NMR Signal Relaxation

Engel and Hertz (1968) measured the NMR longitudinal relaxation times of the water-proton, Tie, in many aqueous electrolyte solutions mainly at 25 °C and for some salts also at 0 °C. An expression analogous to the Jones-Dole expression for the viscosities, Eq. (2.35), described the results very well  

They employed the convention that Bnmr(K+) = Bnmr(Cr) to obtain the ionic values. The rotational correlation times of the water molecules near ions are = 

The B-coefficients obtained from viscosity and NMR signal relaxation rates pertain to dilute solutions (they are the limiting slopes towards infinite dilution). However, an equation of the form of Eq. (3.6) for NMR spin-lattice relaxation rates holds up to fairly large concentrations. Chizhik (1997) reported values of relative water molecule reorientation times Tri/Trw at 22 °C, being 1 for Br , I, NH4+, NOs, and Ns , 1.0 for K+, and 1 for Li+, Na+, Mg +, Ca +, Sr +, Ba +, F , CH, H3O+, S04 , and COs, in more or less agreement with the signs of the Bnmr in dilute solutions. Table 3.1. 

These ionic B values corresponded well with the ionic B values according to Engel and Hertz and also to Abraham et al. [10, 21]. The B values are limiting slopes, but the ratios of the NMR signal relaxation times, proportional to the ratios VT KllT, have the same signs up to large concentrations as reported by Chizhik [22]. 

Such H NMR measurements of longimdinal relaxation times, could be applied only to diamagnetic ions, and hence for (paramagnetic) transition metal cations, the O NMR spin-lattice relaxation of D O molecules in aqueous salt solutions was measured and reported by Yoshida et aL [11]. Again, setting B (K ) = (CF) 

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CCR can easily be explained in a simplified form all coherences between nuclear spins, that finally give rise to NMR signals, relax (decay) with a certain rate, and eventually disappear. In dipolar relaxation, the relaxation of a spin is mediated by the fluctuating electromagnetic field caused by adjacent... 

The most difficult materials to study by NMR microscopy are those with short T2 or T2 relaxation times and/or with low concentrations of the nudear spins, which normally result in poor NMR signal intensities. One possibility for improving the image quality is to adapt the shape and size of the rf coils to the size of the objects in order to achieve the best possible filling factor and therefore the best sensitivity [1]. In addition, methods with short echo or detection times have been developed, such... 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

NMR signals are highly sensitive, via a number of different mechanisms, to the physical and chemical properties of porous materials. Using the set of NMR-based measurement methods that we have developed, it is possible to non-invasively and non-destructively characterize both the microstructural properties of the materials and relaxation properties of fluids imbibed into these materials. 

Figure 1 Schematic representation of the 13C (or 15N) spin-lattice relaxation times (7"i), spin-spin relaxation (T2), and H spin-lattice relaxation time in the rotating frame (Tlp) for the liquid-like and solid-like domains, as a function of the correlation times of local motions. 13C (or 15N) NMR signals from the solid-like domains undergoing incoherent fluctuation motions with the correlation times of 10 4-10 5 s (indicated by the grey colour) could be lost due to failure of attempted peak-narrowing due to interference of frequency with proton decoupling or magic angle spinning.

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