Journal of Colloid And Interface Science (v.303, #2)
Directed crystallisation of zinc oxide on patterned surfaces
by Dennis Palms; Craig Priest; Rossen Sedev; John Ralston; Gerhard Wegner (pp. 333-336).
Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 μm and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.On a patterned SAM surface of high and low surface energy regions nucleation and crystal growth of zinc oxide occurs selectively on the lower energy regions.
Keywords: Zinc oxide; SAMs; Crystallisation; Nucleation; Mineralisation; Photolithography
Adsorption mechanisms of selenium oxyanions at the aluminum oxide/water interface
by Derek Peak (pp. 337-345).
Sorption processes at the mineral/water interface typically control the mobility and bioaccessibility of many inorganic contaminants such as oxyanions. Selenium is an important micronutrient for human and animal health, but at elevated concentrations selenium toxicity is a concern. The objective of this study was to determine the bonding mechanisms of selenate (SeO2−4) and selenite (SeO2−3) on hydrous aluminum oxide (HAO) over a wide range of reaction pH using extended X-ray absorption fine structure (EXAFS) spectroscopy. Additionally, selenate adsorption on corundum ( α-Al2O3) was studied to determine if adsorption mechanisms change as the aluminum oxide surface structure changes. The overall findings were that selenite forms a mixture of outer-sphere and inner-sphere bidentate-binuclear (corner-sharing) surface complexes on HAO, selenate forms primarily outer-sphere surface complexes on HAO, and on corundum selenate forms outer-sphere surface complexes at pH 3.5 but inner-sphere monodentate surface complexes at pH 4.5 and above. It is possible that the lack of inner-sphere complex formation at pH 3.5 is caused by changes in the corundum surface at low pH or secondary precipitate formation. The results are consistent with a structure-based reactivity for metal oxides, wherein hydrous metal oxides form outer-sphere complexes with sulfate and selenate, but inner-sphere monodentate surface complexes are formed between sulfate and selenate and α-Me2O3.Selenium oxyanions adsorption on two aluminum oxide minerals was studied with Se K-edge XANES and EXAFS spectroscopy. It was learned that the bonding mechanisms directly relate to the surface structure of the aluminum oxide minerals.
Keywords: Adsorption; Selenate; Selenite; Aluminum oxides; Se; K; -edge EXAFS; Surface complexation
Comparison of dye adsorption by mesoporous hybrid gels: Understanding the interactions between dyes and gel surfaces
by Zhijian Wu; Laijiang You; Hong Xiang; Yan Jiang (pp. 346-352).
Without using any templating agents, mesoporous hybrid gels were prepared using mixtures of tetraethoxysilane (TEOS) with n-propyltriethoxysilane (PTES), bis(trimethoxysilyl)hexane (TSH), or bis(trimethoxysilylpropyl)amine (TSPA) as precursors. Fourier transform infrared (FTIR), N2 adsorption/desorption, thermogravimetry (TG), point of zero charge (PZC), and water vapor adsorption measurements were used to characterize the gels. The adsorption of methyl orange (MO), methyl red (MR), bromocresol purple (BP), phenol red (PR), neutral red (NR), and brilliant blue FCF (BBF) by the gels in both 0.01 M HCl and 0.01 M NaOH solutions was compared comprehensively. The gel derived from TEOS/TSH (with(CH2)6 groups, Gel 2) has the largest specific surface area (695 m2 g−1), the smallest pore volume (0.564 cm3 g−1), and the smallest average pore size (3.7 nm). The gels derived form TEOS/PTES (with(CH2)2CH3 groups, Gel 1), and TEOS/TSPA (with(CH2)3NH(CH2)3 groups, Gel 3) have similar textual properties. The PZC of Gels 1, 2, and 3 was estimated to be 6.28, 6.20, and 6.88, respectively. Gel 3 has the highest PZC due to the presence ofNH groups. In general, Gel 2 shows the highest dye adsorption among all the gels in both acidic and basic solutions. All the dyes except NR have much lower adsorption in basic solutions than in acidic solutions. In acidic solutions Gels 1 and 2 have similar adsorption trends for the dyes, except for BP, with NR having the highest adsorption, and PR the lowest adsorption. Gel 3 presents a different trend from Gels 1 and 2, with BBF having the highest adsorption, and MR the lowest adsorption. In basic solutions the order of dye adsorption by all the gels is shown to follow the sequence NR≫MR∼BBF>MO>BP∼PR. The adsorption results can be explained by considering the textural properties of the gels and the interactions between the gel surfaces and the dyes, which include hydrogen bonding, electrostatic, and hydrophobic interactions.
