Journal of Colloid And Interface Science (v.309, #1)
Amplified light scattering and emission of silver and silver core–silica shell particles
by Olavi Siiman; Andrei Jitianu; Marjan Bele; Patricia Grom; Egon Matijević (pp. 8-20).
Side versus forward light scattergrams, and fluorescence (488 nm excitation) intensity versus particle count histograms were gathered for bare, R6G-coated, and silica–R6G-coated silver particles of 150–200 nm diameter, one-by-one by flow cytometry. Fluorescence emission intensity of the composite particles monotonically increased and then reached a plateau with greater R6G concentrations, as measured by flow cytometry. Fluorescence amplification factors of up to3.5×103 were estimated by reference to measurements on core–shell particles with silica instead of silver cores. Huge surface enhanced Raman scattering (SERS) intensities, at least 1014-fold greater than normal Raman scattering intensities, were observed with 633 nm excitation for molecules such as rhodamine 6G (R6G) on the same single particles of silver. Although routine transmission (TEM) and scanning (SEM) electron microscopies showed gross structures of the bare and coated particles, high-resolution field emission scanning electron microscopy (FE-SEM), revealed Brownian roughness describing quantum size and larger structures on the surface of primary colloidal silver particles. These silver particles were further characterized by extinction spectra and zeta potentials. Structural and light scattering observations that are reported herein were used to tentatively propose a new hierarchical model for the mechanism of SERS.The light scattering of small particles of silver in the range, 100 to 200 nm in diameter, can be distinctly observed by conventional flow cytometer. It was also possible to detect fluorescence emission intensity from rhodamine 6G on silver particles or on silver–silica core–shell particles.
Keywords: Silver particles; Dye; Silica-coated; Flow cytometry; Raman scattering; Fluorescence emission
Determination of surface potentials from the electrode potentials of a single-crystal electrode
by Nikola Kallay; Tajana Preočanin; Trpimir Ivšić (pp. 21-27).
Determination of surface potentialsΨ0 at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single-crystal electrode (SCrE) is described. The proposed method is based on the surface complexation model and evaluates the surface potential at the isoelectric point, i.e., at pHiep. This value is used for calculation ofΨ0 values from the measured electrode potentials. Both 1-p K and 2-p K models produced the same result so that the procedure does not depend on the assumed mechanism of the surface charging. It is proposed to determine the pristine point of zero charge pHeln and the isoelectric point pHiep, and use these data to set the scale of surface potentials. The value of pHeln can be obtained at a sufficiently low ionic strength where pHpzc coincides with pHiep. The method is demonstrated on the example of the anatase single-crystal electrode. From the shifts of pHiep and pHpzc with respect to the pristine point of zero charge pHeln it was concluded that Cl− ions exhibit higher affinity for association with positively charged surface groups than ClO−4 ions. Also, preferential surface association of Na+ cations compared to both anions was detected. The slopes of theΨ0(pH) functions were found to be significantly lower in magnitude with respect to the Nernst equation, which is due to the high degree of counterion association at the surface caused by their relatively high concentration.
Keywords: Surface potential; Surface charge; Single-crystal electrode; Surface complexation model; Electrical interfacial layer; Anatase
Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates
by Ionel Popa; Brian P. Cahill; Plinio Maroni; Georg Papastavrou; Michal Borkovec (pp. 28-35).
The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on silica substrates quickly saturates, and the resulting surface coverage increases with salt level in accord with the random sequential adsorption (RSA) model.
Keywords: Polyelectrolyte; Adsorption; Reflectometry; Silica
Bioaerosol detection and characterization by surface-enhanced Raman spectroscopy
by Atanu Sengupta; Navpreet Brar; E. James Davis (pp. 36-43).
