Dissipative Particle Dynamics Simulation of Nanoparticle Diffusion in a Crosslinked Polymer Network.

Dissipative particle dynamics simulation is conducted to investigate the diffusion of a nanoparticle in a crosslinked polymer network based on a bead-spring model. Focusing on cases where the particle is comparable in size to the network mesh, we find from rigid networks that the excluded-volume and hydrodynamic interaction effects associated with solvent beads lead to lubricity, which assists the particle to slip through an opening into the adjacent cell. For flexible networks, the hopping mechanism for particle escape becomes less pronounced with higher network flexibility due to either a smaller spring constant or slacker strands, each consisting of more beads. This behavior could be explained by the larger cell size fluctuation and its slower relaxation, whereby large enough openings temporarily formed are longer-lived, increasing the chance for particle hopping.

Nanovoid-Enhanced Thin-Film Composite Reverse Osmosis Membranes Using ZIF-67 Nanoparticles as a Sacrificial Template.

In this work, nanovoid-enhanced thin-film composite (TFC) membranes have been successfully fabricated using ZIF-67 nanoparticles as the sacrificial template. By incorporating different amounts of ZIF-67 during interfacial polymerization, the resultant TFC membranes can have different degrees of nanovoids after self-degradation of ZIF-67 in water, consequently influencing their physiochemical properties and separation performance. Nanovoid structures endow the membranes with additional passages for water molecules. Thus, all the newly developed TFC membranes exhibit better separation performance for brackish water reverse osmosis (BWRO) desalination than the pristine TFC membrane. The membrane made from 0.1 wt % ZIF-67 shows a water permeance of 2.94 LMH bar-1 and a salt rejection of 99.28% when being tested under BWRO at 20 bar. This water permeance is 53% higher than that of the pristine TFC membrane with the salt rejection well maintained.

Continuous UiO-66-Type Metal-Organic Framework Thin Film on Polymeric Support for Organic Solvent Nanofiltration.

For the first time, continuous polycrystalline UiO-66-NH2 thin film supported by a cross-linked Matrimid substrate was successfully fabricated via in situ solvothermal synthesis at room temperature for organic solvent nanofiltration. The integrated structure of the formed UiO-66-NH2 selective layer was inferred by various characterizations including X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. We have demonstrated that pretreatment of the substrate by an organic ligand, the number of solvothermal synthesis cycles, and the reaction time play important roles in MOF film formation. The newly developed UiO-66-NH2 membrane possesses high surface hydrophobicity and mean pore size of 0.89 nm in diameter. It shows an exceptional rejection of 96.33% to Rose Bengal with moderate ethanol permeance of 0.88 L m-2 h-1 bar-1. Benefiting from the extraordinary chemical stability of Zr-MOF crystals, the UiO-66-NH2 membrane shows excellent stability in different solvents, implying their great potential for real applications. This work provides useful insights into the fabrication of continuous UiO-66-type MOF membranes on polymeric substrates, which are very promising in practical separations involving organic solvents.

Polyelectrolyte Translocation through a Tortuous Nanopore.

Although nanopores have shown tremendous promise for use in DNA sequencing, the rate of translocation through most pores studied previously is too rapid for the genetic information to be read accurately. In this study, dissipative particle dynamics simulations were employed to investigate the feasibility of using tortuous nanopores to control the rate of polyelectrolyte translocation. Unlike many previous studies, our simulation method incorporates the effects of hydrodynamic and electrostatic interactions and the spatial variation of electric field strength. The average translocation time, ⟨τ⟩, increases with the pore length and tortuosity but decreases as the pore width increases. For the longest pore investigated, the introduction of tortuosity results in ⟨τ⟩ increasing by as much as 187% as compared to a straight pore. The temporal variation of bond tension indicates that slower translocation in tortuous nanopores is caused by inhibition of tension propagation.

Flow-Induced Translocation of Star Polymers through a Nanopore.

The flow-induced translocation of star polymers through a cylindrical nanopore has been studied using dissipative particle dynamics (DPD) simulations. The number of arms, f, was varied with the total number of monomers, N, kept constant. The effect of simulating the capture of the polymer into the pore upon the mean translocation time, <τt>, has been investigated by varying the chain's initial location. The results indicate that the incorporation of the capture process results in a reduction of <τt> by up to 15%. This is because the chain's initial location affects the extent of its stretching along the flow direction during translocation. <τt> exhibits nonmonotonic variation with f, in agreement with recently reported results for electric field-driven translocation of star polymers. Its value is larger and shows greater variation with f when the solvent quality is better. For the same value of f, the capture occurs faster in a good solvent. In addition, <τt> is greater for a semiflexible chain than its flexible counterpart as the time required for the branch point to enter the nanopore is longer in the former case.

