Bernd Tesche's research while affiliated with Universität Stuttgart and other places

Publications (135)

Article
The preparation, characterization, and catalytic properties of Pd nanoparticles supported on mesoporous organic-inorganic hybrid materials are described for continuous-flow aerobic oxidation of alcohols using supercritical carbon dioxide (scCO(2)) as a mobile phase. The nanoparticles were generated "bottom-up" from molecular precursors that were pr...
Article
We report here a detailed study on the surface topology of well-known ordered mesoporous silica (SBA-15, MCM-41, and KIT-6) and a series of nanocast Co 3O 4, Co 3O 4/CoFe 2O 4 composites by high resolution scanning electron microscopy (HR-SEM). Images of the MCM-41 structure were obtained at a resolution of the pore size, as well as a real space im...
Article
In situ Fourier transform infrared (FTIR) spectroscopy was employed to characterize the adsorption behavior (as a function of pressure or time) and surface species of CO2 molecules on pure, phosphated, and phosphonated CaO. Carbonate and bicarbonate species were found to form on the pure oxide, whereas on the phosphated and phosphonated oxide sampl...
Chapter
IntroductionMicroscopy and SpectroscopyNew Imaging and Analytical Techniques: Tomography (Stereology)Conclusions and OutlookSymbols and Abbreviations
Article
Silver nanoparticles (Ag NPs) stabilized by a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), have been synthesized by the reduction of silver ions with NaBH(4) in aqueous solutions. The obtained Ag NPs are very stable at room temperature due to the extended coil conformation of the PNIPAM chain at temperatures below its volume phas...
Article
of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007
Article
of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007
Article
A micelle-based method to synthesize dispersed polyaniline (PANI)-Au composite particles by direct oxidation of aniline using AuCl4- as the oxidant is presented. The obtained composite particles have a core-shell structure, where Au nanoparticles of 20 nm mean diameter are encapsulated by PANI of well-defined tetrahedron shape with 150 nm average e...
Article
Pure, phosphated, and phosphonated CaO samples were prepared and characterized by X-ray powder diffractometry, FTIR spectroscopy, scanning electron microscopy, and energy-dispersive X-ray microprobing. Surface acid-base properties were probed by in situ FTIR spectroscopy of adsorbed CO (at 85 K), CDCl3 (at RT), CO2 (at RT), and methyl butynol decom...
Article
The presence of structural defects in ETS-10 titanosilicate plays an important role in its adsorption capacity and catalytic activity, e.g., in photocatalysis of organic molecules. In the present work, we used postsynthesis treatments with hydrogen peroxide to introduce different levels of defects in the ETS-10 structure. The number of defects indu...
Article
The phase shift induced by thin amorphous carbon films with thicknesses between 1 and 16 nm was measured by electron holography in a transmission electron microscope. Large phase shifts Delta phi are observed as the thickness of the amorphous C films decreases which cannot be described by the well-known equation Delta phi = CE V0t (V0: mean inner C...
Article
Videomicroscopy as a tool for investigating olefin gas phase co-polymerization is presented in this paper. This technique enables the simultaneous detection of the individual growth of a large number of catalyst particles. The focus is to study the kinetic behaviour of different types of Ziegler- and metallocene catalysts and to demonstrate that vi...
Article
By using laser-induced heating, we prepared Au-Ag nanoalloys via three different procedures: (i) mixture of Au nanoparticles and Ag(+) ions irradiated by a 532 nm laser, (ii) mixture of Au and Ag nanoparticles irradiated by a 532 nm laser, and (iii) mixture of Au and Ag nanoparticles irradiated by a 355 nm laser. Procedure i is advantageous for the...
Article
Full-text available
Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
Article
Reverse w/o microemulsions composed of methyl methacrylate (MMA) forming the oil phase, nonionic surfactants, and water are used for the synthesis of transparent SiO2/PMMA nanocomposites. An inorganic precursor, tetraethoxysilane (Si(OEt)(4), TEOS), is hydrolyzed in the reverse micelles containing aqueous ammonia. During the hydrolysis of TEOS, pol...
