Internal surfaces of porous media studied by nuclear magnetic resonance cryoporometry

The filling processes of water and cyclohexane in porous silica (with a characteristic pore size of 60 Angstrom) are investigated using the nuclear magnetic resonance (NMR) technique of cryoporometry. In this technique, the liquid was frozen in the pores before the temperature was raised gradually; melting the smallest particles first and then particles of increased size. The volume of the molten liquid present was measured using the height of a T-2 spin echo. The experiments were performed with filling fractions ranging from 10% to 100%. The results showed distinctly different behaviors of the fluids, which depended on the surface adhesion. It was found that water (a fluid which wets the pore surface) forms small puddles-much smaller than the smallest pore size-at low filling fractions. These puddles grow in size as more water is added until all the pore volume is filled. Cyclohexane (a non-wetting fluid) on the other hand, does not form small puddles but completely tills the pores with a preference for the smaller pores. Water is found to give more accurate results for the pore size distribution than cyclohexane, in 60 Angstrom silica.