Mechanisms behind concentration profiles illustrated by charge and concentration distributions around ions in double layers

The mechanisms behind the behaviour of concentration and charge density profiles in diffuse electric double layers are investigated quantitatively for 1:1 and 2:2 electrolytes. This is done by analysing various contributions to the mean force that acts on each ion. The forces are obtained from the calculated ionic charge and concentration distributions around individual ions at various positions in the double layer. These distributions are presented graphically which allows an immediate visual illustration of the mechanisms in action. Some features studied are charge inversion in double layers for divalent aqueous electrolytes, overcompensation of surface charge due to large amounts of physisorbed counterions, ion size effects in the double layer structure and various mechanisms that cause deviations from the predictions of the Poisson–Boltzmann approximation. A major objective of the paper is to present the results in a visual form and explain aspects of modern double layer theory in a simple manner.     

Compatibilization of the PBA/PMMA core/shell latex interphase. I. Effect of PMMA macromonomer

In this work poly(methyl methacrylate) (PMMA) macromonomer is used as a compatibilizing agent in a poly(butyl acrylate) (PBA)/PMMA core/shell latex system. The incorporation of the PMMA macromonomer was achieved by copolymerizing it with BA monomer using miniemulsion polymerization. PBA seed latex was also synthesized without the macromonomer present to compare the compatibilizing effects with the PMMA macromonomer. The second stage methyl methacrylate monomer was added semi-continuously to the PBA seed latexes under monomer-starved conditions. Solid-state ¹³C-NMR [H]T₁ρ relaxation studies were used to determine the effect of PMMA compatibilizer on these PBA/PMMA core/shell latex interphase regions. The thickness of the interphase of the core/shell particles prepared with and without the PMMA macromonomer compatibilizing agent are calculated to be in the range of 15–16 nm and 10–12 nm, respectively. Electron microscopy revealed that the seed latex prepared with the PMMA macromonomer achieved a more uniform coverage with the second stage PMMA polymer as compared to the latex synthesized without the compatibilizing agent present. It is concluded that the PMMA macromonomer is effective in increasing the thickness of the interphase region and also the amount of interfacial PMMA. © 1996 John Wiley & Sons, Inc.     

CTAB-assisted fabrication of mesoporous composite consisting of wormlike aluminosilicate shell and ordered MSU-S core

Novel mesoporous composites comprised of aluminosilicate shell with wormhole framework structure and well-ordered MSU-S core have been firstly prepared via treatment of MSU-S with NaAlO₂ solution in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB). The obtained products were thoroughly characterized by XRD, HRTEM, N₂ sorption isotherm, TG-DSC, ²⁷Al MAS NMR and ²⁹Si MAS NMR, etc. Characterization results based on these techniques revealed that the introduction of CTAB during the treatment process played a crucial role in the formation of mesoporous composite, otherwise the parent ordered MSU-S would be completely destroyed. Furthermore, aluminum content in the final product determined by ICP and EDAX was significantly increased compared to the parent MSU-S, thus assuming that CTAB added in this modification process led a key role in the introduction of Al into the final solid via self-assembly with additionally added AlO₂⁻ and partly dissolved silicate species from the parent MSU-S particles. In addition, acidity and catalytic performance of the prepared mesoporous composite were also substantially improved in comparison to the parent MSU-S sample.     

Surface morphology of polypyrrole core/polyacrolein shell latex by atomic force microscopy (AFM)

 Polypyrrole latex (P(P)), synthesized in Redox polymerization of pyrrole, was used as seed for radical polymerization of acrolein initiated with K₂S₂O₈. In this process the polypyrrole core/polyacrolein shell latex (P(P–A)) was obtained. Morphology of the surface of P(P–A) particles was investigated by atomic force microscopy (AFM). It was found that macromolecules of polyacrolein are not randomly distributed on the surface of polypyrrole but form patches. Apparently, attraction between macromolecules of poly-acrolein in the surface layer is high and the enthalpy of formation of polyacrolein macromolecule clusters is sufficient to compensate, at least, the negative entropy change due to ordering of these macromolecules. Thickness of the polyacrolein layer on the surface of polypyrrole particles, which were covered only partially with polyacrolein, was equal to only 1.6 nm (standard deviation σ= 0.2 nm) and thus, it was reasonable to assume that it corresponded to the monolayer coverage. Received: 30 April 1997 Accepted: 25 August 1997     