Article
Full-text available
The electrocatalytic activity of Pt-nanoparticles used in fuel cells increases by 34% upon going from the usual Pt/Vulcan XC72 to support systems such as Pt/Printex XE2 which have a relatively rough surface structure.
Article
In context of investigations of doped sodium alanate as a hydrogen storage material, an investigation combining a TEM-EDX study and XAFS measurements has been carried out on doped sodium alanate, using titanium tetrabutylate (Ti(OBun)4), colloidal titanium nanoparticles (Ti*) or TiCl3 as doping agents. It was found that the dehydrogenated wet-chemi...
Article
of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
Article
Das Wachstum vieler einzelner Katalysatorpartikel kann mithilfe der Videomikroskopie simultan bis hin zum Polymerkorn beobachtet und kinetisch erfasst werden. Exemplarisch wird die Gasphasenethylenpolymerisation von 40 Partikeln in einem Fensterautoklaven mit automatisierter Bildbearbeitung verfolgt. Das Bild zeigt eine Überlagerung dreier Polymeri...
Article
Picture this: The simultaneous growth of many single grains of catalyst culminating in polymer particles can be observed by videomicroscopy, and single-grain kinetics can be studied. The gas-phase polymerization of ethylene on 40 particles was examined in an autoclave equipped with a glass window by automatic image processing. The picture shows the...
Article
Industrial Liquid Pool Propene Polymerization with Metallocene/Mao/Silica Catalyst Systems - Volume 9 Issue S03 - Stefan Knoke, Frank Korber, Gerhard Fink, Bernd Tesche
Article
The role of Promoters in Tungstated Zirconia Catalysts - Volume 9 Issue S03 - B. Tesche, P. Lukinskas, S. Kuba, B. R. K. Graselli, Spliethoff, H. Knözinger
Article
Industrial Liquid Pool Propene Polymerization With Metallocene/Mao/Silica Catalyst Systems - Volume 9 Issue S02 - B. Tesche, S. Knoke, F. Korber, G. Fink
Article
The isomerization of straight-chain alkanes is effectively catalyzed by tungstated zirconia promoted by Pt or Pt and Fe. In the present paper, it is attempted to better understand the role of these promoters. The materials were characterized by CO chemisorption and by transmission electron microscopy (TEM) in combination with EDX, and their catalyt...
Article
The synthesis of colloidal Pt particles obtained from the reaction between platinum acetylacetonate (Pt(acac)(2)) and Al(CH3)(3) was monitored, and the formation of the Pt colloid via "reductive stabilization" was established by a combination of techniques including NMR, anomalous small-angle X-ray scattering (ASAXS), X-ray absorption spectroscopy...
Article
A biochar was prepared by pyrolysis of casein. A helium and mercury porosimeter were used to measure the true and apparent densities of the chars respectively, elemental and IR analysis were used to characterize the chemical composition of char. A SEM was used to observe the char surfaces in order to verify the presence of porosity. The biochar has...
Article
The polypropylene growth on metallocene/MAO catalysts supported on a silica carrier has been examined by scanning electron microscopy. The kinetic induction period in early polymerization stages can be explained by the formation of a discrete polymer layer only on the outer surface of the catalyst particle. Differences in the fragmentation process...
Article
Results from aromatic aminations and Kumada couplings, together with spectroscopic analyses (TEM, EDX, ICP-AES, React-IR), reveal that catalysis using nickel-on-charcoal (Ni/C) is most likely of a homogeneous rather than heterogeneous nature. In the course of a reaction with Ni/C, nickel bleed from the support was calculated to be as high as 78%. H...
Article
For the initial steps of propylene bulk phase polymerization with a silica-supported metallocene/MAO-catalyst the processes of polymer growth, particle expansion and carrier fragmentation are investigated. Gravimetric analysis of the kinetics is given. Light optical examination of the particles shows a significant particle expansion during the firs...
Article
The reversible Mg2FeH6 and the mixed Mg2FeH6-MgH2 hydride systems [Eqs. (1) and (2)] turned out to be highly suitable materials for thermochemical thermal energy storage at around 500 degreesC, for example as the storage of solar or excess industrial heat. The systems are characterized by a high gravimetric and volumetric thermal energy density in...
