Über die Kristallisationsgeschwindigkeit in unterkühlten binären Gemischen

Zusammenfassung Bei den Systemen Acetanilid:2,4-Dinitrophenol, Azobenzol:Benzil und Anästhesin:Benzil wurden die Isothermen der Kristallisationsgeschwindigkeit bei Unterkühlung bestimmt. Sowohl die Isotherme der Kristallisationsgeschwindigkeit der primären als auch die der sekundären Kristallisation besteht bei Unterkühlung aus zwei Teilkurven; während sich die ersteren schneiden, liegen die letzteren — mit mehr oder weniger großem Abstand — übereinander. Die Kurven für die primäre Kristallisation sinken in einem mittleren, vom Unterkühlungsgrad abhängigen Konzentrationsbereich unter die Kurven für die sekundäre Kristallisation, so daß sie zum Schnitt kommen. Der zwischen den Schnittpunkten liegende Kurvenast stellt die Isotherme für die Kristallisationsgeschwindigkeit der quasi-eutektischen Kristallisation dar. Das innerhalb dieses Kurvenastes liegende Minimum entspricht dem Schnittpunkt der beiden Primärkristallisations-Isothermen; es besitzt in der Regel eine von der eutektischen abweichende, vom Unterkühlungsgrad abhängige Konzentration. Das Verhalten der Isothermen steht im Einklang mit den Ergebnissen früherer Untersuchungen über die quasi-eutektische Synkristallisation. Die vonTammann undBotschwar an zwei der behandelten Systeme durchgeführten Untersuchungen werden diskutiert.

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Summary In the systems acetanilid:2,4-dinitrophenol; azobenzene:benzil; and anesthesin: benzil, the isotherms of the crystallization velocity with undercooling were determined. Both the isotherms of the crystallization velocity of the primary and the secondary crystallization with undercooling consists of two branch curves; whereas the former intersect, the latter lie above one another—with more or less distance between. The curves for the primary crystallization, in an intermediate concentration range, which is dependent on the degree of undercooling, sink below the curves for the secondary crystallization, so that they intersect. The branch of the curve lying between the points of intersection represents the isotherm for the crystallization velocity of the quasi-eutectic crystallization. The minimum situated within this branch of the curve corresponds to the intersection point of the two primary crystallization isotherms; as a rule, its concentration differs from that of the eutectic and is dependent on the degree of undercooling. The behavior of the isotherms is in agreement with the findings of previous studies of the quasi-eutectic syncrystallization. The studies byTammann andBotschwar of two of these same systems are discussed.

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Résumé Les isothermes des vitesses de cristallisation sont déterminées par refroidissement pour les systèmes acétanilide: dinitro-2,4 phénol, azobenzènebenzile, et anesthésine: benzile. Les isothermes de vitesse de cristallisation tant primaire que secondaire, se composent de deux parties de courbes, mais, tandis que les premières se coupent, les secondes sont l'une au-dessous de l'autre à intervalle plus ou moins grand. Les courbes de première cristallisation s'abaissent dans un domaine moyen de concentration dépendant du gradient de refroidissement au-dessous des courbes de seconde cristallisation et viennent, par conséquent, les couper. La branche de courbe située entre les points de contact représente l'isotherme pour la vitesse de cristallisation quasi-eutectique; le minimum de cette branche de courbe correspond au point de sécance des deux isothermes de cristallisation primaire; il possède, en général, une concentration anormale par rapport à l'eutectique et dépendant du taux de refroidissement. Le comportement des isothermes est en accord avec les résultats d'une recherche précédente sur la syncristallisation quasieutectique. On discute des recherches deTammann etBotschwar sur deux des systèmes examinés.

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Mit 3 Abbildungen.     

Preferential hydration of lysozyme in water/glycerol mixtures: a small-angle neutron scattering study

In solution small-angle neutron scattering has been used to study the solvation properties of lysozyme dissolved in water/glycerol mixtures. To detect the characteristics of the protein-solvent interface, 35 different experimental conditions (i.e., protein concentration, water/glycerol fraction in the solvent, content of deuterated compounds) have been considered and a suitable software has been developed to fit simultaneously the whole set of scattering data. The average composition of the solvent in the close vicinity of the protein surface at each experimental condition has been derived. In all the investigated conditions, glycerol resulted especially excluded from the protein surface, confirming that lysozyme is preferentially hydrated. By considering a thermodynamic hydration model based on an equilibrium exchange between water and glycerol from the solvation layer to the bulk, the preferential binding coefficient and the excess solvation number have been estimated. Results were compared with data previously derived for ribonuclease A in the same mixed solvent: even if the investigated solvent compositions were very different, the agreement between data is noticeable, suggesting that a unique mechanism presides over the preferential hydration process. Moreover, the curve describing the excess solvation number as a function of the solvent composition shows the occurrence of a region of maximal hydration, which probably accounts for the changes in protein stability detected in the presence of cosolvents.     

Physico-chemical and structural properties of hydrogels formed by chitosan, in the presence and absence of poly(vinylpyrrolidone) and sodium decylsulfate

The structure of chemically-crosslinked chitosan and chitosan-poly(vinylpyrrolidone) (PVP) hydrogels is investigated by means of the combined use of small-angle neutron scattering (SANS), electron paramagnetic resonance spectroscopy (EPR), intradiffusion, and swelling degree measurements. These hydrogels may be described in terms of an inhomogeneous structure composed by polymer-rich and polymer-poor regions. The polymer-rich regions, whose correlation distance zeta is ranged between approximately 600 and approximately 850 A, are, in turn, characterized by the presence of a network formed by the chemical crosslinks, with a mean correlation distance xi approximately 90 A. The structures of chitosan and chitosan-PVP hydrogels have also been analyzed in the presence of sodium decylsulfate micelles that could provide a multidomain system useful, in principle, for drug delivery applications. Both SANS and EPR measurements show that sodium decylsulfate micelles do not significantly interact with both the gels. Finally, intradiffusion and swelling degree measurements show an improved hydrophilicity of chitosan-PVP gels, even further magnified by the presence of C10OS surfactant.     

