Zeolites and mesoporous silicas are porous materials with important applications in catalysis, gas storage, and separation. Zeolite crystals form in the presence of cationic surfactants that act as structure directing agents (SDAs). The way SDAs control the nucleation and polymorphs selection in zeolites is not fully understood. The formation of mesoporous silica is templated by liquid crystalline mesophases that result from frustrated attraction between silicates and long-chain SDAs. Experiments indicate that surfactants C n H 2n+1 (CH 3 ) 3 N + with n > 6 yield mesoporous silicas, and the one with n = 6 produces a zeolite. This suggests that the driving force toward mesophase formation is also present for small organocations, but is overcome by the ability of silica to wrap a crystal lattice around them. Here we use molecular dynamics simulations to investigate whether the existence of metastable mesophases can play a role in the nucleation and polymorph selection of zeolitic crystals. As a proof of concept, we investigate the phase behavior of simple mesogenic mixtures of SDAs and a network former T that favors tetracoordinated crystals. We represent the network-former T by Stillinger−Weber models of water and silicon, in lieu of silica, because a computationally efficient silica potential that would allow for the spontaneous nucleation of zeolites in molecular dynamics simulations is not yet available. The mixtures of T and SDA produce a rich phase diagram that encompasses the sII clathrate and at least six zeolites, including sigma-2 (SGT). We find that the nucleation of SGT is not assisted by a mesophase. The nucleation of the other five zeolites of this study, however, is facilitated by the existence of metastable mesophases that decrease the nucleation barriers and direct the selection of the crystal polymorph. Together with the experimental support for mesophases in mixtures of silicates and SDAs, our results for model systems suggest that metastable mesophases could play a prominent role in promoting the nucleation and polymorph selection of some zeolites.

A growing number of crystalline and quasi-crystalline structures have been formed by coating nanoparticles with ligands, polymers, and DNA. The design of nanoparticles that assemble into mesophases, such as those formed by block copolymers, would combine the order, mobility, and stimuli responsive properties of mesophases with the electronic, magnetic, and optical properties of nanoparticles. Here we use molecular simulations to demonstrate that binary mixtures of unbound particles with simple short-ranged pair interactions produce the same mesophases as block copolymers and surfactants, including lamellar, hexagonal, gyroid, body-centered cubic, face-centered cubic, perforated lamellar, and semicrystalline phases. The key to forming the mesophases is the frustrated attraction between particles of different types, achieved through control over interparticle size and over strength and softness of the interaction. Experimental design of nanoparticles with effective interactions described by the potentials of this work would provide a distinct, robust route to produce ordered tunable liquid crystalline mesophases from nanoparticles.

The importance of nonclassical nucleation pathways in the formation of complex crystals has become apparent in recent years. Nonclassical pathways were unraveled for, among others, the crystallization of proteins, colloids, and clathrates. In those cases, the formation of a metastable fluid with density close to the crystal decreases the crystallization barrier. Recent simulations indicate that mesophases can facilitate the nucleation of zeolites. Here, we use molecular simulations to investigate the role of a gyroid mesophase on the crystallization of a model zeolite from liquid. The nucleation pathway is always nonclassical. At warmer temperatures, the mechanism proceeds in two well-defined steps: nucleation of a metastable gyroid followed by its crystallization into a zeolite. At colder temperatures, the second barrier becomes negligible, and the crystallization occurs in one step. This second scenario is also nonclassical, as the critical nucleus for the crystallization has the structure of the gyroid and seamlessly transforms into a zeolite as it grows past its critical size. To our knowledge, this is the first report of a nonclassical mechanism of crystallization mediated by a mesophase.

