Surface Charge Density of Silica Suspended in Water-Acetone Mixtures

The net proton surface charge density of silica suspended in water-acetone mixtures was studied by potentiometric titration. LiCl and NaCl were used as background electrolytes at concentrations of 10(-1), 10(-2), and 10(-3) mol L-1. The results showed that acetone lowers the net proton surface charge density of silica and that the greater the decrease, the greater the acetone concentration. The surface charge density of silica also is very sensitive to the nature of the background electrolyte, LiCl producing much lower surface charge densities than NaCl. The concept of free energy of transfer (DeltaG0t) of an electrolyte between two different solvents was applied to explain the results in a qualitative manner. Copyright 1999 Academic Press.     

Mesoporous benzene-silica hybrid materials with a different degree of order in the wall structure: an IR comparative study

Recent joint IR and computational work (Onida et al. J. Phys. Chem B 2005) has allowed a detailed characterization of the isolated silanols at the surface of highly ordered benzene-silica hybrid material. In the present paper, a similar characterization is provided for a less ordered sample. The comparison permits the assignment of IR features to the interaction of silanols either with one another or with benzene rings of the structure. The extent of structural imperfections appears to be limited, for example, no more than pairs of interacting silanols are found, readily healed by thermal treatment. Evidence is also provided that probe molecules with simultaneous H-acceptor and H-donor properties (benzene, methylacetylene) may interact with both the acidic proton in silanols and the electronic cloud in the framework aromatic rings.     

Complex aggregates of silica microspheres by the use of a polymer template

Evaporation of a droplet of silica microsphere suspension on a polystyrene and poly(methyl methacrylate) blend film with isolated holes in its surface has been exploited as a means of particles self-assembly. During the retraction of the contact line of the droplet, spontaneous dewetting combined with the strong capillary force pack the silica microspheres into the holes in the polymer surface. Complex aggregates of colloids are formed after being exposed to acetone vapor. The morphology evolution of the underlying polymer film by exposure to acetone solvent vapor is responsible for the complex aggregates of colloids formation.     

Role of the surface of silica gel in the radiation chemical conversions of adsorbed hydrocarbons

The action of gamma radiation on the systems benzene-silica gel and hexane-silica gel has been studied by EPR for different ratios of the components. It is shown that H-atoms detached from the surface OH groups during irradiation react with the adsorbed hydrocarbons to give C₆H ₇ ^. (C₆H₆D in the case of deuterated silica gel) and C₆H ₁₃ ^. radicals. It is ascertained that the SiO^. radicals also formed with the H-atoms during irradiation react with the adsorbate. Confirmation that H-atoms and SiO^. radicals participate in the radiation chemical conversions of adsorbed hydrocarbons is obtained by experiments with silica gels previously dehydrated at various temperatures. A possible mechanism of energy migration within the silica beads is discussed.     

Radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. VI. Temperature dependence and effects of additives on methyl methacrylate–silica gel system

To elucidate the reaction mechanism of radiation-induced polymerization of methyl methacrylate adsorbed on silica gel, the temperature dependence and effects of acetone and pyridine were investigated. It was found that even at ?78°C the polymerization rate was quite fast. The amounts of high molecular weight GPC peaks of both graft polymers and homopolymers increased with increasing irradiation temperature. The activation energy of the adsorbed state polymerization was low compared with that of bulk polymerization. The low molecular weight peaks of homopolymers decreased with acetone addition but were almost unaffected by pyridine. The low molecular weight peaks of homopolymers were thus polymerized by an anionic mechanism. In the methyl methacrylate–silica gel system both radical and anionic polymerization take place at the same time in formation of graft polymers and homopolymers. A reaction mechanism for the methyl methacrylate–silica gel system was proposed based on the results obtained to date.     

Acetone oxidation using ozone on manganese oxide catalysts

Supported manganese oxide catalysts were prepared by the impregnation of alumina foam blocks washcoated with alumina and silica. The manganese content based on the weight of the washcoats was 10 wt % calculated as MnO2. Fourier transform profiles of the Mn K-edge EXAFS spectra for these samples gave three distinctive peaks at 0.15, 0.25, and 0.32 nm and were close to the profiles of Mn3O4 and beta-MnO2. The number of surface active sites was determined through oxygen chemisorption measurements at a reduction temperature (Tred = 443 K) obtained from temperature-programmed reduction (TPR) experiments. Acetone catalytic oxidation was studied from room temperature to 573 K, and was found to be highly accelerated by the use of ozone on both catalysts with substantial reductions in the reaction temperature. The only carbon-containing product detected was CO2. The alumina-supported catalyst was found to be more active than the silica-supported catalyst in acetone and ozone conversion, with higher turnover frequencies (TOFs) for both reactions. The pressure drop through the foam was low and increased little (0.003 kPa/10 000 h(-1)) with space velocity. In situ steady-state Raman spectroscopy measurements during the acetone catalytic oxidation reaction showed the presence of an adsorbed acetone species with a C-H bond at 2930 cm(-1) and a peroxide species derived from ozone with an O-O bond at 890 cm(-1).     

