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71.
Frunza L Schönhals A Frunza S Parvulescu VI Cojocaru B Carriazo D Martín C Rives V 《The journal of physical chemistry. A》2007,111(24):5166-5175
The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma0 in detail gives some hint that sigma0(T) has also an unusual saddlelike form. 相似文献
72.
Preparation, characterization, and Zn(2+) adsorption behavior of chemically modified MCM-41 with 5-mercapto-1-methyltetrazole 总被引:2,自引:0,他引:2
Pérez-Quintanilla D Sánchez A del Hierro I Fajardo M Sierra I 《Journal of colloid and interface science》2007,313(2):551-562
A mesoporous silica (MCM-41) has been chemically modified with 5-mercapto-1-methyltetrazole using the homogeneous route. This synthetic route involved the reaction of 5-mercapto-1-methyltetrazole with 3-chloropropyltriethoxysilane prior to immobilization on the support. The resulting material (MTTZ-MCM-41) has been characterized by powder X-ray diffraction, nitrogen gas sorption, FT-IR and MAS NMR spectroscopy, thermogravimetry, and elemental analysis. The solid was employed as a Zn(II) adsorbent from aqueous solutions at room temperature. The effect of several variables (stirring time, pH, metal concentration, addition of ethanol, presence of other metals in the medium) has been studied using batch and column techniques. Flame atomic absorption spectrometry was used to determine the Zn(II) concentration in the filtrate or in the eluted solution after the adsorption process. Results obtained indicate that under the optimum conditions (pH 8 and 2 h stirring time), the maximum adsorption value for Zn(II) was 1.59+/-0.01 mmol/g, whereas the adsorption capacity of the unmodified mesoporous silica was about 0.010+/-0.001 mmol/g. On the other hand, the Zn(II) adsorption on the MTTZ-MCM-41 was independent of the presence of ethanol and other metals (Cu(II), Mn(II), Ca(II), and Mg(II)) in the medium. Finally, experiments carried out in order to study the regeneration capacity of the MTTZ-MCM-41 revealed that the adsorption capacity of this material was maintained after 3 cycles of the adsorption/desorption process. 相似文献
73.
Juan Carlos Sandoval‐Santana Victor Guadalupe Ibarra‐Sierra Jos Luis Cardoso Alejandro Kunold Pedro Roman‐Taboada Gerardo Naumis 《Annalen der Physik》2019,531(8)
Time‐driven quantum systems are important in many different fields of physics as cold atoms, solid state, optics, etc. Many of their properties are encoded in the time evolution operator or the effective Hamiltonian. Finding these operators usually requires very complex calculations that often involve some approximations. To perform this task, a systematic scheme that can be cast in the form of a symbolic computational algorithm is presented. It is suitable for periodic and non‐periodic potentials and, for convoluted systems, can also be adapted to yield numerical solutions. The method exploits the structure of the associated Lie group and a decomposition of the evolution operator on each group generator. To illustrate the use of the method, five examples are provided: harmonic oscillator with time‐dependent frequency (Paul trap), modulated optical lattice, time‐driven quantum oscillator, a step‐wise driving of a free particle, and the non‐periodic Caldirola‐Kanai Hamiltonian. To the extent of the authors' knowledge, whereas the exact form of Paul trap's evolution operator is well known, its effective Hamiltonian was until now unknown. The remaining four examples accurately reproduce previous results. 相似文献
74.
Damián Pérez-Quintanilla Alfredo Sánchez Isabel del Hierro Mariano Fajardo Isabel Sierra 《Mikrochimica acta》2009,165(3-4):291-298
A mesoporous silica has been chemically modified with 5-mercapto-1-methyltetrazole. The newly synthesized material was characterized by powder X-ray diffraction, N2 adsorption, FT-IR, 13C-NMR spectroscopy and elemental analysis, and used to preconcentrate Pb(II) from aqueous solutions. The effect of several variables on the adsorption capacity (i.e. stirring time, pH, interfering ions, presence of other heavy metals in the medium, etc.) has been studied using batch and column techniques. The adsorption capacity of the material followed the order: Pb(II) >> Cu(II) > Cd(II) >>Mn(II) > Ni(II) > Co(II). In column experiments a pre-concentration factor of 200 was obtained for Pb(II). Spiked tap water, mineral water and river water were used for the preconcentration and determination of Pb(II) by flame atomic absorption spectrometry, and a 101–103% recovery was obtained. The limit of detection and quantification values of the method were found to be 2.22·10?6 mM and 8.20·10?6 mM, respectively. The relative standard deviation for four preconcentration experiments was found to be ≤9% in all cases. 相似文献
75.
