The Effect of Aggregation in Aqueous Aerosil Dispersions on Their Structure and Mechanical Properties

The dynamics of the structure disintegration in the A-175 aerosil dispersions in water and sodium dodecylbenzenesulfonate (SDBS) aqueous solutions was studied by the rotational viscometry. The aqueous dispersions of aerosil were found to be characterized by a high degree of aggregation. At the volume content of the dispersed phase = 0.063–0.082, the flow of the dispersions is described in terms of the theory of highly aggregated systems. The effect of SDBS on the flow behavior of aerosil dispersions is complex. There is a range of SDBS concentrations (C = 2.87 × 10^–6–1.44 × 10^–5 mol/l) where it has a noticeable structuring effect. Aerosil dispersions in micellar SDBS solutions (C > 3.0 × 10^–3 mol/l) demonstrated the behavior of slightly structured systems characterized by the quasi-Newtonian flow.     

Effect of high pressure on the nematic–isotropic transition in aerosil–liquid crystal composites

We report the first high pressure investigations of the nematic–isotropic transition in the composites of a liquid crystal compound with hydrophilic aerosil particles. The low concentrations of the aerosil particles used create soft gels of the composites. As expected T_N–Iso, the nematic–isotropic transition at room pressure exhibits a non-monotonic variation with increasing aerosil concentration. This non-monotonic behaviour is seen in the isobaric scans over the wide range of pressures studied, and its “magnitude” is dependent on the pressure applied. The surprising result of the present investigations on these nanocolloidal systems is that the slope of the pressure–temperature boundary also exhibits a non-monotonic dependence with the aerosil concentration, which qualitatively is similar to that of the transition temperature variation. Employing the transition enthalpy values determined at room pressure using differential scanning calorimetric scans collected at low heating rates, we calculate the transition volume dependence on the aerosil concentration. The study adds a new dimension, namely, the influence of pressure on liquid crystalline transitions in restricted geometries.     

Electronic donor-acceptor interactions of anthracene with a dehydroxylated surface of silica gel and aerosil

The ability of a dehydroxylated surface of silica gel and aerosil to enter into donor-acceptor interactions with poly-acenes was demonstrated by methods of steadystate and kinetic spectrofluorometry. The anthracene molecule forms EDA complexes with electron-acceptor sites of silica gel and aerosil in the ground state (charge-transfer complex, CTC) and the excited state (exciplex). At a degree of coverage of the silica gel and aerosil surfaces by polyacene molecules of 10^–1–10^–3% of a monolayer, energetic and structural inhomogeneity of the OH groups of silica gel appears. Silica gel possesses stronger electron acceptor sites; the electron-acceptor sites of aerosil are more accessible to EDA interactions and more monotypic. Vacancy defects of the SiO₂ matrix are suggested as accepting sites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 693–698, November–December, 1986.     

IR spectroscopic study of adsorption of ammmonia on Brønsted acid centers of a fluorinated surface of silica

The adsorption of aqueous ammonia solution vapor on a fluorinated surface of aerosil was studied by IR spectroscopy. The adsorption of NH₃ is accompanied by the appearance of absorption bands at 740, 1450, and 3330 cm^–1 in the IR spectra. The surface compounds were identified from the results of a study on the thermal degradation of adsorbed complexes and an analysis of literature data. The IR absorption bands at 1450 and 3330 cm^–1 correspond to deformational and stretching vibrations of the N-H bonds of the ammonium cation, The 740 cm^–1 band was assigned to the vibrations of the Si-F bonds in the complex anion, containing a fragment of SiO₂ surface, a fluorine atom, a hydroxyl group and a molecule of water. In this surface-coordinated compound, the silicon atom has a coordination number of six.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 747–751, November–December, 1986.     

Photophysics of pyrene adsorbed on aerosil

The spectral luminescence properties of pyrene adsorbed on the surface of aerosil have been investigated at different thermal activation temperatures (453, 773, and 1073°K) and different degrees of surface coverage (0.01–10% of monolayer). The differences from the properties of pyrene adsorbed on silica gel are dictated by the high polarity of the aerosil surface, the less significant rearrangement of its hydroxyl coverage in thermal treatment, and its pronounced energetic discrete homogeneity at low degrees of coverage.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 121–125, January–February, 1987.     

Interaction between Globular Proteins and Silica Surfaces

The adsorption of bovine serum albumin on silica of three different types, i.e., aerosil, macroporous silica gel, and ground natural quartz, was studied. The IR spectra of this protein adsorbed on aerosil were measured and analyzed. It was shown that the carbonyl groups of albumin macromolecules interact with vicinal hydroxyl groups while imido groups, with individual hydroxyl groups of silica surface. The geminal hydroxyl groups of the surface behaved as single adsorption sites with respect to albumin. The number or such sites on quartz surface was estimated. The IR spectra indicated that the adsorption of albumin macromolecules caused the dehydration of aerosil surface and the appearance of a small amount (to 10%) of unfolded -regions in the secondary structure of the adsorbed protein, while the -helical macromolecular structure remains preserved as a whole. Changes in the tertiary structure of the protein resulted from the adsorption were discussed. Protein macromolecules folded into globules were shown to be tilted with respect to the adsorbent surface.     

