Effect of ZnO nanoparticles doping on structural,dielectric and interfacial behavior of polyoxyethylene (20) sorbitan/ethylene glycol lyotropic phases

ABSTRACT

We are reporting on the interaction of zinc oxide (ZnO) nanoparticles (NPs) with the lyotropic phase comprises of Polyoxyethylene (20) sorbitan monolaurate and protic solvent ethylene glycol. The concentration of the NPs has been varying from 0.05 to 0.5 wt%. Multiwall lamellar and inverse phases have been observed at lower and higher concentration of ZnO NPs doping. Interestingly, the organization of ZnO NPs on the periphery and inside the periphery of ring-like structures has been observed at lower and higher concentration of the dopant, respectively. Such organization of the NPs can be explained considering interfacial interaction amid host and dopant and may also attribute to the adsorption mechanisms of surfactant. Effects of NPs doping on the dielectric dynamics has also been examined. About 32.6% decrease in the dielectric permittivity has been noticed at higher NPs doping. Such decrement in permittivity could be a result of the screening of the ZnO NPs dipole moment by the adsorption of surfactant molecules on their surface. Relaxation and optical parameters of the non-doped and doped mixtures have also been discussed.     

Conserving the linear momentum in stochastic dynamics: Dissipative particle dynamics as a general strategy to achieve local thermostatization in molecular dynamics simulations

Stochastic dynamics is a widely employed strategy to achieve local thermostatization in molecular dynamics simulation studies; however, it suffers from an inherent violation of momentum conservation. Although this short‐coming has little impact on structural and short‐time dynamic properties, it can be shown that dynamics in the long‐time limit such as diffusion is strongly dependent on the respective thermostat setting. Application of the methodically similar dissipative particle dynamics (DPD) provides a simple, effective strategy to ensure the advantages of local, stochastic thermostatization while at the same time the linear momentum of the system remains conserved. In this work, the key parameters to employ the DPD thermostats in the framework of periodic boundary conditions are investigated, in particular the dependence of the system properties on the size of the DPD‐region as well as the treatment of forces near the cutoff. Structural and dynamical data for light and heavy water as well as a Lennard–Jones fluid have been compared to simulations executed via stochastic dynamics as well as via use of the widely employed Nose–Hoover chain and Berendsen thermostats. It is demonstrated that a small size of the DPD region is sufficient to achieve local thermalization, while at the same time artifacts in the self‐diffusion characteristic for stochastic dynamics are eliminated. © 2016 Wiley Periodicals, Inc.     

Dissipative Particle Dynamics Simulations of Polymer Brushes: Comparison with Molecular Dynamics Simulations

Summary: The structure of polymer brushes is investigated by dissipative particle dynamics (DPD) simulations that include explicit solvent particles. With an appropriate choice of the DPD interaction parameters , we obtain good agreement with previous molecular dynamics (MD) results where the good solvent behavior has been modeled by an effective Lennard–Jones potential. The present results confirm that DPD simulation techniques can be applied for large length scale simulations of polymer brushes. A relation between the different length scales and is established.

Polymer brush at a solid–liquid interface.     

Freezing or wrapping: the role of particle size in the mechanism of nanoparticle-biomembrane interaction

Understanding the interactions between nanoparticles (NPs) and biological matter is a high-priority research area because of the importance of elucidating the physical mechanisms underlying the interactions leading to NP potential toxicity as well as NP viability as therapeutic vectors in nanomedicine. Here, we use two model membrane systems, giant unilamellar vesicles (GUVs) and supported monolayers, to demonstrate the competition between adhesion and elastic energy at the nanobio interface, leading to different mechanisms of NP-membrane interaction relating to NP size. Small NPs (18 nm) cause a "freeze effect" of otherwise fluid phospholipids, significantly decreasing the phospholipid lateral mobility. The release of tension through stress-induced fracture mechanics results in a single microsize hole in the GUVs after interaction. Large particles (>78 nm) promote membrane wrapping, which leads to increased lipid lateral mobility and the eventual collapse of the vesicles. Electrochemical impedance spectroscopy on the supported monolayer model confirms that differently sized NPs interact differently with the phospholipids in close proximity to the electrode during the lipid desorption process. The time scale of these processes is in accordance with the proposed NP/GUV interaction mechanism.     

