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1.
将铈β-二酮络合物(Ce(tmhd)4)的氯仿溶液与花生酸(AA)的氯仿溶液以不同摩尔比混合并铺展在纯水亚相上,得到其与AA的混合单分子膜.对混合单分子膜的成膜特性(π-A等温线和体系超额自由能)进行了探讨,发现混合单分子膜的超额自由能为负值,混合过程为热力学自发过程,且在配比为1∶ 2时其绝对值最大,体系最稳定,并进一步讨论了混合单分子膜可能的凝聚态结构.在配比为1∶ 2时,研究了混合单分子膜的静态弹性和动态弹性.  相似文献   

2.
方堃  邹纲  何平笙 《化学学报》2003,61(12):1997-2001
以铈、钆的β二酮螯合物Ce(tmhd)_4和Gd(tmhd)_3(tmhd = tetramethylheptanedionate)为表面离子,与花生酸(AA)在水面上共铺展,可 形成具有良好相容性、稳定性和可压缩性的混合Langmuir膜,这是由于稀土螯合物 与AA间发生了新的配合,而且Gd(tmhd)_3与AA间的相互作用更强。用垂直法将它们 的三组分混合Langmuir膜沉积,制得了具有良好周期性结构的Y型三组分混合LB膜 。以它作前驱物,经过紫外臭氧(UVO)和热处理,制得了CeO_2-Gd_2O_3(CGO)超 薄陶瓷膜。X射线光电子能谱(XPS)表明,由于两种稀土螯合物和AA间的结合强度 不同,在沉积LB膜时表面离子发生了选择性转移,而且温度对表面离子的转移选择 性有影响。这对制备组分和厚度可控的应用薄膜很有意义。  相似文献   

3.
The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic-anionic surfactants at the air/water interface were investigated by measurements of surface pressure-area per alkyl chain (pi-A) and surface potential-area per alkyl chain (DeltaV-A) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C(12) to C(16) were used to form mixed surfactant monolayers on the water subphase at 21 degrees C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the DeltaV-A isotherms were generally more sensitive than the pi-A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic-anionic surfactants were closely related to the alkyl chain lengths of the components.  相似文献   

4.
In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.  相似文献   

5.
This paper reports the pi-A isotherms and spectroscopic characteristics of mixed Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic carbazole (CA) molecules mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). pi-A isotherm studies of mixed monolayer as well as the remarkable change in collapse pressure of the mixed monolayer isotherms definitely show that CA is incorporated into PMMA and SA matrices. However, CA is stacked in the PMMA/SA chains and forms microcrystalline aggregates, as is evidenced from the scanning electron micrograph picture. The nature of these aggregated species in the mixed LB films has been revealed by UV-vis absorption and fluorescence spectroscopic studies. The presence of two different kinds of band systems in the fluorescence spectra of the mixed LB films have been observed. This may be due to the formation of low-dimensional aggregates in the mixed LB films. Intensity distribution of different band systems is highly sensitive to the microenvironment of two different matrices as well as also on the film thickness.  相似文献   

6.
由有机LB膜技术发展了一种制备组分、厚度可控的无机超薄陶瓷膜的方法.以Zr、 Y的β-二酮络合物的作为"表面离子"代替传统的亚相离子,沉积它们与花生酸的混合LB膜.并将它作为前驱物,经臭氧处理和热处理,成功制得了Y2O3稳定的立方相ZrO2超薄膜(YSZ).用X射线衍射(XRD)、 X射线光电子能谱(XPS)等手段研究了YSZ薄膜的相结构和其组成.结果表明,超薄陶瓷膜中Zr与Y的含量比率控制得很好,且形成Y2O3稳定的立方相ZrO2.说明这种方法可以成功地用来制备组分和膜厚均可控的纳米陶瓷膜.  相似文献   