Molecular–Continuum Model for the Cyanide Ion Adsorption from Aqueous Solutions on Copper Metals

Quantum-chemical calculations of the cyanide ion adsorption from aqueous solutions on copper metals are performed for the first time in a combined molecular–continuum model of polar solvent. The calculations use the cluster model of the surface and are carried out by the density functional in the B3LYP version. The effect of the adsorption system's polar dielectric environment is considered in a self-consistent reactive field model, namely, the SCIPCM model. The dielectric cavity is built in SCIPCM self-consistently with the particle's electron density distribution in solution. Calculations show that the CN^– adsorption energy decreases in the sequence Au > Cu > Ag. The calculated energy agrees best with the experimental data when the molecular–continuum model is used, rather than the simpler molecular and continuum models.     

Interaction of water-insoluble quaternary ammonium salts with the hydrated silica surface

PMR spectroscopy of adsorbed molecules upon freeze-out of the liquid phase was used to study the interaction of high-molecular-weight quaternary ammonium salts differing in the structure of the hydrophilic part of the molecule and the distance between the nitrogen atoms with the hydrated aerosil surface. This approach not only yields data on the change in the hydration shell of the adsorbents upon insertion of complex organic molecules with several types of reaction sites but also permits evaluation of the efficiency of the adsorption interactions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 6, 376–379, November-December, 1996.     

Changes in local electronic state densities in aerosil modification

INDO, MINDO/3, and MNDO semiempirical methods have been used in the cluster approximation to calculate the changes in local state density for Si and 0 atoms in surface layers of modified aerosil with the formation of Si-X groups (X = NH₂, CH₂Cl, CH₂NH₂, OC₆H₅), and also on the adsorption of the antibiotics levomycetin and benzyl penicillin. Surface modification alters not only the local densities but also the combined integral state density. Studies have been made on the effects of the boundary conditions and spatial disordering in the clusters on the characteristics of the valency-band subbands. The calculations are compared with x-ray emission spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 483–487, September–December, 1992.     

AFM investigations of a glassy cholesteric liquid crystal with hydrophobic aerosil particles

AFM investigations of a glassy heterogeneous system consisting of an oligomeric cholesteric liquid crystal and the hydrophobic aerosil R812 were carried out. With increasing aerosil concentration, a suppression of the characteristic cholesteric surface pattern was observed. Typical separated aerosil aggregates appear in the samples. Their size and form change from small lumps through bigger rod-like entities to large crystallite-like aggregates of aerosil particles. This matches with observations of light scattering of systems with low molecular mass liquid crystals and of the memory effect. The pitch of the cholesteric fingerprint pattern slightly decreases with increase in the aerosil concentration.     

Formation of peroxide compounds on the oxidation of lignin by oxygen in an organic solvent

The results are given of a determination of the steady-state concentrations of peroxide compounds on the oxidation of lignin by oxygen in dioxane. An iodometric method of determining peroxides was used, with spectrophotometric control of the amount of iodine formed. It was established that the achievable steady-state concentration of peroxide compounds on the oxidation of lignin is 0.28–0.44 wt. % of O_act (oxygen pressure 1 atm, temperature 50–80°C). With a rise in the temperature, the steady-state concentration of peroxide compounds decreased. The addition of water (30 vol. %) to the organic solvent led to an increase in the rate of accumulation of peroxide compounds. It has been shown that the presence of alkaline and acidic catalysts exerts no appreciable influence on the achievable steady-state concentration and the rate of accumulation of peroxide compounds during the oxidation of lignin.Bratsk Industrial Institute. All-Union Scientific Production Combine of the Pulp and Paper Industry, Leningrad. Translated from Khimiya Prirodnykh Soedinenii, Nos. 3,4, pp. 413–417, May–August, 1992.     

Molecular self-diffusion on pyrogenic silica

Pulse-gradient NMR has been used to determine self-diffusion coefficients for acetone, acetonitrile, benzene, hexane, methanol., and trimethylchlorosilane on aerosil. A comparison is made with the corresponding values for silica gel.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 1, pp. 99–103, January–February, 1989.     

