V.M. Gun’ko

Effect of adsorption of nitroaromatic compounds on the characteristics of bound water layers in aqueous suspensions of activated carbons

Abstract The impact of chemical treatment (oxidation, reduction) of microporous carbon prepared from plum stones on the structural parameters was analysed using the nitrogen adsorption method. The effects of pre-adsorption of 2,4,6-trinitrotoluene (TNT) and 1-chloro-2,4-dinitrobenzene (CDNB) on the characteristics of the interfacial water layers in the aqueous suspensions of these materials were explored using 1 H NMR spectroscopy with freezing out of the bulk water between 210 and 273 K. Substantial variations in the Gibbs free energy of the interfacial water in the aqueous suspension of treated carbons were observed, especially for a reduced sample with pre-adsorbed nitroaromatics, due to changes in the hydrophilic–hydrophobic properties of the systems.

Structural, textural and adsorption characteristics of nanosilica mechanochemically activated in different media

The structural, textural, and adsorption characteristics of mechanochemically activated (MCA) fumed silica A-300 as dry or water, ethanol, or water/ethanol-wetted powders (0.5 g of a solvent per gram of silica) in a ball mill for 1-6 h were studied in comparison with those of the initial powder. The MCA treatment enhances bulk density (ρ(b)) of the powder (from 0.045 g/cm(3) for the initial silica to 0.4 g/cm(3) for 6 h-MCA-treated water-wetted silica) depending on medium type and MCA time (t(MCA)). Stronger effects are observed for the MCA treatment of water-wetted silica than of dry or ethanol- or water/ethanol-wetted samples. The MCA treatment weakly affects the specific surface area (S(BET)). However, void (pore) size distribution, porosity, particle aggregation and size distribution in aqueous suspension, behavior of interfacial water, properties of poly(vinyl alcohol)/silica composites and adsorption of gelatin depend more strongly on the t(MCA) and ρ(b) values. Some of the observed changes in the characteristics (e.g., gelatin adsorption) depend on the ρ(b) value but are independent of the medium type used on the MCA. Other characteristics are nonlinear functions of both t(MCA) and ρ(b) values.

Adsorption of polar and nonpolar compounds onto complex nanooxides with silica, alumina, and titania.

Adsorption of low-molecular adsorbates (nonpolar hexane, nitrogen, weakly polar acetonitrile, and polar diethylamine, triethylamine, and water) onto individual (silica, alumina, titania), binary (silica/alumina (SA), silica/titania (ST)), and ternary (alumina/silica/titania, AST) fumed oxides was studied to analyse the effects of morphology and surface composition of the materials. Certain aspects of the interfacial phenomena dependent on the structural characteristics of oxides were analysed using calorimetry, (1)H NMR, and Raman spectroscopies, XRD, and ab initio quantum-chemical calculations. The specific surface area S(BET,X)-to-S(BET,N(2)) ratio (X is an organic adsorbate) changes from 0.68 for hexane adsorbed onto amorphous SA8 (degassed at 200 degrees C) to 1.85 for acetonitrile adsorbed onto crystalline alumina (degassed at 900 degrees C). These changes are relatively large because of variations in orientation, lateral interactions, and adsorption compressing of molecules adsorbed onto oxide surfaces. Larger S(BET,X)/S(BET,N(2)) values are observed for mixed oxides with higher crystallinity of titania or/and alumina phases in larger primary nanoparticles with greater surface roughness and hydrophilicity. Polar adsorbates can change the structure of aggregates of oxide nanoparticles that can, in turn, affect the results of adsorption measurements.

Confined space effects driving to heterogenization of solutions at the interfaces

Water and concentrated solutions of acids (HCl, H3PO4, and H3PO3) or H2O2 are clustered at the interfaces of nanosilicas and multi-layer graphene oxide (MLGO) as shown by low-temperature 1H NMR spectroscopy. The acid solutions in contact with silica or MLGO powders placed in nonpolar (CCl4) or weakly polar (CDCl3) media form nanodomains and clusters of several types and sizes with different contents of water and solutes. This differentiation of the solutions is stronger at a surface of more compacted nanosilicas because of enhanced confinement effects in narrower pores (voids) between adjacent nanoparticles. For MLGO, the confined space effects are relatively weak but the formation of ice crystallites during freezing of the suspension changes the material texture. Obtained results are of importance for deeper insight into the mechanism of the interfacial behavior of complex solutions under different confined space effects and due to the influence of organic co-solvents.

