On calculating a polymer's enthalpy of formation with quantum chemical methods

Calculating the enthalpy of formation of a polymer with ab initio methods requires two choices. The first decision is whether to use oligomeric extrapolation or periodic boundary conditions to model the extended system, and the second choice is between formation reactions to be modeled, for example, formation from atoms, formation from standard states, or formation from some set of molecular systems. Utilizing trans-polyacetylene and polyethylene as examples, the oligomeric and periodic techniques are contrasted, leading to a discussion of the larger than minimal unit cell required when frequency calculations only include in-phase vibrations, that is, only the k = 0 frequencies, in an enthalpy of formation calculation. The accuracy of calculating the enthalpy of formation, in light of density functional theory's increased error with larger systems and with respect to various reference states, is also discussed. The calculation of the enthalpy of formation for a polymer is most accurate when the reference states are chosen carefully and most efficient when using periodic boundary conditions.     

分子模拟方法考察泡沫生成能力

采用分子模拟的方法研究表面活性剂的泡沫生成能力, 以界面形成能作为考察泡沫体系中液膜界面积的量化依据, 研究了泡沫液膜厚度、表面活性剂分子界面密度以及表面活性剂类型对泡沫液膜界面形成能计算的影响. 通过与实验结果相对应, 建立了界面形成能和泡沫生成能力之间的联系.     

Kinetics of the formation of silver dimers: early stages in the formation of silver nanoparticles

Silver clusters too small to support a plasmon band possess interesting fluorescence properties as well as being a convenient route to studying the early stages of nanoparticle formation. Fluorescent silver clusters are synthesized in toluene solution, and the formation is monitored herein by laser flash photolysis (LFP). Kinetic analysis of the formation of the Ag clusters is consistent with the formation of the smallest possible clusters, silver dimers (Ag(2)), whereby a mechanism for the formation of these clusters is provided as well as the first reported extinction coefficient and association constant for Ag(0) to form Ag(2). The formation of Ag(2) clusters is contrasted with the formation of Ag nanoparticles in aqueous media, and the particular stability and selectivity toward Ag(2) in this system is also studied using LFP.     

热环合反应速率的量子理论

应用多声子无辐射跃迁理论研究热环合反应, 给出热环合反应速率的一般公式。在高温条件下得出反应速率和活化能公式。所得公式可适用于多个环合点的热环合反应。     

辽河渣油热转化和加氢裂化过程中生焦行为的研究

考察了辽河渣油热转化和浆液床催化加氢反应过程中生焦行为。研究生焦趋势与渣油物理化学组成的关系，初期生焦机制及其对后生焦的影响，并用高倍显微镜、傅立叶变换红外光谱技术对所生焦进行表征，结果发现，随加工苛刻度的增加，反应体系中生成的焦油细分散向簇状分散过渡、实姓焦对后期生焦有一定促进作用，在低压热转化过程中更为显著，生焦趋势不仅与反应条件有关而且与渣油物理化学性质相关，还与反应物系对生焦先躯物的胶溶能     

Hydrothermal solidification of diatomaceous earth with analcime formation

Diatomaceous earth (DE) solidifies hydrothermally with analcime formation. With analcime formation, strength development was much greater than that with calcium silicate hydrate (CSH) formation. NaOH addition conditioned analcime formation because NaOH solution not only promoted dissolution of quartz and montmorillonite but also provided Na⁺ to form the analcime. Curing temperature and time affected analcime formation, and, in this study, over 6 h (at 200 °C) and 175 °C (for 12 h), analcime seemed to form readily. After hydrothermal treatment, the skeletons of diatoms can still be seen in the solid after analcime formation but can hardly be found after CSH formation.     

End-to-end vs interior loop formation kinetics in unfolded polypeptide chains

The conformational search for favorable intramolecular interactions during protein folding is limited by intrachain diffusion processes. Recent studies on the dynamics of loop formation in unfolded polypeptide chains have focused on loops involving residues near the chain ends. During protein folding, however, most contacts are formed between residues in the interior of the chain. We compared the kinetics of end-to-end loop formation (type I loops) to the formation of end-to-interior (type II loops) and interior-to-interior loops (type III loops) using triplet-triplet energy transfer from xanthone to naphthylalanine. The results show that formation of type II and type III loops is slower compared to type I loops of the same size and amino acid sequence. The rate constant for type II loop formation decreases with increasing overall chain dimensions up to a limiting value, at which loop formation is about 2.5-fold slower for type II loops compared to type I loops. Comparing type II loops of different loop size and amino acid sequence shows that the ratio of loop dimension over total chain dimension determines the rate constant for loop formation. Formation of type III loops is 1.7-fold slower than formation of type II loops, indicating that local chain motions are strongly coupled to motions of other chain segments which leads to faster dynamics toward the chain ends. Our results show that differences in the kinetics of formation of type I, type II, and type III loops are mainly caused by differences in internal flexibility at the different positions in the polypeptide chain. Interactions of the polypeptide chain with the solvent contribute to the kinetics of loop formation, which are strongly viscosity-dependent. However, the observed differences in the kinetics of formation of type I, type II, and type III loops are not due to the increased number of peptide-solvent interactions in type II and type III loops compared to type I loops as indicated by identical viscosity dependencies for the kinetics of formation of the different types of loops.     