Instrumentation has been developed to detect and characterize airborne pollen and bacteria rapidly by injecting a bioaerosol into a nanocolloidal suspension of silver particles using a micropump. The biological particles were mixed with the silver colloid in order to deposit the metallic particles on the surface of the bioanalyte. The silver/bioanalyte suspension was pumped through a light scattering cuvette, and the enhanced Raman spectrum was recorded. Surface-enhanced Raman spectra are presented for tree pollen (cottonwood and redwood pollen) and a bacterium ( Escherichia coli), and the E. coli spectra are compared with results obtained from the literature and with results obtained previously by mixing various concentrations of the bioanalyte with the silver colloid. Although the system has not been optimized to maximize the Raman spectra, it is shown spectra can be obtained rapidly. Some assignments of the chemical bonds associated with the spectra are based on previously published results for bacteria and pollen.Surface-enhanced Raman spectroscopy has been used to characterize airborne biological particles by contacting the aerosol with a nanocolloidal suspension of silver. The mixture is pumped into a light scattering cuvette to obtain the Raman spectrum.
Keywords: Bacteria; Pollen; Nanocolloidal silver; Surface-enhanced Raman spectroscopy
Influence of poly-l-lysine on the biomimetic growth of silica tubes in confined media
by Clémentine Gautier; Pascal J. Lopez; Jacques Livage; Thibaud Coradin (pp. 44-48).
Pore channels of polycarbonate membranes were recently used as biomimetic models to study the effect of confinement on silicate condensation, leading to the formation of silica tubes exhibiting a core–shell structure. In this work, we preimmobilized poly-l-lysine on the membrane pores, leading to modification of the tube shell formation process and variation in core particle size. These data strengthen previous assumptions on the role of confinement on silica growth, i.e., interfacial interactions and perturbation of the diffusion coefficient. They also suggest that this approach is suitable to investigate in more detail the contribution of confinement effects on silica biomineralization.
Keywords: Biomimetism; Silica; Confinement; Poly-; l; -lysine; Sodium silicate
XPS analysis of chemical functions at the surface of Bacillus subtilis
by François Ahimou; Christophe J.P. Boonaert; Yasmine Adriaensen; Philippe Jacques; Philippe Thonart; Michel Paquot; Paul G. Rouxhet (pp. 49-55).
The surface chemical composition of nine strains of Bacillus subtilis was determined by X-ray photoelectron spectroscopy. Regressions between elemental concentrations and concentrations associated with different components of C1s, N1s, and O1s peaks provided a more precise validation of the procedure used for peak decomposition and allowed the assignment of the peak components to be completed or strengthened. The component of the O1s peak appearing around 531.2 eV was shown to contain a contribution of oxygen from phosphate groups (PO, PO−), the other contribution being due to oxygen involved in amide functions. The surface negative charge may be fully attributed to phosphate groups, despite the observation of two types of zeta potential vs pH curves. The strains exhibiting a sharp variation of the zeta potential (range of −35 to−55 mV) between pH 2 and 4.7 were characterized by a high phosphate surface concentration and by an excess (about 25%) of phosphate with respect to the sum of potassium, an exchangeable cation, and protonated nitrogen, attributed to protein or to alanine involved in teichoic acids.The procedure used to decompose XPS peaks of bacterial surfaces is validated and the assignment of peak components is completed and strengthened. The balance of ionized groups is related to electrokinetic properties.
Keywords: Bacterial surfaces; Bacillus subtilis; X-ray photoelectron spectroscopy; Surface analysis; Zeta potential; Electrophoretic mobility; Phosphate
On possible microscopic origins of the swelling of neutral lipid bilayers induced by simple salts
by Marian Manciu; Eli Ruckenstein (pp. 56-67).