Interfacial behavior of silicone oils interacting with nanosilica and silica gels

The interfacial behavior of silicone oils Oxane 1000 and Oxane 5700 (polydimethylsiloxanes, PDMS) interacting with dried or hydrated (hydration h=0.005 or 0.1g/g) silica gels Si-60 and Si-100 or nanosilica A-400 was studied using low-temperature (1)H NMR spectroscopy over the 210-310 K range. Broadening of the melting temperature range toward both sides from the freezing point is observed for silicone oils confined in mesopores (2-15 nm in radius) of silica gel particles (0.2-0.5 mm in size) or voids (1-100 nm) between silica nanoparticles (5-10 nm in size) in their aggregates. This effect is a consequence of the phase state heterogeneity, since both liquid and solid-like fractions of adsorbed PDMS are observed over a large temperature range. The adsorbed PDMS heterogeneity depends on the pore size distribution (confined space effect), and it is lower for silica gel Si-100 possessing broader pores than Si-60. An increase in the amounts of adsorbed polymer and water diminishes the effects of confined space on PDMS because a fraction of the polymers is located in broader pores or out of pores (voids). This leads to relative decrease in interactions of PDMS with the silica surface. (1)H NMR spectra of PDMS and n-decane bound to silica gels reveal much stronger heterogenization of adsorbed PDMS (depending on the polymer length) than that of the alkane.

Interactions of poly(dimethylsiloxane) with nanosilica and silica gel upon cooling-heating.

To control the properties of poly(dimethylsiloxane) (PDMS, Oxane 1000) as a bio-inert material, the characteristics of Oxane 1000 were compared for PDMS alone and interacting with silica gel Si-100 and nanosilica PS400. Low-temperature (1)H NMR spectroscopy, applied to static samples at 200-300 K, and differential scanning calorimetry (DSC) at 153-393 K were used to analyze the properties of PDMS and composites. The NMR study shows that liquid and solid-like fractions of PDMS co-exist over a broad temperature range. The cooling-heating cycles give hysteresis loops of intensity of (1)H NMR signals of methyl groups of a liquid fraction of PDMS vs. temperature depending on the silica type. The loop width differs for PDMS alone and bound to silicas, and the samples preheated at 420 K are characterized by much narrower loops. DSC measurements of the samples show a significant difference in the thermograms on the first and second DSC scans that depend on the silica type. For PDMS confined in pores of silica gel, 3D spatial structure of the polymers can be more ordered than that of PDMS located in thin layers at a surface of nanosilica. Therefore, both melting endotherms and crystallization exotherms are observed for PDMS/silica gel. However, for PDMS/nanosilica, both thermal features are much weaker and observed during only the first DSC scan.

Adsorption of inflammatory cytokines and endotoxin by mesoporous polymers and activated carbons

Adsorption of E. coli lipopolysaccharide (LPS) and an inflammatory cytokine TNF-α from model solutions on uncoated ‘hyper-crosslinked’ polystyrene polymers MN200 and MN500 and activated carbons Carboxen 1003 and Carboxen 1010 has been studied. It has been shown that TNFα can be efficiently removed by both non-functionalised MN200 and MN500 functionalised with cation exchange functional groups. On the contrary, surface chemistry of the resins is important in LPS adsorption, MN500 being significantly more efficient than MN200. Presence of mesopores in Carboxen 1003 correlates with its higher adsorption capacity towards LPS in comparison with purely microporous Carboxen 1010. It has been suggested that hydrophobic interactions play an important role in TNFα adsorption, whereas in LPS adsorption electrostatic interactions dominate the process.

Effect of temperature and a weakly polar organic medium on water localization in slit-like pores of various sizes in microporous activated carbon

The joint adsorption of water and methane in pores of the synthetic carbon adsorbent with specific surface area of 3463 m2/g was studied by 1H NMR spectroscopy. It was shown that during adsorption of water and methane in pores of activated carbon AC86 under conditions of low surface filling the adsorbate molecules are localized not only in micropores, but in mesopores as well, and are revealed in the spectra as several signals with different values of the chemical shift. A substantial drop in the magnetic shielding effect of the carbon surface was detected upon a reduction in temperature; this is explained by the presence of two energy minima on the curves of the adsorption potential energy (close to the walls and in the center of pores) and allows the migration of adsorbed molecules from the walls to the center of pores, which are characterized by a lower shielding effect.