Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: dynamics and scaling of the intermediate scattering function

Intermediate scattering functions are measured for colloidal hard sphere systems using both dynamic light scattering and x-ray photon correlation spectroscopy. We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behavior found by Segré and Pusey [Phys. Rev. Lett. 77, 771 (1996)] but challenged by Lurio et al. [Phys. Rev. Lett. 84, 785 (2000)]. We observe a scaling behavior over several decades in time but not in the long-time regime. Moreover, we do not observe long-time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of long-time diffusion coefficients at these scattering vectors.     

Single-file diffusion through inhomogeneous nanopores

Strict one-dimensional diffusion, due to geometrical confinement in a nanopore, of an assembly of particles forbids overtaking by each other, giving rise to single-file diffusion (SFD). Smooth carbon nanotube is the epitome of SFD. However, natural nanoporous materials are far from smooth; morphologically, the nanopores' inner surface may provide an inhomogeneous environment for diffusion to occur, giving rise to subnormal diffusion even for an isolated particle diffusing through this fractal landscape. The realm of fractional diffusion (FD) falls under this paradigm. In order to understand the characteristics of SFD through inhomogeneous nanopores, here, we introduce a fractional SFD (FSFD) formalism that deals with a combination of these two phenomena, namely, SFD of particles, each of which are moving subdiffusively in one dimension. For an infinite system, we obtain the mean square displacement (MSD) of the combined entity and our analysis is based on FD equation for particles moving in concert where the single-file correlation is established through reflection principle. For a finite system, we calculate the transport probabilities based on continuous time random walk model. While both the diffusion mechanisms (SFD and FD) acting separately are responsible for slow dynamics at long times, their combined effect leads to ultraslow diffusion. For example, while the long time asymptote of MSD of SFD scales as sqr rt of t, that for FSFD is sqr rt of t(alpha), where alpha is the measure of the extent of inhomogeneity. These findings, which are believed to occur in a natural inhomogeneous nanopore, is also important for design and fabrication of nanofluidic devices through which the fluid delivery can be engineered.     

Diffusion NMR methods applied to xenon gas for materials study

We report initial NMR studies of (i) xenon gas diffusion in model heterogeneous porous media and (ii) continuous flow laser-polarized xenon gas. Both areas utilize the pulsed gradient spin-echo (PGSE) techniques in the gas phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients--a brief overview of this area is provided in the Introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t) (an indicator of mean squared displacement), to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. We find that D(t) of imbibed xenon gas at short diffusion times is similar for the mixed bead pack and a pack of the smaller sized beads alone, hence reflecting the pore surface area to volume ratio of the smaller bead sample. The approach of D(t) to the long-time limit follows that of a pack of the larger sized beads alone, although the limiting D(t) for the mixed bead pack is lower, reflecting the lower porosity of the sample compared to that of a pack of mono-sized glass beads. The Pade approximation is used to interpolate D(t) data between the short- and long-time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20-200 mm s-1). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm s-1 owing to the high diffusivity of gases compared with liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data, namely flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.     

Efficient computation of the first passage time distribution of the generalized master equation by steady-state relaxation

The generalized master equation or the equivalent continuous time random walk equations can be used to compute the macroscopic first passage time distribution (FPTD) of a complex stochastic system from short-term microscopic simulation data. The computation of the mean first passage time and additional low-order FPTD moments can be simplified by directly relating the FPTD moment generating function to the moments of the local FPTD matrix. This relationship can be physically interpreted in terms of steady-state relaxation, an extension of steady-state flow. Moreover, it is amenable to a statistical error analysis that can be used to significantly increase computational efficiency. The efficiency improvement can be extended to the FPTD itself by modelling it using a gamma distribution or rational function approximation to its Laplace transform.     

Amphiphilic poly(oxazoline)s – synthesis and application for micellar catalysis

Homogeneous and heterogeneous catalytic processes compete on industrial scale for more then three decades [1]. The advantages of homogeneous catalysis are beside others high activity, mild reaction conditions, high selectivity and no diffusion problems. The disadvantages of such systems include difficulties with recycling of the catalyst and product separation [2].     