It was recently suggested that the swelling of neutral multilipid bilayers upon addition of a salt can be simply explained only by the electrolyte screening of the van der Waals attractions, while assuming that the hydration force and the repulsion due to thermal undulations of membranes are unaffected by the salt. While we agree that the screening of the van der Waals interactions plays a role, we suggest that the increase in the hydration force upon addition of a salt has also to be taken into account. In a statistical model, which accounts for the membrane undulations, parameters could be found to explain the multibilayer swelling even when the van der Waals attraction is considered unaffected by the electrolyte screening. These results point out that the decrease by a factor of three of the Hamaker constant upon addition of a salt, suggested recently to be responsible for the swelling of neutral multilipid bilayers, is perhaps too large, and a smaller decrease in Hamaker constant, coupled with the above mentioned effects might explain the swelling.By coupling the membrane undulations and the hydration and van der Waals interactions, one explains the effect of addition of a salt on the swelling of multilamellar lipid bilayers.
Keywords: Lipid bilayers; Swelling; Effect of salt
Direct aerosol synthesis of carboxy-functionalized iron oxide colloids displaying reversible magnetic behavior
by Pedro Tartaj; Teresita Gonzalez-Carreño; Aldo F. Rebolledo; Oscar Bomatí-Miguel; Carlos J. Serna (pp. 68-71).
A simple and rapid synthetic strategy for fabricating carboxy-functionalized iron oxide colloidal particles displaying reversible magnetic behavior is reported. The method is based on the pyrolysis of aerosols generated from ethanol/water solutions containing iron inorganic salts and mono- or polysaccharides. Essential to the success of the method are the use of hybrid (organo-inorganic) aerosols and the temperature of pyrolysis. The resulting material could be used in advanced biotechnological applications such as the magnetically assisted chemical separation of biocompounds.
Keywords: Aerosol; Biomedicine; Magnetic nanoparticles; Functionalization; Colloids
Homogeneous precipitation of layered Ni(II)–Cr(III) double hydroxides
by Matías Jobbágy; Miguel A. Blesa; Alberto E. Regazzoni (pp. 72-77).
An adequate account of the hydrolytic properties of Cr3+ and Ni2+ allows setting the conditions for homogeneous nucleation of layered Ni(II)–Cr(III) double hydroxides; water exchange and hydrolysis rate constants indicate that, at very high rates of base dosing, formation of heteronuclear Cr(III)–Ni(II) hydroxo species should prevail over precipitation of active Cr(OH)3. This is realized by the urea method under microwave-assisted hydrothermal conditions. This approach yields crystalline Ni1− xCr x(OH)2(CO3) x/2⋅ nH2O (x≈0.32–0.36) in less than 5 min at 453 K; higher degrees of crystallinity are obtained at higher temperatures and/or longer aging times. Formation of Ni(II)–Cr(III) LDHs upon microwave-assisted hydrothermal aging of freshly coprecipitated Ni(OH)2+Cr(OH)3 mixtures takes longer, due to a different operating mechanism. The implications of the advanced rationale for the design of synthesis procedures are stressed. It is proposed that homogeneous nucleation of Ni(II)–Cr(III) LDHs involves the edge-on condensation of planar heteronuclear Cr(III)–Ni(II) hydroxo trimers. Ordered aggregation of primary particles leads to the final platelet crystals, a process that involves the exchange of CO2−3 ions dangling at the crystallites' edges by bridging OH−.An adequate account of the kinetics of urea hydrolysis and of the hydrolytic properties of Cr3+ and Ni2+ allows setting the conditions for homogeneous nucleation of layered Ni(II)–Cr(III) double hydroxides.
Keywords: Layered double hydroxides; Hydrotalcites; Homogeneous precipitation; Nickel hydroxide; Chromium hydroxide
Zinc oxide colloids with controlled size, shape, and structure
by Mihaela Jitianu; Dan V. Goia (pp. 78-85).
Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum. Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase. Depending on the experimental conditions, the size of the final polycrystalline particles formed by the aggregation of nanosize entities varied from 100 to 300 nm. The reaction temperature affected both the size of the nanosize precursors and their arrangement in the final particles. At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 °C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms. The aspect ratio and the internal structure of the latter could be altered by changing the nature of the zinc salt, the addition rate, and the initial concentration of the reactants. Based on the findings of the study a two-stage mechanism for the formation of uniform polycrystalline particles with well-defined geometric shapes is proposed.
