全文获取类型
收费全文 | 1402篇 |
免费 | 41篇 |
国内免费 | 31篇 |
专业分类
化学 | 1054篇 |
晶体学 | 18篇 |
力学 | 66篇 |
综合类 | 1篇 |
数学 | 74篇 |
物理学 | 261篇 |
出版年
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 17篇 |
2019年 | 20篇 |
2018年 | 20篇 |
2017年 | 20篇 |
2016年 | 30篇 |
2015年 | 16篇 |
2014年 | 33篇 |
2013年 | 106篇 |
2012年 | 106篇 |
2011年 | 42篇 |
2010年 | 29篇 |
2009年 | 52篇 |
2008年 | 66篇 |
2007年 | 59篇 |
2006年 | 64篇 |
2005年 | 74篇 |
2004年 | 52篇 |
2003年 | 42篇 |
2002年 | 31篇 |
2001年 | 41篇 |
2000年 | 38篇 |
1999年 | 31篇 |
1998年 | 29篇 |
1997年 | 32篇 |
1996年 | 34篇 |
1995年 | 39篇 |
1994年 | 29篇 |
1993年 | 27篇 |
1992年 | 40篇 |
1991年 | 19篇 |
1990年 | 20篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 19篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有1474条查询结果,搜索用时 15 毫秒
121.
Interaction of sodium dodecyl sulfate (SDS) with the cationic polyelectrolyte poly(ethyleneimine) (PEI) was investigated in this study. Turbidity measurements were performed in order to analyze the interaction and complex formation in bulk solution as a function of polymer concentration and pH. Surface tension measurements were made to investigate the properties of SDS/PEI/water mixtures at air/solution interface. Results revealed that SDS/PEI complexes form in solution depending on the surfactant and polymer concentration. A decrease was observed in surface tension values in the presence of SDS/PEI mixtures compared to the values of pure SDS solutions. Both solution and interfacial properties exhibited pH dependent behavior. A shift was seen in the critical micelle concentration of SDS solutions as a function of PEI concentration and solution pH. Monovalent and divalent salt additions showed some influence on the interfacial properties of SDS solutions in the presence of PEI. 相似文献
122.
M. Sh. Ramadan S. A. Senior G. M. Nabil E. A. Hamed 《Journal of Dispersion Science and Technology》2013,34(12):1752-1760
Reaction rate for alkaline hydrolysis of the substrates 3,5-dinitro-2-chloro benzotriflouride (DNCBTF) (1) at 30°C and 2,4-dinitrochloro benzene (DNCB) (2) at 50°C separetely with NaOH as nucleophile is carried out spectrophotometrically in mixed aqueous-acetonitrile solvents. In each system, cationic surfactant as dodecyltrimethyl ammonium bromide (DoTAB), or anionic one as sodium dodecyle sulfate (SDS) is used in wide range of concentrations to study the effect of micelle on the reaction rate. The micellar effect is explained in term of modified pseudo phase ion exchange model. Pseudo first order rate constant, kobs is obtained for each of the nucleophile and for both substrates 1 and 2 at all range of XAN · kobs at given [OH?] and in presence of any substrate is found to increase with the increase of DoTAB,while decrease with the increase of SDS as micellar phases. Critical micelle concentrations (CMCs) in similar trend are observed to increase in DoTAB while decrease in SDS systems by increasing acetonitrile (AN) content. Micellar binding constant (KS) between any type of given substrate and the formed micelle, is found to decrease in presence of DoTAB and increase in SDS micellar phases by increasing AN content. Finally, the ratios between pseudo first order rate constants for hydrolysis in micellar phase kM to that in the bulk phase kw for DoTAB and SDS systems are found to be greater than and smaller than unity respectively at any given XAN where the data indicated for (1) is always higher than those for (2). The results concluded that micelle DoTAB is working as a catalyst for the reaction rate, while that for SDS is considered as an inhibitor. 相似文献
123.
C. Carnero Ruiz L. Díaz‐López J. Aguiar 《Journal of Dispersion Science and Technology》2013,34(2):266-273
The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles. 相似文献
124.
Hydrogen-bonded blends based on smectic side group functionalized LC copolymers containing 4-alkyloxybenzoic acid fragments (proton donor) and a non-mesogenic low molecular mass dopant 4-cyanophenyl pyridine-4-carboxylate or 4-methoxyphenyl-d4 pyridine-4-carboxylate (proton acceptor) were obtained. The blends containing 10-35 mol % of low molecular weight dopant form nematic (I-N-SmA) or re-entrant SmA phases (I-SmA-N-SmAre). The temperature dependence of the order parameter S, the birefringence Δn, and the splay K 1 and bend K 3 elastic constants of the nematic phase were studied by 2H NMR spectroscopy and the Fréedericksz method of threshold transitions in a magnetic field. A mechanism for the destruction of the SmA phase and the formation of the nematic phase in the hydrogen-bonded blends is suggested. 相似文献
125.
Darrell J. Spells Christopher Lindsey Larry R. Dalton Shin-Tson Wu 《Liquid crystals》2013,40(12):1529-1532
Methods for synthesizing substituted alkyl and fluoro stilbene-tolane (PTPDP) liquid crystal materials are presented. These compounds show a slightly higher birefringence than the analogous bistolanes, but higher melting points and heats of fusion. Potential applications of these compounds for optical communications and displays are discussed. 相似文献
126.
