NMR 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] ... 

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. 

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). 

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)... 

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). 

FIGURE 1.235 NMR cryoporometry measurements of water and decane at various ratios in 10 nm median pore diameter silica (a) melting curves showing separated decane and water fractions, and (b) PSD for the decane and water, which displaces decane from the narrowest pores. (Adapted from J. Chem. Phys., 120, Alnaimi, S.M., Mitchell, J., Strange, J.H., and Webber, J.B.W., Binary liquid mixtures in porous solids, 2075-2077, 2004. Copyright 2004, with permission from Elsevier.)... 

FIGURE 1.236 The size distributions of the nitrobenzene-rich domains after nitrobenzene-hexane phase separation in porous glasses of 7.5 nm, 24 nm, 73 nm and 127 nm average pore size as obtained by H NMR cryoporometry. The dashed lines show the PSDs obtained for the adsorption of pure nitrobenzene. (Adapted from Current Appl. Phys., 4, Valiullin, R., Vargas-Krusa, D., and Furd, L, 370-372, 2004. Copyright 2004,... 

The H NMR cryoporometry method can be used to measure broad pore sizes (Figure 1.239) if octamethylcyclotetrasiloxane is used as an adsorbate (Ono et al. 2009). Three very different methods such as the nitrogen adsorption, the H NMR cryoporometry, and the mercury porosimetry give the PSDs, which are mutually complementary. These results confirm that organic molecules are better probe compounds than water for the structural characterization of macroporous materials with NMR cryoporometry or relaxometry. 

Si-60 (b) the corresponding distribution functions calculated using NMR cryoporometry with IGT eq., and (c) NMR spectra of water (hydration /i = 0.244 g/g) adsorbed on silylated Si-60 (in chloroform-d medium) at different temperatures. (Adapted from J. Colloid Interface Sci., 308, Gun ko, V.M., Turov, V.V., Zarko, V.l. et ak, Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel, 142-156, 2007h. Copyright 2007, with permission from Elsevier.)... 

Jahnert et al. (2008) studied freezing and melting of H2O and DjO in the cylindrical pores of well-characterized MCM-41 silica materials with pore diameter, d from 2.5 to 4.4 nm using DSC and H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting... 

The interfacial behavior of water/organics (Franks 2000, Chaplin 2011), which can be very important on practical applications of porous materials as adsorbents, catalysts, etc., can be investigated by using both NMR cryoporometry and relaxometry techniques and TSDC method for deeper understanding of structurization of liquids confined in pores of ordered mesoporous silicas. Here, we will analyze results of a study of (i) the structural characteristics of synthesized ordered mesoporous silicas MCM-41, MCM-48, and SBA-15 by XRD, adsorption, and FTIR methods to explain the difference in their adsorption characteristics with respect to polar (water), weakly polar (chloroform), and nonpolar (benzene) adsorbates (ii) the relationships between these characteristics and the behavior of adsorbed pure water and its mixtures with benzene or chloroform-c over a wide... 

Thus, the H, C, and Si NMR spectroscopy was frequently used to study silica thin films, hybrid organosilica, and other films. However, these applications deal with chemical structure of the films. These are no publication with NMR cryoporometry applied to thin silica films. 

FIGURE 2.68 (a) Relative NMR spin-eeho intensities versus temperature for cyclohexane confined within the pores of titania spherical particles (b) pore size distribution curves of the MTx samples determined by H NMR cryoporometry and BJH method (dashed fines) (c) insert with field-emission (FE) SEM image of MT400. (Adapted with permission from Ryu, S.-Y., Kim, D.S., Jeon, J.-D., and Kwak, S.-Y., Pore size distribution analysis of mesoporous TiOj spheres by H nuclear magnetic resonance (NMR) cryoporometry, J. Phys. Chem. C 114,17440-17445, 2010, Copyright 2010 American Chemical Society.)... 

Thus, both NMR cryoporometry and solid-state NMR spectroscopy give useful textural and structural information on mixed porous oxides, and this information can be broadened and deepened with the use of DSC, TG, adsorption, FTIR, Raman, XPS, XRD, and other methods. The authors not always used obtained information for maximum comprehensive description of the materials in terms structure— properties. Frequently, this is due to a small number of used techniques, as well as incomplete analysis of obtained experimental results. 

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