Thermal analysis of vitamin PP Niacin and niacinamide 总被引:1,自引:0,他引:1
Elaine C. P. Moreschi Jivaldo R. Matos Ligia B. Almeida-Muradian 《Journal of Thermal Analysis and Calorimetry》2009,98(1):161-164
Vitamin PP includes two vitamers, niacin and niacinamide which are essential for energy production. Vitamins are sensitive
and losses can occur during shelf life and heating processes. Thermal analysis can provide information about thermal behavior
of each vitamer relating them with time and/or temperature exposure. The vitamers thermal behavior were studied by TG/DTG
and DSC under air and nitrogen atmosphere and the results showed that niacin is more stable than the niacinamide and the decomposition
happens by volatilization at 238 °C while niacinamide melts at 129 °C and volatilize at 254 °C when there is the total mass
loss in the TG/DTG curves. 相似文献
76.
Ramiro Varela Camino R. Vela Jorge Puente María Sierra Inés González-Rodríguez 《Annals of Operations Research》2009,166(1):125-146
We confront a practical cutting stock problem from a production plant of plastic rolls. The problem is a variant of the well-known
one dimensional cutting stock, with particular constraints and optimization criteria defined by the experts of the company.
We start by giving a problem formulation in which optimization criteria have been considered in linear hierarchy according
to expert preferences, and then propose a heuristic solution based on a GRASP algorithm. The generation phase of this algorithm
solves a simplified version which is rather similar to the conventional one dimensional cutting stock. To do that, we propose
a Sequential Heuristic Randomized Procedure (SHRP). Then in the repairing phase, the solution of the simplified problem is
transformed into a solution to the real problem. For experimental study we have chosen a set of problem instances of com-mon
use to compare SHRP with another recent approach. Also, we show by means of examples, how our approach works over instances
taken from the real production process.
All authors are supported by MEC-FEDER Grant TIN2007-67466-C02-01 and by contract CN-05-127 of the University of Oviedo and
the company ERVISA, and by FICYT under grant BP04-021. 相似文献
77.
Alvaro E de la Torre MC Sierra MA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(24):6403-6411
The intermolecular Nicholas reaction of terpene-based scaffolds is an excellent access to natural product hybrid compounds. These intermolecular reactions have a low selectivity and are scarcely efficient for non-conjugated cations, but they are highly efficient to produce new terpene structures through an intramolecular reaction pathway. The use of cations derived from natural product derived [Co(2)(CO)(6)]-enyne complexes is, in contrast, a highly efficient regio- and stereoselective procedure to prepare very complex structures, incorporating diverse densely functionalized or labile moieties. Thus, beta-pinene-diterpene-alkaloid or homohybrids can be accessed in totally stereo-, regio- and siteselective fashion. This approach efficiently discriminates between different propargylic positions by selecting the nature of the alcohol, being the enyne-derived cations the most reactive. The chimera 38 with a steroid-terpene-indole skeleton was prepared in this way. 相似文献
78.
The surface properties of the supramolecular inclusion complex (IC) obtained from the threading of alpha-cyclodextrin (alpha-CD) onto poly(ethylene oxide) (PEO) free in solution are studied. The complexes were characterized by IR, (1)H NMR spectroscopy, and thermal analysis. The variation of the interfacial tension, gamma(int), with inclusion complex (IC) concentration and temperature were determined. The results were compared with those found for PEO under the same conditions. alpha-CD does not present surface activity. To quantify the adsorption process of IC and PEO in aqueous medium, the Gibbs equation was used. The driving force for adsorption of IC at the air/aqueous interface seems to arise from an enthalpic contribution. The wettability of the alpha-CD, PEO, and IC films with two liquids was determined by static contact angle measurements. The hydrophobicity degree was estimated. IC is more hydrophobic than PEO and alpha-CD. 相似文献
79.
Schulz PC Rodríguez JL Minardi RM Sierra MB Morini MA 《Journal of colloid and interface science》2006,303(1):264-271
The interaction between homologous surfactants in mixed micelles was studied by the Regular Solution Theory of mixed micelles. The interaction is independent of the nature of the polar head groups and attractive and the interaction parameter betaM depends linearly on the difference in chain length DeltanC. The interaction becomes ideal at DeltanC=0.75+/-0.06. Above DeltanC approximately 5, the dependence remains linear but the slope increased 2.7 times. The phenomenon is explained as the effect of the reduction of the hydrocarbon/water micelle interface and a better packing of the chains in the micelle core, caused by the inclusion of a shorter homologous surfactants. This reduction can be more effective when DeltanC>or=5. 相似文献
80.
The thermal cycloaddition between alkynyl metal(0) Fischer carbenes and nitrones has been studied computationally within the Density Functional Theory framework. It is found that the [3 + 2] cycloaddition takes place via transition structures that are more asynchronous and less aromatic than their nonorganometallic analogues. These reactions are also found to be completely regioselective in favor of the cycloadduct possessing the Fischer carbene moiety and the oxygen atom of the nitrone in a 1,3-relative disposition. These results are consistent with the role of the Fischer carbene moiety as an electron withdrawing group that enhances the electrophilic character of the alkyne group acting as a Michael acceptor as a dipolarophile. In terms of the isolobal analogy model, it can be concluded that alkynylalkoxy metal(0) carbene complexes act in this reaction as organometallic analogues of organic alkyl-propiolates with enhanced electrophilic character. 相似文献