7.
Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.  相似文献   

8.
The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of the usual nonsubstituted stearic acid. The phase behavior, domain morphology, and two-dimensional lattice structure of 9-, 11-, and 12-hydroxystearic acids are studied, using pi-A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD), to obtain detailed information on the effect of the exact position of the OH-substitution. The pi-A isotherms of all three hydroxyoctadecanoic acids have an extended flat plateau region, the extension of which only slightly decreases with the increase of temperature. At the same temperature, the extension of the plateau region increases and the plateau pressure decreases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid. The absolute -DeltaH and -DeltaS values for the phase transition increase slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid and indicate differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/condensed phase transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region is specific for the position of the OH-substitution of the alkyl chain, just as the lattice structures of the condensed monolayer phase. 11-hydroxyoctadecanoic acid monolayers form centered rectangular lattices with the chain tilt toward the NNN (next nearest neighbor) direction, and 12-hydroxyoctadecanoic acid monolayers have an oblique lattice over the entire pressure range. A special feature of 9-hydroxystearic acid monolayers is the phase transition between two condensed phases observed in the pi-A isotherm of 5 degrees C at approximately 18 mN/m, where the centered rectangular lattice shows a NNN/NN transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region, just as the lattice structures of the condensed monolayer phase, reveal the high specifity of the monolayer feature of the bipolar hydroxystearic acids OH-substituted in the mid position.  相似文献   

9.
10.
Magnetic Langmuir-Blodgett films of four ferritin derivatives with different iron contents containing 4220, 3062, 2200, and 1200 iron atoms, respectively, have been prepared by using the adsorption properties of a 6/1 mixed monolayer of methyl stearate (SME) and dioctadecyldimethylammonium bromide (DODA). The molecular organization of the mixed SME/DODA monolayer is strongly affected by the presence of the water-soluble protein in the subphase as shown by pi-A isotherms, BAM images, and imaging ellipsometry at the water-air interface. BAM images reveal the heterogeneity of this mixed monolayer at the air-water interface. We propose that the ferritin is located under the mixed matrix in those regions where the reflectivity is higher whereas the dark regions correspond to the matrix. Ellipsometric angle measurements performed in zones of different brightness of the mixed monolayer confirm such a heterogeneous distribution of the protein under the lipid matrix. Transfer of the monolayer onto different substrates allowed the preparation of multilayer LB films of ferritin. Both infrared and UV-vis spectroscopy indicate that ferritin molecules are incorporated within the LB films. AFM measurements show that the heterogeneous distribution of the ferritin at the water-air interface is maintained when it is transferred onto solid substrates. Magnetic measurements show that the superparamagnetic properties of these molecules are preserved. Thus, marked hysteresis loops of magnetization are obtained below 20 K with coercive fields that depend on the number of iron atoms of the ferritin derivative.  相似文献   

11.
Hybrid monolayers formed with an floating arachidic acid (AA) anions monolayer binding with a densely packed layered double hydroxides (LDHs) monolayer at an air/LDHs suspension interface has been studied by pi-A isotherms and TEM images. An ordered multilayer film of AA/LDHs has been fabricated by Langmuir-Blodgett technique on various substrates. The photoactive dyes (methyl orange, MO, and Congo red, CR) can be incorporated into the galleries of LDHs in the AA/LDHs hybrid LB film by an ion intercalation method. The results of FTIR and UV-vis absorption spectra can approve the formation of AA/LDHs/dyes composite films. In addition, UV-vis absorption spectra and LAXRD analyses also provide evidence for the good vertical uniformity and stable layered periodic structure of AA/LDHs/dyes films. More interestingly, it was found that the dye molecules intercalated can be induced by a positively charged LDHs sheet to align in a special orientation and form different aggregates: MO molecules form sandwich H-type aggregates, while CR molecules form head-to-tail J-type aggregates. On the basis of these data, a possible model of the AA/LDHs/dyes composite films was proposed. Also, the dye molecules incorporated into AA/LDHs films exhibit excellent configuration stability under the irradiation of UV light because the LDHs matrix offers a more rigid and constrained environment for them.  相似文献   

12.
A dipalmitoylphosphatic acid (DPPA) monolayer at the air/liquid interface is used as a binding layer to incorporate glucose oxidase (GOx) from the subphase. The effects of the adsorption time of GOx on the behavior of the mixed DPPA/GOx monolayer and the relevant structure of the mixed LB film were studied using the characteristics of the pressure-area (pi-A) isotherm, Brewster angle microscopy (BAM), and atomic force microscopy (AFM). The experimental results show that two equilibrium states of GOx adsorption exist in the presence of a DPPA monolayer. The first equilibrium stage occurs at tens of minutes after spreading of DPPA, and a surface pressure of ca. 7.5 mN/m is obtained. The second equilibrium stage approaches slowly, and a higher equilibrium surface pressure (ca. 16 mN/m) was obtained at ca. 8 h after the first stage. The BAM and AFM images show that, after the second equilibrium stage is reached, a more condensed phase and rough morphology are obtained on the mixed DPPA/GOx monolayer, indicating a higher amount of GOx incorporated into the mixed film. For the first equilibrium stage of GOx adsorption, DPPA molecules can still pack regularly and closely under compression, suggesting that GOx molecules are mainly located beneath the DPPA monolayer at the compressed state. A more uniform phase was detected on a film prepared after the first equilibrium stage was reached. The present result indicates that distinct structures and properties of mixed DPPA/GOx films can be prepared from the various stages of GOx adsorption.  相似文献   

