Solid + liquid equilibria of (n-alkane + cyclohexane) mixtures at high pressures

Solid+liquid equilibria (SLE) of [n-alkanes (tridecane, hexadecane, octadecane, or eicosane) + cyclohexane] at very high pressures up to about 1.0 GPa have been investigated in the temperature range from 293 to 363 K using a thermostated apparatus for the measurements of transition pressures from the liquid to the solid state in two component isothermal solutions. The freezing temperature of each mixture increases monotonously with increasing pressure. The eutectic point of the binary systems shifts to a higher temperature and to a higher n-alkane concentration with increasing pressure. The pressure–temperature–composition relation of the high-pressure solid–liquid equilibria, a polynomial based on the general solubility equation at atmospheric pressure, was satisfactorily used. Additionally, the SLE of the binary system (tridecane+cyclohexane) at normal pressure was measured by the dynamic method. The results at high pressure for all systems were compared to that at normal pressure.     

Recovering Invisible Signals by Two‐Field NMR Spectroscopy

Nuclear magnetic resonance (NMR) studies have benefited tremendously from the steady increase in the strength of magnetic fields. Spectacular improvements in both sensitivity and resolution have enabled the investigation of molecular systems of rising complexity. At very high fields, this progress may be jeopardized by line broadening, which is due to chemical exchange or relaxation by chemical shift anisotropy. In this work, we introduce a two‐field NMR spectrometer designed for both excitation and observation of nuclear spins in two distinct magnetic fields in a single experiment. NMR spectra of several small molecules as well as a protein were obtained, with two dimensions acquired at vastly different magnetic fields. Resonances of exchanging groups that are broadened beyond recognition at high field can be sharpened to narrow peaks in the low‐field dimension. Two‐field NMR spectroscopy enables the measurement of chemical shifts at optimal fields and the study of molecular systems that suffer from internal dynamics, and opens new avenues for NMR spectroscopy at very high magnetic fields.     

Very-high-speed liquid column chromatography the system and selected applications

Summary Instrumentation needed to achieve very high performance (plate/s) liquid chromatography is discussed. With recently available systems using relatively short (100–125 mm) columns of conventional internal diameter (4–6 mm) and packed with 3–5 m particles, analysis in very short times (usually 1–3 min) is possible. Coupling such columns in series provide very high performance in a time still significantly less than previously encountered. Performance of such liquid chromatographic systems is illustrated with a number of practical examples including analgesic tablets, cosmetics, soft drinks, antioxidants and polyaromatic hydrocarbons.Enlarged text of a paper presented at the 32nd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9–13, 1981, Atlantic City, New Jersey (Paper No. 462).     

Recent advances and actual challenges in late transition metal catalyzed hydrosilylation of olefins from an industrial point of view

Hydrosilylation of olefins is the key catalytic reaction for the production of industrially important organosilicon compounds such as organofunctional silanes and silicones. Moreover catalytic hydrosilylation is used for crosslinking of silicone polymers to elastomers and silicone-based release coatings, and for coupling of silanes and siloxanes to organic polymers. Industrially relevant aspects of hydrosilylation are dominated by the selectivity, activity (defined by the turnover frequency (TOF)), and stability (defined by the turnover number (TON)) of hydrosilylation catalysts as well as switchable catalyses. Furthermore, the high and volatile price of platinum as the industrially most important catalytic metal is a strong motivation for the reduction of precious metal consumption, such as homogeneous catalyst recycling or increasing the TOF resp. TONs of established hydrosilylation catalysts, or employing lower-priced transition metal catalysts. The selectivity of hydrosilylation determines yield and production costs of functional silanes, e.g. hydrosilylation products of allyl chloride, but is of equal importance for the product quality of silane-modified organic polymers and hybrid polymers. As industrial applications of hydrosilylation curing silicones, such as release coatings and elastomers, continuously move towards higher production speed, this requires catalytic systems capable of very high activity resp. turnover frequencies at temperatures typically above 100 °C, but allowing shelf-stable silicone compositions and therefore requiring suppression of any catalytic activity at ambient conditions prior use. This form of switchable catalysis employs carefully designed catalytic systems, which are activated by heating or photoactivation in a very short period of time, demanding very high standards of industrial hydrosilylation chemistry.     

