Vladimir I. Zarko

Aqueous suspension of fumed oxides: particle size distribution and zeta potential

Abstract Comparative analysis of the results of the investigations, which were performed by means of photon correlation spectroscopy (PCS) and electrophoresis, of aqueous suspensions of individual fumed oxides (SiO2, TiO2, Al2O3), binary X/SiO2 (X=TiO2, Al2O3, GeO2, C) and ternary Al2O3/SiO2/TiO2, fumed oxides modified by organic and organosilicon compounds or interacting with dissolved polymers, surfactants, etc., allows us to deeper understand the dependencies of structural and other characteristics of the dispersions on features of primary particles, their morphology, the nature of the surfaces of unmodified and modified individual and mixed oxides varied due to the synthetic techniques and the concentrations of the oxide phases or modifiers, concentrations of oxides in the liquid media, adsorption of soluble polymers, influence of surfactants, polyelectrolytes, inert electrolytes, etc. Some results related to chemically similar oxides but prepared by using other synthetic methods or data obtained by means of other experimental methods, e.g. DRS, TSD, NMR, etc., were used only in minor amounts in order to elucidate some aspects of the aqueous suspensions of fumed oxides studied by using PCS and electrophoresis methods. The main reasons of non-linear dependencies of the particle size distributions (PSD) and ζ potential on the concentration of fumed oxides in the aqueous suspensions, on the amounts of the guest X phases in X/SiO2 or the modifiers, on the pH values, etc., can be (a) a large polydispersity of PSD of both unmodified and modified fumed oxides linked with a four-stepwise structural hierarchy of fumed oxide particles, which can keep (or rearrange) in the suspensions despite different pretreatments of initial silica, X/SiO2 or their dispersions; and (b) a significant non-uniformity of the surfaces of mixed oxides due to features of the grafted or dopant phase distribution on/in the matrix depending on the guest oxide concentrations and the synthetic techniques.

Features of fumed silica coverage with silanes having three or two groups reacting with the surface

Abstract Fumed silicas modified by 3-methacryloyloxypropyltrimethoxysilane (MAPTMS), methacryloyloxymethylenemethyl diethoxysilane (MMDES), and 3-aminopropyltriethoxysilane (APTES) were studied by means of TPD-DTG, IR, 1 H NMR, electrophoresis, photon correlation spectroscopy, and theoretical methods. The modification of fumed silica by these silanes strongly changes the ζ (pH) potential and the particle size distribution in aqueous suspensions depending on the amounts and the structure of a modifier. MMDES gives the better results in comparison with its mixture with APTES or individual MAPTMS in order to reduce the particle (agglomerate) size of modified silica at the modifier concentrations over the 0.05–0.2 mmol/g range. Grafted silanes reduce the hydrophilic properties of silica depending on the pretreatment conditions. Bound MMDES diminishes the ζ potential in contrast to MAPTMS, which markedly magnifies negative ζ potential at pH>5 due to large amounts of residual SiOR groups hydrolysed in the suspensions.

Behaviour of pure water and water mixture with benzene or chloroform adsorbed onto ordered mesoporous silicas

Structural characteristics of synthesized ordered mesoporous silicas MCM-41, MCM-48 and SBA-15 were studied using XRD, nitrogen adsorption and FTIR methods. Pure water and mixtures with water/benzene and water/chloroform-d adsorbed onto silicas were studied by 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial liquids. Concentrated aqueous suspensions of MCM-48 and SBA-15 were studied by thermally stimulated depolarization current (TSDC) method. Benzene and chloroform-d can displace a portion of water to broad pores from the pore walls and from narrower pores, especially in the case of a large excess of an organic solvent. This process is accompanied by diminution of both interaction energy of water with an adsorbent surface and freezing temperature depression of adsorbed water. The effect of nonpolar benzene on pore water is much stronger than that of weakly polar chloroform-d. Modifications of the Gibbs-Thomson relation to describe the freezing point depression of mixtures of immiscible liquids confined in pores allow us to determine distribution functions of sizes of structures with unfrozen pore water and benzene.

Influence of modification of fine silica by organosilicon compounds on particle-particle interaction in aqueous suspensions

Abstract Fumed silica modified by methacryloyloxymethylenemethyl diethoxysilane (MADES) was studied and compared with unmodified fumed silica using IR spectroscopy, electrophoretic and quantum chemical methods. The electrophoretic mobility of the silica particles in aqueous suspensions depends on the modifier concentration in a non-linear manner. The effective diameter of the particles of finely dispersed silica modified by MADES (about 0.2 μm) in aqueous suspensions is considerably smaller than that for unmodified samples, as agglomerates (about 10 μm) of aggregates (0.1–0.2 μm) are decomposed to aggregates upon the modification of silica due to a reduction of hydrogen-bonding and electrostatic interactions between the modified surfaces of aggregates; in addition, MADES molecules bound to the surface possess low solvation energy.

