Dynamic electrophoresis of a sphere in a spherical cavity: arbitrary surface potential

The boundary effect on the dynamic electrophoretic behavior of a charged entity is examined by considering a sphere in a spherical cavity. The present study extends previous analysis to the case of an arbitrary level of electrical potential where the effect of double-layer distortion can be significant. The governing equations are solved numerically based on a pseudo-spectral method, which is found to be sufficient in solving the corresponding electrophoresis problem when a static electric field is applied. The result of numerical simulation reveals that as the size of a cavity decreases, both the magnitude of the mobility and the inertial force acting on a particle decrease accordingly. Also, while the distortion of the ionic cloud should not be ignored, in general, when the surface potential of a particle is high, its influence on the magnitude and on the phase angle of the mobility is alleviated by the presence of the cavity.     

Relationships between the stability,circular dichroism spectrum and type of substitution in inclusion complexes formed between phenophthalein and different cyclodextrins

The perturbation of the three-site interaction of phenolphthalein on complexation with different cyclodextrins (CDs) has been investigated as a function of the type and degree of substitution of the CD. The UV visible spectra are practically identical, while the circular dichroism spectra change dramatically both in intensities and signs. Parallel to this, the stability constants are influenced not only by the size of the cavity but also by the substitution of its rims, particularly by that of the primary hydroxyls. The two phenomena can be related to allow conclusions concerning the mode of inclusion.     

Complexation of cyclodextrins with flavines in aqueous solutions

Data on the binding mode and thermodynamics of complex formation for various cyclodextrins (CDs) with flavines are summarized. It is shown that the governing factors of complexation are the size, degree of hydration, and hydrophobicity of the guest molecule. It is found that the presence of small hydrophobic substituents in a flavine’s structure increases their affinity toward cyclodextrin cavities, raising the stability of a complex. In contrast, the presence of bulky and polar side groups in a flavine’s structure prevents its inclusion in a macrocyclic cavity and weakens complexation. The size of a CD cavity plays a minor role in the interaction between CDs and flavines, since the inclusion of a guest molecule is only partial.     

Synthesis and molecular modelling studies of resorcin[4]arene-capped porphyrins

Three new resorcin[4]arene-capped porphyrins (3, 5 and 7) different in the porphyrin skeleton, in the linking arms and in the cavity dimensions, have been synthesised. Molecular modelling calculations explored the conformations and the cavity size of the three compounds and showed that their hydrophobic pockets can accommodate one molecule of water or methane (3 and 5), or benzene (7) without any distortion. Notably, the capped porphyrin 5 was able to inhibit the oxidation of Co(II) to Co(III), whereas compound 7 did it only partially.     

固相氯化法氯化聚乙烯晶粒尺寸的X射线衍射研究

晶粒尺寸的测定常用付里叶变换法^[1]和积分宽度法^[2],这两种方法都要求样品相应于某个晶面至少存在三个级别的衍射峰,而对聚合物来说,观察到高级衍射峰的情况是很少见的.方差函数法不要求样品存在三个级别的衍射峰,而且方法简单,曾广泛用于金属材料晶粒尺寸和点阵畸变的测定^[3-5],但在聚合物材料中的应用甚少^[6].本文采用方差函数法对固相氯化法氯化聚乙烯(CPE)和原料聚乙烯(PE)的晶粒尺寸和点阵畸变参数进行了研究,得到了较为满意的结果.     

Nonlinear response of the benzene molecule to strong magnetic fields

The fourth-rank hypermagnetizability tensor of the benzene molecule has been evaluated at the coupled Hartree-Fock level of accuracy within the conventional common-origin approach, adopting gaugeless basis sets of increasing size and flexibility. The degree of convergence of theoretical tensor components has been estimated allowing for two different coordinate systems. It is shown that a strong magnetic field perpendicular to the plane of the molecule causes a distortion of the electron charge density, which tends to concentrate in the region of the C-C bonds. This charge contraction has a dynamical origin, and can be interpreted as a feedback effect in terms of the classical Lorentz force acting on the electron current density.     

Crystallite size in highly drawn polyethylene

The size and distortion of crystallites in samples of linear polyethylene were determined before and after plastic deformation. A slowly cooled sample, a quenched sample, and highly drawn films (draw ratio 16) were investigated by different methods. Wide-angle x-ray patterns were analyzed to study the average size of the crystalline mosaic blocks and their distortion. In addition, the longitudinal crystal thickness (in the chain direction) was evaluated by two other approaches, determination of the long period, and the melting temperature of irradiated samples. The results show clearly that the size of the crystalline mosaic blocks changes substantially with drawing of polyethylene. Not only is the lateral crystal thickness affected, but the longitudinal crystal dimensions also change during the drawing process. By the three independent methods we find that the longitudinal crystal thickness after drawing is independent of the value for the undrawn samples, as was reported earlier by Peterlin. The change in crystallite size after drawing is accompanied by a large decrease in crystal volume to about 10% of the value for the undrawn sample. The degree of distortion in the crystals seems not to be affected by the deformation process. These experimental data can be considered evidence for high chain mobility and for the possibility of rearrangement of chain molecules during the process of plastic deformation.     