Determination of Association Effects in TLC Data for Different Solutes Chromatographed in Methanol-Acetone on Silica Gel

Abstract

The TLC data for different solutes chromatographed in methanol-acetone mobile phase on silica gel at 293 K are analysed by means of the theoretical equations discussed in the previous paper (1). This analysis shows that for the above systems the solute-solvent and solvent-solvent association and the composition of the surface phase play an important role. The composition of the surface phase has been determined by utilizing the excess adsorption data of methanol from acetone on silica gel.     

Effect of surface roughness and softness on water capillary adhesion in apolar media

The roughness and softness of interacting surfaces are both important parameters affecting the capillary condensation of water in apolar media, yet are poorly understood at present. We studied the water capillary adhesion between a cellulose surface and a silica colloidal probe in hexane by AFM force measurements. Nanomechanical measurements show that the Young's modulus of the cellulose layer in water is significantly less (~7 MPa) than in hexane (~7 GPa). In addition, the cellulose surface in both water and hexane is rather rough (6-10 nm) and the silica probe has a comparable roughness. The adhesion force between cellulose and silica in water-saturated hexane shows a time-dependent increase up to a waiting time of 200 s and is much (2 orders of magnitude) lower than that expected for a capillary bridge spanning the whole silica probe surface. This suggests the formation of one or more smaller bridges between asperities on both surfaces, which is confirmed by a theoretical analysis. The overall growth rate of the condensate cannot be explained from diffusion mediated capillary condensation alone; thin film flow due to the presence of a wetting layer of water at both the surfaces seems to be the dominant contribution. The logarithmic time dependence of the force can also be explained from the model of the formation of multiple capillary bridges with a distribution of activation times. Finally, the force-distance curves upon retraction show oscillations. Capillary condensation between an atomically smooth mica surface and the silica particle show less significant oscillations and the adhesion force is independent of waiting time. The oscillations in the force-distance curves between cellulose and silica may stem from multiple bridge formation between the asperities present on both surfaces. The softness of the cellulose surface can bring in additional complexities during retraction of the silica particle, also resulting in oscillations in the force-distance curves.     

由硅酸酯合成单分散二氧化硅中碳的化学形态

单分散二氧化硅是指尺寸分布十分狭窄的二氧化硅颗粒．单分散颗粒在科学研究及工业应用中得到了广泛的应用［1］．单分散二氧化硅由正硅酸有机酯在氨催化下于醇溶液中水解缩合得到．硅酸酯的水解和缩合反应可用如下反应描述．总的反应式为：nSi（OR）4＋2nH20→nSiO2＋4nROH1956年Kolbe［2］发现正硅酸乙酯（TEOS）在碱催化下于乙醇溶剂中水解反应有时会形成均一颗粒二氧化硅以来，许多学者对这一反应体系进行了较为广泛的研究，提出了双分子缩合成核机理、单分子叠加生长机理、表面反应控制生长机理、扩散控制生长机理和微晶核团聚生…     

Photooxidation of acetone on TiO2(110): conversion to acetate via methyl radical ejection

It is generally held that radicals form and participate in heterogeneous photocatalytic processes on oxide surfaces, although understanding the mechanistic origins and fates of such species is difficult. In this study, photodesorption and thermal desorption techniques show that acetone is converted into acetate on the surface of TiO2(110) in a two-step process that involves, first, a thermal reaction between acetone and coadsorbed oxygen to make a surface acetone-oxygen complex, followed second by a photocatalytic reaction that ejects a methyl radical from the surface and converts the acetone-oxygen complex into acetate. Designation of the photodesorption species to methyl radicals was confirmed using isotopically labeled acetone. The yield of photodesorbed methyl radicals correlates well with the amount of acetone depleted and with the yield of acetate left on the surface, both gauged using postirradiation temperature programmed desorption (TPD). The thermal reaction between adsorbed acetone and oxygen to form the acetone-oxygen complex exhibits an approximate activation barrier of about 10 kJ/mol. A prerequisite to this reaction is the presence of surface Ti3+ sites that enable O2 adsorption. Creation of these sites by vacuum reduction of the surface prior to acetone and oxygen coadsorption results in an initial spike in the acetone photooxidation rate, but replenishment of these sites by photolytic means (i.e., by trapping excited electrons at the surface) appears to be a slow step in a sustained reaction. Evidence in this study for the ejection of organic radicals from the surface during photooxidation catalysis on TiO2 provides support for mechanistic pathways that involve both adsorbed and nonadsorbed species.     

