A molecular simulation study of the structure and tribology of polymer brushes: Comparison of behavior in theta and good solvents

Implicit solvent Brownian dynamics simulations of the structure and tribology of opposing polymer-brush covered surfaces have been carried out as a function of surface separation and solvent quality. Consistent with experiment, shear forces were found to be greater under theta solvent conditions than in a good solvent at equal relative separations (normalized by the respective height of the brushes in theta and good solvents). Much higher relative compression is required before the onset of significant shear forces in good solvent compared to theta solvent. The dependence of shear force for a given relative separation on solvent quality can be accounted for by differences in interpenetration of the brushes. When compared as a function of absolute surface separation, greater interpenetration and greater shear force are observed at large separations for the brushes in good solvent than in theta solvent, consistent with the greater brush height in good solvent. At shorter separations, corresponding to moderate to high compression, brush-brush interactions result in significant deformation of the brushes. In this regime, greater interpenetration and greater shear forces are observed in theta solvent at a given separation, in qualitative agreement with experiment.     

溶剂浮选法分离富集葛根中大豆甙元的研究

采用溶剂浮选法分离富集葛根中的大豆甙元。考察了浮选溶剂、氮气流速、试液pH、浮选时间及电解质（KC1）等因素对浮选效率的影响,优选出最佳浮选条件;对最佳条件下的浮选效果进行了评价,并与溶剂萃取法进行了对照,前者明显优于后者。     

Mechanism of decoloration by solvent of thermally degraded poly(vinyl chloride)

The mechanism of decoloration of thermally degraded poly(vinyl chloride (PVC)) by solvents has been investigated systematically. The main results obtained are as follows. Good solvents for PVC, especially tetrahydrofuran, methyl ethyl ketone, and dioxane are effective for decoloration. The solvent peroxide which is formed by autoxidation of solvent contributes to decoloration. The number of double bonds in degraded PVC decreases as the decoloration proceeds and at the same time the solvent peroxide existing in solvent is consumed. Moreover, the existence of solvent fragments in decolored PVC is recognized. From these results, it is most reasonable to conclude that the decoloration mechanism is as follows: the solvent partially is changed to a solvent peroxide by autoxidation, and the solvent peroxide reacts with polyene double bonds of degraded PVC and breaks down conjugated double bonds, and consequently degraded PVC is decolored.     

Scaling and structural properties of a polymer in a simple solvent: A study based on integral equations

We present a statistical mechanical theory for polymer–solvent systems based on integral equations derived from the polymer Kirkwood hierarchy. Integral equations for pair monomer–monomer, monomer–solvent, and solvent–solvent correlation functions yield polymer–solvent distribution, chain conformation in three dimensions, and scaling properties associated with polymer swell and collapse in athermal, good, and poor solvents. Variation of polymer properties with solvent density and solvent quality is evaluated for chains having up to 100 bonds. In good solvents, the scaling exponent v has a constant value of about 0.61 at different solvent densities computed. For the athermal solvent case, the gyration radius and scaling exponent decrease with solvent density. In a poor solvent, the chain size scales as N^v with the value of the exponent being about 0.3, compared with the mean field value of ⅓. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3025–3033, 1998     

溶剂浮选法分离富集茶叶中茶多酚的研究

采用溶剂浮选法分离富集了茶叶中的茶多酚。考察了浮选溶剂、氮气流速、试液pH及浮选时间等因素对浮选效率的影响,优选出最佳浮选条件;对最佳条件下的浮选效果进行了评价,并与溶剂萃取法进行了比对,前者明显优于后者。     

精氨酸的溶剂浮选分离技术及其分离机制

以表面活性剂十二烷基苯磺酸为捕收剂(DBSA),二(2-乙基己基)磷酸酯(P204)为萃取剂,正庚烷为有机溶剂,采用溶剂浮选法对水溶液中精氨酸进行分离富集,并与气浮络合萃取法、泡沫浮选法和溶剂萃取法进行了比较.结果表明,在常温下,0.09 g/L精氨酸水溶液250 mL、初始pH 7.0,DBSA浓度0.15 g/L,正庚烷体积10 mL, P204体积4.5 mL,气体流量200 mL/min,溶剂浮选法分离水溶液中精氨酸的富集比为16.2,回收率为97.2%.溶剂浮选法分离精氨酸的动力学实验结果表明,精氨酸的溶剂浮选过程阶段性明显,大致可分为3个阶段,第一阶段和第二阶段都符合一级动力学方程,第三阶段符合零级动力学方程,探索了溶剂浮选法分离精氨酸的分离机制.     