Biomimetic approaches to control soluble concentration gradients in biomaterials

Soluble concentration gradients play a critical role in controlling tissue formation during embryonic development. The importance of soluble signaling in biology has motivated engineers to design systems that allow precise and quantitative manipulation of gradient formation in vitro. Engineering techniques have increasingly moved to the third dimension in order to provide more physiologically relevant models to study the biological role of gradient formation and to guide strategies for controlling new tissue formation for therapeutic applications. This review provides an overview of efforts to design biomimetic strategies for soluble gradient formation, with a focus on microfluidic techniques and biomaterials approaches for moving gradient generation to the third dimension.     

Investigation into the Formation of Bubbles in Capillary Electrochromatography

The formation of bubbles in capillary electrochromatography (CEC) is well documented: possible origins include Joule heating and variations in EOF velocity on passing from the stationary phase through the frit and into the open tube. Methods for the prevention of bubble formation are discussed which are confirmed by experimental results. Using frit lengths varying from 1 mm to 6 mm it is shown how frit length is directly related to the likelihood of bubble formation and how this is affected by applied voltage. It is shown that the change in applied voltage across a capillary affects the formation of bubbles and also that rebonding octadecylsilane (ODS) onto the silica frit after formation of the frit can minimize the formation of bubbles and how this effects the chromatography. A method is also described for increasing the robustness of silica capillaries using a column coupler along with modifications made to conventional capillary electrophoresis equipment to cater for CEC.     

囊泡形成和破坏的动力学

利用停流装置研究了十二烷基硫酸钠(SDS)和十二烷基三甲基溴化铵(DTAB)复配形成囊泡的过程和囊泡破坏过程的动力学性质, 并结合动态光散射技术和电子透射显微镜探索囊泡形成和囊泡破坏过程的机理. 动态光散射和电子透射显微镜的研究结果表明囊泡的形成过程主要包括四个阶段: 混合胶团→柔性的长棒状聚集体→“非平衡囊泡”→平衡囊泡, 而与其对应的粒度分散度则呈现“单分散性→多分散性”的周期性变化规律. 此外, 动力学结果表明囊泡形成过程很长, 但其活化能不大, 这意味着囊泡形成过程的控制步骤可能不是活化能控制. 而相对于囊泡的形成, 囊泡的破坏过程是十分迅速的.     

Unique organic-inorganic interactions leading to a structure-directed microporous aluminophosphate crystallization as observed with in situ Raman spectroscopy

We report on the direct observation of key organic template-framework interactions leading to the formation of specific aluminophosphate structures. In particular, we show how MeAPO-34 formation was governed by an interaction between the divalent framework substituted metal ion and the template conformation, while for AlPO-5 the structure formation was determined by the template conformation alone. Understanding such interactions therefore appears to be important for the rationalization of microporous material formation.     