Ethylene polymerization and ethylene/1-hexene copolymerization using homogeneous and heterogeneous unbridged bisindenyl zirconocene catalysts

Unbridged bis-substituted-indenyl zirconocene complexes, [(2,4-Me₂Ind)₂ZrCl₂, Met-1; (2,4,6-Me₃Ind)₂ZrCl₂, Met-2], were supported on silica and montmorillonite carriers (resulting in silica-supported catalysts MS-1 and MS-2, and montmorillonite-supported catalyst MT-1). Ethylene polymerization by homogeneous and heterogeneous catalysts showed high activity, affording polyethylenes with high molecular weight. The catalytic activity and the molecular weight of the polymer were improved using the heterogeneous systems. The activities for the ethylene/1-hexene copolymerization by heterogeneous systems were lower than those using homogeneous systems, however, the comonomer was incorporated efficiently into polymer in both the homo- and the heterogeneous systems, and moreover, the microstructure of the copolymer derived from the heterogeneous catalysts showed different characteristics from those resulting from the homogeneous systems. The r_Er_H values of the heterogeneous catalysts (1.82 for MS-1 and 0.70 for MS-2), are quite different from those of their homogeneous analogues (1.25 for Met-1 and 1.26 for Met-2).     

Microporous Silica-supported Poi yoxometal ate Catalyst H3PW12O400/SiO2 with High Activity for Acetal Reaction between Benzaldehyde and Ethylene Glycol

Keggin-type polyoxometalates (POMs) whose representative is H3PW12O40(PW12) exhibits a unique combination of physical and chemical properties. They have been widely used as acid catalysts in homogeneous and heterogeneous reaction systems. However, the catalytic activity of PW12 is so limited due to its very low specific surface area (≤9 m2/g)[1] that polar molecules can only react on its surface.     

钯-膦-酸体系催化烯烃氢酯基化反应研究进展

烯烃氢酯基化反应是羰基合成领域的重要分支之一,长期以来一直备受关注。目前已发展了钯基、铑基、钴基、钌基等众多催化体系,其中钯-膦-酸催化体系因具有反应条件温和、底物普适性良好、催化性能优异等优点而得到最广泛、最深入的研究。因此,我们对均相、非均相的钯-膦-酸催化体系在烯烃氢酯基化反应中的研究应用进行了简要综述,并对催化机理、催化剂组成和性能进行讨论,最后指出稳定性高、可回收性好的非均相钯-膦-酸催化体系将是该领域的研究重点。     

正、负离子表面活性剂混合水溶液的相行为

通过对混合系统相行为的研究，总结出正、负离子表面性剂混合体系普遍存在三上浓度区的规律，即在低浓度与高浓度时为透明均相溶液，中间浓度区为复相体系，而且表明，随极性基体积增大，混合体系形成均相溶液的能力增加；在高浓度区，疏水链长短对混合体系形成均相溶液能力的影响与低浓度区的情况不完全一致，有时会出现疏水链较短的体系形成均相溶液的能力反而较差的“反常”现象。     

氢原子在Ni(100), Ni(111)和Ni(110)面上吸附扩散势能面的结构

本文构造了氢-镍相互作用的5参数Morse势, 用经典的对势方法研究氢原子在Ni(100), Ni(111)和Ni(110)面上的吸附和扩散, 得到氢原子在三个表面上的吸附位、吸附几何、结合能及本征振动等数据, 和实验结果符合得很好。同时, 系统地研究了三个体系的吸附扩散势能面结构。     

One-dimensional models of polymer dynamics. I. Dashpot-spring models applied to a single rotor

The proposion of Clark and Zimm that a dashpot and a spring can be used in place of a set of rotational barriers in polymer dynamics is studied. The simplest possible case is examined here, that of a single rotor. Reasons for altering the Clark–Zimm diffusion equation are presented and an alternative diffusion equation is proposed. The results of both diffusion equations for the correlation function 〈exp[?iθ(0)] exp[iθ(t)]〉 (θ is the angular position of the rotor) are compared with the correlation function for a rotor in an n-fold cosine potential. Although the two diffusion equations differ, both agree well with the n-fold cosine model for barriers above a few k_BT. This agreement is obtained in the absence of adjustable parameters, and motivates the application of these two diffusion equations to polymeric systems.     

