Electrophoretic mobility of the polymer-like micelles of tetradecyldimethylamine oxide hemihydrochloride

Effects of the surfactant concentration C_d and the NaCl concentration C_s on the electrophoretic mobilities U of the well-characterized polymer-like micelles have been investigated by the electrophoretic light scattering, using tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl). At the high ionic strength of 0.1 mol kg⁻¹ NaCl, the electrophoretic mobilities were independent of C_d (5 mM < C_d < 100 mM), despite the concentration-dependent micelle growth of the polymer-like micelles. This suggests that the electrophoretic mobility of the polymer-like micelle at high ionic strengths is independent of the contour length (i.e., the molecular weight), as found on linear polyelectrolytes. Somewhat surprisingly, the entanglements of the polymer-like micelles gave small effect on the electrophoretic mobilities in the examined range of the surfactant concentration above an overlap concentration. The mobilities of the polymer-like micelle decreased with √C_s in a single exponential manner in the range of C_s from 0.02 to 0.3 mol kg⁻¹. It is suggested that the cylinder model can be applied to the electrophoretic mobilities of the polymer-like micelles at high ionic strengths (i.e. a free-draining behavior), since the persistence length of the polymer-like micelle (20 nm) is much larger than the Debye length at high ionic strength.     

GPS接收机多径抑制算法研究

研究了窄相关、多径估计延迟锁相环算法(MEDLL)和基于斜率估计(ELS)的GPS接收机多径抑制算法,重点研究了ELS算法,该算法可以在一定的预相关带宽的前提下,通过对相关峰两边的斜率进行估计,达到抑制多径造成的跟踪误差的效果。仿真结果表明,ELS的算法在性能上比窄相关技术更好,接近于MEDLL算法。该算法相对于窄相关技术资源规模增加不大,比MEDLL算法资源规模和计算量都要小很多,易于在硬件上实时实现。     

Structure and reactivity of GaAs surfaces

First the analytical tools, preparation methods and surface crystallography of clean GaAs surfaces are briefly reviewed. Besides the usual methods of cleaving, ion bombardment and annealing, molecular beam epitaxy is mainly used as a growth method under UHV conditions, and has brought a manifold of differently reconstructed structures on the same crystallographic surface, depending on the exact experimental conditions during growth. Quantitative analysis of the surface composition by AES gives the result that these structures differ only in the amount of As in the topmost layer. From the combination of theoretical LEED analysis, UPS results and arguments considering the different physicochemical nature of Ga and As atoms, rehybridisation of the surface atomic bonds emerges as the driving force for reconstruction: the surface Ga atoms try to assume a trivalent planarsp² and the As atoms a trivalentp³ configuration with three mutually perpendicularp-bonds. The better this rehybridised configuration can be achieved, the better is the chemical stability of the respective structure. The sticking coefficient for oxygen, although generally low, thus varies between ～10^-4 and <10^-9, depending on the crystallographic surface and, on the same surface, on the degree of surface bond saturation given by the respective structure. However, it emerges that, at least on As-depleted polar surfaces, adsorption proceeds via a mechanism of removal of Ga atoms during exposure and adsorption on the defect sites created in this way. The existence of such a complicated mechanism is consistent with the difficulties arising with the preparation of thick stoichiometric oxide layers, the preparation methods and properties of which are reviewed briefly in the last section.     

Output-only identification and dynamic analysis of time-varying mechanical structures under random excitation: A comparative assessment of parametric methods

This article addresses the problem of parametric time-domain identification and dynamic analysis for time-varying (TV) mechanical structures under unobservable random excitation. The methods presented are based on time-dependent autoregressive moving average (TARMA) models, and are classified according to the mathematical structure imposed on the TV parameter evolution as unstructured parameter evolution, stochastic parameter evolution, and deterministic parameter evolution. The features and relative merits of each class are outlined. A representative method from each is then assessed through its application to the identification and dynamic analysis of a laboratory TV structure consisting of a beam with a mass moving on it. The results are mutually compared and contrasted to those obtained through “frozen-configuration” (multiple experiment) baseline identification.     

蒸发光散射检测器HPLC法测定大豆低聚糖

蒸发光散射检测器HPLC法测定大豆低聚糖;HPLC;ELS检测器;梯度洗脱;低聚糖;大豆     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Particle Size Distribution by Combined Elastic Light Scattering and Turbidity Measurements. A Novel Method to Estimate the Required Normalization Factor

Turbidity (T) and Elastic Light Scattering (ELS) measurements can be combined to estimate the particle size distribution (PSD) of a polymer latex. To this effect a “normalization factor” must be employed to adequately scale both measurements. In a real application, such factor is a priori unknown, and it must be estimated for determining the PSD. In a previous publication, a method was proposed to simultaneously estimate the normalization factor and the PSD by solving an inverse nonlinear problem. In this work, a simpler technique is presented which estimates the normalization factor and the PSD by sequentially solving two inverse linear problems. To evaluate the proposed technique, a synthetic example based on a polystyrene latex sample with a bimodal PSD was solved. It is shown that, for slight noise measurements, the errors in the estimation of the normalization factor are quite small and have a low effect on the PSD recovery.     

Sample preparation and current applications of liquid chromatography for the determination of non-structural carbohydrates in plants

This article summarizes the current methods of determination of non-structural carbohydrates (NSCs) in plant samples based on liquid chromatography (LC). NSCs comprise several types of carbohydrates: sugar alcohols (e.g., sorbitol), monosaccharides (e.g., glucose and fructose), disaccharides (e.g., sucrose), oligosaccharides (e.g., raffinose) and polysaccharides [e.g., starch and polyfructans (e.g., inulin)]. NSCs are important in plant metabolism and have to be strictly distinguished from all sorts of structural carbohydrates (e.g., polysaccharide cellulose) that make up the backbone of the plants. Consequently, preservation of structural carbohydrates is a crucial step during sample preparation for NSC determination and is therefore addressed.Sugar alcohols, monosaccharides, disaccharides and those oligosaccharides that are easily soluble in polar solvents can be analyzed directly by high-performance LC. They are also referred to as free carbohydrates (FCs).However, polysaccharides are generally submitted to hydrolyzation into monomers prior to their quantitative analysis. This can be done either chemically, using acids, or enzymatically - both methods are discussed. For identification and quantification of the NSCs after LC separation, the following detectors are used: pulsed amperometry, refractive index, evaporate light scattering and finally, mass spectrometry.     

The dispersion of hydrophilic and hydrophobic flame-hydrolysed silicas in short-chained alkanols

The ultrasonic dispersion of Aerosil 200 and some n-alkyl surface-modified derivatives thereof in alkanols have been examined. The sizes of the aggregates formed during this process were investigated from measurements of sol viscosity and quasi-elastic light scattering (QELS). In the former technique, the aggregate volume ratios (AVR) were calculated at very low volume fractions of silica. The AVR values so obtained indicated that the silica monomers were highly aggregated in all systems, as expected. Differences between the various alkanols were found, however. Decreasing continuum polarity from water to hexan-1-ol produced decreasing AVRs. From hexan-1-ol to octan-1-ol, however, the AVRs increased. The n-alkyl surface-modified silicas gave AVRs substantially different to A 200, related most likely to wetting and solvation of the solid surface during dispersion. QELS measurements were subsequently found to be of limited use for these systems, since the aggregate size distributions in these sols were evidently very broad. The results indicated that these size distribution were non-Gaussian. The use of QELS to investigate systems of such high aggregation appears to be limited.