Article
An organometallic compound is the key intermediate during the formation of colloidal Pt particles from [Pt(acac)2] and Al(CH3)3. Additional studies confirm the formation of stable Pt colloids with an average diameter of 1.2 nm, which are stabilized by organoaluminum components (see scheme). Without the stabilizer a Pt nanopowder with a particle siz...
Article
Eine metallorganische Verbindung als Zwischenstufe tritt bei der Kolloidbildung durch Umsetzung von [Pt(acac)2] mit Al(CH3)3 auf. Weitere Untersuchungen belegen die Bildung stabiler Pt-Kolloide mit einem mittleren Durchmesser von 1.2 nm, die durch aluminiumorganische Bestandteile stabilisiert sind (siehe Schema). Ohne diese Stabilisatoren wird ein...
Article
Photonic crystals are periodic dielectric arrangements on the optical wavelength scale. The periodicity of the material modifies wave propagation, which can even be totally forbidden in certain frequency ranges. This leads to the ability to confine light at defects or complicated defect structures, making photonic crystals suitable for microdevices...
Article
A skeleton structure with a predicted bandgap between the 5th and 6th bands has been theoretically calculated (see Figure). Below the critical refractive index contrast, skeleton structures enable broader pseudogaps in the range of the 5th and 6th bands. The contrast required for this new gap is nearly the same (2.9) as that of the 8–9 gap of resid...
Article
Casting silica nets: By repeated "nanocasting" it is possible to first obtain a negative of the SBA-15 structure, known as CMK-3, which is then used to produce another nanocast by impregnation with tetraethoxysilane and condensation. After calcination to remove the carbon, an ordered mesoporous silica is obtained, called NCS-1 (see scheme). This pa...
Article
Using monodisperse nanocrystalline cobalt (Co) particles in non-polar colloidal dispersions, large areas of symmetric multi-dimensional structures were created using magnetophoretic deposition (MPD). To overcome the van der Waals and magnetic dipole–dipole interactions, the particles were stabilized with hydrophobic amines, phosphines, carboxylates...
Article
Noble metal nanoparticle containing [Me](x)-MCM-41 were synthesized using surfactant stabilised palladium, iridium and rhodium nanoparticles in the synthesis gel. The materials were characterised with XRD, ICP AES, TA, TEM and nitrogen sorption. which showed that the nanoparticles were present inside the pores of MCM-41. The [Me](x)-MCM-41 were fou...
Article
This chapter presents the direct synthesis of noble metal cluster containing MCM-41 using surfactant-stabilized metal nanoparticles. [Me]x-MCM-41 containing nanosized particles of platinum, palladium, rhodium, ruthenium, and iridium are directly synthesised from surfactant-stabilized spherical metal nanoparticles in the synthesis gel and characteri...
Article
Introduction Conventional methods to determine polymerization kinetics rely on a measurement of the monomer consumption or on calorimetric measurements. All these methods are integral methods and they are not appropriate to determine the kinetic of individual catalyst particles. Reichert and coworkers have invented video microscopy as a tool that a...
Article
Homogeneous metallocene catalysts for the polyolefin production show compared with conventional Ziegler systems remarkable advantages, like the possibilities to regulate the microstructure leading to optimized polymer properties. However, for industrial application it is necessary to immobilize the metallocenes on a heterogeneous support. The param...
Article
Mesoporous zirconium oxides were obtained using the polymeric sol–gel procedure. The synthesis was carried out in the presence of two series of surfactants, namely, N(Cn)4Br and N(CH3)3CnBr, with n=8–18, and Cn, the linear alkyl chain. Hydrolysis of zirconium isopropoxide was carried out both under acid and base catalysis, and also using acetylacet...
Article
The galvanic deposition of pure aluminium from fluoride-containing electrolytes has been developed further and for the first time aluminium and magnesium have been deposited from a toluene-solution of a halide-free organo-aluminium complex electrolyte. The rate of incorporation of magnesium can be controlled over a wide range by either adjusting th...