Keywords: Mesoporous hybrid gel; Textural property; Dye adsorption; Hydrogen bonding; Electrostatic interaction; Hydrophobic interaction
Polyelectrolyte adsorption layers studied by streaming potential and particle deposition
by Z. Adamczyk; M. Zembala; A. Michna (pp. 353-364).
Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface ζ as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potentialζ/ζ0 on the PAH coverageΘPAH, calculated using the convective diffusion theory. It was found that for the ionic strength of 10−2 M, the dependence ofζ/ζ0 onΘPAH can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. ForΘPAH>0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.Surfaces modified by adsorbed polyelectrolyte layers can be characterized by streaming potential and particle deposition studies. The results obtained can be interpreted in terms of theoretical model developed for particle covered surfaces. It was shown that these methods are sensitive to the very low polyelectrolyte coverage not attainable by other techniques.
Keywords: Adsorption of polyelectrolytes on mica; PAH adsorption on mica; Streaming potential measurements; Polyelectrolyte layer probing by particle deposition; Deposition kinetics at heterogeneous surfaces; Zeta potential of heterogeneous surfaces
Surface complexation mechanism and modeling in Cr(III) biosorption by a microalgal isolate, Chlorella miniata
by Xu Han; Yuk Shan Wong; Nora Fung Yee Tam (pp. 365-371).
The mechanism involved in the removal of Cr(III) by a green microalgal isolate, Chlorella miniata, was examined based on a series of batch experiments and microscopic analyses, and a mathematical model was proposed. Results showed that Cr(III) biosorption increased with the increase of pH from 2.0 to 4.5, and no significant changes in biosorption outside this pH range. Langmuir isotherm indicated that the maximum Cr(III) sorption capacity of Chlorella miniata was 14.17, 28.72, and 41.12 mg g−1 biomass at pH 3.0, 4.0, and 4.5, respectively. Results from desorption studies, SEM (scanning electron microscopy), TEM (transmission electron microscopy), and EDX (energy-dispersive X-ray spectroscope) analyses confirmed that surface complexation was the main process involved in Cr(III) biosorption. Potentiometric titration revealed that carboxyl (pKa1=4.10), phosphonate (pKa2=6.36) and amine (pKa3=8.47) functional groups on the surface of Chlorella miniata were the possible sites for Cr uptake, and their average amounts were 0.53, 0.39, and 0.36 mmol g−1 biomass, respectively. A surface complexation model further indicated that carboxyl group played the main role in Cr(III) complexation, with a binding constant ofK11=1.87×10−4 andK12=6.11×10−4 for Cr3+ and Cr(OH)2+, respectively. This model also suggested that the hydroxy species was more easily to complex with the cell surface of Chlorella miniata.A microalgae isolate, Chlorella miniata was used to remove Cr(III) in solution and the results indicated the surface complexation but not the well accepted ion exchange was the main biosorption mechanism.
Keywords: SEM; TEM; EDX; Desorption; Potentiometric titration
Effects of copolymer concentration and chain length on the pH-responsive behavior of diblock copolymer micellar films
by Kenichi Sakai; Emelyn G. Smith; Grant B. Webber; Erica J. Wanless; Vural Bütün; Steven P. Armes; Simon Biggs (pp. 372-379).