Keywords: Colloid; Zinc oxide; Double jet precipitation; Aggregation; Primary nanoparticles
Reverse micelle synthesis of rhodium nanoparticles
by James D. Hoefelmeyer; Hongjian Liu; Gabor A. Somorjai; T. Don Tilley (pp. 86-93).
Water-in-oil reverse micelles of butyl ammonium laurate in hexanes that contain sodium hexachlororhodate were reduced with sodium borohydride to yield rhodium nanoparticles. The size of the micelle, determined by dynamic light scattering, was from 3 to 20 nm and varied as the water to surfactant ratio ( W) was changed. The rhodium nanoparticles exhibited a Gaussian size distribution (σ=0.35nm), with average diameters of 1.5, 2.2, and 2.9 nm. The products were characterized with TEM, HRTEM, and X-ray photoemission spectroscopy.Water-in-oil reverse micelles of butyl ammonium laurate in hexanes that contained sodium hexachlororhodate were reduced with sodium borohydride to yield rhodium nanoparticles. Particle size was controlled in the range 1.5–2.9 nm.
Keywords: Rhodium; Nanoparticles; Reverse micelle; Micelle; Lauric acid; Butyl amine; Butylammonium laurate
Gram-scale synthesis of aqueous gold colloids stabilized by various ligands
by Savka I. Stoeva; Alexander B. Smetana; Ch.M. Christopher M. Sorensen; Kenneth J. Klabunde (pp. 94-98).
We have developed a method for the large-scale synthesis of gold nanoparticles (Au NPs) in an aqueous medium stabilized by various water-soluble ligands. Significantly, the narrow size-distribution of the particles is achieved without employing size-selective procedures. The versatility of the procedure is demonstrated for the preparation of three colloidal systems stabilized by different ligands. Transmission electron microscopy (TEM), ζ-potential measurements and UV–vis spectroscopy are used to characterize the three colloidal systems.
Keywords: Gold nanoparticles; Synthesis; Aqueous medium
Flocculation of starch-coated solidified emulsion droplets and calcium carbonate particles
by Agatha Poraj-Kozminski; Reghan J. Hill; Theo G.M. van de Ven (pp. 99-105).
In papermaking, many colloidal particles are added to a pulp fiber suspension to improve paper properties. Given the right conditions, these different colloids can interact and flocculate. Examples of papermaking colloids are fillers and internal sizing agents, which improve opacity and hydrophobicity of paper, respectively. Internal sizing agents (added at the wet end of a paper machine) are commonly solidified emulsion droplets, stabilized by cationic starch and other stabilizers. We studied the interaction of a common internal sizing agent, alkyl ketene dimer (AKD), with calcium carbonate fillers. AKD is a liquid above 50–65 °C (depending on alkyl chain length), which can be emulsified above its melting point in the presence of a stabilizer, resulting, after cooling, in solid colloidal particles close to 1 μm in size. We investigated the interaction of AKD particles, stabilized by cationic starch, with precipitated calcium carbonate (PCC) particles. Pure PCC particles are positively charged, but they become negative in process waters. Flocculation experiments with positively charged AKD and negatively charged PCC were performed using a photometric dispersion analyzer. Instead of the expected heteroflocculation between AKD and PCC, we observed PCC homoflocculation and AKD homoflocculation, results confirmed by SEM. The results are explained by the transfer of starch from AKD to PCC, resulting in PCC flocculation by starch and AKD destabilization due to depletion of the stabilizer.Homoflocculation of solidified emulsion droplets stabilized by cationic starch and homoflocculation of negatively charged calcium carbonate particles, both due to starch transfer from droplets to particles, were observed, but expected heteroflocculation was absent.
