High-pressure cryogelation of nanosilica and surface properties of cryosilicas

Abstract

Gun'ko, V M.; Turov, V V.; Zarko, V I.; Prykhod'ko, G P.; Remez, O S.; Leboda, R; Skubiszewska-Zieba, J; Pahklov, E M.; and Blitz, Jonathan, "High-pressure cryogelation of nanosilica and surface properties of cryosilicas" (2013 AbstractSilica cryogels (cryosilicas) in a powder state were synthesized with different concentrations of fumed silica A-300 (C A-300 = 5-20 wt.%), sonicated in aqueous suspension, then frozen at 14 o C at different pressures in a high-pressure stainless steel reactor (a freezing bomb), and dried in air at room temperature. To analyze the effects of low temperature and high pressure, samples were also prepared at 14 o C or room temperature and standard pressure. The structural and adsorption properties of the powder materials were studied using nitrogen adsorption, highresolution transmission electron microscopy, infrared spectroscopy, thermogravimetry, lowtemperature 1 H NMR spectroscopy and thermally stimulated depolarization current. The structural, textural, adsorption and relaxation characteristics of high-pressure cryogel hydrogels and related dried powders are strongly dependent on the silica content in aqueous suspensions frozen at 1, 450 or 1000 atmospheres and then dried. The largest changes are found with C A-300 = 20 wt.% which are analyzed with respect to the interfacial behavior of nonpolar, weakly polar and polar adsorbates using low temperature 1 H NMR spectroscopy.

Evaporation of polar and nonpolar liquids from silica gels and fumed silica

“…The confined space effects are very strong in narrow pores (nanopores at R p < 1 nm and narrow mesopores at R p < 2 nm) [8,[18][19][20][42][43][44]. Therefore, these effects can be ignored for silica gels Si-60 and Si-100 and fumed silica A-300.…”

“…1a). However, there are several factors affecting a difference in the PSD shapes such as different temperatures (77.4 K for NLDFT) and (360-450 K for TG), some swelling of silica gel beads in water (TG); associative desorption of water (from neighbouring silanols) upon heating in TG contributing the left wing of the PSD (toward smaller R p ), and some features of calculations in the methods based on different phenomena [8,[25][26][27][28][42][43][44]. It is clear from this comparison that the textural characteristics can play an important, determining role in both adsorption of nonpolar nitrogen and desorption of polar water.…”

“…1b). The latter corresponds to voids between spherical nonporous nanoparticles (average diameter 8.3 nm) randomly packed in aggregates (<1 m in size) and agglomerates of aggregates (>1 m) [8,28,[42][43][44]. The average pore radius (determined as the ratio of the first and zero moments of the differential PSD) is 2.1, 4.7, 6.9, and 12.2 nm for Si-40, Si-60, Si-100, and A-300, respectively.…”

“…Note that the PSD of fumed silica does not give full picture for macropores at R p > 25 nm because of incomplete filling of macropores by adsorbed nitrogen [8,24]. Therefore, the pore volume estimated from the nitrogen adsorption (V p = 0.734 cm 3 /g) is much smaller than the empty volume (V em = 1/ b − 1/ 0 = 14.6 cm 3 /g, where b and 0 are the bulk and true densities of fumed silica) in the initial A-300 powder [8,[42][43][44]. However, in the case of evaporation of liquids used with excess (their amount is greater than V p but smaller than V em ), the aggregates of primary nanoparticles and agglomerates of aggregates can be rearranged to form more compacted structures (Fig.…”

“…This was taking into account (with the corresponding weighting coefficients) upon calculation of the evaporation rate and condensation/evaporation coefficients by dividing of adsorbed compounds into three types: free bulk (using G s ), bulk in pores and first monolayer of molecules bound to a silica surface (values of G in Tables S2 and S3) using the values of S BET (amounts of bound liquids in the first monolayer were estimated as tS BET , where t is the monolayer thickness), and pore volume V p (amounts of bulk liquids in pores estimated from the difference V p − tS BET ). As a whole, for similar corrections, the experimental adsorption isotherms of studied compounds or NMR data can be used to determine the Gibbs free energy of adsorption [8,[42][43][44].…”