Probing into the mesoporous structure of carbon xerogels via the low-field NMR relaxometry of water and cyclohexane molecules

Abstract

The liquid morphology of water and cyclohexane molecules, under partially saturated conditions inside the porous structure of two mesoporous carbon xerogels, was investigated using low-field nuclear magnetic resonance (NMR) relaxometry. The two xerogels were prepared under the same conditions, but with a different resorcinol-to-catalyst ratio. Two techniques were used in our investigations: the well-known Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence for transverse relaxation measurements and the Fast Field-Cycling (FFC) relaxometry technique for longitudinal relaxation dispersion measurements. It was found a stronger interaction of cyclohexane molecules with the pore surface as compared with water molecules. This leads to a uniform coverage of cyclohexane molecules on the pore surface for lower filling degrees while, in the case of water filled carbon xerogels, the distribution was non-uniform. Due to the stronger adsorption of cyclohexane molecules on the pore surface, it was possible to identify the micropores of the carbon xerogels predicted by nitrogen adsorption data. The NMR relaxation data have clearly demonstrated that, by increasing the resorcinol-to-catalyst ratio, the size of mesopores increases. Moreover, the enhanced adsorption affinity of carbon xerogels for cyclohexane shows that this type of materials can be employed as adsorbents for the removal of similar organic contaminants from water.