Keywords: Homoflocculation; Heteroflocculation; Internal sizing agents; Alkyl ketene dimer; Fillers; Precipitated calcium carbonate; Cationic starch; Starch transfer
Underlying mechanisms in size control of uniform nanoparticles
by Tadao Sugimoto (pp. 106-118).
Insights are given into underlying mechanisms for size control of uniform nanoparticles in liquid phases. At the outset, instead of the classical nucleation theories, which are hardly applicable to the size control of uniform particles, a fundamental equation for the nucleation of monodisperse particles, derived for their size control on the basis of the LaMer model, is introduced. This equation was derived on three assumptions: (1) There is a mass balance between the supply rate of solute and its consumption rate for nucleation and growth of the generated nuclei; (2) The supply rate of solute is independent of the subsequent precipitation events; (3) The nucleation rate is controlled only by the growth of the preformed nuclei at a fixed supply rate of solute. Thus, this nucleation theory is applicable to a system in which the precursor solute is supplied by slow irreversible generation in a closed system or by continuous feed from outside in an open system. However, it is inapplicable even if only one of these three assumptions is not fulfilled. Examples of applicable and inapplicable systems are listed, and finally discussion is focused on the underlying mechanisms of size control in some unique processes chosen from them, such as hydrolysis-induced precipitation of AgCl nanoparticles, double-jet precipitation of AgCl nanoparticles in a reverse micelle system to resolve the mechanism of particle formation in general reverse micelle systems, and a gel–sol process for the formation of nanoparticles of anatase TiO2.The present nucleation theory for size control was derived on the basis of this LaMer diagram which was originally proposed for schematic explanation of the mechanism of monodisperse particle formation by LaMer and Dinegar in 1950.
Keywords: Size; Nucleation; Nanoparticles; Monodisperse particles; Uniform particles; LaMer; Gel–sol; Double jet; Microemulsion; Reverse micelles
Aggregate morphology and aggregation rate constants for silica dispersions in the presence of salt and polyelectrolyte
by R. Dunleavey-Routh; B. Vincent (pp. 119-125).
It is shown that the addition, over suitable concentration ranges, of mixtures of (nonadsorbing) sodium poly(styrene sulfonate) and potassium chloride, to dispersions of silica particles in water, can lead to very large changes in the sediment height of the resulting aggregates, reflecting similarly large changes in particle packing density within the aggregates. It can also lead to aggregation rates which are considerably faster than the diffusion-controlled rates (by as much as a factor of 2.5), although this enhancement is reduced as the dispersion particle concentration is reduced.
Keywords: Depletion aggregation; Aggregation rates; Aggregate morphology; Sodium poly(styrene sulfonate); Silica particles
Influence of humic acid on the aggregation kinetics of fullerene (C60) nanoparticles in monovalent and divalent electrolyte solutions
by Kai Loon Chen; Menachem Elimelech (pp. 126-134).
The early stage aggregation kinetics of fullerene C60 nanoparticles were investigated in the presence of Suwannee River humic acid and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). In the absence of humic acid, the aggregation behavior of the fullerene nanoparticles in the presence of NaCl, MgCl2, and CaCl2 was found to be consistent with the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloidal stability. In the presence of humic acid and NaCl or MgCl2 electrolytes, the adsorbed humic acid on the fullerene nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. This behavior manifested in a dramatic drop in the rate of aggregation, an increase in the critical coagulation concentration (CCC), and an attained value of less than unity for the inverse stability ratio (or attachment efficiency) at high MgCl2 concentrations. While the increase in the nanoparticle stability was similarly observed in the presence of humic acid at low CaCl2 concentrations, enhanced aggregation occurred at higher CaCl2 concentrations. Measurement of scattered light intensities over time indicated significant aggregation of the humic acid macromolecules in solutions of high CaCl2 concentrations. Transmission electron microscopy (TEM) imaging of the fullerene aggregate structures in the presence of humic acid revealed that bridging of the fullerene nanoparticles and aggregates by the humic acid aggregates is the likely mechanism for the enhanced aggregation at high CaCl2 concentrations.Fullerene nanoparticle aggregates with humic acid and divalent cations.