Interaction of acetone with ammonia and alcohols over a HZSM-5 zeolite: Part 1. Methanol

Temperature-programmed desorption accompanied by conversion (TPDC) of preadsorbed acetone was studied on a HZSM-5 zeolite with Si/Al=13.5. The reactivity of surface species created from acetone with methanol, ammonia and mixtures of these was investigated by analysing the composition of the products released (using a mass spectrometer) and the composition of the surface species (using a ¹³C MAS NMR spectrometer). It was found that ammonia reacts with the carbonyl group of acetone surface species to imino carbocations in both the absence and presence of methanol, and methanol alkylates the acetone surface species whether ammonia is present or absent. The decomposition of the species created from acetone, methanol and ammonia leads to different products than the decomposition of the species formed on the zeolite either from acetone (or methanol) alone or from acetone (or methanol) and ammonia. The strong dehydrogenation function of HZSM-5 resulted in the release of acetonitrile and HCN from acetone and methanol, respectively (in the presence of ammonia), while pyridinium bases (in the presence of all 3 reactants) appeared only in trace amounts.     

Sulfonic acid functionalized mesoporous MCM-41 silica as a convenient catalyst for Bisphenol-A synthesis

Sulfonic acid groups anchored to the surface of mesoporous MCM-41 silica have been identified with S K-edge XANES spectra and the material is an efficient catalyst for the liquid phase condensation of phenol with acetone to form Bisphenol-A with high selectivity.     

Revealing complex formation in acetone-n-alkane mixtures by MAS PFG NMR diffusion measurement in nanoporous hosts

Magic-angle spinning pulsed field gradient nuclear magnetic resonance (MAS PFG NMR) was applied for selective self-diffusion measurements of acetone-n-alkane (C(6) up to C(9)) mixtures in nanoporous silica gel. Two specimens of silica gel with mean pore sizes of about 4 and 10 nm are considered. In the smaller pores, the n-alkane diffusivities are by about one and the acetone diffusivities by about two orders of magnitude smaller than in the larger pores. In addition, the acetone diffusivities in the narrow-pore specimen exhibit a pronounced oscillation with increasing chain length of the solvent n-alkanes: the diffusivities of acetone dissolved in odd-carbon number n-alkanes exceed those of acetone dissolved in even-carbon number n-alkanes by about 50%! These findings reproduce the tendencies observed in previous macroscopic release studies (Phys. Chem. Chem. Phys. 2003, 5, 2476) and suggest the formation of acetone-n-alkane complex-like assemblages in the narrow-pore silica gel.     

Adsorption from Ternary Liquid Mixtures on Partially Dehydroxylated Silica Gel

The formation of mixed adsorbed layers has been tested for ternary mixtures containing a specifically adsorbed component—acetone and binary solvent benzene +n-hepane. The specific excess adsorption isotherms from the liquid phase were measured on silica gel samples partially dehydroxylated. The competition of liquid components for silica surface is discussed on the basis of changes in the mixed solvent composition.     

Spreading of proteins and its effect on adsorption and desorption kinetics

The kinetics of adsorption of lysozyme and alpha-lactalbumin from aqueous solution on silica and hydrophobized silica has been studied. The initial rate of adsorption of lysozyme at the hydrophilic surface is comparable with the limiting flux. For lysozyme at the hydrophobic surface and alpha-lactalbumin on both surfaces, the rate of adsorption is lower than the limiting flux, but the adsorption proceeds cooperatively, as manifested by an increase in the adsorption rate after the first protein molecules are adsorbed. At the hydrophilic surface, adsorption saturation (reflected in a steady-state value of the adsorbed amount) of both proteins strongly depends on the rate of adsorption, but for the hydrophobic surface no such dependency is observed. It points to structural relaxation ("spreading") of the adsorbed protein molecules, which occurs at the hydrophobic surface faster than at the hydrophilic one. For lysozyme, desorption has been studied as well. It is found that the desorbable fraction decreases after longer residence time of the protein at the interface.     

Adsorption of polar substances from binary and ternary mixtures on silica gel

The adsorption isotherms of acetone and methyl ethyl ketone from binary and ternary mixtures in benzene and n-heptane on silica gel were measured. The experimental adsorption data are discussed on the basis of changes of the composition of mixed solvent (benzene + n-heptane) in ternary mixtures. It has been found that the different structures of the surface phase correspond to the system investigated. The marked dependence of the adsorption on the solvent character is demonstrated. For benzene and ternary (ketone + benzene + n-heptane) mixtures a mixed character of the surface phase is observed whose composition is determined by competition of liquid components for silica surface as well as its tendency to complex. Bilayer model of the surface phase gives a good representation of the experimental data for binary systems benzene + ketone.     