Preparation of electrically conducting particles consisting of a polymer core and a metallic copper shell

Micron-sized composite particles consisting of spherical polymer cores and metallic copper shells were prepared. Basic cupric carbonate was coated on sulfonated polymer particles by pH-controlled hydrolysis of cupric nitrate. The thickness of the coating layer was controlled by the number of polymer particles. Addition of CO₂ during aging increased the shell thickness. Electrically conducting particles were obtained by reduction with hydrogen of the composite particles obtained. Received: 9 August 2000 Accepted: 17 July 2001     

New insights into the complexities of shell growth and the strong influence of particle volume in nonblinking "giant" core/shell nanocrystal quantum dots

The growth of ultra-thick inorganic CdS shells over CdSe nanocrystal quantum dot (NQD) cores gives rise to a distinct class of NQD called the "giant" NQD (g-NQD). g-NQDs are characterized by unique photophysical properties compared to their conventional core/shell NQD counterparts, including suppressed fluorescence intermittency (blinking), photobleaching, and nonradiative Auger recombination. Here, we report new insights into the numerous synthetic conditions that influence the complex process of thick-shell growth. We show the individual and collective effects of multiple reaction parameters (noncoordinating solvent and coordinating-ligand identities and concentrations, precursor/NQD ratios, precursor reaction times, etc.) on determining g-NQD shape and crystalline phase, and the relationship between these structural features and optical properties. We find that hexagonally faceted wurzite g-NQDs afford the highest ensemble quantum yields in emission and the most complete suppression of blinking. Significantly, we also reveal a clear correlation between g-NQD particle volume and blinking suppression, such that larger cores afford blinking-suppressed behavior at relatively thinner shells compared to smaller starting core sizes, which require application of thicker shells to realize the same level of blinking suppression. We show that there is a common, threshold g-NQD volume (~750 nm(3)) that is required to observe blinking suppression and that this particle volume corresponds to an NQD radiative lifetime of ~65 ns regardless of starting core size. Combining new understanding of key synthetic parameters with optimized core/shell particle volumes, we demonstrate effectively complete suppression of blinking even for long observation times of ~1 h.     

Prediction of axial concentration profiles in an extractive membrane bioreactor and experimental verification

A steady-state mathematical model has been developed to predict axial-concentration profiles of a pollutant in an extractive-membrane bioreactor (EMB). Typically, the previous models describe the pollutant concentration profiles in the membrane-attached biofilms in a direction perpendicular to the membrane. In contrast, the model presented in this work describes not only the radial profiles, but also the axial profiles along the membrane length. Biofilms of Xanthobacter autotrophicus GJ10 were grown on the surface of silicone rubber tubes. A diffusion-reaction model was employed to describe the diffusion and reaction in the biofilm in the radial direction. Membrane tubes were modelled as a series of mixed tanks to allow the prediction of axial concentrations. The model predictions were verified by experimental data from a range of operating conditions. These included different dissolved oxygen concentrations in biomedium and different wastewater flowrates. Finally, the rate-limiting step in the reactor was determined to be the mass-transfer resistance of the pollutant in the biofilm.     

Kinetics of swelling of polyacrylamide gels: concentration profiles and effect of an imposed strain

The time course of the concentration profile of a spherical gel has been measured during a swelling experiment, and compared to the theoretical expressions. The swelling of a cylindrical gel under imposed strain has been studied both theoretically and experimentally. This provides a new method of determination of the ratio of the shear modulus over the osmotic longitudinal modulus.     

