Cryoporometry

NMR cryoporometry relies on the melting point depression, i.e., the difference in the melting point of crystals with a finite size d, Tm(d), relative to the value of the bulk liquid Tm, which is given by the simplified Gibbs-Thomson equation [16]  

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As state before, TPM remains a secondary technique requiring calibration. The extraordinary progress in the preparation of porous materials with well-controlled pore sizes during the last 20 years is now circumventing this problem. Comparison of PSD derived from gas sorption, TPM, and NMR cryoporometry [18, 19] clearly demonstrates the validity of TPM for the study of nanoporous materials. 

The porosity of fibers can be measured using techniques such as gas-adsorption and cryoporometry, which are sensitive to pore sizes in the sub-micron size range. These techniques are discussed further in Section 8 below. Wetting properties and the surface energy of... 

Furo, I. and Daicic, J., NMR cryoporometry A novel method for the investigation of the pore structure of paper and paper coatings, Nordic Pulp Paper Res. J., 14, 221-225 (1999). 

By using the Carr- Purcell-Meiboom-GiU (CPMG) echo sequence, it is possible to distinguish between the solid and the liquid state by using HR NMR spectroscopy for liquids. This temperature-dependent method is also known as cryoporometry. 

NMR cryoporometry shows the difference in the organization of water clusters adsorbed onto the initial and MCA-A-300 (Figure 1.31) because of the difference in the PSD of these silicas (Figure 1.29d). 

The 5h values depend strongly on water amounts, silica type, and temperature. NMR cryoporometry shows that small water clusters (<1 nm) and nanodomains (up to 20 nm in size) are present at the interfaces of hydrated solid POA and silica/POA powders. Quantum chemical calculations of the H NMR spectra demonstrate the influence of POA/water cluster structure and dissociation of the PO-H bonds on the 6 values. 

Thus, to decrease the free energy of MIX contact between water and hydrophobic silica should be minimal. A high viscosity of the MIX system and absence of bulk water prevent floating of MS particles that can occur at small amounts of both oxides in a diluted suspension. Therefore, primary particles of A-300 and their aggregates form dense shells (micelles) around MS particles with entrapped air that create a barrier preventing contact of water with MS and the formation of separated MS phase. The LT H NMR and NMR cryoporometry methods give useful structural information on the hydrophilic/hydrophobic silica-water mixtures and can be used to characterize the properties of dry water materials. 

Characteristics of Water Unfrozen at T<273 K and Bound to Initial and Wet-MCA-Treated Nanosilica Estimated by NMR Cryoporometry... 

NMR cryoporometry method was developed (Overloop and Van Gerven 1993, Strange et al. 1993, Akporiaye et al. 1994) on the basis of the thermodynamic theory of the phase transition in materials in confined space pioneered by J.W. Gibbs, J. Thomson, and W. Thomson. According to Gibbs-Thomson equation for the melting point depression, A7j , for a small isolated spherical crystal of diameter x in its own liquid can be written as (Jackson and McKenna 1990)... 

Petrov and Furo (2006) analyzed freezing-melting hysteresis in cryoporometry and attributed it to a free energy barrier between metastable and stable states of pore-filling material. Freezing point depression can be given by (Petrov and Furo 2006)... 

The cryoporometry results for the PSDs can be used to estimate contributions of pores of different sizes to the total porosity. The f R) function can be converted into the distribution function/s(R) with respect to the specific surface area ... 

In the case of TSDC cryoporometry, modified Equations 1.48 and 1.52 with as a linear function of temperature... 

Figure 1.206 shows the PSD (or UWCSD) calculated using the NMR and TSDC cryoporometry methods, NMR relaxometry (aqueous suspensions), and nitrogen adsorption method. 

Thus, different versions of NMR and TSDC cryoporometry (as well as DSC thermoporometry (Landry 2005), vide infra) and relaxometry allow one to obtain the structural characteristics not only for powders but also for strongly hydrated systems that is of importance for biomaterials, cells, and other bio-objects, which can be studied in the native state. In other words, these methods are nondestructive methods. 

FIGURE 1.206 Comparison of the pore (voids between nanoparticles) size distributions calculated using four methods (1) NMR cryoporometry and (2) NMR relaxometry, (3) and (4) TSDC cryoporometry (aqueous suspensions at Cj, qq=3-1 wt%), and (5) PSD calculated using the nitrogen adsorption isotherm (SCV/SCR model, see Section 1.1.1). 

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