Article
Ru-MCM-41 and MCM-48 catalysts were prepared via deposition of Ru from various precursors RuCl3.3H2O, [Ru(NH3)6]Cl2 and Ru(acac)3. The samples were characterized using several techniques: adsorption and desorption curves of N2 at 77K, TPO, TPR, XRD, XPS, Mössbauer spectroscopy, and TEM. After deposition and activation the support keeps the structur...
Article
[Ir6(CO)16] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)2(acac)] followed by treatment in CO + H2. [Ir6(CO)15]2− in zeolite NaX was prepared similarly. Each sample was characterized by high‐resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost...
Article
Anknüpfend an die Weiterentwicklung der galvanischen Reinaluminium-Abscheidung aus fluoridhaltigen, aluminiumorganischen Elektrolyten ist es jetzt erstmalig gelungen, Aluminium und Magnesium aus Toluollösungen von halogenfreien, aluminiumorganischen Komplexelektrolyten abzuscheiden. Die Magnesium-Einbauraten sind durch entsprechende Zusammensetzung...
Article
A simple and time-saving method of analyzing images obtained from transmission electron microscopic (TEM) investigation of nanostructured transition metal and metal oxide colloids has been developed. It is based on computer image processing using the routine MORFO of the commercially available IMAGIC-5 software package. This high-throughput method...
Article
The polypropylene growth process on silica supported metallocene catalysts was examined by electron microscopy and kinetic investigations. It was possible to identify different polymerization stages with different characteristic kinetic data in the slurry phase which led to the development of a refined polymer growth model [1-2]. Tomographical reco...
Article
The optical properties and electron microscopy images of mesoporous silica fibers grown by slow acid synthesis were examined. It was found that the mesopores run in a circular direction around the axis.
Article
The polymerization of propene on silica-supported metallocene catalysts is studied using electron microscopy. The propylene growth on silica-supported metallocene catalysts are described by a particle growth model. This model is applied for mathematical simulation of the polymerization process. The polymerizations are carried out in liquid monomer...
Article
Nanoscale palladium and nickel colloids prepared in the presence of certain tetra-n-octylammonium carboxylates contain a high proportion of nonequilibrium trigonal cross section particles. Specifically, those carboxylates which bear a hydroxy function at the α-position, e.g., tetra-n-octylammonium glycolate, exert a strong influence on the shape of...
Article
A bimetallic, fully alloyed Pd/Au colloid of 3.0 nm particle size, obtained by the co-reduction of Pd and Au salts with tetraalkylammonium triethylhydroborate, was embedded in a silica matrix employing a modified sol–gel procedure with THF as the solvent. The removal of the protecting tenside led to a mesoporous texture with a comparatively narrow...
Article
The poly(propylene) growth process on silica supported metallocene catalysts has been examined by means of electron microscopy and kinetic investigations. It was possible to extend the polymerization process and to identify different polymerization phases with different characteristic kinetic data by lowering the reaction temperature in the slurry...
Article
Die seit Jahren intensiv untersuchten homogenen Metallocen–Katalysatoren für die Polyolefin-Produktion weisen gegenüber den konventionellen Ziegler-Systemen deutliche Vorteile bezüglich der Variationsmöglichkeiten der Mikrostruktur und damit der Eigenschaften der erzeugten Polymere auf. Sie müssen allerdings für den industriellen Einsatz auf Träger...
Article
Ru–MCM-41 catalysts were investigated in the diastereoselective hydrogenation of class F prostaglandin intermediates. The catalysts were prepared by deposition of Ru from different precursors (ruthenium acetylacetonate, ruthenium chloride or ruthenium hexaamine chloride). These catalysts were characterized using several techniques: atomic emission...
Article
The main focus of this study is the ethylene/hexene copolymerization with the silica supported metallocene SiO2/MAO/rac-Me2Si[2-Me-4-Ph-Ind]2ZrCl2. Polymerizations were carried out in toluene at a reaction temperature of 40°C–60°C and the cocatalyst used was triisobutylaluminium (TIBA). The kinetics of the copolymerization reactions (reactivity rat...
Article
With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of su...