13.
The interactions of mixed monolayers of two lipids, zwitterionic 1,2-dipalmitoyl-phosphatidylcholine (DPPC) and positively charged 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), with phytohormone indolilo-3-acetic acid (IAA) and selenate anions in the aqueous subphase were studied. For this purpose, isotherms of the surface pressure versus the mean molecular area were recorded. Domain formation was investigated by using Brewster angle microscopy (BAM). The method of grazing incidence X-ray diffraction (GIXD) was also applied for the characterization of the organization of lipid molecules in condensed monolayers. It was found that selenate ions contribute to monolayer condensation by neutralizing the positive net charge of mixed monolayers whereas IAA molecules penetrated the lipid monolayer, causing its expansion/fluidization. When both solutes were introduced into the subphase, a competition between them for interaction with the positively charged lipids in the monolayer was observed.  相似文献   

14.
Mixed monolayers of stearic acid (SA) and octadecylamine (ODA) at the air/water interface were investigated in this article. The miscibility of the two compounds was evaluated by the measurement of surface pressure-area per molecule (pi-A) isothems and the direct observation of Brewster angle microscopy (BAM) on the water surface. The two compounds were spread individually on the subphase (method 1) or premixed first in the spreading solvent and then cospread (method 2). The effect of spreading method on the miscibility of the two compounds was also studied. The results show that the mixed monolayers prepared by method 1 cannot get a well-mixed state. The isotherms of mixed monolayers preserve both characteristics of SA and ODA and exhibit two collapse points. The calculated excess surface area is very small. Besides, distinguished domains corresponding to those of pure SA and ODA can be inspected from the BAM images. Such results indicate that SA and ODA cannot get a well-mixed phase via 2-dimensional mixing. On the contrary, in the mixed monolayer prepared by cospreading, the two compounds exhibit high miscibility. In the pi-A isotherms, the individual characteristics of SA and ODA disappear. The calculated excess area exhibits a highly positive deviation which indicates the existence of special interaction between the two compounds. The low compressibility of isotherm implies the highly rigid characteristic of the mixed monolayer. which was also sustained by the striplike collapse morphology observed from the BAM. The rigid characteristic of SA/ODA mixed monolayer was attributed to the formation of "catanionic surfactant" by electrostatic adsorption of headgroups of SA and ODA or to the formation of salt by acid-base reaction.  相似文献   

15.
Interaction between amphotericin B(AmB) and cell membrane is influenced by different metal cations. In the presence of K+, Na+ or Ca2+ ions, the surface pressure-area isotherms and the elastic modulus of an amphotericindipalmitoylphosphatidylcholine(AmB-DPPC) mixed monolayer were discussed. And the excess free energy and entropies of mixing were calculated according to the surface pressure-area isotherms. The phase transition of the mixed monolayer needed a higher concentration of AmB in the sequence Na+ > pure buffer > K+ > Ca2+. When the molar fraction of AmB(xAmB) was 0.5, the molecular interaction changed from attraction to repulsion and the mixed monolayer turned to ordered state from disorder state under the induction of K+ or Ca2+ ions at all surface pressure in our experiment. At high surface pressure, the disorder of monolayer enhanced in the presence of Na+ ions at xAmB > 0.1. At different molar ratios of AmB, the influences of these metal cations were discrepant. These cations may influence AmB molecules to form pores on the monolayer. It is helpful to understand the reduction of AmB's toxicity as theoretical reference.  相似文献   

