Cryoporometry and relaxometry of water in silica-gels

Both cryoporometry and relaxometry are tools to determine the pore size distribution (PSD) of a porous material with NMR. The melting point depression is described by the Gibbs-Thomson equation, yielding the PSD from cryoporometry. The enhanced relaxivity is caused by the surface of the porous material, yielding the PSD from relaxometry. The description in the classical paper of Brownstein and Tarr is only valid for one pore (size). The extended theory of McCall et al. is needed to describe a heterogeneous coupled porous system. As testing material a series of silica-gels called Nucleosil is chosen with typical pore sizes of 5, 10, 12 and 30 nm. Transverse relaxation time distributions are measured using a CPMG-sequence for every temperature of the cryoporometry measurement. These show a mono exponential behaviour, indicating a strongly coupled porous structure. Using the cryoporometry data, an attempt is made to reproduce the averaged relaxivity. Agreement is found for pores with typical pore sizes between 10 nm and 1 microm. The model is not valid for pores smaller than 10 nm.