Translocation of Star Polyelectrolytes through a Nanopore.

The electric field driven translocation of charged star polymers through a cylindrical nanopore has been studied using dissipative particle dynamics simulations. The critical field strength required to induce translocation depends on both the number of arms and the number of beads per arm. It may therefore be possible to separate star polyelectrolytes of different arm lengths using electric field driven translocation through a nanopore. The average translocation time exhibits nonmonotonic variation with the number of arms for good solvent conditions. During translocation, a star polymer with many arms is stretched along the pore axis to a lesser extent as compared to its linear counterpart. Unlike a linear chain that shows tension propagation with large tensions for bonds about to enter the pore, a star has the tensest bonds closest to the branch point whose connectivity to multiple arms raises difficulty for its entry and passage.

Thin-Film Nanocomposite (TFN) Membranes Incorporated with Super-Hydrophilic Metal-Organic Framework (MOF) UiO-66: Toward Enhancement of Water Flux and Salt Rejection.

Zirconiumv (IV)-carboxylate metal-organic framework (MOF) UiO-66 nanoparticles were successfully synthesized and incorporated in the polyamide (PA) selective layer to fabricate novel thin-film nanocomposite (TFN) membranes. Compared to unmodified pure polyamide thin-film composite (TFC) membranes, the incorporation of UiO-66 nanoparticles significantly changes the membrane morphology and chemistry, leading to an improvement of intrinsic separation properties due to the molecular sieving and superhydrophilic nature of UiO-66 particles. The best performing TFN-U2 (0.1 wt % particle loading) membrane not only shows a 52% increase of water permeability but also maintains salt rejection levels (∼95%) similar to the benchmark. The effects of UiO-66 loading on the forward osmosis (FO) performance were also investigated. Incorporation of 0.1 wt % UiO-66 produced a maximum water flux increase of 40% and 25% over the TFC control under PRO and FO modes, when 1 M NaCl was used as the draw solution against deionized water feed. Meanwhile, solute reverse flux was maintained at a relatively low level. In addition, TFN-U2 membrane displayed a relatively linear increase in FO water flux with increasing NaCl concentration up to 2.0 M, suggesting a slightly reduced internal concentration polarization effect. To our best knowledge, the current study is the first to consider implementation of Zr-MOFs (UiO-66) onto TFN-FO membranes.

Co-micellization behavior of triblock copolymers in the presence of hydrophobic drug molecules: A simulation study.

Dissipative particle dynamics simulation of bead-spring chains is applied to study co-micellization, micellar and drug loading properties of binary triblock copolymer systems to seek a better understanding on selection and utilization of polymer mixtures to improve the drug encapsulation efficiency and loading capacity. Co-micellization is found to still occur in the presence of drug; pure and mixed micelles coexist with some devoid of any drug due to their significantly lower aggregation numbers. The drug encapsulation enlarges the micelle cores, and may induce a morphology transition from spherical to ellipsoidal or rod-like micelles. For the binary mixture of polymer species with a considerable difference in chain length, the majority of the shorter chains are located near the periphery of the hydrophobic core, because they prefer the loop conformation so as to join the micelles in the co-micellization process. Increasing the drug concentration causes more chains to become looped to facilitate the formation of larger micelles for drug accommodation.

Dynamics of Brownian particles in three-dimensional ordered porous media subject to an oscillatory force.

Brownian dynamics simulation has been employed to study the dynamic behavior of particles in three-dimensional ordered porous media subject to a sinusoidal force field. The media comprises interconnected spherical cavities arranged in a simple cubic lattice. The thermal noise assists the particles to undergo cavity hopping, leading to a displacement behavior analogous to stochastic resonance, when the imposed field is strong enough but not aligned with the aperture lines, and the oscillation frequency is not too high. The periodic mean trajectory depends on the strength, frequency, and orientation of the imposed field. At sufficiently large field strength, the periodic particle displacement can become nonsinusoidal due to the strong hindrance and pinning effect of the cavity wall.

Co-micellization behavior in poloxamers: dissipative particle dynamics study.

