Water, spin-lattice relaxation

Fig. 3. Schematic representation of the topological space of hydration water in silica fine-particle cluster (45). The processes responsible for the water spin-lattice relaxation behavior are restricted rotational diffusion about an axis normal to the local surface (y process), reorientations mediated by translational displacements on the length scale of a monomer (P process), reorientations mediated by translational displacements in the length scale of the clusters (a process), and exchange with free water as a cutoff limit.

Fig. 10. The water spin-lattice relaxation rates as a function of magnetic field strength represented as the Larmor frequency for packed samples of calibrated porous glass beads with pore diameter of 75 A at several temperatures. The solid lines are best fits to the theory (67).

Fig. 14. Measured water spin-lattice relaxation rates of a hydrated mortar at w/c = 0.38 as a function of the proton Larmor frequency, for different duration of hydration Oh 34 min ( ), 7h 27 min (O), 8h 45 min ( ), and 9h 40 min ( x ), upwards. The insert represents the data obtained after a hydration time of lOh 32 (+), the labels for the two axis are equivalent to those of the main figure. The continuous lines correspond to the best fits to the theory.

NMR relaxation of liquids such as water in porous solids has been studied extensively. In the fast exchange regime, the spin-lattice relaxation rate of water in pores is known to increase due to interactions with the solid matrix (so-called surface relaxation ). In this case, T) can be described by Eq. (3.5.6) ... 

K. Krynicki 1966, (Proton spin-lattice relaxation in pure water between 0 °C and 100°C), Physica 32, 167-178. 

J. C. Hindman, A. Svirmickas, M. Wood 1973, (Relaxation processes in water. A study of the proton spin-lattice relaxation time),/. Chem. Phys. 59 (3), 1517— 1522. 

Another possible solution to the problem of analyzing multiple-layered membrane composites is a newly developed method using NMR spin-lattice relaxation measurements (Glaves 1989). In this method, which allows a wide range of pore sizes to be studied (from less than 1 nm to greater than 10 microns), the moisture content of the composite membrane is controlled so that the fine pores in the membrane film of a two-layered composite are saturated with water, but only a small quantity of adsorbed water is present in the large pores of the support. It has been found that the spin-lattice relaxation decay time of a fluid (such as water) in a pore is shorter than that for the same fluid in the bulk. From the relaxation data the pore volume distribution can be calculated. Thus, the NMR spin-lattice relaxation data of a properly prepared membrane composite sample can be used to derive the pore size distribution that conventional pore structure analysis techniques... 

Activation volumes were derived from pressure dependent NMR experiments using the equation A E = —kT d In T dp]T, where 7) is the spin—lattice relaxation time. A Evalues for the H and NMR experiments were close to each other as well as to the values based on conductivity. These results imply that the electrical transport is correlated with water molecule rotation. There is a trend of increasing A E with decreasing water content. 

The second role of the chemical exchange phenomena can be seen in Eq. (2) the exchange lifetime competes with the in-complex nuclear spin-lattice relaxation time and can become a limiting factor in the attainable PRE. This aspect of the problem is highly relevant in practical consideration in the case of Gd(III) complexes as a potential contrast agent, because the water exchange in these systems is not too fast. This issue is considered to be outside of the scope of this article and we refer to recent literature on the subject 5,160) and to other contributions in this volume. 

NMRD studies (0.01-30 MHz) on bentonite suspensions showed that the water-proton spin-lattice relaxation rates are dominated by magnetic interactions with paramagnetic centers entrapped in the mineral matrix (89). The 1/Ti values were linearly dependent on the concentration of the... 

Fig. 1. Magnetic field dependences of the proton spin-lattice relaxation time of water in Bioran B30 and Vycor glasses at temperatures above 27°C and below the temperature where the non-surface water freezes ( —25°C and —35°C). The solid lines represent the power law in the Larmor frequency with an exponent of 0.67 (34).

Fig. 2. Water proton spin-lattice relaxation dispersion in Cab-O-Sil M- samples with two different degrees of compression. The solid lines were computed using the model as in Ref. (45).

