Influence of solvent properties on cation-macrocycle complexation: Cesium cryptates

The chemical shift of the¹³³Cs resonance has been measured as a function of the mole ratio of cryptands C222, C222B, C221, and C211 to the Cs⁺ ion inN-methylformamide and dimethylacetamide solutions. The overall formation constants of the cesium cryptates were calculated from the NMR titration curves. From these and previously reported data it is clear that solvent properties such as dipole moment, donicity, dielectric constant and hydrogen bonding ability affect macrocyclic ligand-cation complexation equilibria through specific solvation of the cation, the ligand and the complex. A model of M⁺ · L bonding in solution, involving a rather intimate electrostatic attraction between the cation and the local electron distribution of the macrocycle accompanied by little perturbation of the charge distributions of the separated partners, is presented. In addition to the specific interactions previously noted, the presence of the solvent mitigates the ion-multipole forces which would otherwise govern the long-range attraction.This paper is dedicated to the memory of the late Dr C. J. Pedersen.Deceased.     

C2 oxygenates from chloromethane and carbon monoxide

Acetaldehyde (CH₃CHO) and acetic acid (CH₃COOH) are major products of the thermal reaction of carbon monoxide and methyl chloride under certain reaction conditions.     

New cytotoxic cembranolides from the soft coral Lobophytum michaelae

Eleven new cytotoxic cembranolides, michaolides A-K (1-11), and crassolide (12) were isolated from the CH(2)Cl(2) extract of the Formosan soft coral Lobophytum michaelae. Their structures were established by extensive spectral analysis. The cytotoxicity of the isolates against selected cancer cells was measured in vitro.     

Internal electric field distribution within a micro-cylinder-shaped particle suspended in an absorbing gaseous medium

The present study is concerned with photophoresis of a microsized long cylinder-shaped particle suspended in an absorbing gas medium. To facilitate the analysis, an infinite cylinder subjected to an intensive light beam is considered as the physical model. The electromagnetic energy can be absorbed by the particle and turned into thermal energy heating surface unevenly, which results in a net momentum transfer between gas molecules and the particle to drive particle in photophoretic motion. Effects of the governing parameters on the absorbed energy distribution in the cylindrical particle are investigated. The results demonstrate that increasing either the radius or absorptivity of the cylinder enhances the energy absorbed on the illuminated side and tends to generate positive photophoresis; while an increase in the refractivity of the particle tends to enhance the internal electric field intensity and shift the absorption peak on the shaded side toward the particle center. Increase in medium absorptivity reduces the energy reaching at the particle, which significantly degrades the level of energy absorption and therefore weakens the photophoretic mobility of the particle. It is also found that, at the same conditions, the source function peaks in long cylinder-shaped particles are generally lower than those in spheres due to the weaker light refraction of the cylindrical shape.     

Diffusion and sorption of cyclohexane vapor in SBS and SBR copolymers

Diffusion and sorption of cyclohexane vapor in styrenebutadiene copolymers were used to study the morphology of an SBS triblock copolymer (29.3 wt% styrene) cast from different solvents. The weight gains or losses of this SBS and several SBR (5, 23, 45, and 85 wt% S content) samples were continuously monitored at 40°C and at penetrant pressures of 30, 60, 90, and 120 Torr. Significant differences exist in the equilibrium sorption among the different samples. Possible reasons for this observation are explored. The one corroborated by dynamic mechanical measurements attributes the differences to varying interphase volume fractions. The interphase region separating the PB and PS phases in SBS was assumed to be a continuous profile of equivalent SBR with different S contents. Experimental results, coupled with simple calculations, indicated that the overall interphase volume fractions in the SBS are 0.28 for the MEK/THF-cast (1:9 volume ratio of MEK and THF) sample, 0.15 for the toluene-cast sample, and 0.06 for the cyclohexane-cast sample. Important observations consistent with these results are: (1) the existence of a relative minimum (at around 20–30 wt% S content) in the plot of the equilibrium sorption of cyclohexane vs S content in rubbery SBR; and (2) a relative maximum in the plot of the mutual diffusion coefficient of cyclohexane in SBR vs the concentration of cyclohexane in SBR.     