IR spectroscopic study of the reaction of polyalkylhydrosiloxanes with a silica surface

Using IR spectroscopy, we have studied the reaction of polyalkylhydrosiloxanes with a silica surface and the change in the chemical nature and structure of the modified layer upon its thermal oxidation. Upon modification of aerosil A-175 by polymethyl- and polyethylhydrosiloxane, the modifying agent molecules reacts with the surface OH groups. Some of the surface hydroxyl groups are not involved in chemical reaction with the modifying agent, but judging from the position of the corresponding OH-stretch band (3400 cm^–1) they experience the perturbing effect of the polyorganosiloxane film. Thermal oxidation leads to an increase in the number of the lateral bonds in the polyorganosiloxane-modified film; the maximum degree of crosslinking is observed in the case of the aerosil treated with polymethylhydrosiloxane.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 6, pp. 753–755, November–December, 1989.     

Catalytic and asymmetrizing activity of mixed copper-nickel catalysts,deposited on aerosil,in the hydrogenation of acetylacetone

Conclusions A mixed Cu-Ni (11) catalyst, deposited on aerosil and modified with D-(+)-tartarie acid, was obtained, which is active in theselective stereoselective hydrogenation of acetylacetone to the (-)-ketol. The optical yield of the ketol is 9.1% at 133°.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 680–682, March, 1977.     

29Si NMR study of the surface of pyrogenic silica modified by methylchlorosilanes

Cross-polarization ²⁹Si NMR spectra have been used for aerosil modified by methylchlorosilanes to identify surface organosilicon compounds and their reactions during hydrolysis and methanolysis.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, No. 3, pp. 378–383, May–June, 1986.     

Silica Matrix with anhydride groups for immobilization of enzymes

We have studied the process of preparing a silica support with anhydride groups. The process includes introducing vinyl groups onto the surface with subsequent radiative copolymerization of maleic anhydride with them. The maximum degree of grafting of the functional groups of the matrix occurs for -irradiation doses of 20–30 Mrad. The carboxyl groups grafted to the aerosil surface, formed upon hydrolysis of the support, can be converted to anhydride by calcination at 403 K for 3 h.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 6, pp. 750–753, November–December, 1989.     

Mass-spectrometric investigation of the kinetics of hydrolysis of methoxysilyl groups on the surface of silica under the conditions of linear temperature increase

The mass-spectrometric investigation of the kinetics of chemisorption under the conditions of linear temperature increase was tested on the example of the hydrolysis of methoxysilyl groups on the surface of aerosil; a constant vapor pressure of the modifier was maintained above the adsorbent. It has been established that the above-mentioned reaction proceeds in the kinetic region according to an impact mechanism, and that its rate is limited predominantly by the entropy factor.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 634–637, September–October, 1985.     

Photonics of perylene on the surface of SiO2

Stationary and kinetic spectrofluorometry and diffuse reflection spectroscopy have been used to study the photophysics and photochemistry of perylene, adsorbed on the surface of porous (silica gel) and nonporous (aerosil) silica after thermal activation in air (at 453, 773, and 1073 K) as function of the degree of surface coverage.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 2, pp. 231–235, March–April, 1988.     

Sol-Gel Formation of Reticular Methyl-Silicate Materials by Hydrogen Peroxide Decomposition

A new method for the formation of reticular silicate and organically modified silicate is introduced. Monoliths were prepared by incorporating a few percent hydrogen peroxide in the sol-gel starting solution. For example, incorporation of 6–10% (v) hydrogen peroxide in base catalyzed sol-gel precursors of methyl-Ormosil yielded macroporous monoliths with a bi-modal pore size distribution. The average characteristic pore diameters were approximately 1.2 nm and 0.7 m, depending on the sol-gel precursors used and the preparation protocol. The specific surface area was approximately 160 m2/g, contributed mainly by the microporous structure. A similar preparation procedure without hydrogen peroxide yielded only fractured or powdery materials. Presumably, the decomposition of the hydrogen peroxide yielded microbubbles, which formed templates for the polycondensation reaction. SEM, nitrogen adsorption isotherms and small angle X-ray spectroscopy were used to characterize the reticular materials.     

Thermal decomposition of acetyl propionyl peroxide in acetone-<Emphasis Type="Italic">d</Emphasis><Subscript>6</Subscript>

The kinetics of thermolysis of acetyl propinyl peroxide in acetone-d ₆ in the temperature range 323–373 K was studied using NMR spectroscopy and the effect of chemically induced nuclear polarization. The peroxide decomposes in acetone at rates comparable with the rates of thermolysis in alcohols, yielding numerous products. In the examined temperature range, the solvent molecules act as efficient donors of deuterium atoms, forming acetylmethyl-d ₅ radicals which recombine to a significant extent with the peroxide radicals. A scheme of the processes involved in decomposition of the peroxide was suggested. The parameters of the Arrhenius equation for the peroxide decomposition were determined.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004, pp. 1847–1854.Original Russian Text Copyright © 2004 by Skakovskii, Stankevich, Tychinskaya, Shirokii, Choban, Murashko, Rykov.This revised version was published online in April 2005 with a corrected cover date.