On Coarse-Graining by the Inverse Monte Carlo Method: Dissipative Particle Dynamics Simulations Made to a Precise Tool in Soft Matter Modeling

We present a promising coarse-graining strategy for linking micro- and mesoscales of soft matter systems. The approach is based on effective pairwise interaction potentials obtained from detailed atomistic molecular dynamics (MD) simulations, which are then used in coarse-grained dissipative particle dynamics (DPD) simulations. Here, the effective potentials were obtained by applying the inverse Monte Carlo method [Lyubartsev and Laaksonen, Phys. Rev. E. 52, 3730 (1995)] on a chosen subset of degrees of freedom described in terms of radial distribution functions. In our first application of the method, the effective potentials were used in DPD simulations of aqueous NaCl solutions. With the same computational effort we were able to simulate systems of one order of magnitude larger than the MD simulations. The results from the MD and DPD simulations are in excellent agreement.     

聚合物PVP与表面活性剂AOT相互作用的介观模拟

用耗散颗粒动力学模拟(DPD)方法研究了聚乙烯吡咯烷酮(PVP)与2-乙基己基琥珀酸酯磺酸钠(AOT)之间的相互作用.在三维模拟格子中,聚合物链均方末端距〈r²〉随着表面活性剂浓度的增加呈现一种首先减小,接着增加,然后又减小的趋势.构型和结构分析表明,AOT的加入能够引起聚合物链的二面角分布发生改变,这意味着AOT与PVP产生了相互作用.同时表面活性剂/聚合物体系的聚集形态也可以在DPD三维模拟格子中直观显现出来.     

Prediction of the Reverse Micellar Extraction of Papain Using Dissipative Particle Dynamics Simulation

Reverse micellar extraction is a promising technology for large-scale protein purification, but its molecular interaction mechanisms have not been thoroughly characterized. In this study, a dissipative particle dynamics (DPD) molecular simulation method was employed to study the interactions among the surfactant, organic phase, water, and proteins on the mesoscopic scale. This study simulated the self-assembly process of the reverse micelle extraction of papain. The results showed that the papain could be extracted by a CTAB/isooctane/n-hexanol system, which was validated by extraction experiments. The optimized extraction recovery was 76.9 %. This study elucidates the molecular process of the reverse micellar extraction of proteins and provides a method to predict its efficacy.     

Modeling of polyethylene and poly (L-lactide) polymer blends and diblock copolymer: chain length and volume fraction effects on structural arrangement

Dissipative particle dynamics (DPD), a mesoscopic simulation approach, has been used to investigate the chain length effect on the structural property of the immiscible polyethylene (PE)/poly(L-lactide) (PLLA) polymer in a polymer blend and in a system with their diblock copolymer. In this work, the interaction parameter in DPD simulation, related to the Flory-Huggins interaction parameter chi, is estimated by the calculation of mixing energy for each pair of components in molecular dynamics simulation. The immiscibility property of PE and PLLA polymers induces the phase separation and exhibits different architectures at different volume fractions. In order to observe the structural property, the radius of gyration is used to observe the detailed arrangement of the polymer chains. It shows that the structure arrangement of a polymer chain is dependent on the phase structure and has a significantly different structural arrangement character for the very short chains in the homopolymer and copolymers. The chain length effect on the degree of stretching or extension of polymers has also been observed. As the chain length increases, the chain exhibits more stretching behavior at lamellae, perforated lamellae, and cylindrical configurations, whereas the chain exhibits a similar degree of stretching or extension at the cluster configuration.     