The pH-responsive behavior of adsorbed diblock copolymer films of PDMA–PDEA (poly(2-(dimethylamino)ethyl methacrylate)– block–poly(2-(diethylamino)ethyl methacrylate)) on silica has been characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D), an optical reflectometer (OR) and an atomic force microscope (AFM). The copolymer was adsorbed at pH 9 from various copolymer concentrations; QCM-D measurements indicate that the level of desorption when rinsed at pH 9 depends on the initial copolymer concentration. The adsorbed films produced at pH 9 generally have low charge densities; adjusting the solution pH to 4 results in a significant protonation of the constituent copolymers and a related interfacial structural change for the copolymer film. OR studies show no significant change during pH cycling, while QCM-D measurements indicate that the adsorbed mass and dissipation alter dramatically in response to the solution pH. The difference between the QCM-D adsorbed masses and dissipation values at pH 4 and 9 were found to be dependent on the initial copolymer concentration. This is due to differences in the initial conformations within the adsorbed copolymer layers at pH 9. The effect of the PDMA chain length on the pH-responsive behavior has also been studied; both the QCM-D adsorbed mass and dissipation of PDMA54–PDEA24 (shorter PDMA block) at pH 4 and 9 were observed to be greater than those of PDMA9X–PDEA2Y (longer PDMA block). This suggests that the normal extension of the adsorbed PDMA54–PDEA24 copolymer films is more significant than that of the PDMA9X–PDEA2Y films on silica.The pH-responsive behavior of PDMA–PDEA diblock copolymer adsorbed on silica has been characterized as a function of the copolymer concentration and the PDMA chain length.
Keywords: pH-Responsive diblock copolymer; Adsorption; Desorption; QCM-D; Optical reflectometry
Kinetics and adsorption behavior of carboxymethyl starch on α-alumina in aqueous medium
by Yanxiao Chen; Shaoying Liu; Gongying Wang (pp. 380-387).
The adsorption of carboxymethyl starch (CMS) at the α-alumina/aqueous solution interface has been investigated through adsorption studies, electrokinetics mobility measurements, and FTIR spectroscopy. Zeta potential measurements show that the addition of CMS results in a more dramatic increase in the absolute zeta potential in the alkaline region, as well as a shift of the isoelectric point to lower values, indicating the adsorption of CMS from the aqueous solution onto the alumina surface. The positive hydrophilic surface sites of alumina are responsible for the adsorption of CMS molecules. The adsorption of CMS is possible after charge reversal by the addition of excess CMS. Nearly 30 min of contact time are found to be sufficient for the adsorption of CMS to reach equilibrium. CMS adsorption follows a Langmuir isotherm with adsorption capacities of 91.74 mg CMS per gram of α-alumina. For the adsorption of CMS, pseudo-second-order chemical reaction kinetics provides the best correlation with the experimental data. FTIR analysis indicated that CMS forms outer complexes with alumina surfaces depending on the shifting of the asymmetric and symmetric bands.The adsorption mechanism of carboxymethyl starch at acidic pH values was investigated on α-alumina through electrokinetic and the Fourier transform infrared (FTIR) spectroscopy. The experimental results of the specific energy of interaction (ΔG0), the pseudo-second-order model, and FTIR studies indicated that CMS is chemisorbed onto the alumina surfaces by forming a chelate complex.
Keywords: Carboxymethyl starch; Alumina; Zeta potential; Adsorption isotherm; Kinetic model
Study of the surface character as responsible for controlling interfacial forces at membrane–feed interface
by A. Gugliuzza; R. Fabiano; M.G. Garavaglia; A. Spisso; E. Drioli (pp. 388-403).
The role of the interfacial forces was emphasized in interactive processes, involving membrane surface and penetrating molecules. The surface character controlling the dissolution process of some species (CO2, H2O, C3H6O2, C4H8O2, C5H10O2) was evaluated in relation to the supra-molecular chemistry of membranes based on 80PTMO/PA12. Infrared analyses combined with the estimation of the hydrophilic and hydrophobic domains of the membrane surface yielded useful information about the distribution, availability and accessibility of the polar moieties responsible for the penetrant sorption. At the interface, attractive Lewis acid/base interactions such as H-bonding directed the sorption of vapor species into the membranes, whereas quadrupolar CO2 participated in specific Lifshitz–van der Waals interactions with the modified polymers. In both the cases, the presence of additional polar moieties such as carbonyl, sulfonamide, and hydroxyl groups enhanced the affinity of the Pebax-based membranes for the penetrating species considered in this work. As a result, the quantification of the reactivity of a membrane surface for specific molecules may allow predictive models to be constructed and selective membranes to be designed.Membranes with different supra-molecular chemistry have been obtained. A study of the reactivity of the membrane surfaces for specific molecules was performed by using infrared spectroscopy, wetting and sorption techniques. This work is an interpretation of membrane sorption phenomena in relation to the interfacial forces in interactive processes at the membrane–feed interface.