Theophanis M. Sakellarides Triantafyllos A. Albanis 《International journal of environmental analytical chemistry》2013,93(3-4):249-262
Abstract Fly ash and soil mixtures with a range of fly ash content from 1 to 100% were used to study adsorption and desorption of four organophosphorus insecticides, ethyl parathion, methyl parathion, fenitrothion and fenthion, in batch experiments. The object of the study was to develop a treatment process using fly ash as sorbent material to isolate/immobilize organic contaminants from aqueous solutions. The adsorption isotherms fit the Freundlich equation x/m=KdC1/n. The Kd values increase with the increase of the fly ash content. The isotherms seem to fit the S type, in mixtures of soil with a fly ash content from 0 to 10%, which implies that adsorption becomes easier as the concentration in the liquid phase increases. In mixtures of soil with a fly ash content from 25 to 50% the isotherms become L type and correspond to a decrease of site availability as the solution concentration increases. Finally in mixtures of soil with a fly ash content over 50%, C type adsorption was observed which correspond to a constant partition of the insecticides between the bulk solution and the adsorbent. Mass balance estimations show that the mean percent amounts of insecticides for a range of concentration 0.5–15 mg/l, removed by adsorption in the soil sample are 81.56 % for ethyl parathion, 48.97 % for methyl parathion, 67.06 % for fenitrothion and 86.65 % for fenthion. The adsorption increases as the fly ash content increased and reach the 100% in the “pure” fly ash. The adsorbed amounts of insecticides in mixtures of soils with >50% fly ash content, are up to 99%. In contrast, the amounts of desorption in water decrease as the fly ash content increase. The results of this research demonstrate that the fly ash shows a significant capacity for adsorption of organophosphorus compounds from aqueous solution and can be used for pesticide removal process. 相似文献
127.
Separation of complex sugar mixtures on a hydrolytically stable bidentate urea‐type stationary phase for hydrophilic interaction near ultra high performance liquid chromatography
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dorina Kotoni Alessia Ciogli Claudio Villani David S. Bell Francesco Gasparrini 《Journal of separation science》2014,37(5):527-535
A stationary phase bearing both bridged bis‐ureido and free amino groups (USP‐HILIC‐NH2–2.5SP) for high‐speed hydrophilic interaction liquid chromatography separations was prepared using a one‐pot two‐step procedure starting from 2.5 μm totally porous silica particles. Highly polar compounds, such as polyols, hydroxybenzoic acids, and sugars, were successfully analyzed in shorter times and with higher peak efficiency, when compared to results obtained with a bidentate urea‐type column packed with 5 μm particles. Increased sugarophilicity and better peak shape were attested for the USP‐HILIC‐NH2–2.5SP column (100 × 3.2 mm id) when compared with two commercially available UHPLC columns, namely an acquity BEH amide packed with totally porous 1.7 μm microparticles and a HILIC Kinetex column packed with core–shell 2.6 μm particles. Finally, the new column was employed in the separation of complex mixture of sugars (mono‐, di‐, and oligosaccharides) and in the analysis of beer samples. The resulting chromatograms showed good selectivity and overall resolution, while the catalyzing effect of the free amino moieties resulted in excellent peak shapes and in the absence of split peaks due to sugar anomerization phenomena. 相似文献
128.
Reaction kinetics and solubility in water‐organic binary solutions are governed by similar solvation equilibria
下载免费PDF全文
![点击此处可从《Journal of Physical Organic Chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Siim Salmar Morten Vaalma Henry Vider Tiina Tenno Aleksei Kuznetsov Jaak Järv Ants Tuulmets 《Journal of Physical Organic Chemistry》2016,29(3):118-126
The solvation effects observed in water‐organic solutions were studied by combining data for reaction kinetics and dissolution equilibria by means of a linear free‐energy (similarity) analysis. Kinetic data for the pH‐independent hydrolysis of (4‐methoxyphenyl)‐2,2‐dichloroacetate measured in this work and solubility data for naphthalene, and other substrates of low polarity, in aqueous binary mixtures of methanol, ethanol, acetonitrile, dimethyl sulfoxide (DMSO), and 1,4‐dioxane were used. Linear similarity relationships were discovered for these data over the full range of solvent compositions studied. To gain insight into the similarities observed between these different phenomena, molecular dynamics simulations were carried out for naphthalene and an ester in water–acetonitrile solutions. The results revealed considerable preferential solvation of these substrates by the co‐solvent. Linear relationships between the experimental data and the mole fractions of acetonitrile in the solvation shells of substrates were found. Surprisingly, a linear relationship was found between the mole fractions of acetonitrile in the solvation shells of the ester and naphthalene. This linearity indicated that a similar solvation mechanism governs even such different phenomena as dissolution and reaction kinetics. The relationships between the experimental data and the results of the molecular dynamics calculations found in this work explained the solvent effect observed in water‐organic solutions on the molecular level. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
129.
130.
Polymer-stabilized foams and foam films have received considerable attention during the past years. This review paper gives an overview of recent studies dealing with polyelectrolyte/surfactant mixtures, proteins, and microgels adsorbed at single air/water interfaces, in foam films and in macroscopic foams. These polymeric systems have in common that their structure or shape changes when adsorbing at an air/water interface. These structural changes in comparison to their bulk behavior greatly influence the properties of foam films and foams. Regarding the foam stability, formation of adsorbed layers or aggregates plays an important role. The discrepancy between stabilization of macroscopic foams and destabilization of single foam films might be attributed to the blockage of Plateau borders and, therefore, slowed down drainage. Another important parameter is the interfacial viscoelasticity. 相似文献