A new massively parallel version of CRYSTAL for large systems on high performance computing architectures

Fully ab initio treatment of complex solid systems needs computational software which is able to efficiently take advantage of the growing power of high performance computing (HPC) architectures. Recent improvements in CRYSTAL, a periodic ab initio code that uses a Gaussian basis set, allows treatment of very large unit cells for crystalline systems on HPC architectures with high parallel efficiency in terms of running time and memory requirements. The latter is a crucial point, due to the trend toward architectures relying on a very high number of cores with associated relatively low memory availability. An exhaustive performance analysis shows that density functional calculations, based on a hybrid functional, of low‐symmetry systems containing up to 100,000 atomic orbitals and 8000 atoms are feasible on the most advanced HPC architectures available to European researchers today, using thousands of processors. © 2012 Wiley Periodicals, Inc.     

Kinetic studies of colloidal crystallization of thermo-sensitive gel spheres of poly(N-isopropylacrylamide)

Crystal growth rate coefficients, k of the colloidal crystallization of thermo-sensitive gel spheres of poly(N-isopropylacrylamide) were measured from the time-resolved reflection spectroscopy mainly by the inverted mixing method in the deionized state. Crystallization of colloidal silica spheres were also measured for comparison. The k values of gel and silica systems increased sharply as the sphere concentration and suspension temperature increased. The k values of gel system were insensitive to the degree of cross-linking in the range from 10 to 2?mol% of cross-linker against amount of the monomer in mole and decreased sharply when the degree of cross-linking decreased further to 0.5?%. The k values increased as gel size increased. The k values of gel systems at 20?°C were small and observed only at the very high sphere concentration in volume fraction, whereas those at 45?°C were high but smaller than those of silica systems. Induction time (t _i) after which crystallization starts, increased as the degree of cross-linking increased and/or the gel size decreased at any temperatures, when comparison was made at the same gel concentration. The t _i values at 45?°C were high and decreased sharply with increasing sphere concentration, whereas those at 20?°C were high only at the very high sphere concentrations. Significant difference in the k and t _i values between the soft gels and hard silica spheres was clarified. These kinetic results support that the electrical double layers play an important role for the gel crystallization in addition to the excluded volume of gel spheres. It is deduced further that the electrical double layers of the gel system form from the vague interfaces (between soft gel and water phases) compared with those of typical colloidal hard sphere system.     

Vitamin E – a new choice for polyolefin stabilization

Vitamin E based systems provide an alternative way to stabilize polyolefins, instead of the current state-of-the-art stabilizers, for certain niche applications. Its very positive public perception together with its excellent ability for melt flow control at a very low concentration can be utilized to a great extent, particularly for special applications like food and medical packaging. Thus, it's use can lead to an overall reduction in the total amount of additive required compared to a traditional stabilization system in certain applications. Other potential benefits, resulting from the positive attributes of Vitamin E, include low migration, high extraction resistance, very good organoleptics, gel suppression, and longer shelf-life of products. This study examines the performance of Vitamin E based systems in LDPE, LLDPE, HDPE and PP.     

Wiederaufladbare Batterien

As a matter of fact, most of our technical electrochemical energy storage systems operate outside the limitations of thermodynamics. As in the case of rechargeable batteries with aqueous electrolytes (part I of this article), kinetics control the operation and safety also in batteries with nonaqueous electrolytes (this second and final part).A striking example is the lithium ion battery which possesses an operating voltage of >3,5 V and a very high energy density. From a thermodynamic viewpoint such a cell is impossible because the used organic electrolyte is in contact with two lithium insertion electrodes that operate at extreme reducing and oxidizing potentials, respectively. However, a unique mechanism kinetically prevents the decomposition of the electrolyte due to the formation of electronically insulating interphases between electrode and electrolyte that are still permeable to the electrochemically active Li⁺ cations. Lithium ion batteries have already made their breakthrough into the market as small format systems for portable electronics. The only „kinetically shielded”︁ high energy density, however, might be a safety complication for large format batteries, which are assembled for electric vehicle (EV) propulsion. Safety concerns are also valid for high temperature (300°C) batteries such as the sodium-sulfur and sodium-nickel chloride systems. These systems are still in the stage of „experimental batteries”︁, which may find future application in large units for EV's or uninterruptible power systems. The paper is concluded by a performance comparison of various rechargeable battery systems with aqueous and nonaqueous electrolytes. (Possible) applications in consumer electronics and EV's are discussed in more detail.     

