E. M. Pakhlov

National Academy of Sciences of Ukraine, Kievo, Kyiv City, Ukraine

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Publications (63)118.7 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Initial and treated silicas of different origin, such as fumed and precipitated silicas and silica gel, were studied using infrared spectroscopy, DSC, NMR, and adsorption methods. Mechanochemical activation (MCA) of fumed silica A-300 in a ball-mill or a microbreaker affects the structure of aggregates of nanoparticles and agglomerates of aggregates becoming more compacted. Any treatment of ‘soft’ nanosilica affects the interfacial behavior of polar and nonpolar adsorbates because the rearrangement of secondary particles affects the freezing–melting point depression. Clusterization of adsorbates bound in pores causes diminution of heat effects during phase transition (freezing or fusion). The freezing point depression and increasing melting point lead to the hysteresis in freezing–melting of adsorbates bound in pores. The study shows that fumed and precipitated silicas can be more sensitive to external actions, such as wetting–drying and MCA, than silica gels because of less dense packing of nanoparticles in the secondary structures. These effects should be considered upon applications of similar materials in different media.
    No preview · Article · Mar 2016 · Colloids and Surfaces A Physicochemical and Engineering Aspects
  • E. Skwarek · W. Janusz · V. M. Gun’ko · E. M. Pakhlov · V. I. Zarko · K. Gdula
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    ABSTRACT: Hydroxyapatite composite is a very important biomaterial, which can be applied in various life areas. Hydroxyapatite and its composites with fumed oxides ST20, AST1, Al2O3, A90, and A300 were prepared and studied using X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy adsorption, potentiometric titration, quasi-elastic light scattering and zeta potential measurements. The values of pHpzc (the point of zero charge) and pHIEP (the isoelectric point) characteristic of the electrical double layer depend on the type of the fumed oxide matrix. Comparative studies of hydroxyapatite, fumed oxides and composites by adsorption and desorption of nitrogen, scanning electron microscopy and FTIR showed that in most cases composites have properties intermediate between hydroxyapatite and the oxides taken for the synthesis.
    No preview · Article · Jan 2016 · Adsorption
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    ABSTRACT: Composites based on fumed silica with surface hydrophobic groups were synthesized by chemical substitution of silanol groups' proton for trimethylsilyl (TMS) groups, adsorption of poly(dimethylsiloxane) (PDMS) and combined chemical-adsorption methods. Microcalorimetry measurements and values of the wetting contact angle with water (Theta) were obtained to investigate the hydrophobic properties of prepared materials. The hydrophilicity indexes (K-h) were determined from the relationship of heat of immersion in water Q(w) to non-polar decane Q(d). The Theta and Q(w) demonstrated changes with increase of modification degree (d(m)) for composites containing a monolayer of PDMS. All samples with excess of PDMS had high Theta (> 110 degrees) and relatively low Q(w) and K-h. Composites modified with TMS groups demonstrated hydrophobic properties in accordance with K-h at d(m) 0.7, whereas Theta > 90 degrees is observed only for those samples with d(m) = 1.0. The different methods of synthesis may be applied based on the morphological characteristics and hydrophobicity of the samples.
    No preview · Article · Aug 2015 · Adsorption Science and Technology
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    ABSTRACT: High-pressure cryogelation (HPC) of individual (silica, alumina, their mechanical blends) and complex (silica/titania, alumina/silica/titania, AST) nanooxides were studied to reveal the influence of nanooxide composition and gelation conditions on changes in the textural characteristics of the materials. The temperature–pressure behavior of different phases (silica, alumina, and titania) under HPC can result in destroy of complex nanoparticles, especially large (100–200 nm in size) core–shell secondary nanoparticles composed of a number of small primary nanoparticles (~10 nm in size) covered by a shell destroyed under HPC. The main changes in the texture of HPC or gelled nanooxides are due to rearrangement (compaction) of secondary structures such as aggregates of primary nanoparticles and agglomerates of aggregates. This compaction enhances the pore volume but much weakly affect the specific surface area (with one exception of AST) because small primary nanoparticles (10–20 nm in size) are relatively stable during HPC. The HPC materials are characterized by enhanced mesoporosity shifted to macroporosity with decreasing specific surface area and increasing size of primary nanoparticles. Diminution of the freezing temperature from 208 to 77.4 K during HPC results in an increase in pressure (up to 1,050 atm in stainless steel freezing bombs) and enhanced compaction of aggregates and agglomerates but this does not practically affect the primary nanoparticles. The degree of decomposition of secondary core–shell nanoparticles of AST does not practically increase with this decreasing freezing temperature. Graphical Abstract High-pressure cryogelation can destroy core–shell nanoparticles.
