A numerical solution of the coupled heat and mass transfer problem of non-planar solidification and melting of aqueous solutions

During freezing of aqueous solutions containing solutes like sodium chloride or glycerol, pure ice is the first solid phase to form. A numerical model is presented for the case of non-planar (dendritic) freezing, where local thermodynamic equilibrium can be assumed in the whole mushy region, allowing a one-dimensional treatment of the coupled heat and mass transfer in this region. Solute diffusion along the resulting interdendritic concentration gradient in the direction of freezing is taken into account by the model. With regard to the non-linear character of the matching condition between temperature and concentration an iterative method based on the Newton-algorithm was used to solve the system of the common non-linear heat and mass transfer equations, which were derived from the partial differential equations by a finite difference scheme. Varying the geometric and thermodynamic boundary conditions, a parameter study was performed for obtaining more information on heat transfer during thawing of regular-shaped samples (plate, cylinder, sphere). In particular, the warming rate distributions in containers filled with aqueous solution were determined, which are interesting parameters in the field of cryopreservation of biological material.Beim Gefrieren von wäßrigen Lösungen, die Substanzen wie Kochsalz oder Glyzerin enthalten, bildet sich als erste feste Phase reines Eis. Im Falle nichtplanarer (dendritischer) Erstarrung kann innerhalb der gesamten mushy region lokales thermodynamisches Gleichgewicht angenommen werden, wodurch eine eindimensionale Behandlung des gekoppelten Wärme- und Stofftransports in dieser Zone möglich wird. Dabei wird die Diffusion gelöster Stoffe entlang des resultierenden Konzentrationsgefälles im interdendritischen Zwischenraum berücksichtigt. Angeischts der nichtlinearen Anpassungsbedingung zwischen Temperatur und Konzentration wurde ein auf einem Newton-Algorithmus basierendes iteratives Verfahren verwendet, um die gewöhnlichen, nichtlinearen Wärme- und Stofftransportgleichungen zu lösen, die aus den partiellen Differentialgleichungen durch ein Finite-Differenzen-Verfahren gewonnen wurden. Unter Variation der geometrischen und thermodynamischen Randbedingungen wurde eine Parameterstudie mit dem Ziel durchgeführt, genaueren Einblick in das Auftauverhalten von Proben einfacher Geometrien (Platte, Zylinder, Kugel) zu gewinnen. Insbesondere wurden die Erwärmungsraten-Verteilungen in mit wäßriger Lösung geführten Containern bestimmt, die im Zusammenhang mit der Kryokonservierung biologischen Materials von Bedeutung sind.     

Light‐Controlled Formation of Vesicles and Supramolecular Organogels by a Cholesterol‐Bearing Amphiphilic Molecular Switch

A new responsive material composed of an amphiphilic light‐switchable dithienylethene unit functionalized with a hydrophobic cholesterol unit and a hydrophilic poly(ethylene glycol)‐modified pyridinium group has been designed. This unique single‐molecule system shows responsive light‐switchable self‐assembly in both water and organic solvents. Light‐triggered reversible vesicle formation in aqueous solutions is reported. The molecule shows a different behavior in apolar aromatic solvents, in which light‐controlled formation of organogel fibers is observed. The light‐triggered aggregation behavior of this molecule demonstrates that control of a supramolecular structure with light can be achieved in both aqueous and organic media and that this ability can be present in a single molecule. This opens the way toward the effective development of new strategies in soft nanotechnology for applications in controlled chemical release systems.     

A Dithienylethene‐Based Rewritable Hydrogelator

Dithienylethene photochromic switching units have been incorporated into a hydrogelating system based on a tripeptide motif. The resulting hybrid system provided both a photochromic response and the ability to gelate water under acidic and neutral conditions. Fluorescence spectroscopy shows that the dithienylethene units are in sufficient proximity to each other to stack in gel fibers, with the tripeptide unit determining solubility. TEM measurements provided insight into the microscopic structure of the fibers formed.     

1,3-Bis(triphenylphosphonium-ylidyl)cyclotetraphosphane

1,3-Bis(triphenylphosphonium-ylidyl)cyclotetraphosphines The title compounds 3a , b result from the condensation of the ylidyldichlorophosphines Ph₃PCR? PCl₂ 1a , b (R = Me, 3-Tol) with phenyl bis(trimethylsilyl)phosphine 2 . Analysis of the ³¹P-NMR spectra and the X-ray structure investigation of 3a demonstrate a folding of the four-membered ring and an all-trans orientation of the substituents.     

A new process to prepare 3,6-dichloro-2-hydroxybenzoic acid,the penultimate intermediate in the synthesis of herbicide dicamba

Glyphosate [N-(phosphonomethyl)glycine] is a broad spectrum, post-emergent herbicide that is among the most widely used agrochemicals globally. Over the past 30?years, there has been a development of glyphosate-resistant weeds, which pose a significant challenge to growers and crop scientists, resulting in lower crop yields and increased costs. 3,6-Dichloro-2-methoxybenzoic acid (dicamba) is the active ingredient in XtendiMax®, a standalone herbicide developed by Bayer Crop Science to control broadleaf weeds, including glyphosate-resistant species. 3,6-Dichloro-2-hydroxybenzoic acid (3,6-DCSA) is the penultimate intermediate in the synthesis of dicamba. Existing dicamba manufacturing routes utilize a high temperature, high pressure Kolbe-Schmitt carboxylation to prepare 3,6-DCSA. Described in this Letter is a new, non-Kolbe-Schmitt process to prepare 3,6-DCSA from salicylic acid in four chemical steps.     

Structure and dissociation of ylidyl chlorophosphanes

The ylidyl substituent of the chlorophosphane Ph₃P(DOUBLE BOND)CAr(SINGLE BOND)PRCl exerts a strong influence on the P(SINGLE BOND)Cl bond. An X-ray structure investigation of the representative with Ar(DOUBLE BOND)Ph, R(DOUBLE BOND)Me reveals the longest P(SINGLE BOND)Cl bond ever observed for an acyclic chlorophosphine (226.2(1) pm). It is connected to a conformation that allows an effective negative hyperconjugation. The ylidyl chlorophosphanes with an amino group R are covalent in benzene but become more or less ionic in dichloromethane solution. The solvent-dependent dissociation equilibrium can be followed by ³¹P NMR spectra. In case of an enamine-derived ylidyl chlorophosphane, the equilibrium shifts almost completely from the covalent to the ionic side within a rather narrow range of solvent composition (20 to 70 vol % dichloromethane). © 1996 John Wiley & Sons, Inc.