Polymerization of vinyl monomers initiated by KO2–charge transfer agent systems. II

Vinyl monomers having electron acceptor groups such as nitroethylene, acrylonitrile, and acrolein were polymerized by KO₂–charge transfer agent initiator systems in dimethylsulfoxide (DMSO) at 25°C. The new initiator systems were found to be stable for almost 1 month under nitrogen atmosphere. The initial rate of polymerization was so fast that both conversion and molecular weight of the polymers obtained were high. Especially their molecular weight distribution was observed to be very narrow by means of gel permeation chromatography (GPC). The anion radicals generated by one electron transfer from potassium superoxide (KO₂) to charge transfer agents such as naphthalene, benzoquinone, azobenzene, etc., were suitable as initiator for the anionic polymerization of electron acceptor monomers. Study on block copolymerization of nitroethylene with acrylonitrile or acrolein was also attempted.     

Polymer‐supported Ti(IV) and Mn(III) asymmetric alkene epoxidation catalysts

Epoxides represent a very important group of speciality and fine chemicals because they are derived directly from alkenes, a primary petrochemical source, and because of the breadth of opportunity they offer the organic synthetic chemist in terms of the highly selective reactions they undergo, often requiring only very mild conditions. Since most epoxides also bear at least one stereogenic centre the strategic importance of these molecules in synthesis is even higher. The most important asymmetric alkene epoxidation catalyst systems that have been discovered are those reported by Sharpless and his co‐workers utilising tartrate ester complexed Ti(IV) centres¹ and by Jacobsen and his co‐workers utilising chiral Mn(III) salen complexes.² The former system provides high conversions and high enantioselectivity (enantiomeric excess, ee%) in the case of allylic alcohol substrates, while the latter is likewise effective in the case of non‐functional cis‐internal alkenes, especially cyclic systems. Both catalytic systems are homogeneous and exploitation of both involve rather laborious work‐up procedures. Generally no attempt is made to recover and re‐use these catalysts. The potential advantages in converting a process catalysed by a homogeneous metal complex into one involving a heterogeneous polymer‐supported analogue have been well rehearsed.³ Suffice to say that on a laboratory scale supported metal complex catalysts considerably facilitate product work‐up and isolation, while on a large scale such heterogeneous species allow processes to be run continuously using packed or fluidised bed columns with considerable financial advantages both in terms of capital expenditure on plant and with regard to recurrent costs.     

Polypyrrole modified solvent resistant nanofiltration membranes

New solvent resistant nanofiltration (SRNF) membranes with polypyrrole (PPy) modified toplayer were prepared on different types support by in situ pyrrole polymerization. The morphology of the membranes was studied by SEM. The PPy modified membranes were applied in the filtration of organic solvents. All the PPy modified membranes showed a very high retention of the negatively charged RB in different solvent systems, comparable to those of the MPF-50 and STARMEM 122 commercial membranes, but at much higher flux. The extended filtration experiment in strong aprotic DMF of PPy modified membranes showed a clearly stable permeability and retention over 30 h. In addition, the PPy modified membranes showed a much higher flux in THF systems than for earlier reported crosslinked poly(imide) membranes.     

Molecular Dynamics Simulations of Cellulose Oligomers: Conformational Analysis

Summary: The results of classical molecular simulations of cellulose oligomers are presented here. The conformations of the chains in the high temperature melt, room temperature quenched melt and gas phase are compared with respect to various geometrical parameters including square end‐to‐end distances, glycosidic link torsion correlations, ring puckering and hydrogen bonding. The cellulose oligomer melts were relaxed at 800 K with molecular dynamics, and then cooled down in three different ways to obtain dense amorphous systems at 500 K and at room temperature. The sample resulting from the quench (step) shows too much similarity with the melt at 800 K. The two other cooling schemes (ramp, 2ramps) give very similar results for all quantities investigated. The relevance of previous single molecule calculations with respect to the dense amorphous systems is called into question. Comparisons between the chains in the dense systems and those in the gas phase reveal that, even for these relatively short stiff chains, differences exist in the preferred conformations. At high temperatures, where both systems are in equilibrium, the distribution of square end‐to‐end distances are both fairly smooth, but the gas phase clearly prefers more compact conformations. At 300 K, the differences are exacerbated as the equilibrium distribution for the gas phase shows a high proportion of folded conformers, whereas the nonequilibrium quenched systems necessarily retain the extended envelope of the higher temperature. Differences are also evident in the puckering, the rotation of the hydroxymethyl groups and the pattern of hydrogen bonds.

The probability density distribution for the square end‐to‐end distance for octaose in the gas phase (light line) and in the dense phase (dark line) at 300 K.     

