10 research outputs found

    Multidiagnostic analysis of silicate speciation in clear solutions/sols for zeolite synthesis

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    The formation of zeolites in presence of tetraalkylammonium cations from so-called clear solutions using silicon alkoxides is a highly complex process which challenges experimental chemistry. Most clear solutions are better described as clear sols as they contain nanosized silicate particles, which are formed during hydrolysis of the Si source before self-assembly into the zeolite framework. This process spans multiple time- and length-scales and only a combination of different analysis methods allows revelation of molecular level zeolite formation mechanisms. On the example of the early stages of the formation of zeolite beta from clear solutions/sols the different windows of observation of liquid-state 29Si and 27Al nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and mass spectrometry (MS) are demonstrated. Each diagnostic means by itself needs to be carefully assessed for its window of temporal and spatial resolution which can be achieved by exploiting the overlapping information available from their combination

    Revisiting Silicalite‑1 Nucleation in Clear Solution by Electrochemical Impedance Spectroscopy

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    Electrochemical impedance spectroscopy (EIS) was used to detect and investigate nucleation in silicalite-1 clear solutions. Although zeolite nucleation was previously assumed to be a step event, inducing a sharp discontinuity around a Si/OH<sup>–</sup> ratio of 1, complex bulk conductivity measurements at elevated temperatures reveal a gradual decay of conductivity with increased silicon concentrations. Inverse Laplace transformation of the complex conductivity allows the observation of the chemical exchange phenomena governing nanoaggregate formation. At low temperatures, the fast exchange between dissociated ions and ion pairs leads to a gradual decay of conductivity with an increasing silicon content. Upon heating, the exchange rate is slower and the residence time of ion pairs inside of the nanoaggregates is increasing, facilitating the crystallization process. This results in a bilinear chemical exchange and gives rise to the discontinuity at the Si/OH<sup>–</sup> ratio of 1, as observed by Fedeyko et al. EIS allows the observation of slow chemical exchange processes occurring in zeolite precursors. Until now, such processes could be observed only using techniques such as nuclear magnetic or electron paramagnetic resonance spectroscopy. In addition, EIS enables the quantification of interfacial processes via the double layer (DL) capacitance. The electrical DL thickness, derived from the DL capacitance, shows a similar gradual decay and confirms that the onset of nanoaggregate formation is indeed not narrowly defined

    Rivaroxaban versus enoxaparin for thromboprophylaxis after hip arthroplasty.

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    Rivaroxaban versus enoxaparin for thromboprophylaxis after hip arthroplasty

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    BACKGROUND: This phase 3 trial compared the efficacy and safety of rivaroxaban, an oral direct inhibitor of factor Xa, with those of enoxaparin for extended thromboprophylaxis in patients undergoing total hip arthroplasty. METHODS: In this randomized, double-blind study, we assigned 4541 patients to receive either 10 mg of oral rivaroxaban once daily, beginning after surgery, or 40 mg of enoxaparin subcutaneously once daily, beginning the evening before surgery, plus a placebo tablet or injection. The primary efficacy outcome was the composite of deep-vein thrombosis (either symptomatic or detected by bilateral venography if the patient was asymptomatic), nonfatal pulmonary embolism, or death from any cause at 36 days (range, 30 to 42). The main secondary efficacy outcome was major venous thromboembolism (proximal deep-vein thrombosis, nonfatal pulmonary embolism, or death from venous thromboembolism). The primary safety outcome was major bleeding. RESULTS: A total of 3153 patients were included in the superiority analysis (after 1388 exclusions), and 4433 were included in the safety analysis (after 108 exclusions). The primary efficacy outcome occurred in 18 of 1595 patients (1.1%) in the rivaroxaban group and in 58 of 1558 patients (3.7%) in the enoxaparin group (absolute risk reduction, 2.6%; 95% confidence interval [CI], 1.5 to 3.7; P<0.001). Major venous thromboembolism occurred in 4 of 1686 patients (0.2%) in the rivaroxaban group and in 33 of 1678 patients (2.0%) in the enoxaparin group (absolute risk reduction, 1.7%; 95% CI, 1.0 to 2.5; P<0.001). Major bleeding occurred in 6 of 2209 patients (0.3%) in the rivaroxaban group and in 2 of 2224 patients (0.1%) in the enoxaparin group (P=0.18). CONCLUSIONS: A once-daily, 10-mg oral dose of rivaroxaban was significantly more effective for extended thromboprophylaxis than a once-daily, 40-mg subcutaneous dose of enoxaparin in patients undergoing elective total hip arthroplasty. The two drugs had similar safety profiles. (ClinicalTrials.gov number, NCT00329628
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