Similarity solutions of mixed convection heat and mass transfer in combined stagnation and rotation-induced flows over a rotating disk

The objective of the present study is to develop a novel similarity model for analysis of mixed convection heat and mas transfer in combined stagnation and rotation-induced flows over a rotating disk. Thermal and concentration (solutal) buoyancy effects stemmed from temperature and concentration gradients in rotational as well as gravitational forces fields are all taken into account. The influences of the forced flow, disk rotation, thermal buoyancy, buoyancy ratio and the fluid properties, i.e. Prandtl and Schmidt numbers, on the flow, temperature and concentration fields and the associated friction factors, heat and mass transfer rates are investigated. The present results reveal the effects of various buoyancy modes with combined forces on the transport phenomena in rotating-disk flows, and the analysis is also useful in understanding the mechanisms of mixed convection in the class of rotating fluids. Received on 30 December 1997     

Dynamical responses of non-isothermal flow between two independently rotating disks with time-dependent wall conditions

The present work develops a theoretical model of rotational convection and uses it to investigate the dynamical responses of the flow and heat transfer between two disks rotating at different rates under the influences of time-dependent disturbances. The unsteady non-isothermal flow model is formulated by extending a recently developed steady-state similarity model of axi-symmetric rotational convection. In the new model all the rotation-induced buoyancy forces are considered. Using one disk as reference, effects of the time-dependent changes in wall temperature or rotating rate of the other disk on the flow and heat transfer are explored. Various rotational modes with asymptotic or fluctuating change in boundary condition of temperature or disk rotation are studied. The present time-dependent model for this non-isothermal rotating flow is numerically solved by a finite-difference method. By using the present results, the complicated flow and heat transfer mechanisms with thermal-flow coupling in the class of time-dependent rotational convection are manifested.     

Numerical study of liquid film cooling along an inclined plate

A numerical method of analyzing liquid film cooling along an inclined plate is presented. A marching procedure is employed for solution of the equations of mass, momentum, energy and concentration in the flow. Results for heat and mass transfer characteristics are presented for air-water system. The effects of the inclined angle , free-stream temperatureT , free-stream velocityu , and inlet film thickness on the heat and mass transfer along the gasliquid interface are examined in detail. Results show that an increase in free-stream temperature and velocity causes an increase in interfacial temperature while an increase in inclined angle and inlet film thickness causes a reduction in interfacial temperature. Additionally, the predicted results with the transport in the liquid film treated are contrasted with those with the transport in the liquid film untreated, showing that the assumption of an extremely thin film is inappropriate for a larger .

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Numerische Untersuchung der Flüssigfilm-Kühlung entalang einer geeigneten Platte

Zusammenfassung Es wird eine numerische Methode zur Untersuchung der Flüssigfilm-Kühlung entlang einer geneigten Platte vorgestellt. Die Lösung der Bilanzgleichung für Masse, Impuls, Energie und Konzentration in der Strömung erfolgt mit Hilfe eines expliziten Verfahrens. Ergebnisse für das Wärme- und Stoffaustauschverhalten werden bezüglich des Systems Luft — Wasser mitgeteilt. Im einzelnen befaßt sich die Untersuchung mit der Ermittlung des Einflusses von Neigungswinkel , FreistrometemperaturT , Freistromgeschwindigkeitu und Eintrittsfilmdicke auf den Wärme- und Stoffübergang entlang der Gas-Flüssigkeitsgrenzfläche. Die Ergebnisse zeigen eine Abnahme der Grenzflächentemperatur bei ansteigender Freistromtemperatur und -geschwindigkeit und eine Erhöhung, wenn Neigungswinkel und Eintrittsfilmdicke zunehmen. Zusätzlich folgt aus den Berechnungen, daß bei größeren Filmdicken die Annahme eines extrem dünnen Films unter Vernachlässigun g der vollständigen Transportmechanismen im Flüssigkeitsfilm zu falschen Ergebnissen führt.

     

A parallel-plate rheometer for the measurement of steady-state and transient rheological properties

A parallel-plate rheometer (PPR) has been constructed for use with the Materials Test System (MTS) for the measurement of fast transient and steady-state responses of polymeric liquids. This particular combination exploits the versatility and capability of the MTS programmable drive system, while the design of the PPR ensures the ideal simple shearing configuration. Instrument compliance, material inertia and mechanical lag in both the input and output components of the system are minimal, thus avoiding defects of most existing rheometers. Reproducible data on a 20% polyisobutylene-in-decalin solution have been obtained for various complicated flow programs. This test system is thus suitable for extensive rheological characterization of complex viscous liquids.