No abstract

Structural characterization of different silicas (ordered mesoporous silicas MCM-41, MCM-48, and SBA-15, amorphous silica gels Si-40, Si-60, and Si-100, and initial and wetted-dried fumed silica A-300) and bio-objects (fibrinogen solution, yeast cells, wheat seeds, and bone tissues) has been done using two versions of cryoporometry based on integral Gibbs-Thomson (IGT) equation for freezing point depression of pore liquids measured by 1H NMR spectroscopy (180-200 < T < 273 K) and thermally stimulated depolarization current (TSDC) method (90 < T < 273 K). The IGT equation was solved using a self-consisting regularization procedure including the maximum entropy principle applied to the distribution function of pore size (PSD). Comparison of the PSDs calculated by using the cryoporometry and nitrogen adsorption methods for the mentioned silicas demonstrates that IGT equation provides satisfactory fit which is better than that obtained with nonintegral Gibbs-Thomson (GT) equation (based on the GT equation) proposed by Aksnes and Kimtys. The NMR- and TSDC-cryoporometry methods applied to probe biosystems give clear pictures of changes in the structural characteristics caused, e.g., by hydration and swelling of wheat seeds and yeast cells, coagulation and interaction of fibrinogen with solid nanoparticles in the aqueous media, and human bone tissue disease.

The present study focuses on the mechanism of swelling and evaluates interactions between solvents of different chemical characters (polar-ethanol, nonpolarn-heptane) and commercially available porous Amberlite polymers (XAD4, XAD16, XAD7HP) by temperatureprogrammed desorption (TPD). The first two polymers are the product of copolymerization of styrene and divinylbenzene. Despite having the same chemical composition, they differ in pore size and volume. The Amberlite XAD7HP is composed of an acrylic matrix and has lower pore volume and specific surface area than XAD16 and XAD4. All three resins have the ability to swell, though the per cent of polymeric network expansion during this process varies depending on the solvent used (e.g. in tetraethyl orthosilicate, XAD4 and XAD16 spherical particles increase in volume by 20-30%, while XAD7HP particles can expand by more than 120%). The TPD experiment was performed in dynamic linear and quasistatic heating mode. Based on thermogravimetric data, the desorption energy of selected liquids and pore size distribution in the swollen state were estimated. The obtained results are discussed in terms of both mathematical modelling and low-temperature nitrogen adsorption-desorption experiment.

Fumed silicas unmodified (S BET = 378 m2/g) and modified (379−285 m2/g) by hexamethyldisilazane reacting with silanols (concentration of grafted trimethylsilyl groups C TMS = 0.09−0.79 mmol/g) were studied by means of the NMR, IR, differential thermogravimetry, atomic force microscopy, microcalorimetry, adsorption, and theoretical methods. Variation in the surface composition and changes in the structural characteristics of primary and secondary particles lead to the nonlinear dependence of the Gibbs free energy of interfacial water, the heat of immersion of silicas in water, and the chemical shift δH(T) of water adsorbed from air on C TMS, despite a nearly linear decrease in water adsorption (at p/p 0 ≈ 0.8 for 24−72 h) with C TMS. An increase in C TMS causes alterations in the structure of the hydrogen bond network in the interfacial water and other properties of water strongly (700−300 mg/g silica) and weakly (1400−500 mg/g) bound to the silica surfaces.

No abstract

No abstract

No abstract

No abstract