Hydrodynamic interaction in polymer solutions simulated with dissipative particle dynamics

The authors analyzed extensively the dynamics of polymer chains in solutions simulated with dissipative particle dynamics (DPD), with a special focus on the potential influence of a low Schmidt number of a typical DPD fluid on the simulated polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid can lead to underdevelopment of the hydrodynamic interaction in polymer solutions. The authors' analyses reveal that equilibrium polymer dynamics in dilute solution, under typical DPD simulation conditions, obey the Zimm [J. Chem. Phys. 24, 269 (1956)] model very well. With a further reduction in the Schmidt number, a deviation from the Zimm model to the Rouse model is observed. This implies that the hydrodynamic interaction between monomers is reasonably developed under typical conditions of a DPD simulation. Only when the Schmidt number is further reduced, the hydrodynamic interaction within the chains becomes underdeveloped. The screening of the hydrodynamic interaction and the excluded volume interaction as the polymer volume fraction is increased are well reproduced by the DPD simulations. The use of soft interaction between polymer beads and a low Schmidt number do not produce noticeable problems for the simulated dynamics at high concentrations, except for the entanglement effect which is not captured in the simulations.     

Dissipative Particle Dynamics Study of Interfacial Properties and the Effects of Nonionic Surfactants on Hydrocarbon/Water Microemulsions

The aim of this work was to apply dissipative particle dynamics (DPD) mesoscopic simulations to study the interfacial orientation and the effect of the nonionic surfactant, hexaethylene glycol monododecyl ether (C₁₂E₆), on different (oil (dodecane)/water) microemulsion systems. The Hildebrand-solubility-parameter model and Flory–Huggins/Hansen-solubility-parameter (FH/HSP) model were combined to evaluate the DPD interaction parameter (a_ij) where the solubility parameters (δ_i) as DPD input parameters were preliminary validated by all-atom molecular dynamics (MD) results and experimental data. The interfacial property dependence of dodecane/water/C₁₂E₆ system on the oil/water (o/w) ratio and on the concentration of surfactant and orientation at the interface were investigated. It was found that the surfactant addition reduced the IFT of o/w interfaces and this reduction was more efficient for water-in-oil microemulsions (o/w ≤ 1).     

Theory for collective dynamics and optical response of metallic nanoparticles under light-induced force and fluctuations

Due to the strong optical response of localized surface plasmon (LSP) in metallic nanoparticles (NPs), the light-induced force (LIF) is also strong and can be used for the control of their dynamics even at room temperature. However, properties of LIF are still unclear under the collective effects of LSP in multiple NPs. In this article, I discuss the fundamental properties of LIF exerted on metallic NPs taking into account photomediated interaction between NPs, and light-induced dynamics of NPs in fluid medium (for example, water) in the presence of the thermal fluctuations. Remarkably, it has been clarified that the collective optical response of LSP can be greatly modulated through the dynamical pattern formation process of NPs by LIF.     

Molecular dynamics simulations of nanoparticles in dense isotropic nematogens: the role of matrix-induced long-range repulsive interactions

We have carried out molecular dynamics simulation studies of binary mixtures of spherical nanoparticles (NPs) in a matrix of dense isotropic rod-shaped nematogens, with the size of the nematogen length being similar to that of the NP diameter. NPs at even low concentrations were found to shift the isotropic-nematic (I-N) transition significantly to higher pressure at a given temperature, indicative of long-range perturbation of the nematogenic matrix by the NPs. The NPs were found to be dispersed in the dense isotropic nematogenic matrix over a wide range of NP concentrations due to long-range (compared with the molecular size of the nematogens) repulsion caused by NP-induced local order fluctuations and reduced local orientational correlation in the isotropic nematogenic matrix, in contrast to the phase separation predicted and observed in other studies where the particles were much larger or smaller than the nematogens. Furthermore, since the repulsion observed in the NP-nematogen mixtures is only microscopically long range (on the order of about ten molecular lengths of the nematogens), globally ordered clustering observed in mixtures of colloidal particles in nematic matrices resulting from macroscopically long-range interaction is not observed in our simulations.     