Fig. 7. H water proton relaxivity i.e., the nuclear spin-lattice relaxation rate per mM of metal, plotted as a function of the magnetic field strength expressed as the proton Larmor frequency for aqueous solutions of manganese(H) and iron(HI) ions at 298 K. (A) 0.10 mM manganese(II) chloride in 2.80 M perchloric acid (B) 0.1 mM aqueous manganese(H) chloride at pH 6.6 (C) 0.5 mM iron(HI) perchlorate in 2.80 M perchloric acid (D) 0.5 mM iron(IH) perchlorate in water at pH 3.1 (F) 2.0 mM Fe(HI) in 2.0 M ammonium fluoride at pH 7, which causes a distribution of species dominated by [FeFe]"-.

Fig. 8. The water-proton spin-lattice relaxation rates vs. magnetic field strength plotted as the Larmor frequency at 282 K for hexacyanochromate(II) ion ( ), trioxalatochromate(III) ion ( ), and trimalonatochromate(III) ion (A). The lines were computed using translational diffusion models developed by Freed with and without the inclusion of electron spin relaxation effects 54,121).

Fig. 11. The nuclear magnetic spin-lattice relaxation rate for water protons as a function of the magnetic field strength reported as the proton Larmor frequency at 298 K forVolclay ( ), Polargel ( ), and Magnahrite (A) (68).

Fig. 13. The nuclear magnetic spin-lattice relaxation rate for water protons as a function of magnetic field strength reported as the proton Larmor frequency at 298 K for 5% suspensions of the particulate stabilized in a 0.5% agar gel presented as the difference plot (A) Zeolite 3A (B) Zeolite 13X (C) Zeolite NaY (D) kaolin with 7 s added to each point to offset the data presentation (E) Cancrinite with 9 s added to each point to offset the data presentation and (F) 0.5% agar gel profile with 10 s added to each point. The solid lines are fits to a power law (68).

Fig. 17. The water proton spin-lattice relaxation rates as a function of magnetic field strength reported as the proton Larmor frequency in aqueous 1.8 mM samples of bovine serum albumin. The lower data set was taken on the solution, the open circles taken after the sample had been cross-linked with glutaraldehyde to stop rotational motion (89).

Adsorbed water was observed to have a large effect on the F spin-lattice relaxation time for fluorine-doped aluminas in the dilute and intermediate concentration range of fluorine (0.3-8 wt. % F). An increase in Ti by a factor of 2 to 3 was observed in these samples when adsorbed water was removed from the solid by heating between 200-300°. The effect was completely reversible addition of oxygen-free water back to the solid resulted in recovery of the original (shorter) relaxation time. This effect was observed by the measurement of the in phase and ir/2 out of phase components of the dispersion derivative at resonance dx /6Ho at high rf power, from which effective values of Ti may be calculated 46). Values of Ti were also obtained by saturation of the resonance absorption derivative. 

Clearly by working with typical spatial resolutions of approximately 30-50 pm, individual pores within the material are not resolved. However, a wealth of information can be obtained even at this lower resolution (53,54,55). Typical data are shown in Fig. 20, which includes images or maps of spin density, nuclear spin-lattice relaxation time (Ti), and self-diffusivity of water within a porous catalyst support pellet. In-plane spatial resolution is 45 pm x 45 pm, and the image slice thickness is 0.3 mm. The spin-density map is a quantitative measure of the amount of water present within the porous pellet (i.e., it is a spatially resolved map of void volume). Estimates of overall pellet void volume obtained from the MR data agree to within 5% with those obtained by gravimetric analysis. 

In weakly solvating solvents interionic interactions between organic molecular ions can lead to fixation of the ionic end of the molecule. The Tl values of pertinent and neighboring 13C nuclei become smaller. In contrast, strongly solvating solvents such as water and alcohols inhibit interionic interaction and lead to an enhanced mobility of the ions solvated by ion-dipole interactions 13C spin-lattice relaxation is consequently slower in such solvents. Thus the T, values of n-butylammonium trifluoroacetate increase with the polarity of the solvent, as shown in Table 3.19 [148]. 

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