NMRcryoporometri

NMR cryoporometry is a non-destructive porosity characterization technique that determines the pore size distribution of a material by monitoring the melting behavior of a liquid confined in its pores with nuclear magnetic resonance (NMR). It relies on the fact that liquids confined in nanopores exhibit a depressed melting point compared with the bulk liquid.

During measurement, the sample is saturated with a probe liquid, cooled to solidify the liquid within the pores, and then slowly warmed. The fraction of liquid, as detected by NMR signals (commonly the amplitude or relaxation properties of the 1H signal), increases as pores melt. The observed melting temperature for pockets of a given size is related to the pore radius via the Gibbs–Thomson equation, allowing a pore size distribution to be inferred from the temperature-dependent liquid fraction.

Typical procedure elements include the selection of a probe liquid with a well-defined bulk melting point and known interaction with the pore wall; careful saturation and removal of excess liquid; controlled heating with temperature stabilization; acquisition of NMR data as a function of temperature; and data processing to convert the temperature axis to pore radius using a calibration, sometimes with derivative techniques to obtain a differential pore-size distribution.

Advantages of the method include its non-destructive nature, applicability to a broad range of pore sizes (nanometer scale) and compatibility with solid samples under ambient pressure. Limitations include dependence on the probe liquid–surface interactions, the need for calibration, potential adsorption effects at the pore walls, and the requirement for NMR instrumentation. NMR cryoporometry complements other porosimetry methods such as gas sorption and mercury intrusion.