Investigation of eutectic transformation of Zn−Al alloy by DTA

The paper discusses the investigation of crystallization of metals and alloys by differential thermal analysis (DTA). It was assumed that this method allows determination of the mechanism and kinetics of volumetric crystallization underiso conditions (e.g. anisothermal) on the basis of the parameters of the equation expressing an integral form of the DTA curve. From DTA, a course of eutectic transformation was determined for a technical Zn?Al alloy containing 4wt% Al. Investigations were carried out under continuous cooling at various rates and the kinetics parameters were determined with the KEKAM equation:-ln(1-x)=kl _n     

Chemical equilibrium model for the thermodynamic properties of mixed aqueous micellar systems: Application to thermodynamic functions of transfer

Mixed micelles can be formed in water between various pairs of hydrophobic solutes such as surfactants, alcohols and hydrocarbons. These systems can often be studied through the thermodynamic functions of transfer of one of the solutes, usually kept near infinite dilution, from water to an aqueous solution of the other solute. When mixed micelles are formed, these functions change significantly, and often go through extrema, in the region where the binary system micellizes or undergoes some microphase transition.Three main effects are responsible for the observed trends: pair-wise interactions between both solutes in the monomeric form, a distribution of the reference solute between the aqueous and micellar phases and a shift in the monomer-micelle equilibrium in the vicinity of the reference solute. Simple equations can be derived for these three effects which can account for the sign and magnitude of the observed trends using parameters which are derived for the most part from the two binary systems.     

Intramolecular and liquid structure of 2,2,2-trifluoroethanol by X-ray diffraction

Intramolecular and liquid structure of 2,2,2-trifluoroethanol (TFE) have been investigated by x-ray diffraction at 25°C. The structural parameters for the skeleton of the molecules in the liquid phase are similar to those in the gas phase. The conformers of TFE molecules in the liquid phase are discussed. The O...O distance at about 284 pm and additional F...O one at about 302 pm were found to be characteristic for the first neighbor interactions. Various models (dimers and trimers) have been examined for analyzing the first neighbor structure. The liquid structure was explained in terms of small clusters consisting of two to three molecules rather than of a more extended polymeric network.Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, P. O. Box 17, H-1525, Hungary.     

Thallium-205 NMR spectroscopy. Solvation of the thallous ion in dilute aqueous-amide and amide-amide binary solvent systems of formamide,N-methylformamide,andN-ethylformamide

Solvation of the thallous ion in dilute solutions of six binary solvent systems (formamide/water,N-methylformamide/water,N-ethylformamide/water, formamide/N-methylformamide, formamide/N-ethylformamide, andN-methylformamide/N-ethylformamide) was studied with²⁰⁵Tl NMR spectroscopy. An attempt was made to separate solvation effects related to the electrondonating ability (Lewis basicity) of the solvents from effects resulting from structural changes in the solvation sphere. Structural effects were found to be greatest in theN-methylformamide/water system and least in theN-methylformamide/formamide system.     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Magnetic field effects detected by exciplex fluorescence and preferential solvation in binary solvents. The effect of temperature

At 20–70 °C, the temperature has almost no effect on the semisaturation field,B _1/2, or on the magnetic effects detected by exciplex fluorescence (pyrene/N,N-dimethylaniline) in binary benzene-DMSO mixtures. In individual solvents (ethanol, methanol) heating leads to a noticeable increase in the magnetic effect, whileB _1/2 decreases. The results obtained corroborate a previously proposed hypothesis that polar microclusters are formed in binary solvents with components of different polarity. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1821–1823, October, 1993.     