选矿产品矿物自动分析的光片制备

基于扫描电子显微镜的矿物自动分析仪(Quantitative Evaluation of Minerals by Scanning Electronic Microscopy)、MLA (Mineral Liberation Analyser)和AMICS(Advanced Mineral Identification and Characterization System)可用于测定选矿产品中目的矿物的粒度和单体解离度,为确定合理的磨矿细度以及优化选矿工艺流程提供依据。环氧树脂光片的制备是矿物自动识别和测量的最关键的一环,其代表性直接关系到后续数据测量的准确性和真实性。对于金属矿产品来说,由于选矿产品中矿物颗粒粗细不均、密度差异较大,在环氧树脂胶结固化过程中矿物颗粒会产生明显的分异作用并互相黏连,造成分析结果失真。实验证明,把样品与晶质石墨混均,然后加入环氧树脂以及固化剂搅拌混合倒入圆柱状模具进行冷镶嵌,待样品固化后再沿圆柱体的纵向进行切割,并对其切割面进行粗磨、细磨、精磨以及抛光,就可以制备出样品分散性好、分布均匀、表面光滑平整的具有代表性的环氧树脂光片。     

选矿产品矿物自动分析的光片制备

基于扫描电子显微镜的矿物自动分析仪(Quantitative Evaluation of Minerals by Scanning Electronic Microscopy)、MLA(Mineral Liberation Analyser)和AMICS(Advanced Mineral Identification and Characterization System)可用于测定选矿产品中目的矿物的粒度和单体解离度,为确定合理的磨矿细度以及优化选矿工艺流程提供依据。环氧树脂光片的制备是矿物自动识别和测量的最关键的一环,其代表性直接关系到后续数据测量的准确性和真实性。对于金属矿产品来说,由于选矿产品中矿物颗粒粗细不均、密度差异较大,在环氧树脂胶结固化过程中矿物颗粒会产生明显的分异作用并互相黏连,造成分析结果失真。实验证明,把样品与晶质石墨混均,然后加入环氧树脂以及固化剂搅拌混合倒入圆柱状模具进行冷镶嵌,待样品固化后再沿圆柱体的纵向进行切割,并对其切割面进行粗磨、细磨、精磨以及抛光,就可以制备出样品分散性好、分布均匀、表面光滑平整的具有代表性的环氧树脂光片。     

A portable pulsed cavity ring-down transmissometer for measurement of the optical extinction of the atmospheric aerosol

A small portable system is described which is used to directly determine the optical extinction of the atmospheric aerosol. The requisite highly sensitive measurement of the optical extinction is accomplished simultaneously at two wavelengths in the near-infrared (1064 nm) and visible (532 nm), using the pulsed cavity ring-down (CRD) approach. The measurement at the two wavelengths can aid in separating the scattering and absorption components of the optical extinction. Rayleigh equivalent optical extinction of approximately 10 x 10(-6) m(-1) from particulate matter in the atmospherically important 0.1-2.5 pm diameter size range (fine particle accumulation mode) can be readily observed with short (<5 s) integration times. Optical extinction is inversely related to the visual range, and so the instrument provides a direct measurement of this particulate-related air quality indicator. The instrument can also provide particle size range-selected multiwavelength optical property measurements, which can be inverted to provide valuable information about the extant airborne particulate distribution.     

An efficient computational model to predict the synthetic utility of heterocyclic arynes

Think before you act: a computational approach is reported for evaluating the synthetic potential of heterocyclic arynes. Routine and rapid calculations of arene dehydrogenation energies and aryne angle distortion predict the likelihood that a given hetaryne can be generated, as well as the degree of regioselectivity expected in a reaction between a given hetaryne and a nucleophilic trapping agent.     

杯芳烃衍生物在分析化学中的应用

杯芳烃衍生物是以杯芳烃为“分子平台”，经过不同的衍生化而得。根据杯芳烃的空腔大小，构象及其衍生官能团与客体分子间的适应程度，杯芳烃衍生物可实现对金属离子及有机分子的选择性识别。本文介绍了芳烃衍生物在分析化学领域，包括萃取分离，液膜分离，色谱分析及光分析中的应用情况。     

Absorption and fluorescence spectra in diode laser spectrometry affected by relaxation oscillations

Relaxation sidebands were observed in the cavity mode spectra of laser diodes. Strong sidebands amounting to some percent of the central mode intensity affect the resolution and structure of absorption and fluorescence spectra. Very weak sidebands which are not visible in cavity mode spectra or by absorption spectrometry can be identified by saturation of a transition.     

