1,721,028 research outputs found
A Ceglie Messapica si è gettato un seme
No full textL'articolo sottolinea alcuni aspetti dell'esperienza di formazione alla scrittura avviata a Ceglie Messapica sotto il segno della valorizzazione dell'autobiografismo
Resolving complex mixtures by means of pulsed field gradient NMR experiments
No full textWe present a method to analyse pulsed gradient spin-echo (PGSE) NMR data from a mixture of compounds sharing the same NMR resonance (e.g. polymer solutions or mixtures of aliphatic compounds). If all the spin-bearing species undergo Brownian motion, their contribution to the experimental echo decay is exponential (i.e. e(-sD), with s a function of the parameters of the PGSE-NMR experiment and D the self-diffusion coefficient). For the case of more than one diffusing species at a given chemical shift, the (normalized) echo attenuation is the Laplace transform of the distribution function of the self-diffusion coefficients. The Laplace transform can be reduced to a Fredholm integral equation of the first kind in the variable z proportional to e(-sD) (in the interval [0,1]). Applying the algorithm previously developed by us (L. Ambrosone, A Ceglie, G. Colafemmina and G. Palazzo, J. Chem. Phys. 1999, 110, 797) we solve the integral equation, obtaining the distribution function of the diffusion coefficients. The method is tailored for small data sets (10-30 points) typical of PGSE-NMR measurements. Moreover, the relevant variable (z) being an exponential function of the self-diffusion coefficient, it allows insight on the ne structure of the diffusion spectrum. The method was successfully tested on a three-component solution and on an aqueous solution of seven PEG oligomers. In the latter case an estimate of the molecular mass distribution function was obtained. The reported results indicate that such an approach permits determination of self-diffusion coefficients differing by 15% with a high accuracy (6-3%)
NMR studies of food emulsions: the dispersed phase self-diffusion coefficient calculated by the least variance method
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Removal of chromate from water by a new CTAB-silica gelatin composite
No full textA novel composite able to remove hexavalent chromium Cr(VI) from aqueous solutions was obtained by adding the silica precursor
tetraethoxysilane (TEOS) to the hexadecyltrimethylammonium bromide (CTAB) microemulsion-based gel. A physical characterization of the
new matrix revealed high stability of the silica gelatin composite in water at high temperatures and at neutral pH. Good efficiency in removing
chromate from neutral solutions was also demonstrated by the adsorption kinetics. In particular, the adsorption data of chromate obtained with
the CTAB–silica gelatin composite at 25 ◦C and pH 7.5 are described by the Freundlich isotherm model. The specific role of CTAB in the silica
gelatin composite was also evaluated by comparing the kinetics of the anionic AOT–silica gelatin composite to the CTAB one. The data collected
clearly showed that the positively charged surfactant was necessary to efficiently adsorb Cr(VI) from aqueous solutions. SEM and pulsed gradient
spin–echo NMR analysis of the composite demonstrated that the silicon is well assembled in the gelatin network, in which water molecules
maintain a high mobility. The diffusion coefficient of water in this system was shown to remain close to the value of pure water. Finally, X-ray
analyses of the elemental content in the CTAB–silica gelatin composite indicated no difference in terms of percentage of silica distributions in
different areas of the matrix and suggested that chromium adsorption could take place in internal areas
The role of the cosurfactant in the CTAB/water/n-pentanol/n-hexane water-in-oil microemulsion: 1. Pentanol effect on the microstructure
No full textThe microstructure of the quaternary water-in-oil microemulsion CTAB/water/n-pentanol/n-hexane has been investigated by means of the pulsed gradient spin-echo NMR technique over a wide range of composition. The composition of the continuous organic phase and of the interfacial phase has been determined through the analysis of the n-pentanol self-diffusion coefficient. The size of the reverse aggregates has been evaluated from the CTAB self-diffusion coefficient. The correlation of the reverse micellar size with interfacial composition has therefore been possible. Results coming from both water dilution lines and interface dilution lines have been analyzed according to suitable models. A "master plot", i.e., a graphical representation that allows us to display the data collected at all the possible compositions of the four components system, is also proposed
NMR for free and restricted diffusion studies in soft matter systems: applications and perspectives
No full textNMR study of AOT microemulsion with acetone in the presence of chlorophyll a: distribution of acetone and role of chlorophyll
No full textReverse micelles or water-in-oil microemulsions with proteins or molecules involved in photosynthesis such as Chlorophyll a (Chl a) are interesting as membrane mimetic systems. The multicomponent AOT/iso-octane/water-acetone system has been proposed as a carrier of photoactive Chl a. We have focused our attention on this system to characterize its microstructure through NMR techniques. We noted a considerable structural modification in the AOT/iso-octane/water system caused by acetone addition. The acetone partition between the organic bulk and the micellar phase was determined. Furthermore, a competition between Chl a and acetone to penetrate into the outer surfactant palisade was suggested. The solubilizate penetration controls the redistribution of the components between the bulk and the surfactant layers. The aggregation state of the photoactive Chl a dihydrate dimer, proposed as a model for the hypothesized primary donor of the PSII (photosystem II of higher plants) reaction center, is strongly modified when such a microemulsion is used
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