Colloidal crystals of core-shell type spheres with poly(styrene) core and poly(ethylene oxide) shell

Elastic modulus and crystal growth kinetics have been studied for colloidal crystals of core–shell type colloidal spheres (diameter = 160–200 nm) in aqueous suspension. Crystallization properties of three kinds of spheres, which have poly(styrene) core and poly(ethylene oxide) shell with different oxyethylene chain length (n = 50, 80 and 150), were examined by reflection spectroscopy. The suspensions were deionized exhaustively for more than 1 year using mixed bed of ion-exchange resins. The rigidities of the crystals range from 0.11 to 120 Pa and from 0.56 to 76 Pa for the spheres of n = 50 and 80, respectively, and increase sharply as the sphere volume fraction increase. The g factor, parameter for crystal stability, range from 0.029 to 0.13 and from 0.040 to 0.11 for the spheres of n = 50 and 80, respectively. These g values indicate the formation of stable crystals, and the values were decreased as the sphere volume fraction increased. Two components of crystal growth rate coefficients, fast and slow, were observed in the order from 10⁻³ to 10¹ s⁻¹. This is due to the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell size spheres, nor difference between those of core–shell spheres and silica or poly(styrene) spheres. The results are very reasonably interpreted by the fact that colloidal crystals are formed in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers, and their formation is not influenced by the rigidity and internal structure of the spheres.     

Inner filter effect and the onset of concentration dependent red shift of synchronous fluorescence spectra

The phenomenon of concentration dependent red shift, often observed in synchronous fluorescence spectra (SFS) of monofluorophoric as well as multifluorophoric systems at high chromophore concentrations, is known to have good analytical advantages. This was previously understood in terms of large inner filter effect (IFE) through the introduction of a derived absorption spectral profile that closely corresponds to the SFS profile. Using representative monofluorophoric and multifluorophoric systems, it is now explained how the SF spectral maximum changes with concentration of the fluorophore. For dilute solutions of monofluorophores the maximum is unchanged as expected. It is shown here that the onset of red shift of SFS maximum of both the mono as well as the multifluorophoric systems must occur at the derived absorption spectral parameter value of 0.32 that corresponds to the absorbance value of 0.87. This value is unique irrespective of the nature of the fluorophore under study. For monofluorophoric systems, the wavelength of derived absorption spectral maximum and the wavelength of synchronous fluorescence spectral maximum closely correspond with each other in the entire concentration range. In contrast, for multifluorophoric systems like diesel and aqueous humic acid, large deviations were noted that could be explained as to be due to the presence of non-fluorescing chromophores in the system. This work bridges the entire fluorophore concentration range over which the red shift of SFS maximum sets in; and in the process it establishes the importance of the derived absorption spectral parameter in understanding the phenomenon of concentration dependent red shift of SFS maximum.     

核壳结构微球的制备方法与展望

核壳结构微球作为一种功能性聚合材料已得到广泛的关注,本文介绍了国内外关于核壳结构微球制备方法的研究进展,包括种子聚合法、大分子单体法、自组装法、逐步异相凝聚法,并对各种方法的反应机理和影响因素进行了阐述;进而对该领域研究的热点工作进行了概述性的展望。     

Simultaneous determination of particle size and density in polydisperse colloidal samples by sedimentation field-flow fractionation

Sedimentation Field-Flow Fractionation (SFFF), which is a one-phase chromatographic system, is shown to be a rapid and convenient method for the simultaneous determination of particle size and density in polydisperse colloidal samples. This can be done by a methodology based on the variation of the carrier solution density using various aqueous glycerol solutions having different concentrations. A polydisperse polyvinyl chloride (PVC) latex sample was used as a model colloid. The average diameters and densities found by the proposed methodology are in good agreement with those obtained from the normal. SFFF procedure using a carrier of constant density or given by the manufacturer. Number average, d_N, and weight average, d_w, particle diameters were determined for the PVC sample at various carrier solutions containing glycerol up to 40 % (w/w) in order to show whether sample aggregation has occurred or not in the presence of glycerol. In all cases, the ratio d_w/d_N was found to be constant, showing that PVC aggregation has not occurred. Therefore our results, obtained by the proposed methodology in SFFF, are accurate, showing that this approach has considerable promise for characterizing complex colloidal materials.     

