NMR cryoporometric measurements of porous silica: A method for the determination of melting point depression parameters of probe liquids

Nuclear magnetic resonance (NMR) cryoporometry is a non-invasive method for determining the pore size distributions of materials such as porous silica. Cryoporometry has several advantages over other porometric techniques. It is able to measure the melting process in a series of discrete steps, whereas transient heat flow techniques, such as differential scanning calorimetry (DSC), have a minimum rate of measurement, and, secondly, NMR cryoporometry can analyze pore shapes with any geometry, where nitrogen porosimetry is complicated for samples with spherical pores with narrow necks. However, one key drawback of the method is that, for any one liquid observed in any one material, there is a lack of consensus in the two parameters, k(c) and 2sl, used to convert experimental NMR melting point depression data into a pore size distribution. By considering two decades worth of literature data, values for both were obtained for water in porous silica supports, in particular an estimate of a non-freezing layer between the solid ice and the inner surface of the pore. These values were used to produce pore size distributions for three silica materials, SBA-15 and KIT-6, both with cylindrical pores but possessing different structures, and SBA-16, which has spherical pores. This represents the first time KIT-6 has been characterized by the NMR method. Furthermore, this work demonstrates a general method for obtaining values for k(c) and 2sl which can be applied to any liquid for which suitable literature data is available.