过渡金属修饰对铜箔电催化还原CO2的性能影响

CO2还原是一种解决温室效应以及能源短缺问题的有效方式.目前对于水溶液体系中的CO2还原,主要有光催化、电催化以及光电催化等方法,其中还原CO2法可在室温下进行,并较易实现大规模应用.由于金属电极在CO2电催化还原过程中表现较高电流密度和催化性能,使得目前研究的热点集中于金属电极的修饰改性.金属Cu与H2, CO结合能力适中,并且对生成碳氢化合物具有较好的催化性能,因此其在催化CO2还原中具有较大潜力.以往对于Cu的研究主要集中在表面修饰、调控表面结构以及制备合金等方向,其中对金属进行氧化后再还原的处理也是提高其催化活性的一种有效手段.氧化后还原得到的铜具有较大的粗糙度,且暴露的活性位点更多,对CO2还原具有较好的催化活性.我们对铜箔在空气氛围下、300oC焙烧5 h,然后恒电位还原,再进行过渡金属Ni、Zn、Au的修饰,研究所得样品电催化还原CO2性能.电极的表面形貌用扫描电镜表征, CO2还原的液相和气相产物分别用核磁和在线气相色谱进行检测.
　　修饰后电极的形貌没有发生太大变化,仍具有十分粗糙的表面结构.通过线性扫描伏安曲线可以看出,修饰Zn、Au后电流密度较未修饰前有明显增加,但是由于CO2还原过程中不可避免地伴随析氢副反应,因此,我们通过计算产物的法拉第效率来表征修饰后的电极对产物选择性的改变：未修饰时,在?1.2至?1.6 V均可检测到甲酸的生成,电位负于?1.4 V时可以检测到乙醇和正丙醇. Ni的修饰明显提高了甲酸的法拉第效率,也促进了正丙醇的生成.?1.3 V时甲酸的法拉第效率为26.0%,?1.5 V时液相产物的法拉第效率为34.3%.在线气相色谱结果发现, Ni的修饰也明显提高了CO的法拉第效率,在?1.4 V下, CO的法拉第效率为44.6%.这可能是由于Ni (r =0.1246 nm)的原子半径比Cu (r =0.1278 nm)更小,因此Ni的修饰会使Cu发生晶格收缩、导致d带中心下移而降低了CO的结合能,从而更易生成CO和HCOOH;而修饰Ni后对CO2还原产物正丙醇的提高可能是由于Ni的引入促进了C–C键的形成.修饰Zn后,甲酸的产率明显下降,在?1.6 V下甲酸的法拉第效率只有14.8%,但是乙醇与正丙醇的法拉第效率分别为1.6%与2.0%,相较于未修饰的电极略有提高.修饰Au后,液相产物甲酸及醇类的法拉第效率明显下降,在?1.5 V下,甲酸的法拉第效率只有7.9%,且只检测到少量的乙醇,未检测到正丙醇的生成,这可能与Au修饰后的电极对CO2还原中间体CO的吸附较弱有关,生成的CO中间体更易从表面脱附,而难以被进一步还原.     

Determination of hydroxyl radical photoproduction rates in natural waters.

The photochemical formation rates of hydroxyl radicals (OH radicals) in river water and seawater were determined by a simple, rapid and sensitive benzene probe method, in which phenol formed by the reaction between benzene and photochemically-generated OH radicals was analyzed by on-line preconcentration HPLC. The OH radical formation rates from well-known OH radical sources, such as nitrate, nitrite and hydrogen peroxide, were in good agreement with those reported previously. River water samples containing high concentrations of nitrate and nitrite were found to show high OH radical formation rates. Ten to 80% of the OH radical formation in river water and seawater was due to the photolysis of nitrate and nitrite, but OH radical formation from hydrogen peroxide was negligible. The OH radical formation from unknown sources other than nitrate, nitrite and hydrogen peroxide was strongly correlated to the amount of fluorescent matter.     

Complex formation thermodynamics of some metal(II) phenyl-2-pyridylketoximates

The formation of phenyl-2-pyridylketoximates of nickel(II), zinc(II), cadmium-(II), mercury(II) and lead(II) was studied potentiometrically in a medium of 40% (w/w) acetone and 60% (w/w) water. The values of step formation constants, free energy, entropy and enthalpy of complex formation were calculated. The formation of the mercury chelate was found to be entropy driven. The formation of phenyl-2-pyridylketoximates of other metals of the first transitional series could not be studied owing to experimental difficulties.     

Thermodynamic characteristics of complex formation reactions between d- and f-elements and alkylated dipyrrolylmethene in dimethylformamide

Thermal effects of the reactions of complex formation between d- and f-elements and 3,3??,5,5??-tetramethyl-4,4??-diethyl-2,2??-dipyrrolylmethene in dimethylformamide are determined from data of titration microcalorimetry and the temperature behavior of the complex formation equilibrium constants. The entropy and free energy changes of the complex formation reactions are calculated. It is shown that enthalpy-entropy compensation is observed for complex formation processes in all cases. Contributions from the enthalpy and entropy factors to the free energy change of the complex formation are analyzed, allowing us to confirm earlier conclusions as to the effect of the nature of a complexing agent on the relative percentage of covalent and ionic characters of the M-L bond. The dependence of the complex formation enthalpies on the inverse radius of a complexing agent is revealed for the lanthanide complexes.     

