Probing quantized image-potential states at supported carbon nanotubes.

Discrete image-potential states (ISs) are revealed at double-walled carbon nanotubes by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in the distance-voltage z(V) spectroscopy mode. The nanotubes are supported by flat Au(111) substrates. Due to the high sensitivity of the hot IS electrons to local variations of the surface potential, they can be considered as a sensitive probe to investigate interactions with the supporting substrate and impurities or defects at the nanotube surface. ISs provide information on the local electronic structure as well as on the electron dynamics at supported nanotubes.

Surface state of carbon nanotubes and Hansen solubility parameters.

Carbon nanotubes (CNTs) are often described as insoluble or poorly soluble in organic solvents. In a recent study, we have reported that nonfunctionalized CNTs can be solubilized in suitably chosen organic solvents. Furthermore, their solubility could be understood in terms of the Hansen Solubility Parameters (HSPs). The present work addresses further the question of the CNTs solubility by considering a larger range of solvents. A second part is devoted to the application of the HSPs to two types of functionalized CNTs: oxidized and silanized. These results stress the critical role played by the surface state of the CNTs, on the one hand, and the interest in using the HSPs to guide solubility investigations, on the other hand.

Application of the Hansen solubility Parameters theory to carbon nanotubes.

Carbon nanotubes (CNT) are very promising nano-objects due to their exceptional properties. However, their tendency to form bundles as well as their insolubility in common solvents makes them difficult to handle. The main way to solve the problem is chemical or physical CNTs functionalisations, with all the problems inherent to the methods. In this contribution, we present a new approach that allows predicting the solubility of carbon nanotubes in many solvents but also predicting the most appropriate solvents to use for given samples of CNTs. Solubilisation and dispersion being directly connected, the present approach of solubilisation proves also to be efficient in dispersing the CNTs bundles. This contribution is a first step toward the control of carbon nanotube's dispersion in polymers and their homogenous functionalisation. Moreover, we also report here a new method, based on solvents, to separate carbon nanotubes by size, the use of mixture of non-solvents in order to obtain good solvents and the use of mixture of good solvents to obtain higher solubility. The use of mixture of good solvents allowed us to obtain high solubility, up to three times higher then that reported in literature. We have also measured and analysed the solubility of some functionalised carbon nanotubes.

Preparation of organic nanoparticles using microemulsions: their potential use in transdermal delivery.

Organic nanoparticles of cholesterol and retinol have been synthesized in various AOT (Aerosol OT; sodium bis(2-ethylhexyl) sulfosuccinate)/heptane/water microemulsions by direct precipitation of the active principle in the aqueous cores. The nanoparticles are observed by transmission electron microscopy (TEM) using the adsorption of a contrasting agent, such as iodine vapor. The size of the nanoparticles can be influenced, in principle, by the concentration of the organic molecules and the diameter of the water cores, which is related to the ratio R=[H2O]/[surfactant]. The particles remain stable for several months. The average diameter of the cholesterol nanoparticles varies between 3.0 and 7.0 nm, while that of retinol varies between 4.0 and 10 nm. The average size of the cholesterol nanoparticles does not change much either as a function of the ratio R or as a function of the concentration of cholesterol. The constant size of the nanoparticles can be explained by the thermodynamic stabilization of a preferential size of the particles. Chloroform is used to carry the active principle into the aqueous cores. Retinol molecules form J-complexes composed of two or three molecules, as detected by UV-visible spectroscopy.

Formation of an adherent polyacrylonitrile/carbon nanotubes composite film onto a polyacrylonitrile brush electrografted on copper.

An adherent polymer film based on a composite of polyacrylonitrile/multiwall carbon nanotubes (PAN/MWNTs) have been elaborated on a copper substrate. The first layer is an electrografted PAN brush on which of a subsequent layer of PAN/carbon nanotubes composite has been deposited by simple dipping from solution in dimethylformamide (DMF). MWNTs have been previously chemically functionalized with 3-cyanopropyltrichlorosilane to promote de-bundling and homogeneous dispersion of the carbon nanotubes in the composite.

