Dissipative Dynamics Simulations Research Articles

Simulating the behavior of antioxidant to explore the mechanisms of oxidative stability in Pickering emulsion

This study explores effective strategies for bolstering emulsion oxidative stability via optimized interfacial distribution of varying hydrophobicity antioxidants (gallic acid, propyl gallate, octyl gallate) in zein nanoparticle (ZP) stabilized Pickering emulsions. Experimental and simulation methods revealed that antioxidant (AO) with higher hydrophobicity or loaded into ZP demonstrated stronger hydrogen bonding and van der Waals interactions with ZP. This increased interfacial loading of antioxidants resulted in improved oxidative stability in Pickering emulsions. The flow, distribution and orientation of AO, as revealed by dissipative dynamics simulations, highlighted the role of hydrophobic interactions during initial AO migration, influenced by varied alkyl chain lengths. Subsequent interface rearrangements arose from conservative force interactions between the AO's phenol hydroxyl ends and ZP. These findings inform effective interfacial engineering to optimize antioxidant efficiency, guiding practical applications in emulsion systems for improved oxidative stability.

(Invited) Chemical and Structural Factors Affecting the Hydrated Morphology of Anion Exchange Membranes

The hydrated morphology of ion containing polymers is a critically important property affecting ion and water transport. Anion exchange membranes (AEMs) are increasingly of interest as the electrolyte in energy and conversion devices due, in part, to their suitability with non-noble metal catalysts. However, the rather low chemical stability under alkali conditions and only meager ionic conductivity present difficulties in their application. A promising strategy to enhance the conductivity of these materials may be through tailoring of the phase segregation of the polymer. Recently, AEMs based on the triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), and functionalized with various cationic groups have shown satisfactory chemical stability. We are continuing an extensive investigation to understand how various molecular features (degree of functionalization, placement of alkyl spacer(s) on the side chain, choice of the cationic group, and the associated anion) determine the hydrated morphology of functionalized SEBS ionomers. With this goal we are utilizing dissipative dynamics (DPD) simulations, a meso-scale technique, to probe relatively large time and length scales and thereby elucidate the morphology of the polymer systems. The optimized structure and the interaction parameters used in these DPD simulations were calculated using a recently developed methodology employing DFT based electronic structure calculations. The morphology was determined over a broad range of hydration and our results underscore the complexity of the effects of these parameters.

Excited-State Dynamics of a CsPbBr3 Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin-Orbit Coupling Suppresses Carrier Cooling.

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light-emitting devices due to optoelectronic properties that can be tuned/optimized via halide composition, surface passivation, doping, and confinement. Compared to bulk materials, certain excited-state properties in NCs can be adjusted by electronic confinement effects such as suppressed hot carrier cooling and enhanced radiative recombination. Here we use spinor Kohn-Sham orbitals (SKSOs) with spin-orbit coupling (SOC) interaction as a basis to compute excited-state dissipative dynamics simulations on a fully passivated CsPbBr3 NC atomistic model. Redfield theory in the density matrix formalism is used to describe electron-phonon interactions which drive hot carrier cooling and nonradiative recombination ( knonrad). Radiative recombination ( krad) is calculated through oscillator strengths using SKSO basis. From krad and krad + knonrad, we compute a theoretical photoluminescence quantum yield (PLQY) of 53%. Computed rates of hot carrier cooling ( kcooling ≈ 10-1 1/ps) compare favorably with what has been reported in the literature. Interestingly, we observe that hot electron cooling slows down near the band edge, which we attribute to large SOC in the conduction band combined with strong confinement, which creates subgaps above the band edge. This slow carrier cooling could potentially impact hot carrier extraction before complete thermalization in photovoltaics (PVs). Implications of this work suggest that strong/intermediate confined APbX3 NCs are better suited to applications in PVs due to slower carrier cooling near the conduction band edge, while intermediate/weak confined NCs are more appropriate for light-emitting applications, such as LEDs.

Orientational ordering of nanorods of different length in diblock copolymers

ABSTRACTOrientational and positional ordering of nanorods in the lamellae phase of diblock copolymers has been investigated using a simple theoretical model and dissipative dynamics simulations. Orientational order parameter and local concentration profiles of nanorods are calculated numerically and extracted from computer simulations data for different values of the nanoparticle length and different number of the interaction sites in the model nanorod. The predictions of the molecular theory are compared with the results of computer simulations. It has been found that the nanorods are orientationally ordered in the boundary region between the domains and the orientational order parameter changes its sign at the domain wall. At the same time there exists some quantitative discrepancy between theory and computer simulations which is partially removed when a similar model of a nanorod is employed both in the molecular theory and in coarse-grained molecular dynamics simulations.