Adrenaline recognition in water

Host molecule 1 displays a high affinity in water towards catecholamines and especially related structures such as beta-blockers with extended aromatic pi-faces (up to 7x10(3) M(-1) for each single complexation step or 5x10(7) M(-2) for both steps). The amphiphilic structural design leads to an extensive self-association of host molecules through their aromatic flanks. Above a cmc (critical micelle concentration) of 3x10(-4) M, host 1 forms micelles that produce a favorable microenvironment for hydrophobic interactions with the included guest molecules. Electrostatic attraction of the ammonium alcohol by the phosphonate anions is thus combined with hydrophobic contributions between the aromatic moieties. Ionic hydrogen bonds with polar OH or NH groups of the guest enforce the non-covalent interactions, and finally lead to increased specificity. Both its affinity and its selectivity towards adrenergic receptor substrates are greatly enhanced if the receptor molecule 1 is transferred from water into a lipid monolayer. Catecholamines and beta-blockers lead to drastically different effects at concentrations approaching the micromolar regime. Especially beta-blockers with minute structural changes can be easily distinguished from each other. In both cases, extensive hydrophobic interactions with a self-associated and/or self-organized microenvironment are largely responsible for the observed high efficiency and specificity.     

A High Sensitivity Fluorescent Chemo-sensory System Based on β-Cyclodextrin Dimer Modified with Dansyl Moieties

-Cyclodextrin dimer linked with ethylenediamine at the upper rim of the cyclodextrin has been synthesized and then modified with two dansyl moieties inthe presence of N,N'-dicyclohexylcarbodiimide. The sensing ability and bindingproperty of the title compound were investigated for steroids and terpenoids. Thefluorescence intensity of this dimer was decreased when a host–guest complex was formed. The value I/I⁰, where I⁰ and I are fluorescence intensitiesin the absence and presence of a guest and I is I⁰- I, was used as a parameter of sensitivity. This host exhibited a much higher sensitivity and selective molecular recognition ability for bile acids such as ursodeoxycholic acid andchenodeoxycholic acid and terpenoids such as (-)-borneol than the dansyl-modifiedcyclodextrins reported previously including -cyclodextrin dimer. The behaviors of the appended moieties of the host during the formation of host–guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The ICD intensityof this dimer was decreased on accommodation of a guest and this spectral pattern of the title dimer was opposite to that of bis dansyl-modified -cyclodextrin monomer. Theguest-induced variations in the fluorescence and ICD intensities suggest that this dimer formed a 1 : 1 host–guest complex and the appended moieties act as a hydrophobic cap.     

甲醇燃料车尾气净化催化剂的研究Ⅰ.甲醇深度氧化用钯催化剂上吸附物种的原位红外研究

利用原位红外技术研究了ＣＨ３ＯＨ、ＣＯ、Ｏ２等在Ｐｄ／γＡｌ２Ｏ３上的吸附情况及ＣＨ３ＯＨ和Ｏ２共吸附时表面物种的变化，结果表明：表面纯净的Ｐｄ／γＡｌ２Ｏ３催化剂对甲醇具有很好的解离吸附性能，在无氧条件下，ＣＨ３ＯＨ可在其上脱氢产生ＨＣＨＯ及ＣＯ。Ｐｄ对Ｏ２的吸附力强，氧均以解离方式吸附于Ｐｄ／γＡｌ２Ｏ３上。当吸附的［ＣＨ３Ｏ］与［Ｏ］在Ｐｄ／γＡｌ２Ｏ３上相互作用时，可产生ＨＣＨＯ、ＣＯ、甲二氧基、甲酸根等中间物种     

Ionic Association and Mobility IV. Ionophores in Dimethylsulfoxide at 25°C

The results of conductance measurements on pyridinium picrate, tetraphenylo-sonium picrate, potassium picrate, tetraphenylantimony picrate, tetrapropylam-monium, tetrafluoroborate, tetramethylammonium hexafluorophosphate ion association noncoulombic interaction in dimethyl sulfoxide (DMSO) at 25°C in the concentration range 1–15×10^–4 M are reported. The data were analyzed by the Justice modification of the Fuoss–Hsia equation. Except for pyridinium picrate all salts studied were found to be associated.Application of the Justice Barthel–Bjerrum model of ion association permitted calculation of the noncoulombic portion of the potential of mean force, W _±. Ionic limiting conductances were calculated for six ions using known values of previously determined transport numbers. A table of most current limiting ionic conductances for a variety of ions in DMSO at 25°C has been established.