Site competition during coadsorption of acetone with methanol and water on TiO2(110)

The competitive interaction between acetone and two solvent molecules (methanol and water) for surface sites on rutile TiO(2)(110) was studied using temperature-programmed desorption (TPD). On a vacuum-annealed TiO(2)(110) surface, which possessed ~5% oxygen vacancy sites, excess methanol displaced preadsorbed acetone molecules to weakly bound and physisorbed desorption states below 200 K. In contrast, acetone molecules were stabilized on an oxidized surface against displacement by methanol through formation of acetone diolate species. The behavior of acetone with methanol differs from the interactions between acetone and water which are less competitive. Examination of acetone + methanol and acetone + water multilayer combinations shows that acetone is more compatible in water-ice films than in methanol-ice films, presumably because water has greater potential as a hydrogen-bond donor than does methanol. Acetone molecules displaced from the TiO(2)(110) surface by water are more likely to be retained in the near-surface region, in turn having a greater opportunity to revisit the surface, than when methanol is used as a coadsorbate.     

Selective Ion Pair Adsorption of Cobalt and Copper Salts on Cationically Produced Poly(1,3-divinylimidazolid-2-one)/Silica Hybrid Particles

For quantitative metal salt adsorption, poly(1,3-divinylimidazolid-2-one)/silica [poly BVU(cat.)/silica] particles with different polymer contents have been synthesized by a cationic surface polymerization of 1,3-divinylimidazolid-2-one onto silica. Preliminary experiments with the metal ion salts CoCl(2), CoI(2), CuCl(2), and FeCl(3) on poly-BVU(cat.)/silica particles, a radically produced poly-BVU(rad.) resin, and a cationically produced poly-BVU(cat.) resin have been carried in acetone solution to check the suitability of the adsorbents. The adsorption mechanism for Co(2+) and Cu(2+) on poly-BVU(cat.)/silica is in accordance with the Langmuir model for monolayer adsorption as shown by quantitative adsorption measurements by means of UV/vis spectroscopy. An ion pair adsorption mechanism is suggested for CoCl(2), CoI(2), and CuCl(2) on poly-BVU(cat.)/silica because both environments cationically produced poly-BVU and residual silanol groups are required for linking the cation and anion. ESR spectroscopic results of CoCl(2)-, CuCl(2)-, and FeCl(3)-poly-BVU(cat.)/silica hybrid adsorbates show selective adsorption for Co(2+) and Cu(2+). However, two different adsorption sites are indicated for Fe(3+) on poly-BVU(cat.)/silica. Copyright 2001 Academic Press.     

Asymmetric orientation of toluene molecules at oil-silica interfaces

The interfacial structure of heptane and toluene at oil-silica interfaces has previously been studied by sum frequency generation [Z. Yang et al., J. Phys. Chem. C. 113, 20355 (2009)]. It was found that the toluene molecule is almost perpendicular to the silica surface with a tilt angle of about 25°. Here, we have investigated the structural properties of toluene and heptane at oil-silica interfaces using molecular dynamics simulations for two different surfaces: the oxygen-bridging (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces of quartz (silica). Based on the density profile, it was found that both heptane and toluene oscillate on silica surfaces, with heptane showing more oscillation peaks. Furthermore, the toluene molecules of the first layer were found to have an asymmetric distribution of orientations, with more CH(3) groups pointed away from the silica surface than towards the silica surface. These findings are generally consistent with previous experiments, and reveal enhanced molecular structures of liquids at oil-silica interfaces.     

Synthesis and Characterization of Microporous Silica Prepared with Sodium Silicate and Organosilane Compounds

Microporous silica gels were prepared in the pH range of 3–4 using sodium silicate as a silica source. Surface polarity of these gels was modified by grafting hydrophobic groups into the silica gel matrix with the help of hydrophilic solvents (acetone, acetonitrile, ethanol and methanol) and alkoxysilane compounds containing nonhydrolyzable alkyl groups. The porous framework and hydrophobicity of the silica gels were evaluated using nitrogen adsorption/desorption and water adsorption measurement techniques. All the measured isotherms were found to be type I which is indicative of microporosity. The surface area and microporosity of these samples were estimated by analyzing the measured nitrogen adsorption/desorption data using BET, Langmuir and Dubinin-Radushkevich (D-R) adsorption isotherms. The micropore size distribution was determined from their nitrogen adsorption isotherms using the slit-pore model of the Horvath-Kawazoe equation. Silica gels with high surface area (over 500 m²/g) as well as high microporosity (over 0.2 cc/g) were obtained at gelation pH of 3.50 from the water-solvent system.