1,721,018 research outputs found
A novel approach for determining the droplet size distribution in emulsion systems by generating function
No full textWe present a novel approach that allows us to figure out the size distribution of an isolated collection of droplets of a liquid dispersed in another one (emulsion) using NMR pulsed gradient spin echo (PGSE) measurements. The echo decay coming from the ensemble of droplets with different radii was related to the particle size distribution function and to the echo attenuation of an isolated sphere. This latter term was evaluated according to the second cumulant approximation, while the unknown distribution was approximated using a generating function series. The coefficients of the series are evaluated, using a least squares procedure, fitting the obtained relationship to the experimental data. We find that when the volume fraction distribution is contained in the range of lengths determined by the parameters of the PGSE measurement, the experimental data (echo attenuation) contain enough information to describe the distribution function in all the details. The method has been tested on both experimental and simulated data and appears to be powerful and flexible. It is able to work with highly asymmetric distributions, still giving accurate results
Three immobilized enzymes acting in series in layer by layerassemblies: Exploiting the trehalase-glucose oxidase-horseradishperoxidase cascade reactions for the optical determination oftrehalose
No full textA new optical biosensor for trehalose determination has been realized immobilizing three glycoenzymeson a transparent support. Trehalase, glucose oxidase and horseradish peroxidase have been alternatedwith layers of Concanavalin A by a “layer-by-layer” (LbL) deposition. The driving force of this assembly isthe biospecific complexation between Concanavalin A and sugar residues in the glycoenzymes. As con-firmed by UV–vis spectroscopy, the LbL deposition allowed a high ordinate architecture with high loadingof enzymes. After the assembly, the functionality of immobilized enzymes was spectrophotometricallyproven, demonstrating also that they can act in series catalyzing cascade reactions.The prepared biosensor was used to optically detect trehalose, giving a LOD of 10 M and a linearresponse up to 4 mM, and it showed also good time stability. The trehalose content in a real sample(eyewash) was successfully determined by the biosensor
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
Role of Functional Groups and Surfactant Charge in Regulating Chlorophyll Aggregation in Micellar Solutions
No full text
General Methods for Determining the droplet size distribution in emulsion system
No full textWe present a general method that allows us to figure out the size distribution of an isolated collection of droplets of dilute emulsion system using nuclear magnetic resonance pulsed gradient spin echo measurements. We show that the expression to obtain the volume fraction distribution function is equivalent to a Fredholm integral equation of the first kind. We prove, using the Dirac notation, that a solution of this equation can be easily found if its kernel has a complete biorthogonal system of eigenvectors. Two numerical procedures are discussed. The first, termed indirect, is based on the expansion of the unknown distribution function in the eigenfunctions of the kernel. The second one, called direct, uses the properties of shifted Legendre polynomials to integrate numerically the integral equation and evaluates the unknown distribution by means of a constrained least square procedure. The computational limits are analyzed. To extract the distribution's form directly by experimental data we have constructed a generating function using the shifted Jacobi polynomials. The procedures have been tested on simulated and experimental data and appear to be a powerful and flexible method to obtain the size distribution function directly by the experimental data
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
No full textThe impact of alkanes on the structure of Triton X100 micelles
No full textHere we investigate the structural evolution of TX100 micelles upon loading with several linear and cyclic alkanes by DLS, PGSE-NMR, 2D NOESY NMR, viscosity measurements, and molecular dynamic simulations.
Our results confirm that TX100 alone forms spherical, onion-like micelles made of several partially interpenetrating surfactant layers where the polyethylene glycol chains are in contact with the
tetramethyl-butyl-phenyl moieties. Loading with non-penetrating oils larger than decane induces a decrease in micellar size and hydration because the alkane molecules compete with both water and tetramethyl-butyl-phenyl groups for the polyethylene glycol chains. This results in the partial peeling of the “onion” and in the dehydration of polyethylene glycol chains so that the micelles increase in number and decrease in size upon alkane loading. In contrast, small and penetrable oils (mainly cyclo-alkanes) first swell the onion-like micelles (inducing an increase in size) and only above a critical oil/surfactant ratio does the oil induce the weakening of the multilayer structure and the dehydration of polyethylene glycol chains found in long linear alkanes
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