Chapter
The main focus of this study is the polypropylene polymerization with silica supported metallocene catalysts and especially the investigation of the influence of particle diameter on polymerization activity as well as the changing product properties in dependence on pellet size. The metallocene used was rac-Me2Si[IndR2]2ZrCl2 which was supported on...
Article
The structures of iron-promoted sulfated zirconia (FSZ), manganese-promoted sulfated zirconia (MSZ) and iron- and manganese-promoted sulfated zirconia (FMSZ) were investigated by ultra-violet-visible (UV-vis) diffuse reflectance, ESR, laser Raman, and X-ray photoelectron (XP) spectroscopies and by transmission electron microscopy with energy disper...
Article
Commercial titanium powder, after pretreatment with chlorosilanes, can be used as an “off-the-shelf” McMurry reagent for various types of carbonyl coupling reactions. Electron microscopic studies (SEM, TEM, HRTEM, and electron diffraction) of the activated titanium now reveal the morphological basis for the observed reactivity, which stems from a p...
Article
Catalytically active microporous thin film membranes were prepared by dip coating of an asymmetric ceramic support membrane in a Na2PtCl6containing sol based on Ti(OiPr)4. After drying and calcination, the membrane (thickness of the top layer, 0.2–0.4 μm) shows separation properties typical for microporous membranes with low defect concentration (n...
Article
Charcoal-supported Pd-Pt catalysts based on Pd-Pt/NOct4Cl colloidal alloys have superior activity and selectivity in the oxidation of glucose to gluconic acid compared with industrial heterogeneous Pd-Pt catalysts. According to transmission electron microscopy, X-ray diffraction/Debye function analysis, X-ray photoelectron spectroscopy, X-ray absor...
Article
Nanostructured palladium clusters, stabilized by a monomolecular coat of sufactants N(C8H17)4Br or N(C18H37)4Br, self-assemble on carbon surfaces in an ordered manner with the formation of hexagonal close packed structures.
Article
Suspensions of coated superparamagnetic particles (magnetic fluids, MF) in AC magnetic fields have a pronounced specific absorption rate (SAR) per mass compared to multidomain particles. The aim of the present study was to investigate cellular uptake and the biological effects of AC magnetic field excited bio-compatible magnetic fluids on human car...
Article
The entrapment of lipases in hydrophobic sol-gel materials of RSi(OCH3)3 or mixtures of RSi(OCH3)3 and Si(OCH3)4 results in heterogeneous biocatalysts having dramatically enhanced enzyme activities as measured by the esterification of lauric acid by n-octanol in isooctane. These materials have been characterized by solid state NMR studies, revealin...
Article
Mechanical activation of solids leads to an increased internal energy caused by the introduction of defects. This can result for example in a reduced surface melting temperature of activated particles, which in turn may affect their sintering behavior. Analogously the spreading behavior of MoO3 over Al2O3 may depend on the mechanical treatment duri...
Article
Without appreciable migration into the pores of the support (Al2O3), preformed and size-selectively synthesized metal clusters (e.g. Pd or Ni) and bimetal clusters (e.g. Pt/Sn) stabilized by ammonium salts can be placed specifically on the outer surface and immobilized (see schematic representation on the right; black: metal cluster, gray: support...
Article
A Pt black produced by reduction of H2PtCl6 by hydrazine hydrate contains no potassium impurity. Electron microscopy shows more regular polyhedral particles in this catalyst, as opposed to the rounded ones seen after reduction with HCHO in the presence of KOH [Z. Paál, H. Zimmer, J. R. Gunter, R. Schlögl, M. Muhler, J. Catal. 119, 146 (1989)]. The...
Article
Vanadia-promoted Rh/SiOâ-catalysts have been prepared by impregnation followed by calcination at 573, 773, 973, and 1173 K. The structure and morphology of these materials in the oxidized state, and after low (523 K) and high (773 K) temperature reduction, were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), m...
Article
Thin planar model alumina films can be successfully used as supports for metal species in the size range 1 nm, provided the electron optical conditions (defocus setting) are optimized. Os3(CO)12-clusters were anchored on the model support intact and the triatomic clusters ( 0.6 nm diameter) could be made visible. Metal aggregation in H2-atmospheres...
