The elementary reactions for incorporation into crystals.

The growth rates of crystals are largely dictated by the chemical reaction between solute and kinks, in which a solute molecule severs its bonds with the solvent and establishes new bonds with the kink. Details on this sequence of bond breaking and rebuilding remain poorly understood. To elucidate the reaction at the kinks we employ four solvents with distinct functionalities as reporters on the microscopic structures and their dynamics along the pathway into a kink. We combine time-resolved in situ atomic force microscopy and x-ray and optical methods with molecular dynamics simulations. We demonstrate that in all four solvents the solute, etioporphyrin I, molecules reach the steps directly from the solution; this finding identifies the measured rate constant for step growth as the rate constant of the reaction between a solute molecule and a kink. We show that the binding of a solute molecule to a kink divides into two elementary reactions. First, the incoming solute molecule sheds a fraction of its solvent shell and attaches to molecules from the kink by bonds distinct from those in its fully incorporated state. In the second step, the solute breaks these initial bonds and relocates to the kink. The strength of the preliminary bonds with the kink determines the free energy barrier for incorporation into a kink. The presence of an intermediate state, whose stability is controlled by solvents and additives, may illuminate how minor solution components guide the construction of elaborate crystal architectures in nature and the search for solution compositions that suppress undesirable or accelerate favored crystallization in industry.

Particle dispersion through porous media with heterogeneous attractions.

Porous media used in many practical applications contain natural spatial variations in composition and surface charge that lead to heterogeneous physicochemical attractions between the media and transported particles. We performed Stokesian dynamics (SD) simulations to examine the effects of heterogeneous attractions on quiescent diffusion and hydrodynamic dispersion of particles within geometrically ordered arrays of nanoposts. We find that transport under quiescent conditions occurs by two mechanisms, diffusion through the void space and intermittent hopping between the attractive wells of different nanoposts. As the attraction heterogeneity increases, the latter mechanism becomes dominant, resulting in an increase in the particle trajectory tortuosity, deviations from Gaussian behavior in the particle displacement distributions, and a decrease in the long-time particle diffusivity. Similarly, under flow conditions corresponding to low Péclet number (Pe), increased attraction heterogeneity leads to transient localization near the nanoposts, resulting in a broadening of the particle distribution and enhanced longitudinal dispersion in the direction of flow. At high Pe where advection strongly dominates, however, the longitudinal dispersion coefficient is insensitive to attraction heterogeneity and exhibits Taylor-Aris dispersion behavior. Our findings provide insight into how heterogeneous interactions may influence particle transport in complex 3-D porous media.

Confined Dynamics in Spherical Polymer Brushes.

We investigate the dynamics of polymers grafted to spherical nanoparticles in solution using hybrid molecular dynamics simulations with a coarse-grained solvent modeled via the multiparticle collision dynamics algorithm. The mean-square displacements of monomers near the surface of the nanoparticle exhibit a plateau on intermediate time scales, indicating confined dynamics reminiscent of those reported in neutron spin-echo experiments. The confined dynamics vanish beyond a specific radial distance from the nanoparticle surface that depends on the polymer grafting density. We show that this dynamical confinement transition follows theoretical predictions for the critical distance associated with the structural transition from confined to semidilute brush regimes. These findings suggest the existence of a hitherto unreported dynamic length scale connected with theoretically predicted static fluctuations in spherical polymer brushes and provide new insights into recent experimental observations.

pH response of sequence-controlled polyampholyte brushes.

We use molecular simulation to investigate the pH response of sequence-controlled polyampholyte brushes (PABs) with polymer chains consisting of alternating blocks of weakly acidic and basic monomers. Changes in the ionization state, height, lateral structure, and chain conformations of PABs with pH are found to differ qualitatively from those observed for polyelectrolyte brushes. Grafting density has a relatively modest effect on PAB properties. By contrast, monomer sequence strongly affects the pH response, with the extent of the response increasing with the block size. This trend is attributed to strong electrostatic attractions between oppositely charged blocks, which lead to an increase in chain backfolding as block size increases. This behavior is consistent with that observed for polyampholytes with similar monomer sequences in solution in previous studies. Our study shows that monomer sequence can be used to tune the pH response of weak PABs to generate stimuli-responsive surfaces.

Long-Wavelength Fluctuations in Quasi-2D Supercooled Liquids.

