接枝微粒PMAA/SiO2在水介质中对杀虫剂抗蚜威的吸附机理

以硅胶表面接枝聚甲基丙烯酸(PMAA)的复合型功能微粒PMAA/SiO2为固体吸附剂, 对水介质中的抗蚜威进行了静态吸附实验, 通过考察温度、pH值及盐度(NaCl浓度)对吸附容量的影响, 重点研究了PMAA/SiO2对抗蚜威的吸附机理. 为确认所提出的机理的正确性, 还采用紫外光谱吸收法, 研究了单体甲基丙烯酸与抗蚜威之间的相互作用, 也考察了在非水介质CCl4中PMAA/SiO2对抗蚜威的吸附作用. 研究结果表明, PMAA/SiO2对抗蚜威具有强的吸附作用, 吸附的驱动力是氢键、静电以及疏水相互作用三种作用的协同, 其中主驱动力是静电相互作用. 温度升高, 吸附容量减小; 盐度增大, 吸附容量降低; 当pH<8时, 吸附容量随pH升高而增大; 当pH>8时, 吸附容量随pH升高而减小; 当pH=8 时,吸附容量最大.     

Simulation study on the translocation of a partially charged polymer through a nanopore

The translocation of a partially charged polymer through a neutral nanopore under external electrical field is studied by using dynamic Monte Carlo method on a simple cubic lattice. One monomer in the polymer is charged and it suffers a driving force when it locates inside the pore. Two time scales, mean first passage time τ(FP) with the first monomer restricted to never draw back into cis side and translocation time τ for polymer continuously threading through nanopore, are calculated. The first passage time τ(FP) decreases with the increase in the driving force f, and the dependence of τ(FP) on the position of charged monomer M is in agreement with the theoretical results using Fokker-Planck equation [A. Mohan, A. B. Kolomeisky, and M. Pasquali, J. Chem. Phys. 128, 125104 (2008)]. But the dependence of τ on M shows a different behavior: It increases with f for M < N/2 with N the polymer length. The novel behavior of τ is explained qualitatively from dynamics of polymer during the translocation process and from the free energy landscape.     

Diffusive flux of nanoparticles through chemically modified alumina membranes

Surface chemistry plays an important role in determining flux through porous media such as in the environment. In this paper diffusive flux of nanoparticles through alkylsilane modified porous alumina is measured as a model for understanding transport in porous media of differing surface chemistries. Experiments are performed as a function of particle size, pore diameter, attached hydrocarbon chain length and chain terminus, and solvent. Particle fluxes are monitored by the change in absorbance of the solution in the receiving side of a diffusion cell. In general, flux increases when the membranes are modified with alkylsilanes compared to untreated membranes, which is attributed to the hydrophobic nature of the porous membranes and differences in wettability. We find that flux decreases, in both hexane and aqueous solutions, when the hydrocarbon chain lining the interior pore wall increases in length. The rate and selectivity of transport across these membranes is related to the partition coefficient (K(p)) and the diffusion coefficient (D) of the permeating species. By conducting experiments as a function of initial particle concentration, we find that K(p)D increases with increasing particle size, is greater in alkylsilane-modified pores, and larger in hexane solution than water. The impact of the alkylsilane terminus (-CH(3), -Br, -NH(2), -COOH) on permeation in water is also examined. In water, the highest K(p)D is observed when the membranes are modified with carboxylic acid terminated silanes and lowest with amine terminated silanes as a result of electrostatic effects during translocation.     

Polyacrylamide gel method: synthesis and property of BeO nanopowders

Effects of monomer (AM) concentration, monomer/crosslinker (AM/MBAM) ratio and salt concentration on the thermal behavior of precursor gel and the properties of BeO nanopowder synthesized by polyacrylamide gel method were investigated. The decomposition process of precursor gel was also studied. The decomposition process of precursor gel is that, first, the extraction of free and crystallized water, and then the thermal degradation of polymeric network under temperature higher than 600 °C, final, the decomposition of nanoscale beryllium sulfate to BeO nanopowder. As the monomer concentration increases, the calcination temperature of precursor gel decreases due to more compact network structure of gel and thus smaller size of salt in nanocaves in gel. The average particle size of nanopowder reduces correspondingly. The AM/MBAM ratio also has significant effect on the thermal behavior of precursor gel and the average particle size of product. When the ratio of AM to MBAM is 6, the calcination temperature of precursor gel is the lowest, the average particle size of powders is the smallest, because the network structures of gel is the tightest and thus the sizes of salts in precursor gels are the smallest. As the AM/MBAM ratio deviates from this value, the network structures of gel becomes looser and thus the size of salt in precursor gel becomes larger, so the calcination temperature increases and the average particle size of powders becomes larger certainly. For the same reason, both the calcination temperature and the average particle size of powders increases with increasing the salt concentration. The synthesis conditions have no effect on the particle size distribution of the final product due to the natural random distribution of porosity in gel.     

