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Cet article montre les potentialités de la relaxation nucléaire dans l'étude de la microdynamique de liquides moléculaires dans différents systèmes confinés et poreux, et en particulier aux interfaces solide-liquide. Nous avons mis en évidence une réduction et une anisotropie de la dynamique de réorientation moléculaire de liquides à la surface de pores nanomètriques dans différentes conditions de température et de pression. La technique de RMN par cyclage de champ magnétique nous a également permis de mettre en évidence des mouvements de diffusion anormaux de l'eau dans des billes de verres aux pores calibrés. We demonstrate the potential of nuclear relaxation for studying the microdynamics of molecular liquids in confined and porous systems, with particular emphasis on solid-liquid interfaces. We have observed both a reduction and an anisotropy in the molecular reorientation dynamics of liquids on nanometric pore surfaces under different temperature and ressure conditions. The magnetic field cycling NMR technique has also allowed us to observe abnormal diffusion of water among glass beads with calibrated pores.

A new approach is demonstrated for investigating charge and spin diffusion as well as surface and bulk recombination in unpassivated doped semiconductors. This approach consists in using two complementary, conceptually related, techniques, which are time-resolved photoluminescence (TRPL) and spatially resolved microluminescence (μPL) and is applied here to p+ GaAs. Analysis of the sole TRPL signal is limited by the finite risetime. On the other hand, it is shown that joint TRPL and μPL can be used to determine the diffusion constant, the bulk recombination time, and the spin relaxation time. As an illustration, the temperature variation of these quantities is investigated for p+ GaAs. [ABSTRACT FROM AUTHOR]

Discusses quenched molecular reorientation and angular momentum for liquids confined to nanopores of silica glasses prepared by the sol-gel process. Natural abundance spin-lattice relaxation times of liquids; Theoretical model for anisotropic molecular reorientation and angular velocity.

The relative roles of surface and topological effects on the nuclear relaxation rates T-11, T-12, and T-11ρ of polar or nonpolar liquids in porous sol-gel silica glasses are identified via their very different pore size and frequency dependences. On the basis of theory, experimental relaxation rates, and molecular dynamics simulations for the modeled porous systems, the 1/Ti’s are interpreted in terms of a linear combination of bulk, confinement, and surface effects: 1/Ti = 1/Tibulk + ai/R2+ bi/R, where R is the average pore size and ai and bi are given in terms of the usual relaxation parameters of the studied molecular species. This simple expression which allows the determination of the relative roles of surface and topological effects has been used to fit the observed 1H NMR relaxation rates as a function of pore size and frequency for methylcyclohexane, nitrobenzene, pyridine, and toluene both for nonmodified and surface modified porous silica glasses. Using this method, the surface (∝1/R) and pure geometrical (∝1/R2) relaxation contributions are evaluated and the surface and translational correlation times are calculated. More generally, the experimental data allow us to explain the following seemingly paradoxical results obtained for confined liquids: (i) The pure confinement effect is independent of the polarities of the liquid molecules in pores and is very sensitive to the frequency. (ii) The finding of the frequency variation of T-11 and T-11ρ both for polar or nonpolar liquids confined to small pores, shows that the geometrical confinement effects dominate over the surface interaction effects at low frequency and for small pores. [ABSTRACT FROM AUTHOR]

A theory of dipolar relaxation by translational diffusion of a nonwetting liquid confined in model porous media is presented. We obtain expressions of the rates of spin-lattice relaxation 1/T1, spin–spin relaxation 1/T2, and spin-lattice relaxation in the rotating frame 1/T1ρ, which depend on the average pore size d. The frequency variations of these rates are intermediate between the two-dimensional and three-dimensional results. At small frequency they vary logarithmically for small d and tend progressively to a constant with increasing d. For small pore sizes we obtain quadratic confinement dependences of these rates (∝1/d2), at variance with the linear (∝1/d) relation coming from the biphasic fast exchange model usually applied for a wetting liquid in porous media. We apply such a theory to the 1H NMR relaxation of methylcyclohexane liquid in sol-gel porous silica glasses with a narrow pore-size distribution. The experiments confirm the theoretical predictions for very weak interacting solvent in porous silica glasses of pore sizes varying in the range of 18.4–87.2 Å and in the bulk. At the limit of small pores, the logarithmic frequency dependencies of 1/T1ρ and 1/T1 observed over several decades of frequency are interpreted with a model of unbounded two-dimensional diffusion in a layered geometry. The leveling off of the 1/T1ρ low-frequency dependence is interpreted in terms of the bounded two-dimensional diffusion due to the finite length L of the pores. An estimate of a finite size of L=100 Å is in excellent agreement with the experimental results of the transmission electron microscopy study of platinium-carbon replicated xerogels. [ABSTRACT FROM AUTHOR]

We propose a theory of dipolar spin relaxation for lamellar geometry where one of the spin species diffuses on an infinite plane while the other diffuses within an infinite layer of finite thickness. Analytical expressions of the frequency dependence of the spectral densities and spin relaxation rates are given. The theoretical frequency variation of the longitudinal relaxation rate appears to be drastically dependent of the anisotropic diffusion ratio and on the thickness of the layer. An application of this theory to lamellar lyotropic liquid crystal gives the spatial distribution of an organic counterion solute in the bilayer from the frequency variation of the 31P longitudinal relaxation rates of the surfactant. This gives some information about the surfactant-organic counterion interaction and the hydration degree of the phosphate group. [ABSTRACT FROM AUTHOR]

The dynamic effects of pair correlation functions (pcf) on spin relaxation by translational diffusion in infinite two-dimensional fluids are considered explicitly through a Smoluchowski equation, for the usual conditional probability, with appropriate boundary conditions, especially at the contact separation of the interacting pair of molecules. The solution of this equation by finite difference techniques permits the calculation of time correlation functions, spectral densities, and spin-relaxation rates associated with a dipolar relaxation mechanism between the spin-bearing molecules. Comparison of the two-dimensional spin-relaxation results obtained with different pcf is presented. The spectral densities and spin-relaxation rates are indeed found to be significantly altered by the pcf. For example, for a nonuniform pcf, the two-dimensional spectral densities, at (and above) the Larmor frequency ω0, are greater for translational correlation times τt that are an order of magnitude faster than τt=ω-10 which provides the maximum spectral density in bulk theory. This fast motion result is consistent with the two-dimensional dynamical results found by other techniques. Moreover for a nonuniform pcf and a single translational correlation time, two well-defined minima are found in the variation of the spin-lattice relaxation time with the diffusion coefficient. This could be very useful for interpreting the spin-relaxation data of diffusing complexes in clays, intercalation compounds, and bilayers. [ABSTRACT FROM AUTHOR]

The theoretical model of frequency and temperature dependence of surface relaxivity rho(sub 1,2) in porous media is outlined. The nuclear spin-relaxation experiments, at various temperatures and Larmor frequencies for water and oil confined in a series of calibrated porous media and various natural rocks are presented and their potential impact on well logging applications is studied.