Morphology of porous media studied by nuclear magnetic resonance line shapes and spin–echo decays

Abstract

The line shape and spin–echo decays of liquids confined in small pores have been studied as a function of saturation. The samples used were of silica with nominal pore diameters of 60, 200, and 500 Å and had been saturated either with cyclohexane or water. The proton nuclear magnetic resonance linewidth and peak shift were measured as a function of saturation. For water samples the peak shift increased and linewidth decreased up to a critical saturation level above which the peak shift decreased and linewidth increased. This critical saturation point occurs when the surface layer is complete, and is related to the morphology of the pores. It provides evidence to suggest that the internal surface of the 200 Å silica sample was more irregular than the 60 Å sample. For the cyclohexane samples the peak shift increased and the linewidth decreased with saturation. The spin–echo amplitude decays were found to be faster than those seen in previous work. This is attributed to diffusion through a range of field gradients caused by susceptibility variations within the system. The nature of such variations depends on the saturation level. The decays were fitted to a stretched exponential function. Values for the average field gradients and the stretching parameter provide a measure of the range of gradients found and have been deduced for each experiment. Most information on the internal pore morphology is found by studying the systems at low levels of water content. The average field gradient and the stretching exponent may be related to the pore morphology.