Metal–Nonmetal Transition in Fluid Cesium: A Many-Body Dissipative Particle Dynamics Simulation Approach

A many-body dissipative dynamics simulation approach is presented for inclusion of many-body interactions in fluid cesium. Employing this many-body potential, simulations are done, in the grand canonical ensemble, to calculate the vapor–liquid equilibria over a wide range of temperatures (from the melting temperature to the critical temperature). The calculated coexisting liquid and vapor densities and the vapor pressures are in close agreement with experiment. The metal-nonmetal transition is examined in terms of cluster formation in low density cesium with increasing pressure. Our analysis of cluster formation along the critical isotherm shows that at pressures noticeably higher than the critical pressure very large spherical clusters are formed in the simulation box. The cluster sizes decrease with decreasing pressure to the critical pressure. At the critical pressure, only small clusters are seen in the simulation box. The cluster structures also change noticeably as the metal–nonmetal transition appr...

Cholesterol Extraction from Cell Membrane by Graphene Nanosheets: A Computational Study.

The health risk associated with high cholesterol levels in the human body has motivated intensive efforts to lower them by using specialized drugs. However, little research has been reported on utilizing nanomaterials to extract extra cholesterol from living tissues. Graphene possesses great potential for cholesterol extraction from cell membranes due to its distinct porous structure and outstanding surface adhesion. Here we employ dissipative dynamic simulations to explore pathways for cholesterol extraction from a cell membrane by a sheet of graphene using a coarse-grained graphene nanosheets (CGGN) model. We first demonstrate that the self-assembly process among a single layer of graphene and a group of randomly distributed cholesterol molecules in the aqueous environment, which provides a firm foundation for graphene-cholesterol interactions and the dynamic cholesterol extraction process from the cell membrane. Simulations results show that graphene is capable of removing cholesterol molecules from the bilayer membrane. The interaction between graphene and cholesterol molecules plays an important role in determining the amount of extracted cholesterol molecules from the cell membrane. Our findings open up a promising avenue to exploit the capability of graphene for biomedical applications.

Conformational Behavior of a Single Polyelectrolyte Chain with Bulky Counterions

We report a dissipative dynamic simulation study of a polyelectrolyte chain conformational behavior with small and bulky counterions. The size of the small counterions was equal to the size of a monomer unit, while the bulky counterions were modeled as big spherical particles with twice the radius of a monomer unit. Two cases of the charge location within the bulky counterions were studied: (i) in the center (symmetrical case) or (ii) on the counterion surface (asymmetrical case). The strength of electrostatic interactions, λ, as well as the fraction of charged beads on the polymer chain, f, was varied. At high charge fractions the chains with both small counterions and symmetrical bulky counterions collapsed at large λ. In contrast, the chain with asymmetrical bulky counterions did not collapse at all; instead, it adopted swollen conformations with counterions strongly attached to the chain backbone. At low charge fractions and high λ the multiplet structures observed in the systems with small and symmet...

Study of the Mechanisms of Filler Reinforcement in Elastomer Nanocomposites

We performed a large-scale dissipative dynamics simulation to study the structural changes in unfilled and filled elastomers during uniaxial deformation, which helped to shed some light on the underlying reasons of filler reinforcement in rubber nanocomposites. Equilibrium stress–stain curves for different cross-linker concentrations and filler content were obtained, and their features were compared to the experimental data. Dependences of segmental orientation on deformation and true stress were studied; these dependences are discussed in the light of theoretical predictions and available experimental data. The structural changes in the deformed state were studied as well, namely, the dependences of mean end-to-end distance on the subchain length. For the filled elastomers it was found that in the matrix there are several sets of subchains with distinct properties. Part of the subchains which are not connected to the filler particles are deformed slightly more than in the unfilled matrix; the subchains connected to the filler particles are deformed significantly more. This “separation” is the main reason for reinforcement; its influence on the properties of the filled systems is discussed. In addition, the effect of the network topology (randomly cross-linked, end-linked, ideal diamond-like) on the mechanical properties of unfilled elastomers was studied.