Molecular dynamics simulations studies of nanoparticles in an isotropic liquid crystal matrix: Single particle behavior and pairwise interactions

We report results of molecular dynamics simulation studies of the behavior of spherical nanoparticles (NPs) in a dense isotropic nematogen matrix comprised of soft spherocylinders (SSCs). The SSCs exhibit a tendency for frustrated planar anchoring at the NP surface that results in a long-range (compared to the size of the NPs and SSCs) reduction in local orientational ordering and increased fluctuations in local orientational ordering compared to the pure isotropic phase of the SSCs. The potential of mean force between two nanoparticles exhibits a novel long-range repulsive tail separated from short-range molecular packing peaks by a shallow local minimum in free energy. The long-range repulsion is caused by NP-induced ordering fluctuations while the shallow minimum results from increased local ordering within the confinement region in between two NPs. The influence of the NPs on local orientational order in the nematogen matrix and the nematogen-induced interaction between NPs are found to depend strongly on the size of the NPs.     

Effect of the Nanoparticle Functionalization on the Cavitation and Crazing Process in the Polymer Nanocomposites

The nanoparticle(NP) functionalization is an effective method for enhancing their compatibility with polymer which can influence the fracture property of the polymer nanocomposites(PNCs). This work aims to further understand the cavitation and crazing process, hoping to uncover the fracture mechanism on the molecular level. By adopting a coarse-grained molecular dynamics simulation, the fracture energy of PNCs first increases and then decreases with increasing the NP functionalization degree α while it shows a continuous increase with increasing the interaction ε_(pA) between polymer and modified beads. The bond orientation degree is first characterized which is referred to as the elongation. Meanwhile, the stress by polymer chains is gradually reduced with increasing the α or the ε_(pA) while that by NPs is enhanced.Furthermore, the percentage of stress by polymer chains first increases and then decreases with increasing the strain while that by NPs shows a contrast trend. Moreover, the number of voids is quantified which first increases and then decreases with increasing the strain which reflects their nucleation and coalescence process. The voids prefer to generate from the polymer-NP interface to the polymer matrix with increasing α o r ε_(pA).As a result, the number of voids first increases and then decreases with increasing α while it continuously declines with the ε_(pA). In summary, our work provides a clear understanding on how the NP functionalization influences the cavitation and crazing process during the fracture process.     

表面活性剂与聚合物相互作用的动力学模拟

用扩散颗粒动力学模拟方法（Dissipative Particle Dynamics，DPD）模拟了 中性聚合物与离子型表面活性剂的相互作用。在分子水平上研究了介于微观和宏观 上的一些性质，直观地用三维图形描绘了聚合物在表面活性剂溶液中的聚集形成， 并通过聚合物的末端的变化表征了聚集过程。结果发现：随着表面活性剂浓度的增 加，聚合物呈现自由伸缩→形成松散的棒状结构→再出现胶束状珍珠链结构→最终 在六角状和层状相中分布的过程。DPD模拟方法能够直观地得到聚合物在表面活性 剂溶液中的聚集形态。     

重质油胶体聚集结构的耗散粒子动力学模拟

重质油是以沥青质为胶核分散于饱和油分中形成的极其复杂的胶体体系.本文采用耗散粒子动力学(DPD)方法研究重质油的胶体结构及其影响因素.根据重质油各组分的分子结构特征,构建了描述重质油组分的粗粒化模型化合物.模拟结果表明,本文构建的粗粒化模型能很好地反映重质油的胶体聚集结构.沥青质分子结构对胶体聚集结构有序性有显著影响,较高稠合程度的芳香环结构将使胶束结构有较高的有序性,烷基侧链则表现出分散作用.重质油中的胶质具有胶溶作用,胶质与沥青质的浓度比存在一个极限,当小于这个极限时,重质油将出现聚沉.     