Factors affecting H2O absorption of the epoxy tetraglycidyl-4,4′-diaminodiphenyl methane cured with diaminodiphenyl sulfone

It was found that the amount of water absorbed at room temperature in cured tetraglycidyl-4,4′-diaminodiphenyl methane/diaminodiphenyl sulfone epoxy resins increases as the curing time or temperature increases while the amount of tetrahydrofuran-soluble extractables and the room temperature density decreases. These data suggest that the free volume increases with the extent of cure and the resins become more accessible to water. While the driving force for water absorption is the electrostatic attraction between water and the functional groups in the epoxy, the results suggest that equilibrium H₂O absorption is determined primarily by unoccupied volume of the epoxy resin.     

Theoretical study on the translocation of partially charged polymers through nanopore

The translocation time τ of partially charged polymers through a neutral nanopore is calculated using Fokker–Planck equation with adsorbing–adsorbing boundary conditions. For the polymer with one charged monomer, we find that τ is dependent on the position of the charged monomer and on the magnitude of the driving force f inside the nanopore. When the charge is located at the front half of the polymer chain, τ is larger than that of neutral polymer and increases with f. When the charge is located at the back half, it is smaller than that of the neutral polymer and decreases with increasing f. We have also studied the behavior of a symmetrical polymer with two like charges located symmetrically in the chain and that of an asymmetrical polymer with two unlike charges. Moreover, we have calculated the translocation time for a general condition of polymer with two randomly distributed charges. All results show that τ is dependent on the positions of charges in the polymer chain and on the magnitude of the driving force. The results can be explained qualitatively by the free‐energy landscape of polymer translocation. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1017–1025     

Flow microcalorimetry of adsorption of anionic alkyl surfactants at the solid/liquid interface

Flow microcalorimetry was used to study the adsoption of anionic alkyl surfactants from aque--ous solutions onto silica. It is found that for alkyl sulfate systems the strength of adsorption interactionincreases with increases of the alkyl chain length and decreases as temperature rises. The adsorptiondepends only on monomer concentration of the solution even above the critical micelle concentration(cmc). The assumption is made that the adsorption involves only a transfer of monomers from bulkto surface phase. A different adsorption mechanism is operative for the alkyl carboxylate.     

Different elution modes and field programming in gravitational field-flow fractionation. Effect of channel angle

Gravitational field-flow fractionation (GrFFF) has been shown to be useful for separation and characterization of various types of micrometer-sized particles. It has been recognized however that GrFFF is less versatile than other members of FFF because the external field (Earth's gravity) in GrFFF is relatively weak and is not tunable (constant), which makes the force acting on the particles constant. A few approaches have been suggested to control the force acting on particles in GrFFF. They include (1) changing the angle between the Earth's gravitational field and the longitudinal axis of the channel, and (2) the use of carrier liquid having different densities. In the hyperlayer mode of GrFFF, the hydrodynamic lift force (HLF) also act on particles. The existence of HLF allows other means of changing the force acting on the particles in GrFFF. They include (1) the flow rate programming, or (2) the use of channels having non-constant cross-section. In this study, with polystyrene latex beads used as model particles, the channel angle was varied to study its effect on elution parameters (such as selectivity, band broadening and resolution) in the steric or in the hyperlayer mode of GrFFF. In addition, the effects of the channel thickness and the flow rate on the elution parameters were also investigated. It was found that, in the steric mode, the resolution decreases as the flow rate increases due to increased zone broadening despite of the increase in the selectivity. At a constant volumetric flow rate, both the zone broadening and the selectivity increase as the channel thickness increases, resulting in the net increase in the resolution. It was also found that the retention time decreases as the channel angle increases in both up- and down-flow positions. The zone broadening tends to increase almost linearly with the channel angle, while no particular trends were found in selectivity. As a result, the resolution decreases as the channel angle increases.     

