already present larger crystals; in antisolvent crystallization processes, where mixing at the inlets locally causes high supersaturations, primary nucleation is understood to be the main mechanism. We aim to show here that secondary nucleation is the dominant nucleation mechanism, even under conditions that are generally considered to be dominated by primary nucleation mechanisms. Measurements of primary and secondary nucleation rates under similar industrial crystallization conditions of sodium bromate in water, sodium chloride in water, glycine in water and isonicotinamide in ethanol show that the secondary nucleation rate is at least 6 orders of magnitude larger in all these systems. Furthermore, seeded fed-batch and continuous antisolvent crystallizations of sodium bromate under high local supersaturation, seeded with crystals of a specific handedness, result in a close to chirally pure crystalline product with the same handedness. This shows that indeed, enantioselective secondary nucleation is the dominant mechanism in these antisolvent crystallizations. It is even possible to use the enantioselective secondary nucleation mechanism to control the product chirality in such a process, making antisolvent crystallization a viable crystallization-enhanced deracemization technique, having a superior productivity compared to other crystallization-enhanced deracemization methods. Our finding of a dominant secondary nucleation mechanism, rather than primary nucleation, will have a strong impact on nucleation control strategies in industrial crystallization processes.

Structure and kinetics in colloidal films with competing interactions.

Using computer simulation we explore how two-dimensional systems of colloids with a combination of short-range attractive and long-range repulsive interactions generate complex structures and kinetics. Cooperative effects mean the attractive potential, despite being very short-ranged compared to the repulsion, can have significant effects on large-scale structure. By considering the number of particles occupying a notional "repulsion zone" defined by the repulsion length scale, we classify different characteristic structural regimes in which the combination of attraction and repulsion leads to different structural and kinetic outcomes, such as compact clustering, chain labyrinths, and coexisting clusters and chains. In some regimes small changes in repulsion range and/or area fraction can change timescales of structural evolution by many orders of magnitude.

Effects of Secondary Metal Carbonate Addition on the Porous Character of Resorcinol-Formaldehyde Xerogels.

A deeper understanding of the chemistry and physics of growth, aggregation, and gelation processes involved in the formation of xerogels is key to providing greater control of the porous characteristics of such materials, increasing the range of applications for which they may be utilized. Time-resolved dynamic light scattering has been used to study the formation of resorcinol-formaldehyde gels in the presence of combinations of Group I (Na and Cs) and Group II (Ca and Ba) metal carbonates. It was found that the combined catalyst composition, including species and times of addition, is crucial in determining the end properties of the xerogels via its effect on growth of clusters involved in formation of the gel network. Combination materials have textural characteristics within the full gamut offered by each catalyst alone; however, in addition, combination materials that retain the small pores associated with sodium carbonate catalyzed xerogels exhibit a narrowing of the pore size distribution, providing an increased pore volume within an application-specific range of pore sizes. We also show evidence of pore size tunability while maintaining ionic strength, which significantly increases the potential of such systems for biological applications.

Gelation mechanism of resorcinol-formaldehyde gels investigated by dynamic light scattering.

Xerogels and porous materials for specific applications such as catalyst supports, CO2 capture, pollutant adsorption, and selective membrane design require fine control of pore structure, which in turn requires improved understanding of the chemistry and physics of growth, aggregation, and gelation processes governing nanostructure formation in these materials. We used time-resolved dynamic light scattering to study the formation of resorcinol-formaldehyde gels through a sol-gel process in the presence of Group I metal carbonates. We showed that an underlying nanoscale phase transition (independent of carbonate concentration or metal type) controls the size of primary clusters during the preaggregation phase; while the amount of carbonate determines the number concentration of clusters and, hence, the size to which clusters grow before filling space to form the gel. This novel physical insight, based on a close relationship between cluster size at the onset of gelation and average pore size in the final xerogel results in a well-defined master curve, directly linking final gel properties to process conditions, facilitating the rational design of porous gels with properties specifically tuned for particular applications. Interestingly, although results for lithium, sodium, and potassium carbonate fall on the same master curve, cesium carbonate gels have significantly larger average pore size and cluster size at gelation, providing an extended range of tunable pore size for further adsorption applications.

Nanopropulsion by biocatalytic self-assembly.

A number of organisms and organelles are capable of self-propulsion at the micro- and nanoscales. Production of simple man-made mimics of biological transportation systems may prove relevant to achieving movement in artificial cells and nano/micronscale robotics that may be of biological and nanotechnological importance. We demonstrate the propulsion of particles based on catalytically controlled molecular self-assembly and fiber formation at the particle surface. Specifically, phosphatase enzymes (acting as the engine) are conjugated to a quantum dot (the vehicle), and are subsequently exposed to micellar aggregates (fuel) that upon biocatalytic dephosphorylation undergo fibrillar self-assembly, which in turn causes propulsion. The motion of individual enzyme/quantum dot conjugates is followed directly using fluorescence microscopy. While overall movement remains random, the enzyme-conjugates exhibit significantly faster transport in the presence of the fiber forming system, compared to controls without fuel, a non-self-assembling substrate, or a substrate which assembles into spherical, rather than fibrous structures upon enzymatic dephosphorylation. When increasing the concentration of the fiber-forming fuel, the speed of the conjugates increases compared to non-self-assembling substrate, although directionality remains random.

Growth kinetics of colloidal chains and labyrinths.

Particles interacting by a combination of isotropic short-range attraction and long-range repulsion have been shown to form complex phases despite the apparent simplicity of the interparticle potential. Using computer simulations we study the behavior of two-dimensional systems of colloids with such an interaction, focusing on how area fraction and repulsion range at fixed repulsion gradient may be used to tune the resulting kinetics and nonequilibrium structure. While the short-range attraction leads to aggregation, the long-range repulsion encourages growth of chains of particles due to repulsive intercluster interactions. Depending on area fraction/repulsion range we observe chain labyrinths, chain-compact aggregate coexistence, and connected networks of chains. The kinetics of cluster growth displays a sequence of connected networks and disconnected cluster or chain systems with increasing repulsion range, indicating the competing roles of connectivity of growing chains and repulsion-driven breakup of chains into compact aggregates. Chain-dominated systems show approximately logarithmic coarsening at late time that we interpret as the result of chains performing random walks in the randomly fluctuating potential landscape created by their neighbors, a situation reminiscent of glassy systems.

Deposition and aggregation of au at the liquid/liquid interface.

The mesostructure of nanoparticle stabilized interfacial films, in systems such as particle stabilized emulsions, plays a key, yet little understood, role. We have studied a system of gold nanoparticles being formed and aggregating at a planar oil/water interface using optical microscopy. A rapid "nucleation" step of meso (micron) scale aggregates was observed. Aggregation of the meso aggregates to form a percolating structure is seen to switch from a slow to a fast aggregation regime. Aggregation in the slow regime appears to be reaction limited. We tentatively attribute the fast aggregation regime to a capillary attraction.