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

Morphology of porous media studied by nuclear magnetic resonance

The filling processes of water and cyclohexane in porous silica (40 Angstrom, 60 Angstrom and 112 Angstrom pore size samples) were studied using T-2 nuclear magnetic resonance (n.m.r.) experiments. The silica pores contained water or cyclohexane and the experiments were performed at room temperature and at filling fractions ranging from 0.02 to 1.0 (that is, completely full). Two distinct processes were observed which depended on the hydrophilicity of the silica surface (or the surface adhesion of the liquid). Water was found to collect in small puddles in the silica interstices, and to form a surface layer over the silica before the remaining pore volume was filled, Water in a surface-treated porous silica and cyclohexane in regular porous silica appeared to completely fill the smaller be-Fore the larger pores, and not form a separate surface-coating layer. This work also presents the techniques used to calculate quantitative information about the filling process; specifically, determination of the volume to surface-area ratio of the liquid puddles as well as the number of these puddles, is demonstrated. (C) 1997 American Institute of Physics

Phase equilibria of absorbed liquids and the structure of porous media

The phase equilibria of water within porous silica has been studied by proton and deuteron magnetic resonance, Proton signal amplitude as a function of filling factor was measured, These protons arise from the proton-deuterium equilibrium that is established between the liquid and the absorbed layer on the pore wall, The results for temperatures below 0 degrees C show a maximum as a function of filling factor, theta. This suggests that the pores fill from either a surface layer or from the crevices and interstices into the center. Another experiment used cryoporometry to study the size of crystals formed within the pores as a function of theta and leads to the same conclusion. Copyright (C) 1996 Elsevier Science Inc