Keywords: Membrane surface; Interfacial forces; Sorption; ATR-FTIR; Pebax
Structural and redox properties of mitochondrial cytochrome c co-sorbed with phosphate on hematite ( α-Fe2O3) surfaces
by Nidhi Khare; Carrick M. Eggleston; David M. Lovelace; Steven W. Boese (pp. 404-414).
The interaction of metalloproteins with oxides has implications not only for bioanalytical systems and biosensors but also in the areas of biomimetic photovoltaic devices, bioremediation, and bacterial metal reduction. Here, we investigate mitochondrial ferricytochrome c (Cyt c) co-sorption with 0.01 and 0.1 M phosphate on hematite ( α-Fe2O3) surfaces as a function of pH (2–11). Although Cyt c sorption to hematite in the presence of phosphate is consistent with electrostatic attraction, other forces act upon Cyt c as well. The occurrence of multilayer adsorption, and our AFM observations, suggest that Cyt c aggregates as the pH approaches the Cyt c isoelectric point. In solution, methionine coordination of heme Fe occurs only between pH 3 and 7, but in the presence of phosphate this coordination is retained up to pH 10. Electrochemical evidence for the presence of native Cyt c occurs down to pH 3 and up to pH 10 in the absence of phosphate, and this range is extended to pH 2 and 11 in the presence of phosphate. Cyt c that initially adsorbs to a hematite surface may undergo conformation change and coat the surface with unfolded protein such that subsequently adsorbing protein is more likely to retain the native conformational state. AFM provides evidence for rapid sorption kinetics for Cyt c co-sorbed with 0.01 or 0.1 M phosphate. Cyt c co-sorbed with 0.01 M phosphate appears to unfold on the surface of hematite while Cyt c co-sorbed with 0.1 M phosphate possibly retains native conformation due to aggregation.Mitochondrial cytochrome c is shown to co-sorb with phosphate on iron oxide surfaces. Phosphate appears to stabilize Mcc against adsorption-induced conformation change in some cases.1×1μm2 AC-mode AFM image of adsorbed Mcc molecules and aggregates.
Keywords: Adsorption; Phosphate; Cytochrome; c; Conformation change; Hematite; Oxide
High chemical reactivity of silver nanoparticles toward hydrochloric acid
by Liang Li; Ying-Jie Zhu (pp. 415-418).
High chemical reactivity of Ag nanoparticles was observed in the reaction with hydrochloric acid:Ag (nanoparticles)+HCl→AgCl+H2; the reaction product silver chloride was characterized by X-ray powder diffraction to give a direct evidence for the reaction which has been proved impossible for the bulk Ag.High chemical reactivity of Ag nanoparticles was observed in the reaction with hydrochloric acid:Ag (nanoparticles)+HCl→AgCl+H2; the reaction product silver chloride was characterized by X-ray powder diffraction to give a direct evidence for the reaction which has been proved impossible for the bulk Ag.
Keywords: Silver nanoparticles; Reaction; Hydrochloric acid; Chemical reactivity
A study of the collisional fragmentation problem using the Gamma distribution approximation
by M. Kostoglou; A.J. Karabelas (pp. 419-429).
The nonlinear fragmentation population balance formulation has been elevated in recent years from a prototype for studying nonlinear integro-differential equations to a vehicle for analyzing and understanding several physicochemical processes of technological interest. The so-called pure collisional fragmentation, which is the particular mode of nonlinear fragmentation induced by collisions between particles, is studied here. It is shown that the corresponding population balance equation admits large time asymptotic (self-similarity) solutions for homogeneous fragmentation and collision functions (kernels). The self-similar solutions are given in closed form for some simple kernels. Based on the shape of the self-similar solutions the method of moments with Gamma distribution approximation is employed for transient solution (from initial state to establishment of the asymptotic shape) of the collisional fragmentation equation. These solutions are presented for several sets of parameters and their behavior is discussed rather extensively. The present study is similar to the one has already been performed for the case of the much simpler linear fragmentation equation [G. Madras, B.J. McCoy, AIChE J. 44 (1998) 647].The unknown particle size distribution is approximated with a modified Gamma distribution in order to follow its evolution from an initial state to the large time asymptotic (self-similar) form, which is obtained as solution to the collisional fragmentation equation.
