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Diffusion-controlled Mechanism (diffusion-controlled + mechanism)
Selected AbstractsIn-situ Energy-Dispersive X-ray Diffraction Studies of Crystal Growth and Compound Conversion Under Solvothermal ConditionsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2003Lars Engelke Abstract The results of in-situ energy-dispersive X-ray diffraction under solvothermal conditions performed on isostructural, layered thioantimonates Mn2Sb2S5·L (L = amine) demonstrate the great potential of the method. When the synthesis was carried out at low temperatures with L being 1,3-diaminopropane (DAP), two crystalline intermediate phases were detected which then grew and disappeared when product growth started. Surprisingly, when N -methyl-1,3-diaminopropane (MDAP) was used, no crystalline intermediates could be detected and the induction time was significantly shorter than for DAP. For reactions up to 100 °C and for higher temperatures with , < 0.8 (, is the extent of reaction), the crystallisation is predominantly controlled by nucleation. Further progress of crystallisation (, > 0.8) leads to a change of the dominant process and a diffusion-controlled mechanism is observed. During the reaction with diethylenetriamine (DIEN), three crystalline intermediates were detected prior to product growth. The induction time is longer than for the other two compounds. The crystallisation seems to be diffusion-controlled and is faster than for the DAP and MDAP compounds. In a solution of DIEN, the crystalline phases Mn2Sb2S5·L (L = DAP or MDAP) are transformed into the DIEN product under solvothermal conditions, and a rigorous analysis of the intensities of the reflections suggests a partial dissolution of the crystalline starting materials followed by crystallisation of the DIEN material. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Preparation and characterization of quercetin-loaded polymethyl methacrylate microcapsules using a polyol-in-oil-in-polyol emulsion solvent evaporation methodJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 12 2007Dong-Hwan Lee Flavonoids and related compounds exhibit a wide range of useful pharmacological properties but present challenges related to their stability and solubility in commonly available solvents. In this study, polymethyl methacrylate (PMMA) microcapsules were prepared using a novel polyol-in-oil-in-polyol (P/O/P) emulsion solvent evaporation method as a means of stabilizing the flavonoids, using quercetin as a model flavonoid drug. The morphology of the microcapsules was evaluated using a scanning electron microscope, revealing a spherical shape with a smooth surface. The cross-section image of the PMMA microcapsules prepared with an amphiphilic polymer in the inner polyol phase showed that the microcapsule was filled with several submicron microspheres. The mean diameter varied from 1.03 ± 0.12 ,m to 2.39 ± 0.42 ,m, and the encapsulation efficiency ranged from 12.7% to 26.9%. When free quercetin was stored at 42°C, the residual quercetin content gradually decreased to 18% over 28 days as a result of oxidation. However, when encapsulated in PMMA microcapsules with an amphiphilic polymer in the inner polyol phase, the residual quercetin content decreased to just 82%. In-vitro release studies indicated a sustained release pattern throughout the 36-h study. The release kinetics of the microcapsules with an amphiphilic polymer followed a diffusion-controlled mechanism and the microcapsule without amphiphilic polymer followed an anomalous diffusion behaviour. This study suggests that the novel P/O/P emulsion solvent evaporation method can be applied to the encapsulation of flavonoids. [source] Electrochemical bleaching of kraft bagasse pulp using a new cell designTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2010El-Sayed Z. El-Ashtoukhy Abstract In situ single-stage electrochemical bleaching of kraft bagasse pulp was carried out in a cylindrical agitated vessel fitted with four graphite rod anodes and a cylindrical stainless steel screen cathode, using NaCl as an electrolyte. The effect of current density, pH, NaCl concentration, impeller rotational speed, temperature, and pulp consistency on the rate of bleaching was studied. It was found that the rate of bleaching increased with increasing current density, NaCl concentration, and temperature and decreased with increasing pH and pulp consistency. The effect of temperature was found to fit Arrhenius equation with an activation energy of 0.515,kcal/mol, which denotes a diffusion-controlled mechanism. Energy consumption (EC) calculation showed that EC ranged from 0.225 to 3.11,kWh/kg dry pulp depending on the current density. The strength of bleached pulp was little affected by bleaching lying within an acceptable range. Le blanchiment électrochimique à une étape in situ de pâte de bagasse kraft a été effectué dans un réservoir cylindrique agité doté d'une anode sous forme de tige au graphite (4) et de cathode sous forme de treillis cylindrique en acier inoxydable, utilisant le NaCl comme électrolyte. L'effet de la densité du courant, du pH, de la concentration de NaCl, de la vélocité rotationnelle de la roue et de la température de la consistance de la pâte sur le taux de blanchiment a été étudié. L'effet de la température s'est avéré s'inscrire dans l'équation d'Arrhenius avec une énergie d'activation de 0,515 kcal/mol, ce qui dénote un mécanisme de diffusion contrôlé. Le calcul de consommation d'énergie montre que la consommation d'énergie se situe de 0,225 à 3,11,kWh/kg de pâte sèche, selon la densité du courant. La résistance de la pâte blanchie est peu touchée par le blanchiment et se situe en deçà de la plage acceptable. [source] Dynamic Surface Tensions of Fluorous Surfactant SolutionsCHINESE JOURNAL OF CHEMISTRY, Issue 4 2005Gao Yan-An Abstract Dynamic surface properties of aqueous solutions of cationic fluorous surfactant CF3CF2CF2O(CF(CF3)CF2O)2CF(CF3)CONH(CH2)3N+(C2H5)2CH3I - (abbrev. FC-4 ) were reported. The critical micelle concentration (cmc) (3.6×10 -5 mol/L) and equilibrium surface tensions ,eq were measured by Krüss K12 tension apparatus. Dynamic surface tension ,(t) was measured in the range of 15 ms to 200 s using the MBP tensiometer. The surface excess ,, as a function of concentration, was obtained from equilibrium tensiometry using the Gibbs equation. Data from these experiments were combined to analyze the ,(t) decays according to the asymptotic Ward and Tordai equation. The results show that at the initial adsorption stage, the dynamic surface tension data were all consistent with this diffusion-controlled mechanism, and at the end of the adsorption process, there were some evidences for an adsorption barrier, suggesting a mixed diffusion-controlled adsorption mechanism. Using measured quantities, the barrier strength was estimated as between 25 and 35 kJ/mol at 25 °C. The surface pressure plays an important role in contributing to the barrier. [source] | ||||||||||