Purification and analysis of partially alkylated cyclodextrins by liquid and gas chromatography

Complex mixtures of partially alkylated cyclodextrins can be analyzed by both HPLC and high temperature capillary GC. Because of the limited efficiency of LC, suitable analytical and preparative separations can be achieved only with systems of carefully optimized selectivity. Using LC it has been possible to isolate and purify single cyclodextrin species from very complex mixtures of components which contain unreacted hydroxyl groups in addition to the alkoxy groups. Analysis of the reaction mixtures and of fractions taken from LC separations can be performed with advantage by high resolution capillary GC at high temperatures between 300 and 400 °C. The thermal stability of partially alkylated cyclodextrins in high temperature GC is considerably increased by trimethylsilylation of the free hydroxyl groups. Fast atom bombardment mass spectrometry and proton NMR were used to identify species isolated from the preparative LC separations.     

Mechanisms of antioxidant action: Stabilisation of nitrile rubber by interpenetrating networks based on thiol adducts

Nitrile rubbers containing three thiol antioxidants ((I) to (III)) in bound form have been evaluated as masterbatch concentrates by dilution in unstabilised nitrile rubber both at the latex stage and in the polymer melt. Both procedures give effective substantive systems in the case of the hindered phenol (I) and the aromatic thiol amide (II). Both show a high level of antioxidant retention under very severe technological conditions (oil/air oven cyclical test at 150°C). Antioxidant (III), which has only a minor structural difference from (II), is very much less effective as a substantive antioxidant.     

Living carbocationic polymerization. IV. Living polymerization of isobutylene

Truly living polymerization of isobutylene (IB) has been achieved for the first time by the use of new initiating systems comprising organic acetate-BCl³ complexes under conventional laboratory conditions in various solvents from −10 to −50°C. The overall rates of polymerization are very high, which necessitated the development of the incremental monomer addition (IMA) technique to demonstrate living systems. The living nature of the polymerizations was demonstrated by linear versus grams polyisobutylene (PIB) formed plots starting at the origin and horizontal number of polymer molecules formed versus amount of polymer formed plots. obeys [IB]/[CH₃COOR^t · BCl₃]. Molecular weight distributions (MWD) are very narrow in homogeneous systems whereas somewhat broader values are obtained when the polymer precipitates out of solution . The MWDs tend to narrow with increasing molecular weights, i.e., with the accumulation of precipitated polymer in the reactor. Traces of moisture do not affect the outcome of living polymerizations. In the presence of monomer both first and second order chain transfer to monomer are avoided even at −10°C. The diagnosis of first and second order chain transfer has been accomplished, and the first order process seems to dominate. Forced termination can be effected either by thermally decomposing the propagating complexes or by nucleophiles. In either case the end groups will be tertiary chlorides. The living polymerization of isobutylene initiated by ester · BCl₃ complexes most likely proceeds by a two-component group transfer polymerization.     

Application of heat conduction calorimetry to high explosives

This paper describes some thermal analysis experiments conducted on high explosive samples. These employ differential scanning calorimetry to monitor thermal effects at elevated temperatures (around 200 °C) and heat conduction calorimetry to record thermal effects at much lower temperatures (below 100 °C).The work shows that, due to the generally high thermal stability of many high explosive compositions, heat generation rates are very low, if detectable at all, at normal storage temperatures, even when using a very sensitive instrument. The sensitivity and reproducibility of this technique has been investigated in detail by Wilker et al. [S. Wilker, U. Ticmanis, G. Pantel, Detailed investigation of sensitivity and reproducibility of heat flow calorimetry, in: Proceedings of the 11th Symposium on Chemical Problems Connected with the Stability of Explosives, Sweden, 1998] and shown to be capable of recording heat generation rates of less than a microwatt. This allows continuous measurement of decomposition processes in nitrate ester based propellants at temperatures as low as 40 °C. However, the measurement of very low levels of heat generation is difficult, time consuming and therefore expensive. If the assumption is made that the life limiting process is invariably the slow decomposition of the energetic component, this will frequently lead to very long service lifetime predictions.A number of possible complications are identified. Firstly, due to its low detection threshold, a heat conduction calorimeter may detect other reactions which will not lead to failure, but which may still dominate the heat flow signal. Secondly, the true failure process may generate little energy and be overlooked. In view of these considerations, at present it seems unwise to rely on heat conduction microcalorimetry as the only tool for the assessment of the life of high explosive energetic systems.Based on examples of life terminating processes in high explosives during storage and use, it is clear that decomposition of the energetic material is not invariably the cause of system failure. It is also by no means the only reaction that may take place in, and be observed by, a heat conduction calorimeter.     