Solvent effects on two-dimensional molecular self-assemblies investigated by using scanning tunneling microscopy

Self-assembly structures investigated by using scanning tunneling microscopy (STM) at liquid/solid interface have been a topic of broad interest in surface science, molecular materials, molecular electronics. The delicate balance among the adsorbate–solvent, adsorbate–adsorbate, solvent–solvent interactions would give rise to the coadsorption or competitive deposition of solvent with adsorbate. The solvents at the interface enable dynamic absorption and desorption of the adsorbates leading to the controlled assembly of the molecular architectures. The solvent-induced polymorphism, coadsorption effect, as well as solvent effects on chirality and electronic structures are discussed in this report in view of the polarity, solubility and viscosity of the solvent, the hydrogen bonding formation between solute and solvent, and the solvophobic and solvophilic effects. The systematic studies on the solvent effects would shed light on better control of assembly structures for design of new molecular materials and molecular electronics.     

反相液相色谱中溶剂强度规律的研究

 以溶质计量置换保留模型 (SDM R)为依据 ,通过研究 3种正链醇同系物溶剂置换剂对 14种正链醇同系物溶质色谱保留行为的影响 ,发现计量置换参数Z(对应 1mol溶质被吸附时从溶质与固定相接触处释放出的溶剂的总摩尔数 )、lgI(与 1mol溶质对固定相亲和势有关的常数 )和 j(与 1mol溶剂对固定相亲和势有关的常数 )均随着同系物置换剂相对分子质量的增大而减小 ,并呈现出线性变化 ,表明溶剂强度与溶剂分子的大小有关 ,分子愈大 ,溶剂洗脱能力愈强 ,并遵循同系物规律。     

Amphiphilicity and self-assembly behaviors of polystyrene-grafted multi-walled carbon nanotubes in selective solvents

Multi-walled carbon nanotubes (MWCNTs) can self-assemble as cylindrical bundles in some solvents after polystyrene (PS) grafting. In these three-dimensional regular structures, the tubes are oriented and parallel arranged. Every self-assembly structure has an order head and a comparatively loose tail. In order to find out the role that solvent plays, MWCNTs were dispersed in several organic solvents before and after modification. Based on macroscopic stability of suspensions and microscopic state of nanotubes, compatibility between solvents and MWCNTs can be confirmed. According to compatibility difference between tubes and PS, five chosen solvents are divided into three groups: selective solvent, good solvent, and bad solvent. MWCNT-g-PS can self-assemble only in selective solvents. Tetrahydrofuran and benzene are well compatible with PS, but bad with MWCNTs. Driven by the solvent–philic/solvent–phobic interaction, MWCNT-g-PS self-organized regularly as bundles. Because each part of the MWCNT-g-PS is compatible well with 1,2-dichlorobenzene, gathering tendency of the modified tubes have been enervated by their good compatibility and weak amphiphilicity. Grafted or not, nanotubes reveal poor compatibility with methanol and ethanol. Strong incompatibility and limited amphiphilicity make MWCNT-g-PS agglomerate as quickly and irregularly as raw tubes. An adapted hydrophilic/lipophilic balance system is introduced to qualify compatibility and amphiphilicity of MWCNT-g-PS in each solvent. This novel model not only reveals the relationship between solvent and microscopic state of unmodified/modified tubes, but also signifies the decisive role of solvent in self-assembly behaviors of MWCNT-g-PS.     