Dissipative Particle Dynamics Simulation on Self-Assembly of Comb-Coil Copolymers

Using dissipative dynamics simulation, we studied self-assembly behavior of (A-g-B)-b-A comb-coil copolymers in selective solvents. The comb-coil copolymers, having two competitive length scales, are able to self-assemble into aggre- gates of different types, i.e., type I and type II. For the aggregates of type I, the phase separation occurs between the solvo- phobic and solvophilic blocks, which behave as asymmetric graft copolymers. While in the aggregates of type II, the phase separation takes place between coil and comb blocks, acting as diblock copolymers. The self-assembly of the comb-coil block copolymers in solvents selective to either graft arms or backbone was investigated. The effects of the number and length of graft arms on the self-assembly behavior were examined. In the solvents selective to graft arms, the comb-coil copolymers tend to assemble into spherical micelles of type II, where the comb and coil blocks form the shell and core, respectively. This is due to the fact that the crowd of the comb blocks in the shell can be alleviated by forming high-curvature structures. In addition, such a crowd can also be alleviated by decreasing the length of graft arms and therefore, vesicles were observed when the graft arms are short. In addition, the decrease in the interaction strength between backbone and graft arms (and sol- vents) favors the formation of the aggregates of type II. In the solvents selective to backbones, the comb-coil copolymers incline to form low-curvature aggregates of type II, such as disklike micelles and vesicles. By forming low-curvature struc- tures, the rod-like comb blocks can be tightly packed in the cores of the aggregates. When the comb-coil copolymers form the aggregates of type I in both solvents, the morphologies are very sensitive to the length of the graft arms. For example, in sol- vents selective to graft arms, as the length of graft arms increases, a morphological transformation of large-compound micelle → vesicle → cylindrical micelle → spherical micelle was observed. The simulation results were compared with the avail- able experimental findings reported in the literatures, and an agreement was observed. In addition, the simulations predict some behaviors that have not been observed yet. The present work is helpful for further understanding the competitive self-assembly behavior of the comb-coil copolymers. Keywords comb-coil copolymer; self-assembly; dissipative particle dynamics simulation; micelle; phase separation

Molecular dynamics and dissipative particle dynamics simulations for the miscibility of poly(ethylene oxide)/poly(vinyl chloride) blends

The miscibility of poly(ethylene oxide) (PEO)/poly(vinyl chloride) (PVC) blends are investigated by atomistic molecular dynamics and mesoscale dissipative dynamics simulations. The specific volumes of three PEO/PVC blends (with weight ratio at 70/30, 50/50 and 30/70) as well as pure PEO and PVC are examined as a function of temperature. The glass transition temperatures are estimated to be 251, 268, 280, 313 and 350K for pure PEO, PEO/PVC 70/30, 50/50, 30/70 and pure PVC. Among different energy contributions, the torsion and van der Waals energies exhibit pronounced kinks versus temperature. The Flory–Huggins parameters determined from the cohesive energy densities and the radial distribution functions of the inter-molecular atoms suggest that PEO/PVC 70/30 and 30/70 blends are more miscible than 50/50 blend. This is further supported by the morphologies of PEO/PVC blends, in which the 50/50 blend exhibits segregated domains implying a weak phase separation. Hydrogen bonds are found to form between O atoms of PEO and H atoms of PVC, with a larger degree in PEO/PVC 70/30 and 30/70 blends than in 50/50 blend. The addition of PVC into PEO suppresses the mobility of PEO chains, which is consistent with the experiment observation of decreased crystallization rate as well as crystallization temperature of PEO.

Morphology and Structure Control of Multicompartment Micelles from Triblock Copolymer Blends

Control of the overall morphology and inner structure of multicompartment micelles from binary blends of triblock copolymers in solution was studied by dissipative dynamics simulations. The effects of the block sequence, block ratio, block length, and chain architecture on the morphology and structure of mixed multicompartment micelles were investigated systematically. This work shows that by changing the block composition or chain architecture of one copolymer in the binary blends, the mixed degree, relative distance, and degree of participation of core-forming blocks from two triblock copolymers can be tuned, and diverse morphologies of mixed micelles with alterable domain arrangements and overall size can be obtained. This work shows that blending of copolymers is an effective way to control the morphology and inner structure of multicompartment micelles, providing useful information on the preparation of mixed micelles with unique properties for practical applications.