Manipulation of chain transfer agent and cross-linker concentration to modify latex micro-structure for pressure-sensitive adhesives

N-Dodecyl mercaptan (NDM) chain transfer agent and allyl methacrylate (AMA) cross-linker were used to manipulate latex properties in a starved seeded semi-batch emulsion polymerization of butyl acrylate (BA) and methyl methacrylate (MMA) or with a third monomer, acrylic acid (AA). Latexes with higher gel content and lower sol polymer molecular weight (M_w) were produced by adding only AMA. On the other hand, latexes with lower gel content and M_w were produced by adding only NDM. In addition, at a constant AMA concentration (0.2 phm), the addition of NDM (0.2 phm) decreased gel content, increased molecular weight between cross-linking points (M_c), and decreased M_w. Adding more NDM (to a total of 0.4 phm) further decreased the gel content, while decreasing the tested M_c and increasing M_w. It was also found that using higher concentrations of both AMA and NDM could produce latex with similar gel content, but smaller M_c and M_w, compared to the latex produced at lower concentrations of both NDM and AMA. Regarding the influence of AA, gel content was increased and M_w was significantly decreased with an increase in AA concentration and a decrease in MMA concentration. The performance of the latexes was evaluated for application as a pressure-sensitive adhesive (PSA).     

Preparation and characterization of micron-sized polystyrene/polysiloxane core/shell particles

A procedure has been developed to coat micron-sized polystyrene (PS) spheres with a smooth layer of polysiloxane by a sol–gel process of methyl trimethoxylsilane (MTMS) without using silane coupling agents. The thickness of the shells can be easily varied with different polystyrene seeds and methyl trimethoxysilane feed ratio. When we used PS particles with diameters of 2.09 μm prepared by conventional dispersion polymerization as seeds, the thickness of the polysiloxane shells can be varied from 0.11 to 0.21 μm. The particle size, size distribution, thermal decomposition, and solvent resistance were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), size analyzer, and TG, respectively.     

Synthesis of core/shell structure of glycidyl–functionalized poly(methyl methacrylate) latex nanoparticles via differential microemulsion polymerization

The differential microemulsion polymerization technique was used to synthesize the nanoparticles of glycidyl-functionalized poly(methyl methacrylate) or PMMA via a two-step process, by which the amount of sodium dodecyl sulfate (SDS) surfactant required was 1/217 of the monomer amount by weight and the surfactant/water ratio could be as low as 1/600. These surfactant levels are extremely low in comparison with those used in a conventional microemulsion polymerization system. The glycidyl-functionalized PMMA nanoparticles are composed of nanosized cores of high molecular weight PMMA and nano-thin shells of the random copolymer poly[(methyl methacrylate)-ran-(glycidyl methacrylate)]. The particle sizes were about 50 nm. The ratios of the glycidyl methacrylate in the glycidyl-functionalized PMMA were achieved at about 5–26 wt.%, depending on the reaction conditions. The molecular weight of glycidyl-functionalized PMMA was in the range of about 1 × 10⁶ to 3 × 10⁶ g mol⁻¹. The solid content of glycidyl-functionalized PMMA increased when the amount of added glycidyl methacrylate was increased. The glycidyl-functionalized polymer on the surface of nano-seed PMMA nanoparticles was a random copolymer which was confirmed by ¹H-NMR spectroscopy. The amounts of functionalization were investigated by the titration of the glycidyl functional group. The structure of the glycidyl-functionalized PMMA nanoparticles was investigated by means of TEM. The glycidyl-functionalized PMMA has two regions of T_g which are at around 90 °C and 125 °C, respectively, of which the first one was attributed to the poly[(methyl methacrylate)-ran-(glycidyl methacrylate)] and the second one was due to the PMMA. A core/shell structure of the glycidyl-functionalized PMMA latex nanoparticles was observed.     