Solvent density effects on the solvation behavior and configurational structure of bare and passivated 38-atom gold nanoparticle in supercritical ethane

In exploring the effects of solvent density on the mode and the degree of solvation of the bare and passivated 38-atom gold particle in supercritical ethane, we have extended the molecular dynamics simulations of the system, reported previously,(34) to cover a range of isotherms in the T > T(c) regime, where T(c) is the critical temperature of the solvent. Consonant with our previous observations, the modes of solvation of the bare and the passivated particle, deduced from the radial distribution of the solvent about the metal core center of mass, are found to be vastly different from each other at all solvent densities: while the molecules solvating the bare particle form a well-defined, two-region layer around it, those solvating the passivated particle are loosely dispersed in the passivating layer. For the bare particle, the degree of solvation (vartheta) as a function of solvent density passes through a maximum occurring in the close vicinity of the critical point, consistent with our previous results and in agreement with Debenedetti's theoretical analysis,(22,23) which predicts a solvation enhancement effect in the critical region for systems where the unlike solvent/solute interaction is much stronger than the solvent/solvent interaction. Taking the degree of solvation (vartheta) as a measure of solvent quality, we have investigated how the solvent quality would vary along the solvent-density isotherms. In the solvent-density regime rho > rho(c), the solvent quality is found to be a decreasing function of the density as a result of progressive dominance of the excluded volume effect over the attractive particle/solvent interactions. The particle/solvent affinity is greatly reduced in the presence of the passivating layer, resulting in considerable shrinkage of the good-solvent-quality domain in the supercritical regime. The solvent environment and the presence of the passivating chains produce significant disorder in the equilibrium structure assumed by the nanoparticle core.     

Chain conformations and phase separation in polymer solutions with varying solvent quality

Molecular dynamics simulations are used to investigate the conformations of a single polymer chain, represented by the Kremer-Grest bead-spring model, in a solution with a Lennard-Jones liquid as the solvent when the interaction strength between the polymer and solvent is varied. Results show that when the polymer-solvent interaction is unfavorable, the chain collapses as one would expect in a poor solvent. For more attractive polymer-solvent interactions, the solvent quality improves and the chain is increasingly solvated and exhibits ideal and then swollen conformations. However, as the polymer-solvent interaction strength is increased further to be more than about twice the strength of the polymer-polymer and solvent-solvent interactions, the chain exhibits an unexpected collapsing behavior. Correspondingly, for strong polymer-solvent attractions, phase separation is observed in the solutions of multiple chains. These results indicate that the solvent becomes effectively poor again at very attractive polymer-solvent interactions. Nonetheless, the mechanism of chain collapsing and phase separation in this limit differs from the case with a poor solvent rendered by unfavorable polymer-solvent interactions. In the latter, the solvent is excluded from the domain of the collapsed chains while in the former, the solvent is still present in the pervaded volume of a collapsed chain or in the polymer-rich domain that phase separates from the pure solvent. In the limit of strong polymer-solvent attractions, the solvent behaves as a glue to stick monomers together, causing a single chain to collapse and multiple chains to aggregate and phase separate.     

Mobile phase compensation to improve NMR spectral properties during solvent gradients

A solvent compensation method based on flow injection analysis is used to obtain high quality nuclear magnetic resonance (NMR) spectra during solvent gradients. Using a binary solvent system containing D2O and CD3OD, NMR line broadening and chemical shift changes are observed with a 10% methanol per min solvent composition gradient. However, by creating a second equal but reverse gradient and combining the two solvent gradients before the NMR detector, the composition of solvent reaching the NMR flow cell is kept constant. We demonstrate a system using flow injection analysis of combining solvent gradients and show constant NMR spectral performance as a function of time as the combined flow has a constant solvent composition irrespective of the initial solvent gradient. Using this approach, methods can be developed to measure high quality NMR spectra during on-flow gradient LC-NMR experiments. The ultimate ability of this approach depends on the ability to compensate for the disturbance of the solvent gradient and reverse gradient by a pair of LC columns (the analytical and reverse gradient columns).     