Keywords: Nanoparticles; Fullerene; C; 60; Aggregation; Humic acid; DLS
Study of the colloidal stability of concentrated bimodal magnetic fluids
by J.L. Viota; F. González-Caballero; J.D.G. Durán; A.V. Delgado (pp. 135-139).
In this paper, we describe an investigation of the stability and sedimentation behavior of moderately concentrated suspensions of extremely bimodal magnetite particles, including micro- (diameter 1450 nm) and nano- (diameter 8 nm) units. An original method is used, based on the determination of the time dependence of the inductance of a coil surrounding the suspensions. The method proves to be very useful for the determination of the volume fraction of magnetic material in the sensed volume. The observed changes in the resonant frequency of a parallel LC circuit demonstrate that the addition of the magnetite nanoparticles improves the stability and slows down the settling rate of the mixed suspensions. It is proposed that the observed behavior is the result of competition between two processes. One is the formation of a cloud of nanoparticles around the large magnetite units, by virtue of which the latter are maintained at distances beyond the range of DLVO and magnetic attractive interactions. At long times, these composite units will eventually sediment when some critical size is reached, as the small particles are progressively associated with the large ones. The second mechanism is mainly predominant at short times and is related to the higher viscosity of the dispersion medium (the nanoparticles dispersed in the base fluid) for higher nanoparticle concentrations. The stability of the suspensions is discussed in terms of the competition between the two mechanisms.The addition of increasing concentrations of magnetite nanoparticles produces more open aggregates of the micromagnetite because of the halo effect and maintains the large magnetite units less closely bound and occupying larger volumes.
Keywords: Bimodal magnetite suspensions; Ferrofluids; Halo stabilization; Magnetorheological fluids; Sedimentation; Stability
Low-energy-loss EFTEM imaging of thick particles and aggregates
by Leonardo Fonseca Valadares; Fábio do Carmo Bragança; Cristiane Aparecida da Silva; Carlos Alberto Paula Leite; Fernando Galembeck (pp. 140-148).
Electron spectroscopy imaging is a powerful tool for the elucidation of colloidal particle morphology and microchemistry, but it normally requires the use of very thin samples, typically less than 50 nm, to avoid the effects of multiple scattering. This work shows that many aspects of the internal morphology of thick particles and aggregates and the chemical component distribution are revealed using low-energy-loss electron imaging in the transmission electron microscope, benefiting from multiple scattering as well as small but significant differences in the low-energy-loss spectra of aggregate constituents. Low-loss images reveal morphological details of thick aggregates made out of colloidal polymers (natural rubber and styrene–acrylic latex) and inorganic particles (silica, montmorillonite, and aluminum phosphate) at a spatial resolution close to that achieved in the bright-field images and much better than in the elemental maps, showing the advantages of the simultaneous use of low-loss images and standard thin-cut elemental maps.The low-energy-loss EFTEM technique yields information complementary to chemical composition maps but at higher spatial resolution, allowing the observation of morphology and microchemical details of thick particles and aggregates.
Keywords: Electron energy-loss spectra; EFTEM; ESI-TEM; Aggregate imaging; Thick particle imaging; Chemical composition map
Formation of metallic Ni nanoparticles on titania surfaces by chemical vapor reductive deposition method
by Masaki Yoshinaga; Hideyuki Takahashi; Katsutoshi Yamamoto; Atsushi Muramatsu; Takeshi Morikawa (pp. 149-154).