Synthesis and characterization of particles consisting of a biodegradable poly(l-lactide) core and a functional hydrophilic shell

Block copolymers consisting of poly(solketal acrylate) and poly(l-lactide) were synthesized by combination of atom transfer radical polymerization (ATRP) and ring opening polymerization (ROP) technique. Block copolymerization has been done by two different pathways, simultaneously and sequentially by using a dual functional initiator. Well defined block copolymers were obtained by sequential block copolymerization first implementing ROP of l-lactide followed by ATRP of solketal acrylate. After hydrolysis of the solketal acrylate segments hydrophilic poly(2,3-dihydroxypropyl acrylate) blocks were obtained. The amphiphilic block copolymers were able to self-organize in aqueous solution. Aggregation behavior was studied by means of dynamic and static light scattering. Time dependent enzymatic and hydrolytic degradation of the poly(l-lactide) cores was detected by dynamic light scattering. If enzymatic solutions were used the degradation process proceeded faster and was completed within 4000 min.     

枸杞子中铜的测定及其浓度分布特征

用超声波辅助水提取-醇沉-溶剂萃取方法分离枸杞子得Ⅰ-Ⅷ8个组分,用石墨炉原子吸收分光光度法测定其中铜的浓度,并探讨了铜在枸杞子不同组分中的浓度分布特征。结果表明,枸杞子及其分离组分Ⅰ-Ⅷ中铜的质量分数分别是13.20、4.03、9.01、6.41、3.41、5.66、3.96、16.70、8.20μg/g,枸杞子中的铜呈现不均匀的浓度分布特征,在组分Ⅱ中浓度最高,组分Ⅳ中浓度最低。     

Functional core/shell nanoparticles via layer-by-layer assembly. investigation of the experimental parameters for controlling particle aggregation and for enhancing dispersion stability

Gold nanoparticles (AuNPs) with a size of 13.5 nm were synthesized using well-established methods as described earlier by Turkevich (Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1961, 11, 55-75) and Frens (Frens, G. Nature (London), Phys. Sci. 1973, 241, 20-22) using citrate as the reducing agent. It has already been reported that such AuNPs can easily be coated with polymeric shells using electrostatic layer-by-layer assembly of certain polyelectrolytes. Here, we show which parameters, namely, the polyelectrolyte concentration, the contour length of the polyelectrolyte chain, and the ionic strength, are preventing bridging flocculation during polyelectrolyte adsorption and enhancing the stability of the colloidal dispersion. For the preparation of individually coated particles with high yield, we identified optimal conditions such as the degree of polymerization of the polyelectrolytes used, the polyelectrolyte concentration, the nanoparticle concentration, and the concentration of added NaCl during multilayer buildup. Surprisingly, such functional nanoparticles are obtained with highest yield at a moderate excess of polyions. In contrast to expectations, a larger excess of polyions leads again to slight destabilization of the dispersion. The present findings raise our confidence to establish layer-by-layer deposition as a general method for functionalizing even different nanoparticles using a single method.     

Concentration-dependence of faradaic currents and conductivity in a polystyrene sulfonic latex suspension

The conductivity, κ, in a suspension of polystyrene sulfonic latex without supporting electrolyte showed a linear dependence on the volume fraction, v_f, of the latex for v_f<0.03 with a finite intercept. In contrast, this deviated upward from the linear line for v_f>0.03. These variations were qualitatively consistent with the dependence of the voltammetric reduction current of H⁺ on v_f without supporting electrolyte. The current values were only a few percent of the theoretical diffusion-controlled current that could be observed in the suspension with supporting electrolyte. This fact indicates the electrostatic immobilization of the hydrogen ions by sulfonic latex particles. A plot of the current against κ at common values of v_f showed that the current for v_f>0.07 was smaller than the value predicted from the conductivity. This can be explained in terms of a combination of the increase in electrostatically unbounded H⁺ estimated by conductance measurements and electric migration in which the electrochemical depletion of [H⁺] also causes the depletion of the latex.