Water Permeation Through DMPC Lipid Bilayers using Polarizable Charge Equilibration Force Fields

We investigate permeation energetics of water entering a model dimyristoylphosphatidylcholine (DMPC) bilayer via molecular dynamics simulations using polarizable Charge Equilibration (CHEQ) models. Potentials of mean force show 4.5-5.5 kcal/mol barriers for water permeation into bilayers. Barriers are highest when water coordination within the bilayer is prevented, and also when using force fields that accurately reproduce experimental alkane hydration free energies. The magnitude of the average water dipole moment decreases from 2.6 Debye (in bulk) to 1.88 Debye (in membrane interior). This variation correlates with the change in a water molecule's coordination number.     

Collective mechanical behavior of multilayer colloidal arrays of hollow nanoparticles

The collective mechanical behavior of multilayer colloidal arrays of hollow silica nanoparticles (HSNP) is explored under spherical nanoindentation through a combination of experimental, numerical, and theoretical approaches. The effective indentation modulus E(ind) is found to decrease with an increasing number of layers in a nonlinear manner. The indentation force versus penetration depth behavior for multilayer hollow particle arrays is predicted by an approximate analytical model based on the spring stiffness of the individual particles and the multipoint, multiparticle interactions as well as force transmission between the layers. The model is in good agreement with experiments and with detailed finite element simulations. The ability to tune the effective indentation modulus, E(ind), of the multilayer arrays by manipulating particle geometry and layering is revealed through the model, where E(ind) = (0.725m(-3/2) + 0.275)E(mon) and E(mon) is the monolayer modulus and m is number of layers. E(ind) is seen to plateau with increasing m to E(ind_plateau) = 0.275E(mon) and E(mon) scales with (t/R)(2), t being the particle shell thickness and R being the particle radius. The scaling law governing the nonlinear decrease in indentation modulus with an increase in layer number (E(ind) scaling with m(-3/2)) is found to be similar to that governing the indentation modulus of thin solid films E(ind_solid) on a stiff substrate (where E(ind_solid) scales with h(-1.4) and also decreases until reaching a plateau value) which also decreases with an increase in film thickness h. However, the mechanisms underlying this trend for the colloidal array are clearly different, where discrete particle-to-particle interactions govern the colloidal array behavior in contrast to the substrate constraint on deformation, which governs the thickness dependence of the continuous thin film indentation modulus.     

How does water-nanotube interaction influence water flow through the nanochannel?

Water permeation across various nitrogen-doped double-walled carbon nanotubes (N-DWCNT) has been studied with molecular dynamics simulations to better understand the influence of water-nanopore interaction on the water permeation rate. There exists a threshold interaction energy at around -34.1 kJ/mol. Over the threshold energy, the water flow through N-DWCNT decreases monotonically with the strengthening of the water-nanotube interaction. The effect on the water flow across the channel is found to be negligible when the interaction energy is weaker than the threshold. The water-nanotube interaction energy can be controlled by doping nitrogen atoms into the nanotube walls. Although the van der Waals interaction energy is much stronger than the electrostatic interaction energy, it is less sensitive to the proportion of doped nitrogen atoms. On the other hand, the electrostatic interaction energy weakens after the initial strengthening when the percentage of doped nitrogen atoms increases to ~25%. The doped nitrogen atoms make less influence on the overall electrostatic interaction energy when the proportion is over 25%, due to the repulsions among themselves. Thus, the monotonous strengthening of the van der Waals interaction energy seems to dominate the overall trend of the total interaction energy, whereas the change of the long-range electrostatic interaction energy characterizes the shape of the correlation curve, as the percentage of doped nitrogen atoms increases.     

Pore structure of the packing of fine particles

This paper presents a numerical study of the pore structure of fine particles. By means of granular dynamics simulation, packings of mono-sized particles ranging from 1 to 1000 microm are constructed. Our results show that packing density varies with particle size due to the effect of the cohesive van der Waals force. Pores and their connectivity are then analysed in terms of Delaunay tessellation. The geometries of the pores are represented by the size and shape of Delaunay cells and quantified as a function of packing density or particle size. It shows that the cell size decreases and the cell shape becomes more spherical with increasing packing density. A general correlation exists between the size and shape of cells: the larger the cell size relative to particle size, the more spherical the cell shape. This correlation, however, becomes weaker as packing density decreases. The connectivity between pores is represented by throat size and channel length. With decreasing packing density, the throat size increases and the channel length decreases. The pore scale information would be useful to understand and model the transport and mechanical properties of porous media.     

