What Is the Smallest Zeolite That Could Be Synthesized?

Zeolites with a few unit cells are promising as catalyst and adsorbents. The quest to synthesize the smallest zeolites has recently resulted in 4 to 8 nm nanozeolites, about 2 to 4 unit cells. These findings pose the question of what is the smallest zeolite that could be obtained by hydrothermal synthesis. Here we address this question using molecular simulations and thermodynamic analysis. The simulations predict that amorphous precursors as small as 4 nm can crystallize zeolites, in agreement with the experiments. We find that interfacial forces dominate the structure of smaller particles, resulting in size-dependent compact isomers that have ring and pore distributions different from open framework zeolites. The instability of zeolites smaller than 3±0.5 nm precludes a classical mechanism of nucleation from solution or through assembly of small nanoslabs.

Polymorph Selection in Zeolite Synthesis Occurs after Nucleation.

Zeolites are porous crystals with extensive polymorphism. The hydrothermal synthesis of zeolites is a multistage process involving amorphous precursors that evolve continuously in solubility and local order toward those of the crystal. These results pose several questions: Why does a first-order transition appear as a continuous transformation? At which stage is the polymorph selected? How large are the barriers and critical sizes for zeolite nucleation? Here we address these questions using nucleation theory with experimental data. We find that the nucleation barriers and critical zeolite nuclei are extremely small at temperatures of hydrothermal synthesis, resulting in spinodal-like crystallization that produces a mosaic of tiny zeolitic crystallites that compete to grow inside each glassy precursor nanoparticle. The subnanometer size of the critical nuclei reveals that the selection between zeolite polymorphs occurs after the nucleation stage, during the growth and coarsening of the crystals around the excluded volume of the structure-directing agents.

How Do Surfactants Control the Agglomeration of Clathrate Hydrates?

Clathrate hydrates can spontaneously form under typical conditions found in oil and gas pipelines. The agglomeration of clathrates into large solid masses plugs the pipelines, posing adverse safety, economic, and environmental threats. Surfactants are customarily used to prevent the aggregation of clathrate particles and their coalescence with water droplets. It is generally assumed that a large contact angle between the surfactant-covered clathrate and water is a key predictor of the antiagglomerant performance of the surfactant. Here we use molecular dynamic simulations to investigate the structure and dynamics of surfactant films at the clathrate-oil interface, and their impact on the contact angle and coalescence between water droplets and hydrate particles. In agreement with the experiments, the simulations predict that surfactant-covered clathrate-oil interfaces are oil wet but super-hydrophobic to water. Although the water contact angle determines the driving force for coalescence, we find that a large contact angle is not sufficient to predict good antiagglomerant performance of a surfactant. We conclude that the length of the surfactant molecules, the density of the interfacial film, and the strength of binding of its molecules to the clathrate surface are the main factors in preventing the coalescence and agglomeration of clathrate particles with water droplets in oil. Our analysis provides a molecular foundation to guide the molecular design of effective clathrate antiagglomerants.

The Clathrate-Water Interface Is Oleophilic.

The slow nucleation of clathrate hydrates is a central challenge for their use in the storage and transportation of natural gas. Molecules that strongly adsorb to the clathrate-water interface decrease the crystal-water surface tension, lowering the barrier for clathrate nucleation. Surfactants are widely used to promote the nucleation and growth of clathrate hydrates. It has been proposed that these amphiphilic molecules bind to the clathrate surface via hydrogen bonding. However, recent studies reveal that PVCap, an amphiphilic polymer, binds to clathrates through hydrophobic moieties. Here we use molecular dynamic simulations and theory to investigate the mode and strength of binding of surfactants to the clathrate-water interface and their effect on the nucleation rate. We find that the surfactants bind to the clathrate-water interface exclusively through their hydrophobic tails. The binding is strong, driven by the entropy of dehydration of the alkyl chain, as it penetrates empty cavities at the hydrate surface. The hydrophobic attraction of alkyl groups to the clathrate surface also results in strong adsorption of alkanes. We identify two regimes for the binding of surfactants as a function of their density at the hydrate surface, which we interpret to correspond to the two steps of the Langmuir adsorption isotherm observed in experiments. Our results indicate that hydrophobic attraction to the clathrate-water interface is key for the design of soluble additives that promote the nucleation of hydrates. We use the calculated adsorption coefficients to estimate the concentration of sodium dodecyl sulfate (SDS) required to reach nucleation rates for methane hydrate consistent with those measured in experiments. To our knowledge, this study is the first to quantify the effect of surfactant concentration in the nucleation rate of clathrate hydrates.

Water-like Anomalies and Phase Behavior of a Pair Potential that Stabilizes Diamond.

Water, silicon, silica, and other liquids that favor tetrahedral order display thermodynamic, dynamic, and structural anomalies in the pressure range in which they form tetrahedrally coordinated crystals. The tetrahedral order in these liquids is induced by anisotropic hydrogen bonding or covalent interactions, or, in ionic melts, by an appropriate size ratio of the ions. Simple isotropic two-length scale models have been extensively used to understand the origin of anomalies in complex liquids. However, single-component isotropic liquids characterized to date generally do not stabilize tetrahedral crystals, and in the few cases that they do, it was found that the liquids do not display anomalies in the region of the tetrahedral crystal. This poses the question of whether it is possible for isotropic pair potentials to display water-like phase behavior and anomalies. In this work, we use molecular dynamics simulations to investigate the phase behavior and the existence and loci of anomalies of a single-component purely repulsive isotropic pair potential that stabilizes diamond in the ground state over a wide range of pressures. We demonstrate that, akin to water, silica, and silicon, the isotropic potential of Marcotte, Stillinger, and Torquato (MST) presents structural, dynamic, and thermodynamic anomalies in the region of stability of the tetrahedral crystal. The regions of anomalies of MST are nested in the T-p plane following the same hierarchy as in silica: the region of diffusional anomalies encloses the region of structural anomalies, which in turn contains the region of thermodynamic anomalies. To our knowledge, MST is the first example of pair potential for which water-like anomalies are associated with the formation of tetrahedral order.