Dissipative particle dynamics simulations are applied to investigate co-micellization behavior for binary mixtures of Poloxamers in dilute aqueous solution. In view of block length similarity/dissimilarity, four representative mixture cases are considered: F127/P123, F127/P105, P123/P84, and F127/L64. With appropriate interaction parameters, the simulations enable us to examine the formation of micelles, their types, size, shape, and composition. In the investigated concentration range, we find that pure and mixed micelles, both ellipsoidal, always coexist for all cases. At similar concentrations, both species form pure micelles of their own together with mixed micelles. In the case of F127/L64, it is found that the L64 chains are involved in the mixed micelles, even when the L64 concentration is below its CMC. The fraction of L64 involved in the mixed micelles is lower as compared to the other systems studied. For all cases, the proportion of mixed micelles can be increased when the two polymer species have similar concentrations. Moreover, shorter chains may prefer to straddle the core and corona in the region of ellipsoidal interface that is closer to the center of mixed micelle.

Driven transport of particles in 3D ordered porous media.

Brownian dynamics simulation has been employed to study the behavior of force-driven particle transport in two different types of ordered porous media: (A) interconnected spherical cavities and (B) an array of spherical obstacles, both arranged in simple cubic lattice. The effects of the imposed field strength and direction on the particle mobility and diffusivity are investigated. At a given porosity and imposed field strength, the particle mobility is greater for Case B than for Case A, owing to a higher degree of pore openness for the former. While the normalized mobility always increases with the field strength for Case B, irrespective of the field direction, it decreases for Case A when the field is not aligned with an aperture line. The particle diffusion may become anisotropic and show a nonmonotonic behavior with varying field strength. The diffusivity may increase 10-fold under a field misaligned with an aperture line.

Effect of chain length of PEO on the gelation and micellization of the pluronic F127 copolymer aqueous system.

The effect of adding homopolymer poly(ethylene oxide) (PEO) on the sol/gel behavior of amphiphilic triblock copolymer Pluronic F127 ((EO)98(PO)67(EO)98) in aqueous media is explored. Emphasis is placed on the influence of the PEO molecular weight and concentration on micellization and gelation and the exploration of their correlation. PEO is always found to lower the critical micellization temperature modestly. However, short PEO chains promote the gelation of F127, and long chains delay or even curb gel formation. Micelle size measurements and cryo-TEM micrographs provide evidence for micellar aggregation via the bridging of long PEO chains or depletion flocculation, thereby impeding the ordering of micelles for gel formation.

Drag force of a particle moving axisymmetrically in open or closed cavities.

Hydrodynamic resistance to particle transport arising from the solid mass in porous media is of fundamental importance. We investigate an axisymmetric creeping flow caused by a spherical particle migrating in a spherical cavity or connected cavities of equal size by a boundary element method. Each cavity has either one or two circular apertures, through which a sufficiently small particle can pass. Drag force on the particle is calculated to determine the correction factor to the Stokes law. It is found that when passing through an aperture, the particle experiences a local maximum drag force larger than that located in the cavity center. This force is also greater than that for the particle near the closed end at the same smallest surface-to-surface distance. For connected cavities open to the exterior fluid, the drag force is smaller than that in the corresponding closed system.

Force-driven migration of particles in ordered porous media.

Brownian dynamics simulation has been employed to study the behavior of force-driven particle migration in different ordered porous media comprised of periodically interconnected spherical cavities, representing inverted colloidal crystals. The effects of the imposed field strength and direction on the particle mobility and direction are investigated. The simulation results find that in a weak or intermediate field, the mobility normalized by the value in free solvent behaves in a similar way as the normalized diffusivity when the porosity is varied. Under a strong field, the normalized mobility can increase or decrease with the field strength, depending on the field direction relative to the cavity arrangement. If the imposed field is not aligned with any unobstructed pathway, the mobility tensor may become anisotropic and prolonged particle entrapment may also take place.

Complexation of cationic polyelectrolyte with anionic phospholipid vesicles: Concentration, molecular weight and salt effects.

The influence of cationic poly(diallyldimethylammonium chloride) on the morphology and phase behavior of anionic phospholipid vesicles was investigated using differential scanning calorimetry, fluorescent microscopy and light scattering technique. A wide range of polymer concentration has been examined for the first time. The polycation can bind electrostatically to the vesicles to compensate, neutralize and reverse the vesicular charge, depending on the molar ratio of cationic to anionic group R. For R<1, charge compensation weakened the electrostatic repulsion between the lipid molecules, leading to formation of polymer-modified vesicles, each with an increased number of bilayers. The bilayer exhibits a rising main phase transition temperature from a gel to liquid crystalline state. This behavior persisted until R≈1 around the neutralization condition, where the complexes became largest and precipitate. With R>1, charge reversal took place, the complex size reduced. Interestingly, the main phase transition temperature was found for the first time to shift back towards the original value in the absence of polymer for large enough R. Although the thermal behavior was nearly independent of the polymer molecular weight, the complex morphology could be different.