16.
In this work, surface film balance and Brewster angle microscopy techniques have been used to analyze the structural characteristics (structure, topography, reflectivity, thickness, miscibility, and interactions) of hydrolysates from sunflower protein isolate (SPI) and dipalmitoylphosphatidylcholine (DPPC) mixed monolayers spread on the air-water interface. The degree of hydrolysis (DH) of SPI, low (5.62%), medium (23.5%), and high (46.3%), and the protein/DPPC mass fraction were analyzed as variables. The structural characteristics of the mixed monolayers deduced from the surface pressure (pi)-area (A) isotherms depend on the interfacial composition and degree of hydrolysis. At surface pressures lower than the equilibrium surface pressure of SPI hydrolysate (pi(e)(SPI hydrolysate)), both DPPC and protein are present in the mixed monolayer. At higher surface pressures (at pi > pi(e)(SPI hydrolysate)), collapsed protein residues may be displaced from the interface by DPPC molecules. The differences observed between pure SPI hydrolysates and DPPC in reflectivity (I) and monolayer thickness during monolayer compression have been used to analyze the topographical characteristics of SPI hydrolysates and DPPC mixed monolayers at the air-water interface. The topography, reflectivity, and thickness of mixed monolayers confirm at microscopic and nanoscopic levels the structural characteristics deduced from the pi-A isotherms.  相似文献   

17.
Langmuir and Langmuir-Blodgett monolayers of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of glycine, tyrosine, and phenylalanine were studied using pi-A isotherms and photoelastic modulated FTIR (PEM-FTIR). Based on compression modulus and interaction parameters, mixed monolayers of these compounds with stearylamine (SAM) showed well-organized monolayers compared to mixed systems with stearic acid (SA) and stearyl alcohol (SAL). The pure amphiphiles exhibited fairly well-ordered packing in the films, and in the mixtures, the ordering increased and showed a triclinic packing arrangement. For the phenylalanine amphiphile the packing showed slight disorder compared to the other two compounds. Surface properties of the LB films of these compounds on solid substrates were analyzed using static and dynamic contact angles of a series of liquids. The surface tension of coated substrates reflected clearly the highly acidic character. Fluidlike monolayers having a molecularly rough surface indicated high wettability for n-alkanes. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettability and small hysteresis.  相似文献   

18.
In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.  相似文献   

19.
The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,omega-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)2 : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (micro perpendicular)-area (A) isotherms] and morphological images of CnBP(FC14)2 monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)2 formed a stable monolayer on water at 298.2 K, where these pi-A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the pi-A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the pi-A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)2 molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)2 occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression-expansion cycle curve, and DeltaV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)2 and FC14 monolayers was also confirmed by FM and BAM images.  相似文献   

20.
Recent progress in studies of the main characteristics of supramolecular assemblies formed by interfacial molecular recognition between an amphiphilic monolayer and a non-surface-active species, which is dissolved in the aqueous subphase, by complementary hydrogen bonding and/or electrostatic interaction at the air-water interface is reviewed. Systems consisting of an amphiphilic melamine-type monolayer and an pyrimidine derivative dissolved in the aqueous subphase are representative model systems for molecular recognition on the basis of complementary hydrogen bonding. Most of the studies have been performed with 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers as host component and thymine, uracil or barbituric acid as dissolved non-surface-active pyrimidine derivatives. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of all pyrimidine derivatives dissolved in the aqueous subphase changes drastically and in a specific way the characteristic features (pi-A isotherms, morphology of the condensed phase domains) of the 2C11H23-melamine monolayer. The small condensed phase domains of the pure 2C11H23-melamine monolayer are compact without an inner texture. The monolayers of the supramolecular 2C11H23-melamine entities with thymine or uracil form specifically well-shaped condensed phase domains with an inner alkyl chain texture essentially oriented parallel to the periphery. The completely different morphology of the 2C11H23-melamine-barbituric acid monolayers is characterized by the formation of large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer. The striking differences in the main characteristics between the supramolecular entities are related to their different chemical structures: complementary hydrogen bonding of two thymine or uracil molecules by one 2C11H23-melamine molecule and a linearly extended hydrogen bonding network between 2C11H23-melamine and barbituric acid. The high values of hydrogen bonding energy obtained by quantum chemical calculations on the basis of the semi-empirical PM3 method state the high stability of the supramolecular entities. The GIXD results reveal that the formation of hydrogen-bond based superstructures between the polar head groups of the amphiphilic 2C11H23-melamine monolayer and the non-surface-active pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure of the alkyl chains. The alternative possibility to construct interfacial molecular recognition systems on the basis of acid-base interaction is demonstrated by the experimental results obtained by molecular recognition of the heptadecyl-benzamidinium chloride monolayers with dissolved non-surface-active phenylacetate ions. The formation of supramolecular assemblies causes also drastical changes of the surface features in these systems. Here, the development of a substructure in the condensed phase domains consisting of long filigree strings and the favoured formation of bilayers overgrowing the strings indicates a linearly extended amidinium-carboxylate interfacial structure of the base and acid component in alternating sequence.  相似文献   

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