Polyethylenimine (PEI) is a pH sensitive polymer possessing stretched and coiled conformation at low and high pH, respectively. It is an efficient gene delivery agent. Thus, the interaction of PEI with the biomembrane is very crucial to understand the gene delivery mechanism. In this report, we have investigated the structural properties of PEI and bilayer due to the interaction of PEI with lipid molecules. PEI has coil structure at high pH while at low pH it is elongated. The neutral PEI chain predominately settles itself at the bilayer water interface. We do not find any disruption or pore formation on the bilayer due to interaction of neutral PEI chain. PEI at low pH gets elongated due to electrostatic interaction between charges of the protonated sites. This protonated PEI chain interacts with bilayer membrane, which leads to formation of water/ion channel through the membrane. We have analyzed the structure of the channel and water dynamics along the channel.

Residues spanning distinct regions of the low-complexity domain of the RNA-binding protein, Fused in Sarcoma (FUS-LC), form fibril structures with different core morphologies. Solid-state NMR experiments show that the 214-residue FUS-LC forms a fibril with an S-bend (core-1, residues 39–95), while the rest of the protein is disordered. In contrast, the fibrils of the C-terminal variant (FUS-LC-C; residues 111–214) have a U-bend topology (core-2, residues 112–150). Absence of the U-bend in FUS-LC implies that the two fibril cores do not coexist. Computer simulations show that these perplexing findings could be understood in terms of the population of sparsely populated fibril-like excited states in the monomer. The propensity to form core-1 is higher compared to core-2. We predict that core-2 forms only in truncated variants that do not contain the core-1 sequence. At the monomer level, sequence-dependent enthalpic effects determine the relative stabilities of the core-1 and core-2 topologies.

Block copolymers, surfactants, and biomolecules form lamellar, hexagonal, and gyroid mesophases. Across these systems, the nucleation of lamellar from the disordered liquid is the easiest and the nucleation of gyroid the most challenging. This poses the question of what are the factors that determine the rates of nucleation of the mesophases and whether they are controlled by the complexity of the structures or the thermodynamics of nucleation. Here, we use molecular simulations to investigate the nucleation and thermodynamics of lamellar, hexagonal, and gyroid in a binary mixture of particles that produces the same mesophases as those of surfactants and block copolymers. We demonstrate that a combination of averaged bond-order parameters q̅ and q̅ identifies and distinguishes the three mesophases. We use these parameters to track the microscopic process of nucleation of each mesophase and investigate the existence of heterogeneous nucleation (cross-nucleation) between mesophases. We estimate the surface tensions of the liquid/mesophase interfaces from nucleation rates using classical nucleation theory and find that they are comparable for the three mesophases with values that are about a third of those expected for liquid-crystal interfaces. The driving forces for nucleation, on the other hand, are quite different and increase in the order gyroid < hexagonal < lamellar at any temperature. We find that the nucleation rates of the mesophases follow the order of their driving forces. We conclude that the difficulty to nucleate the gyroid originates in its lower temperature of melting and extremely low entropy of melting compared to those of the hexagonal and lamellar mesophases.

Porous zeolitic crystals made of nanoparticles could have interesting applications in membranes and photonics. However, there is not yet a synthetic pathway to realize these complex architectures using nanoparticles. The challenge arises from the difficulty to control independently the different length scales intrinsic to the ordering of zeolitic crystals: the short-range tiling of the typically tetravalent sites that form the crystal, and the medium-and long-range order and connectivity of the pores. Here we propose a strategy to nucleate and grow zeolitic crystals from a binary mixture of nanoparticles in which one component favors tetravalent coordination and the two types of particles interact through frustrated attraction that promotes the formation of mesophases. We use molecular simulations to demonstrate that this approach produces zeolitic crystals in which the ordering of the pores is controlled by the structure of the mesophase, while the tiling of the network-former particles is controlled by the strength of orientational interactions of the model patchy particles. We investigate the formation of two gyroidal zeolitic crystals and the transformations between them to elucidate the role of the mesophases in the crystallization, polymorph selection, and crystal-tocrystal transformations between zeolitic crystals. This study provides a novel and distinct avenue to assemble porous zeolitic crystals from mesogenic patchy nanoparticles.

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.