We use molecular simulation to characterize the dynamics of supercooled liquids confined in quasi-2D slit geometries. Similar to bulk supercooled liquids, the confined systems exhibit subdiffusive dynamics on intermediate time scales arising from particle localization inside their neighbor cages, followed by an eventual crossover to diffusive behavior as cage rearrangement occurs. The quasi-2D confined liquids also exhibit signatures of long-wavelength fluctuations (LWFs) in the lateral directions parallel to the confining walls, reminiscent of the collective displacements observed in 2D but not 3D systems. The magnitude of the LWFs increases with the lateral dimensions of systems with the same particle volume fraction and confinement length scale, consistent with the logarithmic scaling predicted for 2D Mermin-Wagner fluctuations. The amplitude of the fluctuations is a nonmonotonic function of the confinement length scale because of a competition between caging and strengthening LWFs upon approaching the 2D limit. Our findings suggest that LWFs may play an important role in understanding the behavior of confined supercooled liquids due to their prevalence over a surprisingly broad range of particle densities and confinement length scales.

In Situ Imaging of Faujasite Surface Growth Reveals Unique Pathways of Zeolite Crystallization.

Zeolite crystallization occurs by complex processes involving a variety of possible mechanisms. The sol gel media used to prepare zeolites leads to heterogeneous mixtures of solution and solid states with diverse solute species. At later stages of zeolite synthesis when growth occurs predominantly from solution, classical two-dimensional nucleation and spreading of layers on crystal surfaces via the addition of soluble species is the dominant pathway. At earlier stages, these processes occur in parallel with nonclassical pathways involving crystallization by particle attachment (CPA). The relative roles of solution- and solid-state species in zeolite crystallization have been a subject of debate. Here, we investigate the growth mechanism of a commercially relevant zeolite, faujasite (FAU). In situ atomic force microscopy (AFM) measurements reveal that supernatant solutions extracted from a conventional FAU synthesis at various times do not result in growth, indicating that FAU growth predominantly occurs from the solid state through a disorder-to-order transition of amorphous precursors. Elemental analysis shows that supernatant solutions are significantly more siliceous than both the original growth mixture and the FAU zeolite product; however, in situ AFM studies using a dilute clear solution with a lower Si/Al ratio revealed three-dimensional growth of surfaces that is distinct from layer-by-layer and CPA pathways. This unique mechanism of growth differs from those observed in studies of other zeolites. Given that relatively few zeolite frameworks have been the subject of mechanistic investigation by in situ techniques, these observations of FAU crystallization raise the question whether its growth pathway is characteristic of other zeolite structures.

Nanoparticle dispersion in porous media: Effects of attractive particle-media interactions.

We investigate the effects of physicochemical attractions on the transport of finite-sized particles in three-dimensional ordered nanopost arrays using Stokesian dynamics simulations. We find that weak particle-nanopost attractions negligibly affect diffusion due to the dominance of Brownian fluctuations. Strong attractions, however, significantly hinder particle diffusion due to localization of particles around the nanoposts. Conversely, under flow, attractions significantly enhance longitudinal dispersion at low to moderate Péclet number (Pe). At high Pe, by contrast, advection becomes dominant and attractions weakly enhance dispersion. Moreover, attractions frustrate directional locking at moderate flow rates, and shift the onset of this behavior to higher Pe.

Precrystallization solute assemblies and crystal symmetry.

Solution crystallization is a part of the synthesis of materials ranging from geological and biological minerals to pharmaceuticals, fine chemicals, and advanced electronic components. Attempts to predict the structure, growth rates and properties of emerging crystals have been frustrated, in part, by the poor understanding of the correlations between the oligomeric state of the solute, the growth unit, and the crystal symmetry. To explore how a solute monomer or oligomer is selected as the unit that incorporates into kinks and how crystal symmetry impacts this selection, we combine scanning probe microscopy, optical spectroscopy, and all-atom molecular simulations using as examples two organic materials, olanzapine (OZPN) and etioporphyrin I (EtpI). The dominance of dimeric structures in OZPN crystals has spurred speculation that the dimers preform in the solution, where they capture the majority of the solute, and then assemble into crystals. By contrast, EtpI in crystals aligns in parallel stacks of flat EtpI monomers unrelated by point symmetry. Raman and absorption spectroscopies show that solute monomers are the majority solute species in solutions of both compounds. Surprisingly, the kinetics of incorporation of OZPN into kinks is bimolecular, indicating that the growth unit is a solute dimer, a minority solution component. The disconnection between the dominant solute species, the growth unit, and the crystal symmetry is even stronger with EtpI, for which the (010) face grows by incorporating monomers, whereas the growth unit of the (001) face is a dimer. Collectively, the crystallization kinetics results with OZPN and EtpI establish that the structures of the dominant solute species and of the incorporating solute complex do not correlate with the symmetry of the crystal lattice. In a broader context, these findings illuminate the immense complexity of crystallization scenarios that need to be explored on the road to the understanding and control of crystallization.