Local solvent density augmentation around a solute in supercritical solvent bath: 1. A mechanism explanation and a new phenomenon

A recently proposed partitioned density functional (DF) approximation (Phys. Rev. E 2003, 68, 061201) and an adjustable parameter-free version of a Lagrangian theorem-based DF approximation (LTDFA: Phys. Lett. A 2003, 319, 279) are combined to propose a DF approximation for nonuniform Lennard-Jones (LJ) fluid. Predictions of the present DF approximation for local LJ solvent density inhomogeneity around a large LJ solute particle or hard core Yukawa particle are in good agreement with existing simulation data. An extensive investigation about the effect of solvent bath temperature, solvent-solute interaction range, solvent-solute interaction magnitude, and solute size on the local solvent density inhomogeneity is carried out with the present DF approximation. It is found that a plateau of solvent accumulation number as a function of solvent bath bulk density is due to a coupling between the solvent-solute interaction and solvent correlation whose mathematical expression is a convolution integral appearing in the density profile equation of the DF theory formalism. The coupling becomes stronger as the increasing of the whole solvent-solute interaction strength, solute size relative to solvent size, and the closeness to the critical density and temperature of the solvent bath. When the attractive solvent-solute interaction becomes large enough and the bulk state moves close enough to the critical temperature of the solvent bath, the maximum solvent accumulation number as a function of solvent bath bulk density appears near the solvent bath critical density; the appearance of this maximum is in contrast with a conclusion drawn by a previous investigation based on an inhomogeneous version of Ornstein-Zernike integral equation carried out only for a smaller parameter space than that in the present paper. Advantage of the DFT approach over the integral equation is discussed.     

Gas-chromatographische Überwachung des Extraktionsmittels bei der Wiederaufarbeitung von HTR-Brennelementen

During reprocessing of HTR fuel elements by the THOREX solvent extraction process the tributylphosphate/n-alcane mixture used as extractant is subject to an intensive radiolytic and hydrolytic burden. Main degradation products are di- and monobutylphosphate which in quite low concentrations disturb the solvent extraction by retention of uranium and transfer of some fission products into the aqueous product solution. Both esters are removed from the irradiated solvent by washing with a sodium carbonate solution, after which the solvent may be recycled. For a control of the efficiency of this solvent recovery procedure as well as of the composition of the fresh and recycled solvent in the JUPITER reprocessing facility, gas chromatography will be used. This method allows the determination of di- and monobutylphosphate down to concentration levels of about 25 parts per million. An analysis requires about 100 min.     

溶剂对丙烯环氧化反应活性的影响

研究了溶剂对钛硅分子筛催化丙烯环氧化反应的影响．研究结果表明，随着溶剂极性增加，活性中间过渡态中的过氧键越易断裂，容易把氧原子传递给丙烯，因此丙烯环氧化反应活性增加．由于TS—1催化剂孔道狭小，产生空间位阻效应，醇溶剂分子越大，活性中间过渡态的形成越困难，反应活性越低．甲醇-丙醇混合溶剂中丙酮含量小于50％时，反应活性与纯甲酵做溶剂相近；混合溶剂中丙酮含量增加到80％时，反应活性略降低；丙酮含量进一步增加，反应活性急剧下降．     

Effect of solvent composition on dispersing ability of reaction sialon suspensions

This work focuses on the optimization of the rheological behavior of suspensions considering different solvent compositions. The effects of methyl ethyl ketone (MEK)/ethanol (E) solvent mixtures on reaction sialon suspensions were investigated by measuring sedimentation behavior, adsorption of dispersant, and flow behavior. It was shown that both the flow behavior and the sedimentation behavior strongly depended on selection of solvent composition. Using 3 wt% KD1 as dispersant, well-dispersed colloidal suspensions could be obtained in MEK-rich solvents. The suspensions with 60 vol% MEK/40 vol% E as solvent could be fitted to the Bingham model with very low yield stress, while suspensions with pure MEK or ethanol-rich mixtures as solvent showed pseudo plastic behavior with relatively high yield stress values. A model was proposed to explain the different flow behaviors of suspensions considering the different configurations of dispersant at particles' surfaces.     

