Abstract
The pore size distributions of four controlled pore glasses and two silica gels with nominal diameters ranging from 6–24 nm were determined by measuring the1H nuclear magnetic resonance (NMR) signal from the nonfrozen fraction of confined cyclohexane as a function of temperature, in steps of ca. 0.1–1 K. The intensity curves of the liquid component are well represented by a sum of two error functions. The mean melting point depression of cyclohexane confined in pores with radiusR follows the simplified Gibbs-Thompson equation δT=k p/R with ak p value of 72.4 Knm. To our knowledge, this is the first time that thek p value of cyclohexane has been directly and accurately calibrated by NMR. As expected, thek p value mainly determines the position of the pore size distribution curve, i.e., the mean pore radius. The overall pore size distributions determined by NMR are in reasonable agreement with the results specified by the manufacturer, or measured by us by the N2 sorption technique. Although the melting point depression of confined cyclohexane is found to be less than previously assumed, this compound is still one of the most suitable candidates for NMR-based pore size determinations. However, pore sizes larger than approximately 50 nm in diameter will be difficult to determine accurately by NMR unless adsorbates undergoing larger melting point depressions than cyclohexane can be found.
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Aksnes, D.W., Kimtys, L. 1H NMR studies of cyclohexane confined in mesoporous solids: Melting point depression and pore size distribution. Appl. Magn. Reson. 23, 51–62 (2002). https://doi.org/10.1007/BF03166183
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DOI: https://doi.org/10.1007/BF03166183