NMR in Porous Materials

Restricted Diffusion

Diffusion in pores is quite different than in bulk. Water molecules near pore walls cannot freely move. Therefore they spend more time in one position when they are in proximity of walls than when they are in the center of pores. This can be well seen by diffusion weighted NMR experiment.

A notch (2.6 mm wide) drilled in a plastic glass was filled with water and imaged in by 1D diffusion weighted imaging sequence. When gradients for diffusion weighting were low (b) the 1D profile well followed the shape of the notch (a). As the gradients were increased the signal from the central region of the notch became significantly more attenuated than the signal of water molecules in the proximity of notch walls (e-g). This came to the extreme (h-i) when practically no signal was obtained from the center of the notch and still enough signal from the region next to walls. This was due to restricted motion of water molecules at notch walls.

Water migration in porous materials

Water in pores of many materials is often unwanted since it may reduce their durability and lifetime. From that perspective it is important to understand how water migrates into these materials. Water migration can be accurately monitored via high-resolution magnetic resonance imaging in non-magnetic and non-conductive materials. We were particularly interested in situations where at the beginning of migration the material was completely dry and at the end was saturated with water. Water migration was monitored by 1D profiles of water concentration along the direction of migration.

For the experiment, cylindrically-shaped samples of dental cement with a diameter of 3 mm and a length of 5 mm were used. The long surface of the samples was coated with a varnish to prevent water migration into the cement, while the ends of the samples were left uncoated. One end of the cylinder was exposed to a constant water concentration, and the other end was in contact with the air around the sample.

Images above depict water concentration (image intensity in color coded scale) as a function of depth (vertical axis) and time (horizontal axis) as measured in the experiment on dental cement. The experimental results were analysed by three different mathematical models: an imbibition model, a diffusion model, and a combination of both models. From the results it can be clearly seen that depth of the region saturated with water increases as the square root function of time.

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