Elusive forms and structures of N-hydroxyphthalimide: the colorless and yellow crystal forms of N-hydroxyphthalimide

A comprehensive structural characterization of the colorless and yellow forms of N-hydroxyphthalimide (NHP), the deuterated form (NDP), and the ethoxylated form (ethoxy-NHP) has been carried out using single-crystal X-ray diffraction, FTIR and Raman spectroscopies, and scanning electron microscopy. Both NHP and NDP forms crystallize in the monoclinic space group (P21/c, No. 14). The various forms of NHP differ in the way in which the molecules adjoin one another through their N-hydroxyl groups and how the carbonyls of the isoindole-1,3-dione ring differ through intermolecular hydrogen bonding. Although the hydrogen bonding about the b axis is virtually the same, the isoindole-1,3-dione ring experiences different twists for the two NHP forms. Both the colorless and yellow forms of NHP exhibit strong intermolecular hydrogen bonding between O(3) and H(1). In the yellow form, the N-hydroxyl group is significantly out of the plane (approximately 1.19 degrees ), but the N-hydroxyl group in the colorless form is only approximately 0.06 degrees out of the plane. Both forms of NHP reveal an infinite chain of intermolecular hydrogen-bonded molecules in the direction of the b axis; however, the molecules are ordered differently within the unit cells. The hydrogen-bond geometry for the yellow form of NHP is O(2)-H(1)...O(3), with an angle of 185 degrees , intermolecular distances of O(2)...O(3) = 2.68 A and H(1)...O(3) = 1.70 A, and an intramolecular hydrogen bond of O(1)...H(1) = 1.17 A. The colorless form of NHP shows an intermolecular hydrogen-bond geometry between O(3) and H(1) with a distance of 1.78 A; the O(2)-O(3) distance is 2.71 A. The O(2)-H(1)...O(3) angle is 159 degrees, and the intramolecular distance is O(1)...H(1) = 0.97 A. The N-ethoxy derivative of NHP crystallizes in an orthorhombic space group (Pnma, No. 62) and exhibits no hydrogen bonding, displaying a strong head-to-tail stacking of the planar rings along the needle axis direction.     

An empirical constitutive law for concentrated colloidal suspensions in the approach of the glass transition

Concentrated, non-crystallizing colloidal suspensions in their approach of the glass state exhibit distinct dynamics patterns. These patterns suggest a powerlaw rheological constitutive model for near-glass viscoelasticity, as presented here. The rheological parameters used for this model originate in the mode-coupling theory. The proposed constitutive model provides explicit expressions for the steady shear viscosity, the steady normal stress coefficient, the modulus-compliance relation, and the α peak of G″. The relaxation pattern distinctly differs from gelation.     

Non-invasive identification of chemical compounds by energy dispersive X-ray fluorescence spectrometry, combined with chemometric methods of data evaluation

Chemicals from customers’ returns have to be analyzed before they can be reused as raw materials in production. A procedure for non-invasive qualitative analysis of compounds in a closed container based on energy dispersive X-ray fluorescence (EDXRF) spectrometry is described. EDXRF was chosen as method for non-invasive analysis of chemicals through PE bottle walls without opening the bottle. This analysis aims for a quick proof of correspondence between the declaration of a reagent on the label of the bottle and its content. This analytical result cannot be achieved by a mere evaluation of characteristic element lines in EDXRF-spectra in combination with the method of matrix correction or the method of mean atomic number. These methods take into account only a small part of the total information available in an X-ray spectrum. It is shown here that valuable additional information is extractable from the spectral ranges of the Compton-scattering and Rayleigh-scattering areas by the use of methods of multivariate data analysis, especially by principle component analysis (PCA). Regularized discriminant analysis (RDA) was employed to establish a classification scheme for unknown samples.     

Roseabol A,a New Peptaibol from the Fungus Clonostachys rosea

A new 11 amino acid linear peptide named roseabol A (1) and the known compound 13-oxo-trans-9,10-epoxy-11(E)-octadecenoic acid (2) were isolated from the fungus Clonostachys rosea. Combined NMR and MS analysis revealed that roseabol A (1) contained amino acid residues characteristic of the peptaibol family of peptides such as isovaline, α-aminoisobutyric acid, hydroxyproline, leucinol, and an N-terminal isovaleric acid moiety. The amino acid sequence was established by a combination of NMR studies and tandem MS fragmentation analyses, and the absolute configurations of the constituent amino acids of 1 were determined by the advanced Marfey’s method. Compound 2 showed inhibitory activity against Merkel cell carcinoma, a rare and difficult-to-treat type of skin cancer, with an IC₅₀ value of 16.5 μM.     

The surface structure of Fe3O4(1 1 1) films as studied by CO adsorption

We have studied adsorption of CO on Fe₃O₄(1 1 1) films grown on a Pt(1 1 1) substrate by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS). Three adsorption states are observed, from which CO desorbs at ∼110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at ∼2115-2140, 2080 and 2207 cm⁻¹, respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe₃O₄(1 1 1) surface is terminated by 1/2 ML of iron, with an outermost 1/4 ML consisting of octahedral Fe²⁺ cations situated above an 1/4 ML of tetrahedral Fe³⁺ ions, in agreement with previous theoretical calculations. The most strongly bound CO is assigned to adsorption to Fe³⁺ cations present on the step edges.