Self-diffusion in two-dimensional hard ellipsoid suspensions

We studied the self-diffusion of colloidal ellipsoids in a monolayer near a flat wall by video microscopy. The image processing algorithm can track the positions and orientations of ellipsoids with subpixel resolution. The translational and rotational diffusions were measured in both the laboratory frame and the body frame along the long and short axes. The long-time and short-time diffusion coefficients of translational and rotational motions were measured as functions of the particle concentration. We observed the nondiffusive crossover region in the intermediate time regime due to the caging of neighboring particles. Both the beginning and the ending times of the intermediate regime exhibit power-law dependence on concentration. The long-time and short-time diffusion anisotropies change nonmonotonically with concentration and reach minima in the semidilute regime because the motions along long axes are caged at lower concentrations than the motions along short axes. The time derivatives of mean-square displacements change linearly with the inverse of time in the intermediate time regimes at various particle densities. This indicates that their relaxation functions decay as 1/t which provides new challenges in theory. The effects of coupling between rotational and translational Brownian motions were demonstrated and the two time scales corresponding to anisotropic particle shape and anisotropic neighboring environment were measured.     

Tunable, light-assisted co-generation of CO and H2 from CO2 and H2O by Re(bipy-tbu)(CO)3Cl and p-Si in non-aqueous medium

The light-assisted co-generation of carbon monoxide and hydrogen from carbon dioxide and water is reported. The combination of a homogeneous CO-evolving electrocatalyst and a heterogeneous H(2)-evolving photoelectrode surface provides for tunability of the H(2)/CO ratio. A total Faradaic efficiency of 102 ± 5% and a H(2)/CO ratio of 2:1 were achieved at a low homogeneous catalyst concentration (0.5 mM) in acetonitrile/water mixtures.     

Electric mass transport through homogeneous and surface-modified heterogeneous ion-exchange membranes at a rotating membrane disk

Polarization characteristics of the homogeneous MF-4SK perfluorinated sulfonated cation-exchange membrane and the heterogeneous MK-40 sulfonic acid membrane with its surface modified by a homogeneous film of Nafion are studied at a rotating membrane disk in 0.1 and 0.001 M sodium chloride solutions. Partial current-voltage curves (CVC) are obtained for sodium and hydrogen ions, and limiting current densities in the electromembrane systems (EMS) under study are calculated as a function of the rotation rate of the membrane disk. Contribution from different mechanisms (electrodiffusion, electroconvection, dissociation of water, and the effect of the limiting-current exaltation) to the total ion flow is estimated experimentally and theoretically under conditions that the diffusion layer in the EMS has stabilized in thickness. It is established that surface modification of the heterogeneous MK-40 membrane with a 7 μm layer of a modifying agent almost completely eliminates the dissociation of water molecules, and the properties of the heterogeneous MK-40 membrane approximate those of the homogeneous Nafion membrane. From IR spectra and potentiometric titration curves of the MK-40 and MF-4SK membranes, it is shown that the acidity of the sulfonate groups in these membranes is nearly identical, but a difference in the dissociation rate of water at these membranes is determined by a different character of charge-density distribution and potential near the membrane-solution interphase boundary. By means of the theory of the overlimiting state in EMS, the internal parameters of the systems under investigation are calculated: distribution of space-charge density and electric-field potential in the diffusion layer and in the membrane. Partial CVC are calculated for H⁺ ions for the space-charge region in the phase of the MF-4SK and MK-40/Nafion ion-exchange membranes. Partial CVC with similar characteristics are compared for the heterogeneous monopolar MK-40 and the bipolar MB-2 membranes, which contain sulfonate groups. It is concluded that the membrane surface layer, where the space charge is localized, plays a dominant role in speeding up the dissociation of water in EMS.     

