Slow dynamics of embedded fluid in mesoscopic confining systems as probed by NMR relaxometry

Mesoscopic media such as porous materials or colloidal dispersions strongly influence the dynamics of the embedded fluid. In the strong-adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long-time pathology, enlarging the time domain of the embedded-fluid dynamics towards the low-frequency regime. An interesting way to probe such a slow interfacial process is to use the field-cycling NMR relaxometry. This technique is used here to probe the fluid dynamics in two types of interfacial systems: i) a colloidal glass made of thin and flat particles; ii) a fully saturated porous media, the Vycor glass. Experimental results are critically compared to either a simple theoretical model of NMR dispersion involving elementary steps of the fluid dynamics near an interface (loops, trains, tails) or Brownian-dynamics simulations performed inside 3D reconstructions of these confined systems.

Surface nuclear magnetic relaxation and dynamics of water and oil in macroporous media

Proton nuclear spin-relaxation studies on water- or oil-saturated granular packings and limestone rocks allow estimating surface molecular dynamical parameters. Measurements were performed at various conditions of temperature, magnetic field strengths, and pore size. We show by low field NMR relaxation that changing the amount of surface paramagnetic impurities leads to striking different pore-size dependences of the relaxation times T1 and T2 of liquids in pores. These dependences are well supported by surface-limited or diffusion-limited relaxation models. Surface relaxivity parameters rho(1) and rho(2) are deduced from the pore-size dependence in the surface-limited regime. We evidence the frequency and temperature dependence of the surface relaxivity rho(1) by field cycling NMR relaxation and relevant theoretical models. The typical frequency dependence found allows an experimental separation of the surface and bulk microdynamics in porous media. Several surface dynamical parameters, such as diffusion coefficients, activation energies, time of residence, and coefficient of surface affinity, were therefore determined. The methods presented here give a powerful analysis of the surface microdynamics of confined liquids, which can be applied to the study of oil-bearing rocks.

New ways of probing surface nuclear relaxation and microdynamics of water and oil in porous media

The microdynamics of water and oil in macroporous media with SiO2 or CaCO3 surfaces has been probed at various temperatures by magnetic field-cycling measurements of spin-lattice relaxation rates. These measurements and an original theory of surface diffusion allow us to obtain surface dynamical parameters such as surface correlation times, residence times and diffusion coefficients. A coefficient of affinity of the liquids for the pore surface is deduced.

Investigation of the microporosity of high performance concrete by proton nuclear relaxation

The difference in microporosity features between high and ultra high performance concrete was highlighted by measuring their respective proton spin-lattice relaxation times. A surface fractal dimension was attributed to each formulation and exhibits a correlation with the amount of calcium silicate hydrates.

Micropore size analysis in hydrated cement paste by NMR

We present a time evolution of 1H spin-lattice relaxation rates in the laboratory (1/T(1)) and in the rotating frame (1/T(1rho)) of a synthetic cement paste. The typical results found for both rates allows us to follow the main hydration stages of the cement paste and the refinement of its microporosity. In particular the texturation of the porosity and the structuration of the surface of the material is evidenced.

Surface dynamics of liquids in porous media

We report remarkable differences in the 1H nuclear magnetic relaxation dispersion data (NMRD) between water and other common aprotic solvents such as acetone when in contact with high surface area calibrated microporous chromatographic silica glasses that contain trace paramagnetic impurities located at or close to the pore surface. All these differences have been related to the particular chemical behaviors and dynamics of these liquids at the pore surface. We apply this technique to probe the structure and dynamics of water and oil at the surface of calibrated macroporous systems, where similar surface dynamics effects have been observed. This technique is also applied to follow the first hydration stage of a white cement-paste. Last, we present an analysis of the magnetic field dependence of 1H nuclear relaxation data to exhibit the microporosity of ultra high performance concretes.

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.

Translational diffusion of liquids at surface of microporous materials: new theoretical analysis of field cycling magnetic relaxation measurements

1H spin-lattice relaxation rates of several aprotic polar liquids on calibrated microporous chromatographic glass beads that have paramagnetic ion impurities are recorded over magnetic field strengths using a field-switched magnetic relaxation dispersion spectrometer. The typical bilogarithmic magnetic field dependence of these rates supports quantitatively our theory of nuclear paramagnetic relaxation and gives the translational diffusion at the surface of nanopores. Our results demonstrate that magnetic relaxation dispersion at low magnetic field strengths in high surface area heterogeneous systems may be quantitatively understood in terms of the parameters of the spatial confinement and the local translational dynamics.

Analysis of microporosity and setting of reactive powder concrete by proton nuclear relaxation

The proton spin-lattice relaxation measured at several frequencies leads to a resolved distribution of four Tli for reactive powder concrete (RPC). The typical Tli frequency dependences are quantitatively interpreted by a biphasic fast exchange model and a proton nuclear relaxation of hydrated paramagnetic ions at the surface of the pores. This leads to an estimation of the pore sizes. We present the first application of this nuclear relaxation method to follow in situ the kinetics of the hydration and setting of such material.

Magnetic cross-relaxation and chemical exchange between microporous solid and mobile liquid phases

Nuclear magnetic spin-lattice relaxation rates are reported as a function of magnetic field strength, pH, and temperature for water protons in Sephadex gels. The water proton rates report the relaxation profile characteristic of the solid at high pH values. At lower pH values, the relaxation rate is limited by a slow chemical exchange process that makes the relaxation rate independent of magnetic field at low field strengths. In cases where the proton exchange rate does not limit the total relaxation rate, the magnetic field dependence of the relaxation rate is given by a power law of the type R = Av-B, where B is 0.75 except at pH 11 where the data are better represented with B = 0.5. The exchange events important for the water proton relaxation are represented as a two-stage process in which water within the pores of the gel exchanges protons and magnetization with the solid spin system rapidly compared with the rate at which the water within the pores exchanges with the bulk water phase.

Fractal geometry impact on nuclear relaxation in irregular pores

We apply a fractal description of pore surface irregularity to study the nuclear relaxation of a confined liquid. From the introduction of a length characteristic of diffusive and surface relaxing properties we describe three different relaxation regimes. These regimes show that the nuclear relaxation can be drastically modified by pore surface irregularity.

Quenched molecular reorientation and angular velocity in nanopores

A theoretical treatment shows how the orientational dependent and spin rotation relaxation rates of a confined nonpolar liquid depend on the average pore size. Experimental nuclear relaxation data on carbon disulfide and cyclohexane in a set of calibrated porous glasses support the theory.

Nuclear relaxation of liquids in confinements

We study theoretically the effects of geometrical confinement on the dipolar relaxation of a non-interacting liquid in porous media. Application to the 1H relaxation of methylcyclohexane liquid in porous silica glasses is given. The case of an interacting liquid is considered by molecular dynamics simulations. Geometrical confinement and surface interaction lead to similar frequency behaviour of relaxation rates according to the layering of local density and anisotropy of the molecular mobility.