A nucleophilic substitution reaction between 4-tert-butylbenzyl bromide and a series of iodide salts has been performed in oil-in-water microemulsions based on either a fatty alcohol ethoxylate or a sugar surfactant. The reaction kinetics was compared with the kinetics of the same reaction performed in a microhomogeneous reaction medium, d-MeOH. Previous results showing a particularly high reactivity in the microemulsion based on the fatty alcohol ethoxylate was confirmed. It was shown that in both microemulsions the reaction rate was almost independent of the choice of counterion to iodide. This indicates that complexation of the cation with the surfactant headgroup, which, in particular, could have taken place with surfactants containing oligooxyethylene chains (a “crown ether effect”), seems not to be of importance.
127I NMR studies, as well as quadrupole splitting experiments performed by 2H NMR, indicate that there is a certain accumulation of iodide at the oil–water interface of the microemulsions. It is difficult to draw any quantitative conclusions in this respect, however.
The results obtained in this study, combined with results from previous investigations of the same reaction, indicate that the unexpectedly high reactivity obtained in the microemulsion based on a surfactant containing an oligooxyethylene headgroup is most probably due to the nucleophile being poorly solvated when present in the headgroup layer of such a microemulsion. Poorly solvated anions are known to be highly reactive nucleophiles. 相似文献
Characterization of autocatalytic decomposition reactions is important
for the safe handling and storage of energetic materials. Isothermal differential
scanning calorimetry (DSC) has been widely used to detect autocatalytic decomposition
of energetic materials. However, isothermal DSC tests are time consuming and
the choice of experimental temperature is crucial. This paper shows that an
automatic pressure tracking calorimeter (APTAC) can be a reliable and efficient
screening tool for the identification of autocatalytic decomposition behavior
of energetic materials.
Hydroxylamine nitrate (HAN) is an important
member of the hydroxylamine family. High concentrations of HAN are used as
liquid propellants, and low concentrations of HAN are used primarily in the
nuclear industry for decontamination of equipment. Because of its instability
and autocatalytic decomposition behavior, HAN has been involved in several
incidents. 相似文献
Measurements on the inverse filtered airflow waveform (the "glottal waveform") and of estimated average transglottal pressure and glottal airflow were made from noninvasive recordings of productions of syllable sequences in soft, normal, and loud voice for 25 male and 20 female speakers. Statistical analyses showed that with change from normal to loud voice, both males and females produced loud voice with increased pressure, accompanied by increased ac flow and increased maximum airflow declination rate. With change from normal voice, soft voice was produced with decreased pressure, ac flow and maximum airflow declination rate, and increased dc and average flow. Within the loudness conditions, there was no significant male-female difference in air pressure. Several glottal waveform parameters separated males and females in normal and loud voice. The data indicate higher ac flow and higher maximum airflow declination rate for males. In soft voice, the male and female glottal waveforms were more alike, and there was no significant difference in maximum airflow declination rate. The dc flow did not differ significantly between males and females. Possible relevance to biomechanical differences and differences in voice source characteristics between males and females and across loudness conditions is discussed. 相似文献
A series of novel nonionic surfactants based on fatty acids, each with two hydrophilic and two hydrophobic groups in the molecule (so-called Gemini surfactants), have been synthesized. The hydrophobic part of the surfactant, made from oleylnitrile, has a double bond in the middle of the chain to which the hydrophilic part is attached. One of the hydrophilic groups is a methyl-capped polyoxyethylene chain with 11 or 16 oxyethylene units, whereas the other is a secondary hydroxyl group. The reason for using the nitrile derivative of the fatty acid instead of a more conventional derivative such as an ester or amide is to achieve good hydrolytic stability, which is often demanded for cleaning applications. The cmc values of the surfactants were determined to be 0.2 and 0.4 mM for NIHG550 and NIHG750, respectively, using tensiometry and fluorescence. Pulsed-field gradient spin-echo NMR measurements gave similar but slightly higher values. The micelle size of NIHG750 was estimated to be on the order of 40 ?, as calculated from the self-diffusion coefficient. The dynamic surface tension, gamma(t), was measured in the range 1 ms-1 s using a maximum bubble pressure instrument and analyzed in terms of asymptotic solutions to the Ward and Tordai equation. The results suggest that at the beginning the adsorption is essentially diffusion-controlled. However, close to equilibrium, the DST decays are not consistent with a diffusion-controlled adsorption mechanism. One important result from the present work is that the heterogeminis seem to align better than conventional surfactants at the air-liquid and solid-liquid interfaces. Copyright 2001 Academic Press. 相似文献
The reaction between 4-tert-butylbenzyl bromide and potassium iodide was carried out in microemulsions based on different nonionic surfactants, and the reaction rates were compared with those obtained in two-phase systems with added phase-transfer agent, either a quaternary ammonium salt or a crown ether. The reactions were relatively fast in the microemulsions and extremely sluggish in the two-phase systems without additional phase-transfer agent. Addition of a phase-transfer agent did not accelerate the reaction when a hydrocarbon was used as organic solvent, neither in the two-phase system nor in the microemulsion. When a chlorinated hydrocarbon was used as solvent, phase-transfer catalysis became effective and the rate obtained in the two-phase system with an equimolar amount of phase-transfer agent added was higher than that obtained in the microemulsion. When a catalytic amount of phase-transfer agent was used, the rate in the two-phase system was about the same as the rate obtained in the microemulsion without the phase-transfer agent. The combined approach, that is, use of a microemulsion as the reaction medium and addition of a phase-transfer agent, gave the highest reaction rate. The quaternary ammonium salt (tetrabutylammonium hydrogen sulfate) was a more efficient catalyst in the microemulsion system than the crown ether ([18]crown-6). 相似文献
Enzymatic hydrolysis of a model triglyceride, palm oil, was carried out with lipase fromRhizopus sp. in microemulsions with varying water content. The microemulsions were based on a nonionic surfactant, pentaethylene glycol monododecyl ether (C12 EO5), buffered water solution and an oil component consisting of isooctane and palm oil at a weight ratio of 20:1. The structure of the microemulsions was characterized using Fourier transform pulsed-gradient spin-echo1H NMR. The rate of reaction decreased as the water content of the reaction medium was increased. The self-diffusion coefficient of water, Dw was found to be constant within the interval 1–20% water. The difference in reactivity is believed to be due to a difference in structure of the palisade layer between water and hydrocarbon microdomains. The nonionic surfactant was demonstrated to be unsuitable for enzymatic reactions since only partial hydrolysis was obtained in all experiments. The surfactant, however, did not cause enzyme deactivation, even at very high concentrations. 相似文献
Pretreatment of human cells with near UV radiation (UVA) in fluences exceeding 5 × 104 Jm−2 caused a decrease in the amount of the unscheduled DNA synthesis induced by far UV radiation (UVC). The DNA repair synthesis, as measured by the incorporation of [3H] -thymidine, is reduced by nearly a factor of 2 for a UVA radiation exposure of 1.5 × 105 Jm−2. Since solar UVA fluence rate is rather independent of latitude, this figure corresponds to a UVA exposure time of 50-60 min from noon sunlight in the summer time. 相似文献