Scaled particle theory study of the length scale dependence of cavity thermodynamics in different liquids

It has been noted that the work of cavity creation in water exhibits a crossover behavior, in that its cavity size dependence changes from volume dependence for small cavities to area dependence for larger cavities [Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570]. It is shown here that this behavior can be reproduced using the scaled particle theory in a straightforward manner for six different liquids (water, methanol, ethanol, benzene, cyclohexane, and carbon tetrachloride). It has also been suggested that the crossover is due to a change in the physical mechanism of the process, from one entropy-dominated to another enthalpy-dominated. However, the crossover behavior can be produced using the scaled particle theory without invoking any change in any physical mechanism. Also, the crossover occurs at a length scale of the size of the liquid molecules, as has been pointed out by others. This is the length regime where the work of cavity creation bears little relation to the bulk liquid surface tension. In addition, it is pointed out that cavity creation can always be considered as a purely entropy-driven process, which is usually accompanied by another process with compensating enthalpy and entropy changes.     

Behavior of Ag3 clusters inside a nanometer-sized space of ZSM-5 zeolite

We found from DFT calculations that Ag-Ag orbital interactions as well as Ag-O electrostatic interactions determine the structures of three silver cations inside a nanometer-sized cavity of ZSM-5 (Ag(3)-ZSM-5) in lower and higher spin states. Both interactions strongly depend on the number of Al atoms substituted for Si atoms on the ZSM-5 framework (ZSM-5(Al(n))), where n ranges from 1 to 3. In smaller n, stronger Ag-Ag orbital interactions and weaker Ag-O electrostatic interactions operate. Accordingly, there are significant dependencies of the structures of three silver cations on the number of Al atoms. In lower spin states of Ag(3)-ZSM-5(Al(1)) and Ag(3)-ZSM-5(Al(2)), D(3h)-like triangle clusters are contained inside ZSM-5 whereas their higher spin states have triangle clusters distorted significantly from the D(3h) structure. In lower spin states, the totally symmetric orbital consisting of 5s(Ag) orbitals is responsible for cluster formation, whereas in higher spin states occupation of a 5s(Ag)-based orbital with one node results in significant distortion of the triangle clusters. The distortion can be partially understood by analogies to Jahn-Teller distortion of the bare D(3h) Ag(3)(+) cluster in the triplet spin state. When n is 3, we found that three silver cations are isolated in a lower spin state and that a linear cluster consisting of two silver cations is formed in a higher spin state. Thus, we demonstrate from DFT calculations that the number of Al atoms can control the properties of three silver cations inside a ZSM-5 cavity. Since the structural and electronic features of the enclosed silver clusters can link to their catalytic properties, the DFT findings can help us to understand the catalytic activity of Ag-ZSM-5.     

Infrared reflection-absorption method for the detection of aromatic compounds in aqueous solutions with limited sample volumes.

An infrared reflection-absorption (IR/RA) method was developed to detect aromatic organic compounds in aqueous solutions where the required sample volume can be as low as 50 microL. Two aluminum plates were used to form the sampling cell for the detection of small amount of aqueous samples. One plate was used as an IR reflection substrate and a second plate, in which several holes were drilled, was placed tightly on the top of the reflection plate to form cavities for sampling. The cavities were further coated with hydrophobic film. After the hydrophobic film dried, a certain amount of aqueous sample was injected to the cavity. Analytes in the aqueous solution were attracted into the hydrophobic film through the solid phase micro-extraction principle. After residual water was removed from the cavity, organic compounds absorbed by the hydrophobic film could be sensed using IR radiation based on the reflection-absorption mode. To investigate the applicability of this type of sensing method for small-volume detection, factors such as the volume of the aqueous solution, the sample concentration, size of the cavity and the sensitivity of this method were investigated. An examination of the linear relationship between the signals and the analyte concentrations showed regression coefficients that were generally in the range of 0.992 to 0.999 for the examined analytes in the concentration range of 10 to 100 ppm. Under the condition that the sample volume was 100 microL and based on three-times the spectra noise level, the calculated detection limits for this method were found at around 1 ppm for the examined analytes.     

