Pulsed gradient spin-echo nuclear magnetic self-diffusion coefficients

Basically two nuclear magnetic resonance (NMR) techniques can be used to determine the partition coefficient. The first is based on the Fourier transform NMR pulsed-gradient spin echo (FT-PGSE) self-diffusion techiuque, the other by the NMR paramagnetic relaxation technique. In both techniques the fraction of solute in the micelle, a, is determined and can thus be calculated through Equation 6.10. 

The microstructure of complex fluids such as ILs, surfactant systems, and liquid crystals can be profitably investigated by means of pulsed gradient spin-echo nuclear magnetic resonance (PGSE-NMR) experiments, a technique that allows the determination of the self-diffusion coefficients. 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

Molecular self-diffusion coefficients are experimentally easily accessible and can be measured accurately with various techniques. However, the Fourier transform pulsed field gradient spin-echo NMR (FT PGSE NMR) approach has during the last decade proved to be superior to other approaches for several reasons. In this technique the displacement of nuclear spins in a controlled magnetic field gradient is monitored, and the contributions of different components are resolved by Fourier transformation of the NMR signal (spin echo). 

The different mobilities of monomers versus micelles can be used as a means to determine the value of CMC. The self-diffusion coefficients of the water and surfactant can be measured by pulsed-gradient spin Fourier-transform nuclear magnetic resonance echo (FT-NMR) spectroscopy, following the NMR spectra (13). Hence, a plot of the diffusion coefficient D versus the surfactant concentration shows a change in slope corresponding to the CMC. 

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