A joint use of melting and freezing data in NMR cryoporometry

Freezing and melting behaviour of water, benzene, cyclooctane and cyclohexane confined in different-sized controlled pore glasses (CPG’s) has been studied by NMR cryoporometry technique. It was found that the freezing (ΔTf) and melting (ΔTm) temperature shifts correlate linearly, giving a constant ratio ΔTm/ΔTf over different fluids. This suggests that the ratio is a measure of pore morphology rather than fluid’s properties. The suggestion is supported by the theoretical treatment that predicts for ΔTf to be controlled by the surface-to-volume ratio (S/V) and ΔTm by the pore curvature (dS/dV). Therefore, the ratio ΔTm/ΔTf provides one with the information on the pore shape, unavailable from ΔTf or ΔTm measurements alone. For CPG samples under study, this ratio decreases from 0.67 in 7.5 nm pores, which is expected for spherical geometry, to 0.57 in 27.3 and 72.9 nm pores, which is closer to what is expected for cylindrical, or tubular, pores. The pore size distribution functions obtained from the freezing and melting data were found to be similar in shape and width, which indicates that melting and freezing processes occur in CPG on the same length scale of the pore structure.