The Pacman effect: a supramolecular strategy for controlling the excited-state dynamics of pillared cofacial bisporphyrins

The molecular recognition properties of dizinc(II) bisporphyrin anchored by dibenzofuran (DPD), Zn2(DPD) (1), were evaluated as a strategy for utilizing the Pacman effect to control the excited-state properties of cofacial bisporphyrin motifs. Crystallographic studies establish that DPD furnishes a cofacial system with vertical flexibility and horizontal preorganization. The structure determination of a substrate-bound DPD species, Zn2(DPD)(2-aminopyrimidine) (2), completes a set of structurally homologous zinc(II) porphyrin host and host-guest complexes, which offer a direct structural comparison for the Pacman effect upon substrate complexation. Binding studies reveal that pyrimidine encapsulation by the DPD framework is accompanied by a markedly reduced entropic penalty (approximately 60 J mol(-1)K(-1)) with respect to traditional face-to-face bisporphyrin systems, giving rise to a smaller conformational energy cost upon substrate binding. Transient absorption spectroscopy reveals that substrate encapsulation within the DPD cleft dramatically affects excited-state dynamics of cofacial bisporphyrins. The emission lifetime of host-guest complex 2 increases by more than an order of magnitude compared to free host 1. In the absence of the guest, the excited-state dynamics are governed by torsional motion of the porphyrin rings about the aryl ring of the DPD pillar. Host-guest binding attenuates this conformational flexibility, thereby removing efficient nonradiative decay pathways. Taken together, these findings support the exceptional ability of the DPD system to structurally accommodate reaction intermediates during catalytic turnover and provide a novel supramolecular approach toward developing a reaction chemistry derived directly from the excited states of Pacman constructs.     

Cation and anion substitution effects on the ultrafast dynamics of interionic interaction in imidazolium based ionic liquids

Room-temperature Ionic Liquids(ILs) have numerous unique properties that differ from those of conventional molecular solvents.Although the unique properties of ILs have been suggested to origin from their microscopic interionic interaction,detailed dynamics of interionic interaction of ILs has not been fully understood.Here,with the Femtosecond Optical Heterodyne-Detected Raman Induced Kerr Effect Spectroscopy(fs-OHD-RIKES),we measured the ultrafast dynamics of the interionic interaction of three typical im...     

Bond or cage effect: how nitrophorins transport and release nitric oxide

Most blood-sucking insects possess salivary proteins which, upon injection into the victim's tissue, help them improve their feeding. One group of these salivary proteins takes advantage of the vasodilator properties of NO to perform this task. These proteins are the so-called nitrophorins (NPs). NPs are heme proteins that store and transport NO, which, when released in the victim's tissue, produces vasodilation and inhibition of blood coagulation. It has been proposed that NO binds tightly to NP at a low pH of around 5.6 and that once NPs are injected in the victims tissue, at a pH of approximately 7.4, a conformational change occurs which lowers NO affinity, allowing it to be released. In this work we have studied the NO release mechanism of NP4 at a molecular level using state of the art computer simulation techniques. We have used molecular dynamics (MD) simulations to study NP4 conformational dynamics at both pH values 5.6 and 7.4 and computed the corresponding free energy profile for NO release using a multiple steering molecular dynamics scheme. We also have used hybrid quantum mechanical/molecular mechanics (QM/MM) techniques to analyze the heme-NO structure and the Fe-NO bond strength in the different NP4 conformations. Our results provide the molecular basis to explain that NO escape from NP4 is determined by differential NO migration rates and not by a difference in the Fe-NO bond strength. In contrast to most heme proteins that control ligand affinity by modulating the bond strength to the iron, NP4 has evolved a cage mechanism that traps the NO at low pH and releases it upon cage opening when the pH rises.