Effect of solvent type on formation of PVC‐solvent complex

The effect of solvent type on the structural features of PVC gels prepared from the solutions of dibutyl phthalate(DBP) and butyl benzoate(BB) were investigated. The influence of solvent type, i.e., diester solvent or monoester solvent, on the mechanical responses was characterized by dynamic viscoelastic measurements as a function of temperature, aging time, and PVC concentration. In the case of PVC/DBP aged gels, the transition phenomenon appeared close to 50°C, whereas no such transitions were observed for PVC/BB aged gels. This transition is considered to be associated with polymer‐solvent complex which can be accounted for electrostatic interaction between C=O bond of solvent and H‐C‐Cl part of PVC. While BB solvent molecules can not form polymer‐solvent complex due to no capacity to act as a bridge between PVC molecular chains. FT‐IR spectroscopy and DSC investigation on either PVC/DBP gels or PVC/BB gels was reported and discussed together with the mechanical behavior. Electron micrographs of the dried gel prepared from the critical point drying technique reveal that the gel morphology consists of well developed three dimensional fibrous network structure independent of solvent type.     

Relationships between Allerhand and Schleyer's G scale and other solvent polarity and acidity parameters

Values of Allerhand and Schleyer's solvent G parameter for 36 solvents have been correlated with solvent polarity—polarizability properties as well as parameters of solvent acidity. The equations found permit the extension of the G scale to more solvents and quantification of the influence on G of both solvent polarity—polarizability and acidity.     

醋酸酯淀粉高取代度的测定

淀粉的乙酰化改性是扩大其应用的一个重要手段。测定醋酸酯淀粉取代度 (DegreeofSubstitution ,简称DS)的方法通常有酸碱滴定法、紫外法、核磁共振法及衰减全反射法[1 ] 等 ,后 3种方法需建立标准曲线 ,要求有一系列取代度的样品 ,实验操作较困难。所以实验室更多采用酸碱滴定法。实验室多采用乙醇为溶剂进行醋酸酯淀粉高取代度的测定[1～3] 。这种方法的缺点是皂化时间需 48～ 72h ,并且高取代度醋酸酯淀粉的溶解性不理想。本文对酸碱滴定法测定醋酸酯淀粉高取代度溶剂选择进行了探讨 ,以期建立一种快速、准确地测定醋酸酯淀粉高取代度的…     

Binary–solvent–based ionic–liquid–assisted surfactant‐enhanced emulsification microextraction for the determination of four fungicides in apple juice and apple vinegar

A binary–solvent–based ionic–liquid–assisted surfactant‐enhanced emulsification microextraction method was developed for the separation/preconcentration and determination of four fungicides (pyrimethanil, fludioxonil, cyprodynil, pyraclostrobin) in apple juice and apple vinegar. A nonchlorinated solvent amyl acetate, which has a lower density than water, was used as the extraction solvent, and an ionic liquid 1‐hexyl‐3‐methylimidazolium hexafluorophosphate, which has a high density and low toxicity, was used as a secondary solvent mixed with the extraction solvent. After centrifugation, the binary solvent drop with a relatively high density was deposited on the bottom of the tube. Some parameters influencing the extraction efficiency of analytes such as type of extraction solvent, ratio of ionic liquid, volume of mixed solvent, type and concentration of surfactant, sample pH, NaCl concentration, and vortex time were investigated and optimized. Under the optimized conditions, the proposed method provided a good linearity in the range of 5–200 μg/L. The limits of quantification of the method were in the range of 2–5 μg/L. The relative standard deviations for interday assays were 1.7–11.9%. The method was applied to the determination of pyrimethanil, fludioxonil, cyprodynil, and pyraclostrobin in apple juice and apple vinegar samples, and the accuracy was evaluated through recovery experiments.