    No preview · Article · Apr 2015 · Journal of Sol-Gel Science and Technology
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    ABSTRACT: Fumed silica, initial alumina blend, after mechanochemical activation (MCA), and fumed binary silica/alumina (SA) were studied using infrared spectroscopy, X-ray diffraction (XRD), ultrasoft X-ray emission (USXE) spectroscopy (giving emission bands Si Lα, Al Lα and O Kα related to valence electron transfer onto core levels) and quantum chemistry. The MCA influence on nanoparticle characteristics (sizes, electronic structure) increases with increasing alumina content in the blends due to stronger abrasive effect of alumina nanocrystallites (snagging a surface layer of nanoparticles during MCA) than non-crystalline silica nanoparticles. A difference in Si Lα, Al Lα and O Kα affected by MCA increases with increasing alumina content. It is greater for the top peak of the upper valence band (UVB) than for a lower energy peak in the bottom of the UVB. These spectral changes suggest redistribution of electron density between Si, Al and O atoms depending on alumina content, material type and treatment conditions. The main difference in the properties of the SA blends and binary SA is due to distribution of Si atoms in alumina phase and Al atoms in silica phase in fumed SA (which is amorphous at CAl2O3 ≤ 30 wt.%) in contrast to the SA blends with practically separated silica and alumina nanoparticles.
    No preview · Article · Nov 2014 · Journal of Non-Crystalline Solids
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    ABSTRACT: Unmodified pyrogenic silica PS300 and partially silylated nanosilica samples at a degree of substitution of surface silanols by trimethylsilyl (TMS) groups ΘTMS=27.2% and 37.2% were studied to elucidate features of the interfacial behavior of water adsorbed alone, or co-adsorbed with methane, hydrogen, or trifluoroacetic acid (TFAA). In the aqueous suspension modified PS300 at ΘTMS=37.2% forms aggregates of 50-200nm in size and can bind significant amounts of water (up to ∼5g/g). Only 0.5g/g of this water is strongly bound, while the major fraction of water is weakly bound. The presence of surface TMS groups causes the appearance of weakly associated water (WAW) at the interfaces. The adsorption of methane and hydrogen onto TMS-nanosilica with pre-adsorbed water (hydration degree h=0.05 or 0.005g/g) increases with increasing temperature. In weakly polar CDCl3 medium, interfacial water exists in strongly (SAW, chemical shift δH=4-5ppm) and weakly (δH=1-2ppm) associated states, as well as strongly (changes in the Gibbs free energy -ΔG>0.5-0.8kJ/mol) and weakly (-ΔG<0.5-0.8kJ/mol) bound states. WAW does not dissolve TFAA but some fraction of SAW bound to TMS-nanosilica surface can dissolve TFAA.
