Interaction forces between cellulose microspheres and ultrathin cellulose films monitored by colloidal probe microscopy-effect of wet strength agents

Colloidal probe microscopy was employed to study forces between cellulose surfaces upon addition of a series of cationic copolymers in aqueous solution, as model compounds for wet strength agents. The content of quaternary ammonium groups and primary amines was systematically varied in the cationic polymers, to distinguish between the importance of electrostatical and H-bonding effects. Cellulose microspheres were glued at the apex of tipless microfabricated cantilevers and used as colloidal probes. Ultra thin cellulose films and cellulose fibres were employed as model surfaces. The cellulose films of a thickness of about 5 nm were spin-coated from cellulose solution onto silicon substrates. The root-mean-square-roughness (RMS) was 0.3-0.8 nm. The cationic model polymers were compared to Servamine, a polymer employed as standard wet strength resin in papermaking industries. Force versus separation measurements showed a detailed picture of adhesion and contact breaking. Relatively strong adhesion of the order of 0.3 mJ/m(2) was observed with Servamine within a range of approximately 10 nm. At larger distances weak bond breaking and elastic chain pulling were identified. When approaching the surface one to two small jump-in's possibly related to strong binding of Servamine and subsequent attraction could be found in the case of Servamine. In contrast, all the model copolymers showed only a weak adhesion of 8-30 micro/m(2), i.e., an order of magnitude less than that of Servamine and subsequent elastic rupture domains. The contour length, persistence length and characteristic rupture distances were calculated by means of applying the WLC model. Measurements against cellulose fibres obtained from the production process proved the relevance of the model systems.     

Analysis and Simulation of the Dynamics of a Catalyzed Model Reaction: CO Oxidation on Zeolite Supported Palladium

The particle size effect on the oscillatory behavior during CO oxidation over zeolite-supported Pd catalysts is simulated with the help of a deterministic point model and a stochastic mesoscopic model. The point model is developed on the basis of the well-known Sales–Turner–Maple model, which is modified to consider the slow processes of oxidation and reduction of the Pd bulk as well as the effects of the bulk oxidation on the catalyst activity. It is demonstrated that the point model developed can simulate many experimental trends, e.g., the dependence of the catalytic activity and the waveform of the oscillations on the particle size and the pretreatment of the catalyst, as well as the counterclockwise hysteresis, depending on the reaction rate during the cyclic variation of the CO inlet concentration. The higher activity of the smaller particles can be explained by the attainment of a more reduced state of Pd in smaller particles in the course of the reaction. The stochastic model simulates the reaction by a Markovian chain of elementary stages of the reaction. The model variables are the numbers of reagent atoms. Transition probabilities of the stochastic model are chosen in accordance with the rates of the developed point model. It is shown that intrinsic fluctuations and correlations of stochastic variables can significantly change the reaction dynamics on nm-sized particles by extending the oscillatory region in the parameter space.     

Reactions in microemulsion media. Borohydride reduction of mono- and dicarbonyl compounds

Microemulsions were prepared at 26° from mixtures of hexanes (O), a 50: 1 (w/w) solution (W) of 0.1 M KOH-NaBH₄, and a 1.23:1 (w/w) mixture (S) of hexadecyltrimethylammonium bromide (HTABr) and 1-butanol. A pseudoternary phase map contained a significant microemulsion (μE) region, and μE's A and B (60:35:5 and 20:10:70 S:W:O, respectively) were used for reduction of several monocarbonyl compounds [benzophenone (1a), benzaldehyde (2a), acetophenone (3a), and 1-phenyl-1-octadecanone (4a)], an α,β-unsaturated ketone [trans-4-phenyl-3-buten-2-one (6a)], and a diketone [4-(4'-benzoylphenyl)-2-butanone (7a)]at 26°. For comparison purposes, reductions were also performed in aqueous 2-propanol (2-PrOH A and 2-PrOH B) prepared by the substitution of 2-propanol for the S and O components of μE's A and B. Generally, the reductions were slightly faster in the microemulsion media than in the corresponding aqueous 2-propanol media. The significantly slower reduction of 4a relative to that of 3a in μE B indicated that the interphase is the reactive site. With enone 6a, the influence of microemulsions on the competition between 1,2- and 1,4-reduction was determined. In μE's A and B there was 8% and ll% 1,4-reduction, respectively, whereas in 2-PrOH A and B there was only a trace. With diketone 7a, the reactivity of the aromatic carbonyl group relative to that of the aliphatic carbonyl group increased on going from 2-PrOH A and B to μE's A and B, respectively. For the sodium borohydride reduction of ketones, microemulsion catalysis is more effective than phase transfer catalysis or the use of a tetraalkylammonium borohydride in a hydrocarbon solvent.     

Cascade of coil-globule conformational transitions of single flexible polyelectrolyte molecules in poor solvent

We show that hydrophobic flexible polyelectrolyte molecules of poly(2-vinylpyridine) and poly(methacryloyloxyethyl dimethylbenzylammonium chloride) are trapped and frozen due to adsorption on the mica surface, and the observed AFM single molecule structures reflect the molecular conformation in solution. An increase of the ionic strength of the solution induces the cascade of abrupt conformational transitions due to the intrachain segregation from elongated coil to compact globule conformations through intermediate pearl necklace-globule conformations with different amounts of beads per chain. The length of the necklaces and the number of beads decrease, while the diameter of beads increases with the increase of ionic strength. Coexistence at the same time of extended coils, necklaces with different amounts of beads, and compact globules indicates the cascade of the first-order-type phase transitions.     

Selective quantification of doxycycline in human plasma and urine with optimised chromatography

Summary A method for the selective quantification of doxycycline in human plasma and urine has been developed using HPLC with UV-detection at 350 nm. Analyte and internal standard were extracted from plasma by extraction columns filled with octadecylsilica. Urines were only mixed with diluted acid prio to injection. The influence of pH on peak shapes is discussed as well as comparative investigations on selectivity and peak symmetry on commerical octadecylsilica. The method was successfully applied to the samples of a clinical study with an oral single dose of 100 mg. Precision and accuracydata of the assay and calculated pharmacokinetic parameters are presented.     

Über Blei(II)-oxid- und -hydroxidchloride

Inhaltsübersicht. Bei der Reaktion von tetragonalem Blei(II)-oxid in wäßriger Suspension mit Calciumchlorid oder mit Ammoniumchlorid lassen sich für molare Verhältnisse Cl^–/PbO bis zu 4/1 nur drei kristalline basische Blei(II)-chloride in der festen Phase mit den Pb/Cl-Verhältnissen von 3,5, 2,0 bzw. 1,0 nachweisen. Lead(II) Oxide and Hydroxide Chloride Abstract. In the reaction between tetragonal lead(II) oxide in aqueous suspension and calcium chloride, or ammonium chloride, and for molar ratios of Cl^–/PbO up to 4/1 only three kinds of crystalline basic lead chlorides are detectable in the solid phase with Pb/Cl ratios of 3.5, 2.0 and 1.0, respectively.