Analysis of pore size distribution by 2H NMR

The pore size distributions of four controlled pore glasses with mean diameters ranging from ca. 7.9 to 23.9 nm were analysed by measuring the (2)H NMR signals from the liquid fraction of confined benzene-d(6) and cyclohexane-d(12) as a function of temperature, in steps of ca. 0.1-1 K. The liquid and solid components of the adsorbates were distinguished, on the basis of the spin-spin relaxation time T(2), by employing a spin-echo sequence. The experimental intensity curves of the non-frozen liquids are well represented by a sum of two error functions. The observed melting point depressions are well represented by the simplified Gibbs-Thompson equation DeltaT = k(p)/R where R is the pore radius and k(p) is a characteristic property of the adsorbate. The k(p) value mainly affects the position of the pore size distribution curve, i.e., the mean pore radius, while the slope of the intensity curve determines the width of the distribution curve. In practice, the NMR method can only be used to determine pore sizes with reasonable accuracy in the mesoporous range unless liquids undergoing larger melting point depressions than the ones investigated so far can be found.