Formation and filtration characteristics of solids generated in a high level liquid waste treatment process

The solids formation behavior in a simulated high level liquid waste (HLLW) was experimentally examined, when the simulated HLLW was treated in the ordinary way of actual HLLW treatment process. Solids formation conditions and mechanism were closely discussed. The solids formation during a concentration step can be explained by considering the formation of zirconium phosphate, phosphomolybdic acid and precipitation of strontium and barium nitrates and their solubilities. For the solids formation during the denitration step, at least four courses were observed; formation of an undissolved material by a chemical reaction with each other of solute elements (zirconium, molybdenum, tellurium) precipitation by reduction (platinum group metals) formation of hydroxide or carbonate compounds (chromium, neodymium, iron nickel, strontium, barium) and a physical adsorption to stable solid such as zirconium molybdate (nickel, strontium, barium).     

Air-facilitated three-phase contact formation at hydrophobic solid surfaces under dynamic conditions

The paper presents results documenting the mechanism of facilitation of the three-phase contact (TPC) formation due to gas entrapped during immersion of hydrophobic (Teflon) plates into distilled water and n-octanol solutions. Collisions, bouncing, the time scale of the TPC formation, and bubble attachment to Teflon plates of different surface roughness were studied using a high-speed camera. Processes occurring during the microscopic wetting film formation at the Teflon plates were monitored using the microinterferometric method (Scheludko-Exerowa cell). A strong relation between the time necessary to form a stable TPC and the roughness of the Teflon was observed. The higher the Teflon roughness was the shorter the time for the TPC formation. This effect can be attributed to two factors: (i) local differences in the thickness of the thinning intervening liquid layer (quicker attainment of rupture thickness at pillars of rough surface) and/or (ii) the presence of gas at the hydrophobic surface. Experimental findings, that (i) prolongation of the plate immersion time resulted in quicker TPC formation, (ii) white irregular and disappearing spots (air pockets) were recorded during the wetting film formation, and (iii) high n-octanol concentration caused prolongation of the time of the TPC formation, show that attachment (TPC formation) of the colliding bubble to hydrophobic surfaces was facilitated by air entrapped at the Teflon plates (and re-distributed) during their immersion into water phase. Thus, on collision instead of solid/gas wetting liquid film a thin gas/liquid/gas foam film was formed which facilitated the TPC formation.     

Aggregation between xanthan and nonyphenyloxypropyl beta-hydroxyltrimethylammonium bromide in aqueous solution: MesoDyn simulation and binding isotherm measurement

The aggregation behavior of nonyphenyloxypropyl beta-hydroxyltrimethylammonium bromide (C9phNBr) and xanthan (XC) in aqueous solution was investigated by MesoDyn density functional simulation and binding isotherm measurement. The process of aggregate formation and the aggregate morphology are reported. The formation of aggregates includes three stages and the morphology of XC-C9phNBr aggregates is rodlike or ellipsoidal. The effects of temperature and XC concentration on the aggregation are analyzed. Results indicate that the formation of aggregates is an exothermic process, and their formation becomes more difficult and the formation rate decreases with increasing temperature. The formation of aggregates is also related to XC concentration, and it becomes much more difficult when the concentration of XC is higher than 20 vol %. The simulation results agree with binding isotherms of C9phNBr to XC obtained via the potentiometric titration method, which shows a typical cooperative binding between C9phNBr and XC.     

Molecular mechanism of the photochemical generation of gold nanoparticles in ethylene glycol: support for the disproportionation mechanism

It is found that replacement of the chloride ions in tetrachloroauric acid with bulky bromide ions inhibits the formation of gold nanoparticles in the photochemical reduction in ethylene glycol. However, the addition of silver ions to either the bromide or the chloride auric acid solution is found to enhance the rate of gold nanoparticle formation. These results are found to be accounted for by the previously proposed mechanism (Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811) which involves disproportionation of the chloroauric complexes to generate free gold atoms and chloride ions. The steric effects of the bulky bromide ions inhibit the formation of the Au-Au bond needed in the electron transfer process of the disproportionation reaction. The addition of Ag+ ions results in the formation of insoluble silver halide, which shifts the disproportionation reaction toward the formation of gold atoms and thus the formation of gold nanoparticles.     

Dynamics of carbide formation in iron-supported catalysts of the Fischer–Tropsch process promoted by copper and potassium

The kinetics of the formation of iron carbides during the activation of iron-coated catalyst for Fischer–Tropsch synthesis promoted by copper and potassium, and by carbon monoxide and syngas, is studied. It is established that the presence of copper lowers the initial temperature of hematite reduction to magnetite and leads to the formation of carbide in both CO and СО/Н₂. Potassium slows the rate of magnetite formation, but it accelerates the formation of iron oxide. It is shown that the rate of carbide formation during magnetite reduction for catalysts is half that in the reaction of hematite reduction to magnetite in both CO and СО/Н₂.