Article
0.8% Pt on KL zeolite was prepared by reduction with H2 and NaBH4. Transmission electron microscopy and X-ray diffraction were used for characterization of morphology of the metal particles; acidity of the zeolite framework was measured with IR spectra of adsorbed pyridine. Reduction by NaBHP4 produced rather large, needle-like Pt crystallites on t...
Article
Using SiO2, γ-Al2O3 as typical examples for acidic, amphoteric and basic oxides, the effect of the support nature on the behaviour of Rh was studied by temperature-programmed reduction, transmission electron microscopy and IR spectroscopy of adsorbed CO. Reduction is much easier on SiO2 than on Al2O3 and MgO. The highest metal dispersion is stabili...
Article
Al2O3-supported Os catalysts were prepd. from Os3(CO)12 and from K2[OsO2(OH)4]. The cluster-derived catalyst was modified with K by an addnl. impregnation with K2CO3. The materials were phys. characterized by Ce IR spectroscopy, temp.-programmed redn., and TEM. Isolated surface complexes were the majority species in the Os3-derived catalysts, their...
Article
Electron microscopy studies of thin tin dioxide films (about 80 nm thick) prepared by spraying from a mixture of SnCl4 and ethyl acetate indicate that such films are amorphous at substrate temperatures of 340–350°C, partially crystallized at 360–410°C and polycrystalline at 420–500°C. Grains about 40 nm in size were found for all polycrystalline st...
Article
The determination of size distribution is sought via time-of-flight mass spectroscopy and electron microscopy of neutral silver clusters produced by the inert gas condensation method. By adjusting suitable parameters the mean size could be varied between a few tens and several hundreds of atoms per cluster. In particular, the influence exerted on t...
Article
The gas aggregation technique has been brought up to the point where metal clusters can be produced continuously in the size range from 105 atoms per cluster down to molecules. Knowledge of the parameters controlling the cluster growth allows the shift of the mean cluster size within the above given size range with reasonably narrow size distributi...
Article
Supported molecular metal clusters were prepared by the ligand association reaction involving the phosphine groups of poly(styrene-co-divinylbenzene-co-p-styryldiphenylphosphine) and [H2RhOs3(acac)(CO)10] to give [H2RhOs3(acac)(CO)10Ph2P- (circle latin capital P)]. This molecular analogue was used as a precursor of a supported metal catalyst, its a...
Article
This paper deals with the development of physical fixation and contrasting procedures for the electron microscopic structure analysis of isolated biomolecules. It has been shown on isolated antibodies (IgG) that these preparation methods give better preservation of biomolecules than the commonly practiced chemical fixation and staining techniques....
Article
Electron micrographs and size distribution curves are presented for silver clusters formed by the photoelectrochemical deposition of silver atoms onto the surface of pure single crystals of anthracene. The clusters were resolved to a diameter of 2 nm. For comparison clusters were also deposited using a cluster source in vacuum.
Article
Phosphotungstate (pH 7.4) used for negative staining of membranes from Torpedo electric tissue rich in acetylcholine receptor does not affect binding properties and cation permeability of the receptor and its ion channel. Uranyl salts, frequently used for negative staining, precipitate the receptor-rich membranes making measurements of ligand bindi...
Article
Extended x-ray-absorption fine structure was used to determine the nearest-neighbor distance for silver particles isolated in solid argon. The silver particles were produced in the gas phase with the use of the gas-aggregation technique. In this technique the metal atoms are evaporated in an argon atmosphere in the pressure range 0.1-10 Torr; a coo...
Article
The bimetallic clusters (μ2-H)AuOs3(CO)10PPh3 (I) and (μ2-Cl)AuOs3-(CO)10PPh3 (II) were anchored intact onto phosphine-functionalized silica by phosphine-phosphine ligand exchange. CO could be added reversibly at room temperature onto the electronically unsaturated, immobilized cluster I. The addition reaction was accompanied by metal-metal bond op...
Article
The authors report a new method of monolayer vesicle formation, where vesicle formation is caused by different counterions. With water-insoluble symmetric salts were produced monolayered crystals and micelles which are reported for the first time. These aggregates can reversibly be converted to monolayer vesicles.