    Full-text · Article · Nov 2014 · Journal of Colloid and Interface Science
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    ABSTRACT: To study changes in the characteristics of gelled/dried nanooxides (fumed silicas PS300 and PS100, alumina, silica/titania ST, alumina/silica/titania AST), high-pressure cryogelation, HPCG (at 208 K or 260 K) or gelation (293 K, 1 atm) was carried out using 20 wt.% aqueous suspensions pure or with addition of 0.1 M NaCl. The nanooxide samples were studied after drying at room temperature. Maximal changes in the textural and crystalline characteristics are observed for cryogels with AST prepared at similar to 1000 atm and 208 K due to decomposition of complex nanoparticles. Its specific surface area S-BET increases from 83 to 160 m(2)/g, and the crystallinity degree grows by 11%. Mixing of PS300 and AST (1:1, w/w) in the suspension prevents the decomposition of AST particles in the cryogel since S-BET decreases in comparison with that of the initial blend powders. Addition of NaCl (2.8 wt.% in dried powders) reduces decomposition of AST particles due to changes in HPCG conditions. For binary ST, S-BET increases but much smaller than for AST. HPCG of individual silica and alumina leads to a decrease in the S-BET value; i.e. nanoparticles are not decomposed. A significant increase in the volume of large mesopores (pore radius 5-10 nm < R < 25 nm) and macropores (25 nm < R < 60-100 nm) is observed for gelled/dried samples in comparison with the initial powders. Contribution of nanopores (R < 1 nm) and narrow mesopores (1 nm < R < 5-10 nm) decreases after HPCG. The effects of NaCl and HPCG on the textural characteristics are minimal for ST studied.
    No preview · Article · Aug 2014 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    ABSTRACT: In this study, cellulose-inorganic sorbents based on bleached cotton fibres and ferrocyanides of different d-metals have been prepared and used for separation of 137Cs from simulated aqueous solutions. Comparative analysis has been carried out of the sorption properties of obtained sorption materials towards to micro- and macroquantities of caesium at different liquid to solid phases ratios. Various characterization methods including X-ray analysis, IR spectroscopy, and differential thermal analysis were used to determine the structural properties of the obtained composite sorbents. It has been confirmed that the sorbents obtained at approximately the same content of the inorganic component are characterized by different values of the recovery rate and 137Cs distribution coefficient.
    No preview · Article · Aug 2014 · Journal of Radioanalytical and Nuclear Chemistry
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    ABSTRACT: Silica cryogels (cryosilicas) in a powder state were synthesized with different concentrations of fumed silica A-300 (CA-300 = 5-20 wt%), sonicated in aqueous suspension, then frozen at -14 degrees 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 degrees C or room temperature and standard pressure. The structural and adsorption properties of the powder materials were studied using nitrogen adsorption, high-resolution transmission electron microscopy, infrared spectroscopy, thermogravimetry, low-temperature H-1 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 1450 or 1000 atm and then dried. The largest changes are found with CA-300 = 20 wt% which are analyzed with respect to the interfacial behavior of nonpolar, weakly polar and polar adsorbates using low temperature H-1 NMR spectroscopy.
    No preview · Article · Sep 2013 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    ABSTRACT: This paper focuses on the investigation of nanooxide composites SiO2/TiO2/X (X = SnO2, NiO) synthesized by controlled low�temperature hydrolysis of TiCl4, SnCl4 (followed by thermal treatment of some samples) on nanosilica surface (specific surface area 300 m2/g) and by thermolysis of Ni(NO3)2 ⋅ 6H2O at 600°C. The effect of the nature and concentration of a doping agent on the phase composition of the com� posites and their physicochemical properties is determined. The properties of the synthesized systems are examined by means of XRD analysis, nitrogen adsorption, potentiometric titration, and photon correlation spectroscopy.
