مجله علوم دانشگاه تهران
سال سی و سوم شماره 2 (تابستان 1386)

Today, the development of design and operation of industrial crystallization processes requires data on the kinetics of crystal growth and understanding the effect of operating conditions on kinetics and rate of crystal growth. In this paper, it is aimed to investigate the factors influencing on the mechanism of growth rate of potassium sulfate in the fluidized bed crystallizer. Several important parameters including the height bed, solution PH value and presence of impurity is considered. In fact, the quality (size, shape and purity) of the product is affected by many parameters influencing the growth mechanism such as hydrodynamic conditions, pH of the crystallizing solution, and the presence of additives and impurities. The results show that larger bed heights generate higher growth rates. Also, the variation of the PH value of the solution changes the growth rate significantly. In addition, one can conclude that the presence of small amounts of impurity material such as chromic ions can decrease the growth rate considerably.

4- Tertiarybutyl catechol (TBC) is classified as a two-valance phenolic antioxidant, which can be used in greases, industrial oils, and polyurethanes as polymerization inhibitor and thermal stablizer. This compound scavenges radicals and prevents the chain reaction from proceeding. Also this compound is used in BD (butadiene) unit as an antioxidant. In this paper we report a practical procedure for the synthesis of TBC using catechol and isobutylene in the presence of an ion exchange resin as a catalyst in good yield. Important parameters in the reaction are catalyst, temperature, and reaction time and flow gas. The reaction and product formation are monitored by thin layer chromatography, gas chromatography (GC), FT-IR and FT-HNMR spectroscopy. Reaction yield is 84.9% and products purity after distillation at low pressure above 95% and greater than purity of available standard TBC that equals to 95%. This purity is high enough for industrial application of TBC.

A pullulans classified under the family Dothideales. In present study different strains isolated from nature and the best strain with a large amount of pullulan production (51) is selected for optimization. Amount of pullulan and its purity determind by pullulanase enzyme activity. Sucrose and glucose select as source of carbon and this strain produced 40 g/l pullulan on sucrose. The best yield was obtained on 5% sucrose. Production of pullulan under different nitrogen source (yeast extract, ammonium sulphate, sodium nitrate), pH (4- 9), temperature (25, 28, 32 °C) and shacking (120,180,200 rpm) by this strain were investigated. Under the best condition 48g/l pullulan was produced within 5 days. A small pigment in the medium was observed during the. fermentation.

This study investigated the influence of the molecular weight on the thermal stability of poly (vinyl chloride) (PVC) under nitrogen atmosphere was studied by bromometry titration at various temperatures (160-190oC).The extent of dehydorchlorination (%conversion) was calculated from the ratio of the HCl evolved to the amount available in the polymer. The intrinsic viscosities of solutions to deduce the effect of the molecular weight on the degradation of polymer were calculated.In addition to the course of the dehydrochlorination, the spectral changes in the visible and infrared regions have been investigated. It was found that the number- average molecular weight (45.000, 104000) slightly affected the thermal dehydrochlorination.The experimental results also indicated the relationships between molecular weight and structural defects.

A pullulans classified under the family Dothideales. In present study different strains isolated from nature and the best strain with a large amount of pullulan production (51) is selected for optimization. Amount of pullulan and its purity determind by pullulanase enzyme activity. Sucrose and glucose select as source of carbon and this strain produced 40 g/l pullulan on sucrose. The best yield was obtained on 5% sucrose. Production of pullulan under different nitrogen source (yeast extract, ammonium sulphate, sodium nitrate), pH (4- 9), temperature (25, 28, 32 °C) and shacking (120,180,200 rpm) by this strain were investigated. Under the best condition 48g/l pullulan was produced within 5 days. A small pigment in the medium was observed during the. fermentation.

In this research using starting materials of ferric nitrate, strontium nitrate, citric acid and trimethylamine and by the sol-gel auto-combustion method, was produced strontium hexaferrite powder. The effect of adding the hydrocarbon of tetrabutylammonium bromide and surfactant of n-decyltrimethyl ammonium bromide on the formation temperature and particles size of resultant powder was investigated. FTIR results showed the Occurrence of more complete combustion reaction in the samples with hydrocarbon and surfactant. According to the XRD results, single-phase strontium hexaferrite is formed in sample without additive by calcination at 900 °C, while in samples with hydrocarbon and surfactant formed at 850 and 800 °C, respectively. With addition of surfactant, grains and particles size decreased significantly.