Facile synthesis and optimization of conductive copolymer nanoparticles and nanocomposite films from aniline with sulfodiphenylamine

A new series of electrically conductive pure copolymer nanoparticles was facilely synthesized by using oxidative polymerization of aniline (AN) and sodium diphenylamine-4-sulfonate (SDP) in acidic media in the absence of stabilizer. The variation of the structure of the copolymer particles was comprehensively studied by carefully choosing several important parameters, such as the comonomer ratio, oxidant/monomer ratio, polymerization time and temperature, monomer concentration, acidic medium, and oxidant species. Analytical techniques used include IR and UV-visible spectroscopy, X-ray diffraction, laser particle analysis, atomic force microscopy, and transmission electron microscopy. It was found that the particle size varied significantly with the above-mentioned polymerization parameters, only changes in the salt concentration in the aqueous testing solution had no noticeable effect. The polymerization conditions were optimized for the formation of copolymer nanoparticles with sought-after properties. The doped copolymer particles of AN/SDP (50:50) at an oxidant/monomer molar ratio of 0.5 exhibit a minimum length of 50 nm and a minimum diameter of 44 nm. The bulk electrical conductivity of the copolymer particles increases greatly from 5.90x10(-4) to 1.15x10(-2) S cm(-1) with increasing AN content. Compared with barely soluble polyaniline, the copolymers exhibit a remarkably enhanced solubility in most solvents, including NH4OH and even water, due to the presence of the hydrophilic sulfonic groups. Nanocomposite films of the nanoparticles and cellulose diacetate exhibit a percolation threshold of down to 0.1 wt %, at which the film retains 98% of the transparency, 94% of the strength, and 5x10(7) times the conductivity of a pure cellulose diacetate film.     

Size selectivity in field-flow fractionation: lift mode of retention with near-wall lift force

A simple theoretical model for the size selectivity, S(d), in the lift mode of retention in field-flow fractionation (FFF) is developed on the basis of the near-wall lift force expression. S(d) is made up of two contributions: the flow contribution, S(d,f), arising from the variation of the flow velocity at center of particle due to a change in particle position with particle size, and a slip contribution, S(d,s), arising from the concomitant change in the extent of retardation due to the presence of a nearby channel wall. The slip contribution is minor, but not negligible, and amounts to 10-20% of the overall size selectivity. It contributes to reduce S(d) in sedimentation FFF but to enhance it in flow FFF. S(d) would steadily increase with particle size if the flow profile was linear (Couette flow). Because of the curvature of the flow profile encountered in the classical Poiseuille flow, S(d) exhibits a maximum at some specific particle size. The model predicts a significant difference in S(d) between sedimentation FFF and flow FFF, arising from the different functional dependences of the field force with particle size between these two methods. The predictions are in good agreement with the various S(d) values reported in the literature in both sedimentation and flow FFF. On the basis of the model, guidelines are given for adjusting the operating parameters (carrier flow rate and field strength) to optimize the size selectivity. Finally, it is found that S(d) generally decreases with decreasing channel thickness.     

Structure, surface excess and effective interactions in polymer nanocomposite melts and concentrated solutions

The Polymer Reference Interaction Site Model (PRISM) theory is employed to investigate structure, effective forces, and thermodynamics in dense polymer-particle mixtures in the one and two particle limit. The influence of particle size, degree of polymerization, and polymer reduced density is established. In the athermal limit, the surface excess is negative implying an entropic dewetting interface. Polymer induced depletion interactions are quantified via the particle-particle pair correlation function and potential of mean force. A transition from (nearly) monotonic decaying, attractive depletion interactions to much stronger repulsive-attractive oscillatory depletion forces occurs at roughly the semidilute-concentrated solution boundary. Under melt conditions, the depletion force is extremely large and attractive at contact, but is proceeded by a high repulsive barrier. For particle diameters larger than roughly five monomer diameters, division of the force by the particle radius results in a nearly universal collapse of the depletion force for all interparticle separations. Molecular dynamics simulations have been employed to determine the depletion force for nanoparticles of a diameter five times the monomer size over a wide range of polymer densities spanning the semidilute, concentrated, and melt regimes. PRISM calculations based on the spatially nonlocal hypernetted chain closure for particle-particle direct correlations capture all the rich features found in the simulations, with quantitative errors for the amplitude of the depletion forces at the level of a factor of 2 or less. The consequences of monomer-particle attractions are briefly explored. Modification of the polymer-particle pair correlations is relatively small, but much larger effects are found for the surface excess including an energetic driven transition to a wetting polymer-particle interface. The particle-particle potential of mean force exhibits multiple qualitatively different behaviors (contact aggregation, steric stabilization, local bridging attraction) depending on the strength and spatial range of the polymer-particle attraction.     