Rheological study of hydrophobically modified hydroxyethyl cellulose and phospholipid vesicles.

The rheological behavior of hydrophobically modified hydroxyehtyl cellulose (HMHEC) was studied in the presence of phospholipid vesicles. The effects of sonication, lipid species and concentration on the rheology were investigated at two HMHEC concentrations: 0.3wt.% and 0.7wt.%. It is found that compared to a pure HMHEC solution, the mixture viscosity could be enhanced by more than one order of magnitude for sonicated samples at certain lipid concentrations. The viscosity, which depends on the vesicle size and concentration, first increases with the lipid concentration and then decreases. Besides, the lipid addition increases the plateau modulus. These observed rheological behaviors can be explained by two types of association; some of the hydrophobes aggregate to form micelles, while others are embedded in the vesicle bilayers. By this means, the vesicles can be interconnected, contributing to the viscosity enhancement.

Electrophoretic mobility of a spherical liposome.

The electrophoretic velocity of a spherical liposome with a thin double layer is investigated theoretically for the first time to account for a field-induced redistribution of lipid molecules coupled with ion polarization as well as a capacitive effect when the two leaflets of the bilayer have different surface charges. The formulation also incorporates the influence of water osmotic flow through the bilayer. It is found that the lipid redistribution is stronger for a larger liposome, and can give rise to a smaller electrophoretic mobility. For a sufficiently thin double layer, this effect also increases with increasing strength of the imposed electric field. The capacitive effect is significant for the leaflets having very different charge densities, in particular when the outer leaflet becomes nearly uncharged. For such a case, the induction of a surface potential for the outer leaflet is pronounced, leading to electrophoretic migration of the liposome.

Interaction between poly(acrylic acid) and phospholipid vesicles: effect of pH, concentration, and molecular weight.

Interactions between phospholipid vesicles and poly(acrylic acid) (PAA) are of fundamental importance for pH-sensitive delivery systems in pharmaceutical applications. In this study, we investigated the behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at varied concentrations of PAA with low and high molecular weights. Differential scanning calorimetry and fluorescence microscopy experiments suggested that hydrogen bonding between the carboxyl group of PAA and the phosphodiester group of the lipid is the main driving force for association under acidic conditions. Complexation between DPPC and PAA depends on the PAA molecular weight and concentration. At high enough concentrations of the large molecular weight PAA, the polymer can completely disrupt the bilayers of DPPC. For the small molecular weight PAA, in contrast, the complexation leads to vesicle aggregation without destroying the bilayers. The difference in complex structure was detected by SEM and a UV-visible spectrophotometer. At alkaline pH, complexation did not occur because of strong dissociation of PAA.

A study of effective diffusivity in porous scaffold by Brownian dynamics simulation.

Brownian dynamics simulation has been employed to study the diffusion behavior of nutrient molecules in a random porous scaffold, which models a realistic one made by a gas-foaming technique. Excluded volume interaction between nutrient and the pore wall is only considered and implemented with reflecting boundary condition. Several effects are investigated, including the medium porosity, the scaffold microstructure and the size ratio of nutrient to pore. It is found that unlike in ordered porous media, the effective nutrient diffusivity in random media depends not only on the porosity, but also on degree of pore interconnection and overlapping, which is affected by the pore number concentration. At a given porosity, the diffusivity may increase noticeably when the pore number concentration becomes high enough. This behavior arises when the porosity is not too large (lower than 80%).

Interaction and complexation of phospholipid vesicles and triblock copolymers.

Mixtures of Pluronic (F-127 or L-61) and phospholipid were investigated for a wide range of Pluronic concentrations (0-15 wt %) using dynamic light scattering, differential scanning calorimetry, and fluorescence microscopy. The present study is aimed at better understanding how the amphiphilic triblock copolymers affect the lipid vesicles, particularly in the high-concentration regime. Our results show that L-61 interacts more strongly with phospholipid vesicles than F-127 when the copolymer is at the unimer state in the solution. For high concentrations, F-127 forms mixed micelles with solubilized lipid molecules in the form of bilayer patches. This novel behavior was observed for the first time. In contrast, more hydrophobic L-61 tends to precipitate with the solubilized lipids as large crew-cut mixed aggregates.