Clinoptiloite transforms to analcime under hydrothermal condition in presence of alkaline fluids. The effect of Na+ and K+ concentrations, temperature and run duration are being considered as the variable parameters in the present study. Alkaline solutions were used in concentration of 1.25 up to 7.5mol/lit under temperatures of 100 and 150? C. Results of this study show that analcime is synthesized at 150?C and high concentration of Na+. With increasing Na+ content of the fluids and reaction time, the amount of analcime increases drastically. Powder X-ray diffraction study has revealed an orthorhombic symmetry with the unit-cell parameter of a: 13.731? , b: 13.726? , c: 13.769? .

Infrared and Raman spectra of the nonaaqua yttrium(III) bromate, Y(H2O)9(BrO3)3, compound have been recorded. Force field calculations have been performed for YO93+ moiety, with a tricapped trigonal prismatic oxygen atom arrangement in D3h point group symmetry. The larger stretching force constant was found, 90.7 Nm-1, for the Y-O prism in comparison with the corresponding value in the capped Y-O, 56.5 Nm-1.

A novel method for extraction and analysis of volatile compounds of Artemisia Haussknechtii Boiss, using simultaneous hydro-distillation and headspace liquid microextraction followed by gas chromatography–mass spectrometry (HD-HLPME-GCMS) is developed. Headspace liquid phase microextraction was performed in two modes of static and dynamic. Then, effective parameters of the both optimized. Comparison of these methods with hydro-distillation alone showed that HD-HLPME is fast, simple, inexpensive and effective for the analysis of volatile compounds of aromatic plants. Also, dynamic method represented higher efficiency than static mode. Finally, fifty six compounds were extracted and identified for Artemisia Haussknechtii Boiss. by dynamic-HLPME. The main constituents of the essential oil, extracted by this method, includes camphor (40.83%), 1,8-cineole (26.84%), cis-davanone (4.77%), linalool (4.44%), 4-terpineol (3.62%), beta-fenchyl alcohol (3.52%), borneol (2.87%) and comphene (1.34%).

The structure of the cis-dichlorotetrakis(dimethyl sulfoxide)ruthenium(II) have been studied by means of single-crystal X-ray diffraction. The data are described in orthorhombic unit cell with space group Pccn (No. 56) at 295 K, a = 28.478(4)? , b = 10.900(6)? , c = 11.697(13)? , and V = 3631(4)? 3, Z = 8. The crystal structure of the title compound comprises a nearly octahedral configuration with a Ru-O bond distance of, 2.151? , and the average values of, 2.269? and 2.421? for Ru-S and Ru-Cl, respectively.

This paper describes the preparation of gypsum tlcs and shows the utility of this type of tlcs for a variety of organic compounds and finally obtains the relation between gypsum tlc and column chromatography (filled by gypsum).

We report a first-principles study of electrical transport in a single molecular conductor consisting of ethane-dithiol (C2H4S2) sandwiched between two Au (100) electrodes. We show that the current was increase with increasing the external voltage biases. The projected density of states (PDOS) and transmission coefficients under some external voltage biases are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to the increase of the current. Therefore, we propose that the most origin of electron transport mechanism in molecular devices is caused by the characteristics of the molecule and the electrodes as well as their cooperation. Furthermore, the transport properties of the buthane-dithiol (C4H6S2) molecular wire are also investigated. The results show that the buthane-dithiol molecular wire has a lower conductivity than the ethane-dithiol ones at a given external biases thus; it is a better insulator device for the molecular electronics technology.

In this work a novel method for the determination of tamoxifen in flow-injection systems has been developed. The fast Fourier transform continuous cyclic voltammetry (FFTCV) at gold microelectrode in flowing solution system was used for determination of tamoxifen in its pharmaceutical formulations. The developed technique is very simple, precise, accurate, time saving and economical, compared to all of the previously reported methods. The effects of various parameters on the sensitivity of the method were investigated. The best performance was obtained with the pH value of 2, scan rate value of 50V s-1, accumulation potential of 0 mV and accumulation time of 0.5 s. The proposed method has some advantages over other reported methods such as, no need for the removal of oxygen from the test solution, a picomolar detection limit, and finally the method is fast enough for the determination of any such compound, in a wide variety of chromatographic methods. The method was linear over the concentration range of 1875-78000 pg/ml (r = 0.9980) with a limit of detection and quantitation 37.1 and 200 pg/ml, respectively. We used this method to determination of tamoxifen in tablets.