Interfacial Behavior of n-Decane Bound to Weakly Hydrated Silica Gel and Nanosilica over a Broad Temperature Range

The interfacial and temperature behavior of n-decane bound to weakly hydrated nanosilica A-400 (initial, heated, or compacted) or silica gel Si-60 was studied using low-temperature (1)H NMR spectroscopy applied to static samples that allowed us to observe signals only of mobile decane and unfrozen water molecules. For deeper insight into the phenomena studied, interactions of n-decane, 1-decanol, and water with a set of nanosilicas and silica gels were analyzed using DSC and thermoporometry. Both NMR and DSC results demonstrated that during heating of frozen samples at a heating rate of 5 K/min a portion of decane or decanol remained frozen at temperature higher than the freezing point of bulk liquid (Tf). For decane and decanol adsorbed onto silica gels Si-40, Si-60, and Si-100, the number, position, and intensity of freezing and melting peaks observed in the DSC thermograms over the 170-300 K range during cooling and heating of samples depended on the pore size distribution of silicas as well as on the amounts and type of adsorbates. The position of the main freezing peak of decane for all samples was close to Tf because the alkane amount was greater than the pore volume; i.e., a fraction of decane was bulk liquid. According to (1)H NMR data, a portion of decane, which was in a quasi-crystalline solid state characterized by fast molecular exchange (i.e., short transverse relaxation time) and not observed in the spectra, was greater than a portion of decane frozen at temperatures close to Tf during cooling that appears in the DSC endotherms of heated samples.

Comparison of stability properties of poly(acrylic acid) adsorbed on the surface of silica, alumina and mixed silica-alumina nanoparticles — application of turbidimetry method

The influence of anionic poly(acrylic acid) — PAA addition on the stability of synthesized silica, alumina and mixed silica-alumina suspensions as a function of solution pH was studied. The turbidimetry method was used to monitor the changes of the examined systems stability over time. The calculated stability coefficients enabled estimation of polymer adsorption influence on stability of metal oxide suspension. It was shown that the alumina suspension without the polymer is the most unstable at the pH values 6 and 9, whereas the silica polymer was most unstable at pH 3. PAA with higher molecular weight (240 000) is a relatively effective stabilizer of all investigated adsorbents (except silica at pH 3). These properties of poly(acrylic acid) are highly desirable in many branches of industry (e.g. production of cosmetics, pharmaceuticals, paints) where polymers are widely used as effective stabilizers of colloidal suspensions.

Well-defined oxide core-polymer shell nanoparticles: interfacial interactions, peculiar dynamics, and transitions in polymer nanolayers.

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.

Effect of solution pH on the stability of mixed silica -alumina suspension in the presence of polyacrylic acid (PAA) with different molecular weights

The influence of solution pH (in the range 3–9) on mixed silica-alumina suspension in the absence and presence of polyacrylic acid (PAA) was studied. The composition of the adsorbent was SiO2 (97%) and Al2O3 (3%). The turbidimetry method was applied to record changes in the stability of the investigated systems as a function of time. It was shown that the suspension without the polymer is less stable at pH 3, whereas at pH 6 and 9, the systems were stable. PAA with molecular weights 100 000 and 240 000 at pH 3 (improvement of system stability conditions) and PAA 2 000 at pH 6 (deterioration of suspension stability) have a great effect on the silica-alumina suspension stability. The stabilization-flocculation properties of polyacrylic acid are a result of a specific conformation of its chains on the solid surface where it depends on the solution pH and the polymer molecular weight.

Interaction of unmodified and partially silylated nanosilica with red blood cells

Interaction of red blood cells (RBCs) with unmodified and partially (50%) silylated fumed silica A-300 (nanosilica)was studied by microscopic, XRD and thermally stimulated depolarisation current (TSDC) methods. Nanosilica at a low concentration CA-300 < 0.01 wt.% in buffered aqueous suspension is characterised by a weak haemolytic effect on RBCs. However, at CA-300 = 1 wt% all RBCs transform into shadow corpuscles because of 100% haemolysis. Partial (one-half) hydrophobization of nanosilica leads to reduction of the haemolytic effect in comparison with unmodified silica at the same concentrations. A certain portion of the TSDC spectra of the buffered suspensions with RBC/A-300 is independent of the amounts of silica. However, significant portions of the low-and high-temperature TSDC bands have a lower intensity at CA-300 = 1 wt% than that for RBCs alone or RBC/A-300 at CA-300 = 0.01 wt.% because of structural changes in RBCs. Results of microscopic and XRD investigations and calculations using the TSDC-and NMR-cryoporometry suggest that the intracellular structures in RBCs (both organic and aqueous components) depend on nanosilica concentration in the suspension.

Interaction of fibrinogen with nanosilica

Interaction of human plasma fibrinogen (HPF) with fumed nanosilica A-300 in a phosphate buffer solution (PBS) was studied using 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial water in the temperature range of 210–273 K, TSDC (90 < T < 265 K), adsorption, FTIR, and UV spectroscopy methods. An increase in concentration of HPF in the PBS leads to a decrease in amounts of structured water (frozen at T < 273 K) because of coagulation of HPF molecules. Addition of nanosilica to the HPF solution strongly reduces the amounts of structured water because of adsorption interaction of HPF molecules with silica nanoparticles, self-association of HPF molecules, formation of denser packed hybrid agglomerates with HPF and silica, and lastly, because of conformational changes of HPF. A monolayer adsorption capacity of A-300 corresponds to 156 mg of HPF per gram of silica. The FTIR and UV spectra show that the HPF adsorption on silica leads to structural changes of the protein molecules. These changes and formation of hybrid HPF/A-300 aggregates can increase the rate of clotting that is of importance on nanosilica application as a component of tourniquet preparations.