Metallic Ni nanoparticles were successfully prepared on the surface of titania thin film substrate by a novel method, named as chemical vapor reductive deposition (CVRD) method. The growth of the nanoparticles was based on the specific adsorption and heterogeneous nucleation on the surface of substrate, not via vapor-phase formation and subsequent sedimentation. The nanoparticle size was found to be well controllable between 10 and 30 nm by the preparation time and vapor pressure of metal complex precursor. ESCA and electron diffraction results clearly demonstrated Ni nanoparticles as metallic. Titania thin film with metallic Ni nanoparticles on its surface showed high efficiency in their photocatalysis of hydrogen evolution from decomposition of ethanol.Metallic Ni nanoparticles were successfully prepared on titania thin films by a newly developed chemical vapor reductive deposition method. Obtained Ni/titania catalysts were found to enhance the photocatalytic activity compared to that of titania.
Keywords: Ni nanoparticles; Heteronucleation; Growth control; High dispersibility; Titania thin films; Photocatalysis; Hydrogen; Ethanol decomposition
The influence of cationic impurities in silica on its crystallization and point of zero charge
by Slobodan K. Milonjić; Ljiljana S. Čerović; Djuro M. Čokeša; Slavica Zec (pp. 155-159).
The influence of the heat treatment on the crystal structure change of five (two laboratory prepared and three commercial) silica powders were investigated. Silicas were heat treated in the temperature range from 150 to 1300 °C in air. Also, the influence of the washing procedure with 0.01 mol/L HCl in order to desorb cations present as impurities in silica samples, was studied. The change of the amorphous into the crystal structure ( α-cristobalite) takes place in the range 1050–1300 °C, depending on the chemical purity of silica samples. Points of zero charge, pHpzc, of the same silica samples, ranging from 3.8 to 7.1, were also determined by batch equilibration method. It is shown that the desorption of cations from silica samples, i.e., increase in their purity, leads to a shift in pHpzc toward lower values.Increase in purity of silica samples leads to an increase in crystallization temperature and a shift in pHpzc toward lower values.
Keywords: Silica; Point of zero charge; XRD; ICP-AES; EDXRF
New ceramic–carbon composites for electrodes for electrochemical capacitors
by Marek Kosmulski; Jadwiga Skubiszewska-Zięba; Roman Leboda; Krystyna Marczewska-Boczkowska; Piotr Próchniak (pp. 160-168).
A new class of composite materials is introduced. Fine powders of silica, titania, Y-modified zirconia, and three types of alumina were pressed and sintered to form porous monoliths with relatively uniform pore structure. Carbon was then deposited in the pores of such monoliths by thermal decomposition of dichloromethane, cyclohexene, and glucose. The structure of the carbon deposit was studied by low-temperature nitrogen adsorption and by thermal analysis. The composite materials were used as electrodes in electrochemical capacitors with 1-ethyl-3-methylimidazolium trifluoromethylsulfonate (a low-temperature ionic liquid) as the electrolyte. High capacitances were observed for glucose-derived materials, which had high specific surface areas.
Keywords: Carbon; Electrochemical capacitor; Composite material; Ionic liquid
Graft polymerization of styrene initiated by covalently bonded peroxide groups on silica
by Stefan Bachmann; Hongyu Wang; Klaus Albert; Richard Partch (pp. 169-175).
The graft polymerization of styrene initiated by immobilized peroxide groups was investigated. Three different types of modification reactions were used to introduce peroxide groups which are directly attached onto the surface of two different silica supports. Silanol groups were chlorinated using thionyl chloride or tetrachlorosilane. In another reaction pathway 1,3,5-benzenetricarbonyl chloride enabled the introduction of free acid chloride residues bonded onto the surface of silica. tert-Butyl hydroperoxide (TBHP) was used to transform the chlorosilyl and the acid chloride groups into peroxide residues. In a further reaction step the covalently bonded peroxides initiated the polymerization of styrene to form grafted polystyrene directly attached onto the silica support. Solid-state13C CP/MAS NMR spectroscopy, and thermogravimetric and scanning electron microscope measurements enabled a clear structure and property elucidation of the different bonded phases. The highest amount of grafted polystyrene was achieved employing the acid chloride synthesis pathway with silica–gel, whereas modification of spherical silica only led to minor amounts of grafted polymer. The results contribute to the evolving need to understand particle surface modifications and may have positive impact on development of new HPLC stationary phases for improved elutant resolution.Silica gel coated with covalently grafted polystyrene, initiated by tethered peroxy radicals has been achieved. The core–shell composite particles are characterized by TGA, SEM, and CP/MAS NMR methods.