    Full-text · Article · Sep 2013 · Protection of metals and physical chemistry of surfaces
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    Full-text · Article · Oct 2012 · Applied Surface Science
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    Full-text · Conference Paper · Sep 2012
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    ABSTRACT: A series of photocatalysts based on silica (nanoparticulate) supported titania, ceria, and ceria/zirconia were synthesized and characterized by a variety of techniques including surface area measurements, X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential, surface charge density, and photocatalytic behavior toward methylene blue decomposition. Thermal treatment at 600 °C increases the anatase content of the titania based catalysts detected by XRD. Changes in the infrared spectra before and after thermal treatment indicate that at low temperature there are more SiOTi bonds than at high temperature. As these bonds break upon heating the SiO2 and TiO2 separate, allowing the TiO2 anatase phase to form. This results in an increased catalytic activity for the thermally treated samples. Nearly all titania based samples exhibit a negative surface charge density at pH 7 (initial pH of photocatalytic studies) which aids adsorption of methylene blue. The crystallinity of ceria and ceria/zirconia based catalysts are in some cases limited, and in others non-existent. Even though the energy band gap (Eg) can be lower for these catalysts than for the titania based catalysts, their photocatalytic properties are inferior.
    No preview · Article · Jun 2012 · Applied Surface Science
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    ABSTRACT: Morphological characteristics of CuO- and NiO-doped silica/titania nanocomposites are analyzed using atomic force microscopy and low-temperature nitrogen adsorption. The different types of pores (nano-, meso- and macropores) in the powder composites are modeled as voids between spherical nanoparticles packed in random aggregates. It is shown that specific surface area, total pore volume, average pore radius, incremental pore size distribution and changes in relationships between these characteristics of the composites reveal a non-linear behavior depended on the presence of the dopants. The textural and atomic force microscopy investigations suggest that processes of aggregation differ from each other depending on the type of doping oxide. Fractal dimension, estimated according to Frenkel-Halsey-Hill equation using the adsorption branch, has values about 2.50–2.60 and indicates the aggregate formation.
    Full-text · Article · Jan 2012
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    ABSTRACT: SiO2/TiO2/CuO composites have been synthesized by deposition technique using low temperature hydrolysis of TiCl4 and thermolysis of Cu(CH3CO2)2 · H2O at silica A-300 surface. The properties of the samples have been explored by means of XRD, nitrogen adsorption, potentiometric titration, and photon correlation spectroscopy. As the results indicate, there are both anatase and some amount of rutile formed at CCuO = 1 wt. %. A gradual shift of the point of zero charge (from pH = 3.48 to 6.60) has been observed with increasing CuO content.
    Full-text · Article · Jan 2012
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    ABSTRACT: Nanosilicas (A-50, A-300, A-500)/activated carbon (AC, SBET=1520m2/g) composites were prepared using short-term (5min) mechanochemical activation (MCA) of powder mixtures in a microbreaker. Smaller silica nanoparticles of A-500 (average diameter dav=5.5nm) can more easily penetrate into broad mesopores and macropores of AC microparticles than larger nanoparticles of A-50 (dav=52.4nm) or A-300 (dav=8.1nm). After MCA of silica/AC, nanopores of non-broken AC nanoparticles remained accessible for adsorbed N2 molecules. According to ultra-soft X-ray emission spectra (USXES), MCA of silica/AC caused formation of chemical bonds Si–O–C; however, Si–C and Si–Si bonds were practically not formed. A decrease in intensity of OKα band in respect to CKα band of silica/AC composites with diminishing sizes of silica nanoparticles is due to both changes in the surface structure of particles and penetration of a greater number of silica nanoparticles into broad pores of AC microparticles and restriction of penetration depth of exciting electron beam into the AC particles.
    No preview · Article · Nov 2011 · Applied Surface Science
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    ABSTRACT: The characteristics of composites with PEG (35kDa) - nanooxides (fumed silica A-300, SBET=342m2/g, and alumina/silica/titania (AST) at oxides content of 22, 28, and 50wt%, respectively, SBET=38m2/g) were studied depending on PEG content (CPEG=100, 90, 80, 40, and 20wt%) at different temperatures and various hydration using Fourier transform infrared (FTIR) spectroscopy, photon correlation spectroscopy (PCS), differential scanning calorimetry (DSC), thermally stimulated depolarization current (TSDC), and dielectric relaxation spectroscopy (DRS) methods. In the aqueous suspensions the secondary particle (aggregates of primary particles) size distribution corresponds to larger particles for PEG/AST than PEG/A-300. For composite powders (0.3-3wt% of water) with well distributed macromolecules (according to the FTIR spectra) the relaxation characteristics (temperature of relaxation maxima, activation energy of relaxation, glass transition temperature and activation energy) show that overall interaction is stronger for PEG/A-300 but local interaction in the adsorption layer is stronger for PEG/AST because of much greater SBET of A-300 (first effect) and stronger Brønsted acid sites in AST (second effect). © 2010 Elsevier B.V.