Heteropoly acid as a novel efficient catalyst for Fries rearrangement

Heteropoly acid H3PW12O40 is a very efficient and environmentally benign catalyst for the Fries rearrangement of phenyl acetate in homogeneous or heterogeneous liquid-phase systems at 100-150 degrees C.     

Transport properties in ionic liquids and ionic liquid mixtures: the challenges of NMR pulsed field gradient diffusion measurements

Pulsed field gradient NMR is a powerful method for the measurement of diffusion coefficients in liquids and solids and has begun to attract much attention in the ionic liquids field. However, aspects of the methodology as traditionally applied to solutions may not be uniformly applicable in these more viscous and chemically complex systems. In this paper we present data which shows that the Pulsed Gradient Spin Echo (PGSE) method in particular suffers from intrinsic internal gradients and can produce apparent diffusion coefficients which vary by as much as 20% for different 1H nuclei within a given molecule--an obvious anomaly. In contrast, we show that the Pulsed Gradient Stimulated Echo method does not suffer from this problem to the same extent and produces self-consistent data to a high degree of accuracy (better than 1%). This level of significance has allowed the detection, in this work, of subtle mixing effects in [C(3)mpyr][NTf(2)] and [C(4)mpyr][NTf(2)] mixtures.     

离子液热条件下金属有机骨架化合物的合成

以1-丁基-3-甲基咪唑氟硼酸盐为溶剂, 在离子液热条件下得到两个化合物Zn3(BDC)3(MIA)2(1) 和Zn(HBTZ)(MIA)(BPY)(2). X射线衍射分析表明, 两种化合物的晶体分别属于单斜晶系, C2/c空间群和正交晶系, Fdd2空间群. 化合物1为三维结构, 在除去端基的N-甲基咪唑后, 形成了大约0.9 nm×0.9 nm的一维菱形孔道. 化合物2中的链状次级结构单元则通过π键和氢键作用形成了超分子结构.     

Theoretical Studies of Single Molecule Spectroscopy at Room Temperature

This talk is motivated by recent room-temperature single molecule experiments, which measure the optical spectrum along single molecular trajectories and monitor the molecular dynamics and chemical kinetics of individual reactive systems. These experiments contain new information that requires theoretical models and interpretations. Several aspects of single molecule spectroscopy are analyzed:(1) Event-averaged single molecule quantities are calculated, with the prediction of the echo signal in the joint event probability distribution function^[1]. Similar to the photon echo phenomenon, the single molecule echo signal measures solvent effects on chemical kinetics. (2) The statistics of single molecule blinking events are often correlated to underlying quantum mechanisms. The distribution functions of waiting-time sequences are examined for several quantum processes, including electron transfer, solvent relaxation, laser-induced emission, and single quantum-dot blinking^[2]. (3) Single molecule measurements of heterogeneous diffusion reveal deviations from the Gaussian distribution of Brownian motion. As a quantitative measure, the non-Gaussian indicator decays asymptotically to zero according to 1/t for finite time correlation, but saturates at a plateau value for power-law correlation.     

rac-Me_2Si(Ind)_2ZrCl_2负载型催化剂催化丙烯等规聚合的研究

将rac Me2 Si(Ind) 2 ZrCl2 与甲基铝氧烷 (MAO)预络合后负载于SiO2 上得到了高活性的负载型茂金属催化剂 ,并用于丙烯等规聚合 ,研究了 [Al]/ [Zr]比、聚合温度对聚合活性、产物分子量、聚合动力学的影响 ,同时与相应的均相体系进行比较 .结果表明 ,负载化后 ,活性中心的稳定性提高 ,所得聚合物的分子量也明显高于均相体系 .同时用13 C NMR测定了两种催化体系所制备的等规聚丙烯的微结构 .结果发现 ,负载型茂金属催化剂制得的聚丙烯立构选择性高于均相体系 ,其五元组立构序列 [mmmm]从均相的 82 0 %提高到负载的 86 3 % .同时区域选择性也有所改进 ,2 ,1 插入、1 ,3 插入分别从均相体系的 4 4‰和 1 9‰降到负载体系的 3 7‰和 0 5‰ .