Fluorescence Suppression of 7-Methoxycoumarin upon Inclusion into Cyclodextrins

The fluorescence intensity of 7-methoxycoumarin (7MC) in aqueoussolution is found to significantly decrease upon addition of variouscyclodextrins. This observed phenomenon is described asfluorescence suppression, to distinguish it from fluorescencereduction via bimolecular quenching. The decrease in fluorescenceof 7MC is proposed to be the result of the formation of ahost–guest inclusion complex with cyclodextrin. Since 7MC isa polarity-sensitive fluorophore, which is less fluorescent ina nonpolar environment, its fluorescence decreases uponinclusion into the relatively nonpolar internal cavity of thecyclodextrin. The same equation used for extracting the associationconstant in the case of 1 : 1 host–guest inclusion-induced fluorescenceenhancement is shown to be applicable to the case of fluorescencesuppression. In the case of β-cyclodextrin, the degree of fluorescencesuppression, as well as the value of the binding constant for formationof the inclusion complex, are found to be unaffected by modificationof the cyclodextrin rims, suggesting that the molecule iscompletely included within the β-cyclodextrin cavity. In the case ofγ-cyclodextrin, the degree of fluorescence suppression, butnot the value of the binding constant, is found to be significantlyaffected by modification of the cyclodextrin rims, suggesting thatthe molecule is experiencing a less polar environment in the modifiedγ-cyclodextrin cavity. The binding constant is three timeslarger in β- as compared to γ-cyclodextrin, indicatinga much better size match in the smaller β-cyclodextrin cavity.     

A versatile bis-porphyrin tweezer host for the assembly of noncovalent photoactive architectures: a photophysical characterization of the tweezers and their association with porphyrins and other guests

A bis(Zn(II)-porphyrin) tweezer host with anthracene components as apex and side-arms has been synthesized. Mono- (pyridine) and bidentate (4,4'-bipyridine) guests were used as models for single and double axial coordination inside the cavity, respectively. A series of dipyridylporphyrin guests with different substitution patterns and excited-state energy levels have association constants with the tweezers that are of the order of 10(6) M(-1), which is indicative of complexation with the inside of the cavity. This complexation can only occur upon an important distortion of the cavity that opens the bite by about 30 %. This characteristic, in conjunction with their ability to reduce the bite distance by rotation around single bonds, makes these porphyrin tweezers amongst the most versatile so far reported, with tuning of the bite distance in the range of approximately 5-20 Angstroms. Energy transfer to the free-base guest within the triporphyrin complex is nearly quantitative (95-98 %) and the rates of transfer are consistent with a F?rster mechanism that is characterized by a reduced orientation factor.     

Effect of charged boundary on electrophoresis: Sphere in spherical cavity at arbitrary potential and double-layer thickness

The boundary effect on electrophoresis is investigated by considering a spherical particle at an arbitrary position in a spherical cavity. Our previous analysis is extended to the case where the effect of double-layer polarization can be significant. Also, the effect of a charged boundary, which yields an electroosmotic flow and a pressure gradient, thereby making the problem under consideration more complicated, is investigated. The influences of the level of the surface potential, the thickness of double layer, the relative size of a sphere, and its position in a cavity on the electrophoretic behavior of the sphere are discussed. Some results that are of practical significance are observed. For example, if a positively charged sphere is placed in an uncharged cavity, its mobility may have a local minimum as the thickness of the double layer varies. If an uncharged sphere is placed in a positively charged cavity, the mobility may have a local minimum as the position of the sphere varies. Also, if the size of a sphere is fixed, its mobility may have a local minimum as the size of a cavity varies. These provide useful information for the design of an electrophoresis apparatus.     

Contact angles of liquid drops on super hydrophobic surfaces: understanding the role of flattening of drops by gravity

Measurement of contact angles on super hydrophobic surfaces by conventional methods can produce ambiguous results. Experimental difficulties in constructing tangent lines, gravitational distortion or erroneous assumptions regarding the extent of spreading can lead to underestimation of contact angles. Three models were used to estimate drop shape and perceived contact angles on completely nonwetting super hydrophobic surfaces. One of the models employed the classic numerical solutions from Bashforth and Adams. Additionally, two approximate models were derived as part of this work. All three showed significant distortion of microliter-sized drops and similar trends in perceived contact angles. Liquid drops of several microliters are traditionally used in sessile contact angle measurements. Drops of this size are expected to and indeed undergo significant flattening on super hydrophobic surfaces, even if the wetting interactions are minimal. The distortion is more pronounced if the liquid has a lesser surface tension or greater density. For surfaces that are completely nonwetting, underestimation of contact angles can be tens of degrees. Our modeling efforts suggest that accurate contact angle measurements on super hydrophobic surfaces would require very small sessile drops, on the order of hundreds of picoliters.