Spatiotemporal Coke Coupling Enhances para-Xylene Selectivity in Highly Stable MCM-22 Catalysts.

Identifying zeolite catalysts that can simultaneously optimize p-xylene selectivity and feed utilization is critical to toluene alkylation with methanol (TAM). Here, we show that zeolite MCM-22 (MWW) has an exceptional catalyst lifetime in the TAM reaction at high operating pressure, conversion, and selectivity. We systematically probe the catalytic behavior of active sites in distinct topological features of MCM-22, revealing that high p-xylene yield and catalyst stability are predominantly attributed to sinusoidal channels and supercages, respectively. Using a combination of catalyst design and testing, density functional theory, and molecular dynamics simulations, we propose a spatiotemporal coke coupling phenomenon to explain a multistage p-xylene selectivity profile wherein the formation of light coke in supercages initiates the deactivation of unselective external surface sites. Our findings indicate that the specific nature of coke is critical to catalyst performance. Moreover, they provide unprecedented insight into the synchronous roles of distinct topological features giving rise to the exceptional stability and selectivity of MCM-22 in the TAM reaction.

Signatures of sluggish dynamics and local structural ordering during ice nucleation.

We investigate the microscopic pathway of spontaneous crystallization in the ST2 model of water under deeply supercooled conditions via unbiased classical molecular dynamics simulations. After quenching below the liquid-liquid critical point, the ST2 model spontaneously separates into low-density liquid (LDL) and high-density liquid phases, respectively. The LDL phase, which is characterized by lower molecular mobility and enhanced structural order, fosters the formation of a sub-critical ice nucleus that, after a stabilization time, develops into the critical nucleus and grows. Polymorphic selection coincides with the development of the sub-critical nucleus and favors the formation of cubic (Ic) over hexagonal (Ih) ice. We rationalize polymorphic selection in terms of geometric arguments based on differences in the symmetry of second neighbor shells of ice Ic and Ih, which are posited to favor formation of the former. The rapidly growing critical nucleus absorbs both Ic and Ih crystallites dispersed in the liquid phase, a crystal with stacking faults. Our results are consistent with, and expand upon, recent observations of non-classical nucleation pathways in several systems.

Factors controlling the molecular modification of one-dimensional zeolites.

Interactions between organic molecules and inorganic materials are ubiquitous in many applications and often play significant roles in directing pathways of crystallization. It is frequently debated whether kinetics or thermodynamics plays a more prominent role in the ability of molecular modifiers to impact crystal nucleation and growth processes. In the case of nanoporous zeolites, approaches in rational design often capitalize on the ability of organics, used as either modifiers or structure-directing agents, to markedly impact the physicochemical properties of zeolites. It has been demonstrated for multiple topologies that modifier-zeolite interactions can alter crystal size and morphology, yet few studies have distinguished the roles of thermodynamics and kinetics. We use a combination of calorimetry and molecular modeling to estimate the binding energies of organics on zeolite surfaces and correlate these results with synthetic trends in crystal morphology. Our findings reveal unexpectedly small energies of interaction for a range of modifiers with two zeolite structures, indicating the effect of organics on zeolite crystal surface free energy is minor and kinetic factors most likely govern growth modification.

Solvent Structure and Dynamics near the Surfaces of ß-Hematin Crystals.