    Full-text · Article · May 2010 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    ABSTRACT: Interfacial interactions, chain dynamics, and glass and melting transitions were studied in well-defined core-shell nanoparticles with amorphous silica or crystalline alumina cores and noncrystallizable poly(vinyl pyrrolidone) (PVP) or crystallizable poly(ethylene glycol) (PEG) shells. Varying particle composition caused regular changes in the shell thickness from 1 to 2 nm (monomolecular layer) up to 90 nm. Far- and mid-IR spectroscopy allowed us to register hydrogen bonding and, tentatively, Lewis/Brønsted (LB) interfacial interactions as well as changes in the dynamics and conformational state of the polymer chains as a function of the nanoshell thickness. Their most pronounced peculiarities were found for the monomolecular polymer layers. The LB interactions were stronger with the alumina substrate than silica. DSC analysis was performed, and the data obtained were in agreement with the spectroscopic data. Unlike the bulk polymer, the PVP monolayer was characterized with an extraordinarily large dynamic heterogeneity within the glass transition while broadening the transition range and varying the activation energy by an order of magnitude. The PEG monolayer adsorbed on silica was totally amorphous, whereas a highly crystalline one with the anomalously thin lamellae, down to 3 nm thick, was adsorbed on an alumina surface, presumably as a result of the quasi-heteroepitaxial crystallization process.
    Full-text · Article · Apr 2010 · Langmuir
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    ABSTRACT: Polyurethane-poly(2-hydroxyethyl methacrylate) semi-IPN-silica nanocomposites with low content (0.25 and 3 wt%) of differently functionalized 3-D fumed silica nanoparticles were studied using a combined AFM/DSC/CRS approach over the −100 to 160°C range. The pronounced heterogeneity of the PHEMA and PU glass transitions’ dynamics and the effects of considerable suppression of dynamics and increasing elastic properties by silica additives were shown. It was caused by formation of peculiarly cross-linked structures due to “double hybridization,” in particular via selective covalent bonding of the silica surface, functionalized by ‒OH, ‒NH2 or ‒CH˭CH2 groups, with the matrix constituents. The silica dispersion remained unchanged in these nanocomposites; therefore the relationships between interfacial interactions and dynamics/modulus behavior could be followed.
    No preview · Article · Jan 2010 · Journal of Macromolecular Science Part B
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    ABSTRACT: Adsorption modification of nanosilica allows effective regulation of its physicochemical, structural–mechanical, pharmacological, and biological properties thus broadening its field of application. Liquid-phase methods used for this purpose have several substantial drawbacks mainly of a technological and ecological character. Therefore a new effective and ecologically safe method was developed for adsorption modification of nanosilica with non-volatile organic compounds (bioactive compounds, polymers). The process is carried out in fluidized state in gaseous dispersion medium with the solvent atmosphere. The developed method allows one to achieve a defined degree of surface coverage and to conserve initial dispersity of silica required.
    No preview · Chapter · Dec 2009

Publication Stats

882 Citations
118.70 Total Impact Points

Institutions

  • 2000-2015
    • National Academy of Sciences of Ukraine
      • • Department of Amorphous and Structurally Ordered Oxides
      • • O. O. Chuiko Institute of Surface Chemistry
      Kievo, Kyiv City, Ukraine
  • 1993
    • Ukrainian Academy of Agrarian Sciences
      Kievo, Kyiv City, Ukraine