Note on the dispersion in perfluoropolyether lubricants of multi-walled carbon nanotubes functionalized with (tridecafluoro-1,1 ,2,2-tetrahydrooctyl)trichlorosilane.

Multi-walled carbon nanotubes functionalized with F3C-(CF2)5-(CH2)2-SiCl3, a molecule bearing a fluorinated terminal group is found to favor the carbon nanotubes dispersion in three perfluoropolyether fluids (Galden SV70, Fomblin Y25 and Fomblin YR1800). The stability of the suspension appears to be related to the perfluoropolyether viscosity. With respect to time, Fomblin YR1800, the more viscous fluid, leads to the most stable solutions, followed in order by Fomblin Y25 and Galden SV70. In general the smallest particles stay in suspensions. Stability extending over more than one year is observed in the case of Fomblin YR1800.

Mechanism of formation of inorganic and organic nanoparticles from microemulsions.

This chapter essentially deals with the preparation of nanoparticles using microemulsions. The preparation of inorganic nanoparticles--Ni2B, Pt, Au, Pt-Au, AgX--and the synthesis of organic nanoparticles--cholesterol, rhovanil, rhodiarome--are systematically studied as a function of the concentration of the precursor molecules, the size of the inner water cores, and the manner of mixing the various solutions. Two different behaviors are observed in the various systems. The first case shows a dependence of the nanoparticle size on the various physicochemical parameters. Either a monotonous increase of the size or the presence of a minimum is observed as a function of the concentration of the precursor molecules. This case can be easily explained following the LaMer diagram, where the nucleation of the nanoparticles is separated from the particle growth. The second case does not show any dependence of the nanoparticle size on the physicochemical parameters. The size remains constant in all experimental conditions. The constant character of the size can be explained only by thermodynamic stabilization, where particles with a certain size are better stabilized. It should be emphasized that the size distribution is small in all the cases studied. Finally, the aging of the nanoparticles was also checked, especially for the organic nanoparticles. It is concluded that these particles remain stable for months in the microemulsion.

Dispersion of carbon nanotubes using organic solvents.

Phenyl ethyl alcohol was used for fast and stable dispersion of carbon nanotubes. This solvent, more effective than ethanol and toluene, allows easy dispersion of carbon nanotubes for TEM characterization. For TEM grids prepared at high dilution, it is possible to observe each tube separately. Applying that solvent, it was possible to measure the length, the diameter and the solubility of different carbon nanotubes samples.

129Xe NMR study of Xe adsorption on multiwall carbon nanotubes.

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Hydrocarbon decomposition in alumina membrane: an effective way to produce carbon nanotubes bundles.

Carbon nanotubes were synthesised within the pores of an alumina membrane. The membrane had 200 nm diameter pores and 60 microm thickness, and ethylene was used as carbon source. Membrane dissolution by HF results in a bundle of parallel open tubes, aligned without macroscopic defects. The external diameter of the tubes is uniform and there is no evidence of any amorphous carbon. Wall thickness control was obtained by varying the reaction time, length by the thickness of alumina membrane, and external tube diameter by the membrane pore size. Scanning (SEM) and transmission (TEM) electron microscopy, atomic force microscopy (AFM), X-ray diffraction, thermogravimetric analysis (TG) and surface area evaluation by nitrogen adsorption were used for the characterization of membrane and nanotubes.

On the growth mechanism of single-walled carbon nanotubes by catalytic carbon vapor deposition on supported metal catalysts.

A comprehensive kinetic study was performed to throw light on the formation mechanism of single walled carbon nanotubes (SWNTs) in chemical vapor deposition processes. SWNTs were synthesized by catalytic decomposition of methane or ethylene on supported transition metal catalysts. Kinetic curves (the amount of SWNT as a function of time) were obtained as a function of the nature and the preparation of the supported catalysts, temperature, the fluxes of the gases (the reagent hydrocarbon and the carrying gas), and the partial pressure of the hydrocarbon. The final products were characterized by transmission electron microscopy, Raman spectroscopy, chemical analysis, and thermogravimetric measurements. The fundamental factors determining the SWNT formation are discussed in detail, taking into consideration several observations from the literature as well.