Hematin crystallization, which is an essential component of the physiology of malaria parasites and the most successful target for antimalarial drugs, proceeds in mixed organic-aqueous solvents both in vivo and in vitro. Here we employ molecular dynamics simulations to examine the structuring and dynamics of a water-normal octanol mixture (a solvent that mimics the environment hosting hematin crystallization in vivo) in the vicinity of the typical faces in the habit of a hematin crystal. The simulations reveal that the properties of the solvent in the layer adjacent to the crystal are strongly impacted by the distinct chemical and topological features presented by each crystal face. The solvent organizes into at least three distinct layers. We also show that structuring of the solvent near the different faces of ß-hematin strongly impacts the interfacial dynamics. The relaxation time of n-octanol molecules is longest in the contact layers and correlates with the degree of structural ordering at the respective face. We show that the macroscopically homogeneous water-octanol solution holds clusters of water and n-octanol connected by hydrogen bonds that entrap the majority of the water but are mostly smaller than 30 water molecules. Near the crystal surface the clusters anchor on hematin carboxyl groups. These results provide a direct example that solvent structuring is not restricted to aqueous and other hydrogen-bonded solutions. Our findings illuminate two fundamental features of the mechanisms of hematin crystallization: the elongated shapes of natural and synthetic hematin crystals and the stabilization of charged groups of hematin and antimalarials by encasing in water clusters. In addition, these findings suggest that hematin crystallization may be controlled by additives that disrupt or reinforce solvent structuring.

Nanoparticle dispersion in porous media: Effects of array geometry and flow orientation.

We investigate the effects of array geometry and flow orientation on transport of finite-sized particles in ordered arrays using Stokesian dynamics simulations. We find that quiescent diffusion is independent of array geometry over the range of volume fraction of the nanoposts examined. Longitudinal dispersion under flow depends on the direction of incident flow relative to the array lattice vectors. Taylor-Aris behavior is recovered for flow along the lattice directions, whereas a nonmonotonic dependence of the dispersion coefficient on the Péclet number is obtained for flow orientations slightly perturbed from certain lattice vectors, owing to a competition between directional locking and spatial velocity variations.

Acidic Polysaccharides as Green Alternatives for Barite Scale Dissolution.

Barium sulfate (barite) scale poses significant challenges for processes ranging from water treatment to fossil fuel production. Here, we identify alginate (a polysaccharide derived from brown algae) as a potent, "green" alternative to commercial barite demineralizing agents. Unlike conventional treatments of inorganic scales that require caustic conditions, alginate polymers dissolve barite at near-neutral conditions. In this study, we benchmark the demineralizing efficacy of alginate against a commercial dissolver, diethylenetriaminepentaacetic acid (DTPA), using a combination of bulk dissolution assays, scanning probe microscopy, and molecular dynamics simulations. Time-resolved rates of dissolution measured in a microfluidic device show that demineralization is enhanced more than an order of magnitude under flow. In situ atomic force microscopy reveals that alginate and DTPA exhibit distinct mechanisms of surface dissolution; and surprisingly, their binary combination in alkaline media results in a synergistic cooperativity that enhances the overall rate of barite dissolution. These studies collectively demonstrate a unique approach to demineralization using an inexpensive and abundant biopolymer that enables environmentally friendly treatment of inorganic scales.

Olanzapine crystal symmetry originates in preformed centrosymmetric solute dimers.

The symmetries of a crystal are notoriously uncorrelated to those of its constituent molecules. This symmetry breaking is typically thought to occur during crystallization. Here we demonstrate that one of the two symmetry elements of olanzapine crystals, an inversion centre, emerges in solute dimers extant in solution prior to crystallization. We combine time-resolved in situ scanning probe microscopy to monitor the crystal growth processes with all-atom molecular dynamics simulations. We show that crystals grow non-classically, predominantly by incorporation of centrosymmetric dimers. The growth rate of crystal layers exhibits a quadratic dependence on the solute concentration, characteristic of the second-order kinetics of the incorporation of dimers, which exist in equilibrium with a majority of monomers. We show that growth by dimers is preferred due to overwhelming accumulation of adsorbed dimers on the crystal surface, where it is complemented by dimerization and expedites dimer incorporation into growth sites.

Finned zeolite catalysts.

There is growing evidence for the advantages of synthesizing nanosized zeolites with markedly reduced internal diffusion limitations for enhanced performances in catalysis and adsorption. Producing zeolite crystals with sizes less than 100 nm, however, is non-trivial, often requires the use of complex organics and typically results in a small product yield. Here we present an alternative, facile approach to enhance the mass-transport properties of zeolites by the epitaxial growth of fin-like protrusions on seed crystals. We validate this generalizable methodology on two common zeolites and confirm that fins are in crystallographic registry with the underlying seeds, and that secondary growth does not impede access to the micropores. Molecular modelling and time-resolved titration experiments of finned zeolites probe internal diffusion and reveal substantial improvements in mass transport, consistent with catalytic tests of a model reaction, which show that these structures behave as pseudo-nanocrystals with sizes commensurate to that of the fin. This approach could be extended to the rational synthesis of other zeolite and aluminosilicate materials.