Keywords: 13; C CP/MAS NMR spectroscopy; Peroxide bonded silica; Graft polymerization of styrene; Particle surface modification; Particle encapsulation
Synthesis and stabilization of Prussian blue nanoparticles and application for sensors
by Viktória Hornok; Imre Dékány (pp. 176-182).
Prussian blue (PB) nanoparticles were synthesized by two methods from FeCl2 and K3Fe(CN)6 and from FeCl3 and K3Fe(CN)6 based on the method published by Fiorito et al., and stabilized by different polymers like polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyallylamine hydrochloride (PAH), polydiallyl-dimethyldiammonium chloride (PDDA) and polystyrene sulfonate (PSS). The effect of the monomer/Fe3+ ratio was studied regarding the average particle size and ζ-potential. The forming PB structure was checked by X-ray diffraction. The stabilization was successful for every applied polymer, but the average particle size significantly differs. Particle size distributions were determined by Malvern type nanosizer equipment and by transmission electron microscope (TEM) and zeta potential values were determined for the obtained stabile samples. The results revealed that by using FeCl2 and K3Fe(CN)6 for PB preparation particles with narrow size distribution and average diameter of 1.7 nm occurred but stabilization was necessary. By the other method the dispersion was stabile with 182 nm particles but the particle size exponentially decreased to 18 nm with increasing PVP concentration. Ultrathin nanofilms were prepared on glass support by the alternating layer-by-layer (LbL) method from PB particles and PAH. The morphology of the prepared films was investigated also by AFM. The films were immobilized on interdigitated microsensor electrodes (IME) and tested in sensing hydrogen peroxide and different acids like acetic acid, hydrochloric acid vapors.
Keywords: Prussian blue; Nanoparticles; Stabilization by polymers; Sensors
The isoelectric point/point-of zero-charge of interfaces formed by aqueous solutions and nonpolar solids, liquids, and gases
by Thomas W. Healy; Douglas W. Fuerstenau (pp. 183-188).
From our previous work on the role of the electrostatic field strength in controlling the pH of the iso-electric point (iep)/point-of-zero-charge (pzc) of polar solids we have extended the analysis to predict that the pH of the iep/pzc of a nonpolar solid, liquid or gas-aqueous interface should occur at pH 1.0–3.0, dependant on the value assigned to water molecules or clusters at the interface. Consideration of a wide range of experimental results covering nonpolar solids such as molybdenite, stibnite, paraffin, etc. as well as hydrocarbon liquids such as xylene, decalin, and long chain (>C8) alkane oils, as well as nitrogen and hydrogen gases, all in various simple 1:1 electrolyte solutions confirm the general validity of the result. We further consider various models of the origin of the charge on nonpolar material–water interfaces.We predict that the point of zero charge of hydrophobic solids, liquids, or gases (pH0pzc) is at pH 1.0–3.0, depending on the size of ionizing sites at the interface. Literature results confirm that all hydrophobic materials in water have pzc's approaching this value.4.606RT(pH)pzc=(Fμ)+constant F is the electrostatic field strength of the lattice ForF→0, i.e., for nonpolar materialspHpzc0≈1.0–3.0
Keywords: Electrical double layer; Point of zero charge; Aqueous solution interfaces; Hydrophobic surfaces
Egon Matijević and the Royal Institution
by Sir John Meurig Thomas (pp. 189-191).
The background to the Faraday Evening Discourse given by Egon Matijević in 1989, as well as its impact, is outlined, and so also is his participation in the special Faraday Discussion convened at the Royal Institution in September, 1991 to mark the bicentennial of Michael Faraday's birth.