Continuous-flow optical pumping NMR in a closed circuit system.

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.

Synthesis of monodisperse nimesulide nanoparticles in microemulsions E170/isopropyl myristate/water/n-butanol (or isopropanol).

Nanoparticles of nimesulide have been synthesized in two systems of microemulsion: E170/isopropyl myristate/water/n-butanol (or isopropanol). Nanoparticles are monodisperse. In the two microemulsions, the size of the nanoparticles is comprised between 45 and 60 A and also seems to be independent of the factor R ([water]/[E170]) and of the concentration of the nimesulide solubilized in chloroform. The constancy of the size suggests that the size is controlled by thermodynamic stabilization of the nanoparticles with the surfactant. The nature of the cosurfactant does not have an obvious influence on the nanoparticle size. The nanoparticles are instantaneously formed and stay stable during a long period of time (several months).

Spin relaxation studies of pyrene by 13C nuclear magnetic resonance spectroscopy.

The effect of the repetition and contact times on the intensity of the magic-angle spinning (MAS) spectra of pyrene has been studied. Pyrene, like a great number of fused polyaromatic compounds, has a very large 13C longitudinal relaxation time (T1Z). When using the cross-polarization (CP) MAS technique, the optimum repetition time is 800 s. The simulation of the central line of the pyrene CP-MAS spectra allowed us to determine the chemical shift of each carbon in the solid state. As the T1Z (13C of the total spectrum) of pyrene was observed to be 1780 s, attempts have been made to relax it by the addition of chromium(III) acetylacetonate or by supporting it on a carrier of gamma-Al2O3: the mixing of chromium(III) acetylacetonate to pyrene by solid mixing, or solution mixing followed by evaporation of the solvent, has no effect on its relaxation time; the Haldor Topsøe TK551 catalyst, a gamma-alumina supported Ni-Mo-P catalyst, has been found to be a very efficient spin relaxation agent for the products adsorbed on it. For example, at 25 degrees C, the 10 wt.-% pyrene-TK551 catalyst is 3 x 10(4) times more relaxed than pure pyrene.

A formation mechanism for catalytically grown helix-shaped graphite nanotubes.

The concept of a spatial-velocity hodograph is introduced to describe quantitatively the extrusion of a carbon tubule from a catalytic particle. The conditions under which a continuous tubular surface can be generated are discussed in terms of this hodograph, the shape of which determines the geometry of the initial nanotube. The model is consistent with all observed tubular shapes and explains why the formation process induces stresses that may lead to "spontaneous" plastic deformation of the tubule. This result is due to the violation of the continuity condition, that is, to the mismatch between the extrusion velocity by the catalytic particle, required to generate a continuous tubular surface, and the rate of carbon deposition.

A multifaceted approach to the study of the side-chain conformation in beta-lactamase-resistant penicillins.

The resistance of some penicillins to beta-lactamase enzymes was previously attributed to the nature of their C(6) side chain. In order to find explicitly the influence of the conformation of this side chain in the enzymatic mechanism, we have analyzed by experimental and theoretical methods (X-ray diffraction, NMR, IR, PCILO) the molecular structure of six resistant penicillins and derivatives: oxacillin, cloxacillin, dicloxacillin, flucloxacillin, methicillin, nafcillin, cloxacillin sulfoxide, and oxacillinpenicilloic acid. X-ray crystallography of flucloxacillin and nafcillin is fully described. We observe that the side chains of these penicillins have no influence on the electronic properties of the penam nucleus but are much more rigid than in the sensitive ones. The molecular conformations are mostly governed by the nonbonded Van der Waals interactions and, in the oxacillins, partly by the conjugation between exocyclic groups. The lack of flexibility could result in a distorting effect on the structure of the active site of the beta-lactamase, leading to the deactivation of the enzyme.