Analysis of synthetic peptides by capillary electrophoresis: effect of organic solvent modifiers and variable electrical potentials on separation efficiencies

In this study, procedures based on volatile ammonium acetate buffer electrolytes of high pH value containing different organic solvent modifiers have been developed to achieve very high efficiency separations of histidine-containing synthetic peptides by high-performance capillary electrophoresis (HPCE) employing untreated fused silica capillaries. Different organic solvents, including acetonitrile, methanol and ethanol, at high volume fractions were used to modify the composition of the background buffer electrolyte. With the peptides investigated, it was found that methanol had the greatest effect in terms of enhancement of separation efficiency, as determined from the evaluation of theoretical plate numbers, N, of these HPCE systems. On the other hand, separation selectivities, e.g. the alpha(ij) values, did not change significantly as the volume fraction, psi, of the organic solvents was increased up to psi = 60% (v/v). Under these conditions, very rapid, e.g. 1-2 min, separation times could be still achieved. Compared to the effect of carrying out the separation of these peptides at constant voltage, a dramatic increase in the separation efficiency was also achieved by applying a linear voltage gradient during the HPCE experiment. Under optimal conditions of organic solvent composition and linear voltage gradient ramps, very high peak efficiencies for the studied set of synthetic peptides with N values of approximately 2-3 million theoretical plates per meter could be routinely obtained with fast analysis times. Moreover, these buffer electrolyte conditions are compatible with direct interfacing of the HPCE effluent to electrospray ionisation and ion trap mass spectrometers, thus expanding the analytical capabilities of these HPCE systems.     

葡萄糖、半乳糖和乙醇恒电流氧化过程电位振荡的EQCM研究

采用电化学石英晶体微天平(EQCM)研究了0.5 mol•L^-1 NaOH水溶液中铂电极上葡萄糖、半乳糖和乙醇恒电流氧化过程中伴随的电位振荡行为. 两个糖体系的电位振荡过程伴随EQCM频率的同步振荡响应, 而乙醇体系中相应的频率响应却非常小；三个体系振荡过程的同步动态电阻响应均很小, 表明振荡过程频率响应主要为质量效应. 虽然葡萄糖和半乳糖结构相似, 电位和频率振荡的幅度相当, 但频率波数和周期明显不同, 表明电位振荡行为对两者呈现良好的分子识别能力. 本文也讨论了相关振荡机理和NaOH浓度效应及碱性介质中铂电极电化学过程, 提出了所形成的铂氧化物主要是PtO₂-3H₂O_ad以及两糖体系振荡过程中糖酸根阴离子伴随着高/低电位在铂电极上吸/脱附的新观点.     

Cellulose esters in drug delivery

Cellulose esters have played a vital role in the development of modern drug delivery technology. They possess properties that are not only well-suited to the needs of pharmaceutical applications, but that enable construction of drug delivery systems that address critical patient needs. These properties include very low toxicity, endogenous and/or dietary decomposition products, stability, high water permeability, high T _g, film strength, compatibility with a wide range of actives, and ability to form micro- and nanoparticles. This suite of properties has enabled the creation of a wide range of drug delivery systems employing cellulose esters as key ingredients. The following is a review of the most important types of these systems, and of the critical roles played by cellulose esters in making them work, focusing on more recent developments.     

Liquid-liquid equilibrium of aqueous two-phase systems composed of poly(ethylene oxide) 1500 and different electrolytes ((NH4)2SO4, ZnSO4 and K2HPO4): Experimental and correlation

Phase diagrams of aqueous two-phase systems composed of PEO1500 + salt (di-potassium phosphate + potassium hydroxide or ammonium sulfate or zinc sulfate) + water were determined at (283.15, 298.15, and 313.15) K. All systems produce a large two-phase region; however the influence of temperature on the binodal position seems to be very small. By analyzing the effects of ammonium sulfate or zinc sulfate, it was observed that zinc was more effective in promoting phase separation than ammonium. The consistency of the tie-line data was ascertained by applying the Othmer-Tobias correlation. In this paper, aqueous two-phase systems data for nine ternary systems are correlated by using the NRTL model and UNIFAC for the activity coefficient. The results are very satisfactory, with root mean square deviations between experimental and calculated compositions as low as 0.99 and 1.21%, respectively. However the NRTL model better represents the systems in study, when compared with UNIFAC.