1,721,017 research outputs found
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%)
Dielectric properties of biological cells in the dipolar approximation for the single-shell ellipsoidal model: the effect of localized surface charge distribution at the membrane interface
No full textNumerical simulation of dielectric spectra of aqueous suspensions of non-spheroidal differently shaped biological cells
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A self-diffusion study of poly(ethylene-oxide) alkyl alcohols
No full textThe self-diffusioncoefficients of HDO and some surfactants in aqueous mixtures
at different concentrations, below the critical micelle concentration, have been
determined by means of the NMR, spin-echo pulsed field gradient method. The surfactant solutes chosen were ethylene glycol-pentyl alcohol (diethylene glycol- pentylalcohol, ethylene glycol-hexyalcohol, diethylene glycol-hexyl alcohol, tri- ethylene glycol-hexyl alcohol, tetraethylene glycol-hexyl alcohol, pentaethylene glycol-hexyl alcohol). The interactions in solution are studied by analyzing the solute self-diffusion coefficients extrapolated to infinite dilution. These values are comparedwith those of 1-alkanols.The slope of the self diffusioncoefficients vs. the solute concentration are correlated with the microscopic friction coeffi- cients. A model for interpreting the experimental data is suggested
Density and viscosity studies of poly(ethylene-oxide) alkyl alcohols
No full textDensity and viscosity of binary systems water-nonionic surfactants poly(ethylen-oxide) alkyl alcohols type, [CnH2n+1(OCH2CH2)mOH, CnEm], have been studied. The partial molar volumes in the dilute solution range and the viscosity B-coefficients were calculated. The nonionic surfactants partial molar volumes were compared with those of ethylene glycol and poly(ethylenglycol) (PEG). The comparison shows that the ethoxy unit volume seems to be independent of the particular system. The consequences of this are discussed. A model for interpreting the experimental B values has been proposed. The model treats the macroscopic viscosity as the superimposition of different local effects. The following surfactants have been considered: C5E1, C5E2, C6E1, C6E2, C6E3, C6E4
Diffusion phenomena in ternary systems polymer-nonsolvent-solvent
No full textDiffusion in a boundary between a polymer + solvent soln. and nonsolvent was treated by accounting for the presence of the four diffusion coeffs. that describe the isothermal transport process in a three-component system. Diffusion equations were integrated assuming a concn. dependence of diffusion coeffs. that account for the thermodn. conditions on the cross-diffusion terms. The presence of non-zero cross terms promotes an incongruent diffusion of polymer whose concn. increases at the boundary between the polymer + solvent soln. and the non-solvent. Although the authors' model describes diffusion in the range of homogeneous soln., this incongruent polymer diffusion is a process similar to that promoted by the solvent evapn. from the polymer + solvent film that some authors suggested as an intermediate step before the film immersion into the coagulation bath to obtain good asym. membranes
Empirical correlations between thermodynamic and spectroscopic properties of aqueous solutions of alkan-m,n-diols: excess enthalpies and spin-lattice relaxation times at 298.15 K.
No full textThe heats of diln. in water and nuclear relaxation times at 298.15 were detd. for binary aq. solns. contg. alkan-a,w-diols and for ternary aq. solns. contg. the probe mol. hexane-1,2- or hexane-1,6-diol and one of the members of the homologous series of alkane-1,2-diols and of alkane-a,w-diols, up to heptane-1,7-diol. The results were interpreted through the model of preferential orientation, i.e. the "side-on" model, already proposed to explain the interactions in soln. between various positional isomers of hydroxylated substances. For binary solns., a correlation is presented between the relaxation times and the enthalpic self-interaction coeffs
Dynamic properties of aqueous solutions of ethylene glycol oligomers as measured by the pulsed gradient spin-echo NMR technique at25 °C
No full textSelf-diffusion coefficients for binary systems containing water and an oligomer of ethylene glycol [HO-(CH2CH2-O)nH, n = 1-5], have been measured at 25 °C in the whole concentration range by the pulsed gradient spin-echo NMR technique. Upon adding oligomer to pure water, both the oligomer and water self-di†usion coefficients decrease until a certain composition is reached, after which they remain approximately constant. This evidence is interpreted in terms of molecular interactions, especially hydrogen-bond formation. A relation between mutual and self-diffusion coefficients for these systems is found and commented upon
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