Reply to the 'Comment on "Pressure enhancement in carbon nanopores: a major confinement effect"' by D. van Dijk, Phys. Chem. Chem. Phys., 2020, 22, DOI: 10.1039/C9CP02890K.

In his 'Comment' van Dijk points out that the local pressure at a point r in an inhomogeneous thermodynamic system, like other thermodynamic properties, is not uniquely defined; one must make an operational definition that involves deciding how to assign the intermolecular forces between pairs of molecules to the point r. This non-uniqueness difficulty is well known, and was discussed in our paper. It was discussed in detail in the 1950 paper of Irving and Kirkwood, and in many books and papers since then. We reply to these comments, and note that an average of the local pressure over a region of space may yield a well-defined pressure. We also discuss other possible ways to quantify the adsorption compression effect near an attractive wall. van Dijk also suggests that the non-uniqueness difficulty can be avoided by using the pressure of the uniform bulk fluid in equilibrium with the pore. While this pressure is well-defined, it only reflects the intermolecular forces in the bulk phase, and gives no information about the behavior in the pore.

Tracer transport in attractive and repulsive supercooled liquids and glasses.

The transport of small penetrants through disordered materials with glassy dynamics is encountered in applications ranging from drug delivery to chemical separations. Nonetheless, understanding the influence of the matrix structure and fluctuations on penetrant motions remains a persistent challenge. Here, we use event-driven molecular dynamics to investigate the transport of small, hard-sphere tracers embedded in matrices of square-well particles. Short-range attractions between matrix particles give rise to reentrant dynamics in the supercooled regime, in which the liquid's relaxation time increases dramatically upon heating or cooling. Heating results in a "repulsive" supercooled liquid where relaxations are frustrated by steric interactions between particles, whereas cooling produces an "attractive" liquid in which relaxations are hindered by long-lived interparticle bonds. Further cooling or heating, or compression, of the supercooled liquids results in the formation of distinct glasses. Our study reveals that tracer transport in these supercooled liquids and glasses is influenced by the matrix structure and dynamics. The relative importance of each factor varies between matrices and is examined in detail by analyzing particle mean-square displacements, caging behavior, and trajectories sampled from the isoconfigurational ensemble. We identify features of tracer dynamics that reveal the spatial and temporal heterogeneity of the matrices and show that matrix arrest is insufficient to localize tracers.

Crystallization of Mordenite Platelets using Cooperative Organic Structure-Directing Agents.

Organic structure-directing agents (OSDAs) are exploited in the crystallization of microporous materials to tailor the physicochemical properties of the resulting zeolite for applications ranging from separations to catalysis. The rational design of these OSDAs often entails the identification of molecules with a geometry that is commensurate with the channels and cages of the target zeolite structure. Syntheses tend to employ only a single OSDA, but there are a few examples where two or more organics operate synergistically to yield a desired product. Using a combination of state-of-the-art characterization techniques and molecular modeling, we show that the coupling of N,N,N-trimethyl-1,1-adamantammonium and 1,2-hexanediol, each yielding distinct zeolites when used alone, results in the cooperative direction of a third structure, HOU-4, with the mordenite framework type (MOR). Rietveld refinement using synchrotron X-ray diffraction data reveals the spatial arrangement of the organics in the HOU-4 crystals, with amines located in the large channels and alcohols oriented in the side pockets lining the one-dimensional pores. These results are in excellent agreement with molecular dynamics calculations, which predict similar spatial distributions of organics with an energetically favorable packing density that agrees with experimental measurements of OSDA loading, as well as with solid-state two-dimensional 27Al{29Si}, 27Al{1H}, and 13C{1H} NMR correlation spectra, which establish the proximities and interactions of occluded OSDAs. A combination of high-resolution transmission electron microscopy and atomic force microscopy is used to quantify the size of the HOU-4 crystals, which exhibit a platelike morphology, and to index the crystal facets. Our findings reveal that the combined OSDAs work in tandem to produce ultrathin, nonfaulted HOU-4 crystals that exhibit improved catalytic activity for cumene cracking in comparison to mordenite crystals prepared via conventional syntheses. This novel demonstration of cooperativity highlights the potential possibilities for expanding the use of dual structure-directing agents in zeolite synthesis.