On the Knudsen transport of gases in nanochannels

We investigate the diffusion of gas molecules in nanochannels under the combinational effect of the vibration of the channel, gas-wall binding energy, and channel size through molecular dynamics simulations. It is found that the molecular vibration of the channel plays a critical role in gas transport process when the gas-wall binding energy is strong. For small binding energies, the influence of the flexibility of the wall can be neglected. In rigid channels, the gas self-diffusion coefficient increases with increasing gas-wall binding energy, while it decreases in nonrigid channels. The effect of the channel size on the self-diffusion coefficient is not significant except that a local maximum in the gas self-diffusion coefficient is found in 2 nm channels due to the strong repulsive force caused by the surface curvature of the channels.     

Tunneling through a fluctuating barrier in the presence of a periodically driving field

The zero and finite temperature tunneling dynamics of a periodically driven particle moving in a bistable potential with a fluctuating barrier is studied. We have focused on the influence of barrier fluctuation and thermal modulation on the tunneling processes in the presence of a driving field. At zero temperature, for a fixed strength of the driving field, both the tunneling probability and rate passes through a well-defined minimum when plotted as a function of fluctuation frequency while it reveals a clear maximum as a function of driving frequency. However, at T > 0 the tunneling probability and rate show two maxima as a function of both fluctuation frequency and driving frequency. In both zero and finite temperature, the tunneling rate constant decreases with increasing fluctuation strength. So, the barrier fluctuation may enhance the stability of a periodically driven system. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Polymerization of oil (styrene and methylmethacrylate)-in-water microemulsions

 The polymerization of styrene-in-water and methylmeth-acrylate-in-water microemulsions stabilized by nonionic surfactants was investigated using different initiation techniques. Thermally induced initiation was carried out using potassium persulfate (water soluble) and azobisiso-butyronitrile (AIBN) (oil soluble) at 60° and 50°C, respectively. When the monomer concentration was kept below a certain limit, the particle size of the nanolatex was similar to the droplet size of the microemulsion precursor. At higher monomer concentrations, the latex produced was significantly larger than the microemulsion droplets, as a result of the possible coalescence of the microemulsion droplets during polymerization. By using chemically induced polymerization (hydrogen peroxide+ascorbic acid) at temperatures below the cloud point temperature of the microemulsion or by photochemically induced initiation at room temperature, it was possible to obtain nanolatex particles with similar size to the droplets up to 10% monomer content. In all cases, the particle size was determined using photon correlation spectroscopy (PCS). Electron micrographs of the microlatex particles were taken and these confirmed the measurements obtained by PCS. The molecular weight of the polymers produced was determined by gel permeation chromatography. The average number of polymer molecules per particle was calculated. It was shown in some cases that the nanolatex contained one polymer chain per particle. A mechanism was suggested for polymerization and particle growth. Received: 29 May 1997 Accepted: 28 May 1998     

纳米镍团簇表面性质及能量的分子动力学研究

在单分散金属纳米粒子制备过程中,金属烧结现象需要尽量避免。烧结与诸多因素有关,其中金属纳米粒子的表面性质和能量对烧结作用有着重要影响。本工作利用分子动力学,以4种不同粒径的金属Ni纳米团簇为研究对象,在COMPASS力场下对不同温度下其表面积、扩散性质、表面能以及比表面能等进行了计算。结果显示,随着温度从300K升至1000K,纳米团簇的表面积稍微增加了约5%,表面层扩散系数显著增加了约3个数量级,表面能量升高了约15%,同时表面层与体相的能量差明显增加了近3倍。比表面能定义为增加单位表面积所引起的表面能的增量。计算结果表明,700K时团簇的比表面能比镍熔点处的表面张力高出约3个数量级,预示着团簇烧结具有强大的推动力。比表面能随温度升高以及粒径增大而下降,与热力学原理相一致。     

A channel Brownian pump powered by an unbiased external force

A Brownian pump of particles in an asymmetric finite tube is investigated in the presence of an unbiased external force. The pumping system is bounded by two particle reservoirs. It is found that the particles can be pumped through the tube from a reservoir at low concentration to one at the same or higher concentration. There exists an optimized value of temperature (or the amplitude of the external force) at which the pumping capacity takes its maximum value. The pumping capacity decreases with increasing radius at the bottleneck of the tube.