Keywords: Population balance; Nonlinear fragmentation; Collisional fragmentation; Breakage; Moments method; Self-similar solution
Synthesis of the gold nanodumbbells by electrochemical method
by Chien-Jung Huang; Pin-Hsiang Chiu; Yeong-Her Wang; Cheng-Fu Yang (pp. 430-436).
The gold nanoparticles with novel dumbbells-like structures have been successfully fabricated by an electrochemical method using micelle template formed by two surfactants, the primary surfactant being cetyltrimethylammonium bromide (CTABr) and the co-surfactant being tetradecyltrimethylammonium bromide (TTABr). The shape of gold nanoparticles can be modified to form dumbbell structure by addition of acetone solvent during the electrolysis. The gold nanodumbbells have been determined to be single-crystalline with a face-centered cubic (FCC) structure by X-ray diffraction (XRD) analysis. The lattice constant calculated from this selected-area electron diffraction (SAED) pattern is 4.068 Å. Transmission electronic microscopy (TEM) suggests that shape of gold nanodumbbell is fatter in two ends and thinner in the middle section to compare with the normal nanorod shape. These gold nanodumbbells exhibit aspect ratio ( γ) of around 3.
Keywords: Gold; Nanoparticles; Dumbbell; Surfactant; Micelle; Electrochemical
Fabrication, characterization, and formation mechanism of hollow spindle-like hematite via a solvothermal process
by Jian Lu; Dairong Chen; Xiuling Jiao (pp. 437-443).
As a novel hollow nanostructure, hollow spindle-like hematite with uniform size and morphology was solvothermally synthesized. These hollow polycrystalline particles with the length of 220–300 nm, the width of 70–100 nm, and the wall thickness of ca. 18 nm were characterized by TEM, FE-SEM, XRD, FT-IR, TGA, Mössbauer spectrum, and XPS methods. It was found that these hollow-structured particles were transformed from their original solid spindle particles. During the hollow structure formation process, the interiors of solid particles were preferentially dissolved while the retained exteriors were protected by coordinated sulfate ions. The formation mechanism was proposed as a coordination-assisted dissolution process occurred in a reverse microemulsion system.
Keywords: Hollow nanostructure; Spindle-like hematite; Solvothermal synthesis; Surfactant-assistant synthesis
Aggregation of methyl orange probed by electrical impedance spectroscopy
by H.P. de Oliveira; E.G.L. Oliveira; C.P. de Melo (pp. 444-449).
We present results of an electrical impedance spectroscopy investigation of the evolution of the aggregation of methyl orange (MO) in pure aqueous solutions as the concentration of the dye is varied. By applying the constant phase element (CPE) approximation to model the electrical response of the MO solutions, we have verified that the formation of dimers and oligomers can be recognized by specific signatures in the loss and capacitive components of the dielectric response of the system. We interpret these well-defined changes in the dielectric properties of the solutions as a result of molecular rearrangements caused by the aggregation process that alter the current circulation pathways and the electric dipole distribution. The fact that these specific changes in the dielectric behavior coincide with critical concentrations where dimer and oligomer formation in pure aqueous MO solutions are known to occur suggests that electrical impedance spectroscopy can be a competitive technique for the investigation of aggregation behavior in dyes and surfactants.Discontinuities in the resistance and the second derivative of the resistance (left) and in the capacitance (right) of aqueous solutions as a function of varying methyl orange concentrations are hallmarks of the formation of aggregates.
Keywords: Methyl orange; Aggregation; Electrical impedance spectroscopy; Dimer formation
Dissolution kinetics of nanodispersed γ-alumina in aqueous solution at different pH: Unusual kinetic size effect and formation of a new phase
by Frank Roelofs; Wolfram Vogelsberger (pp. 450-459).
The dissolution of a technical, nanodispersed γ-alumina in water was studied at 25 °C in the pH range3.0⩽pH⩽11.0. The obtained kinetic dissolution curves showed a distinct pH dependency, whereas only forpH⩾4.5 the typical behavior of nanodispersed materials could be observed. X-ray powder diffraction analysis and nitrogen adsorption, as well as IR and UV-Raman spectroscopy, were used to characterize the solid material collected during and at the end of each dissolution experiment. As a result the formation of a new aluminum phase—bayerite—could be proven. The analysis of the equilibrium concentration enabled us to determine the solubility constant of the corresponding phase assuming a pH-dependent species distribution. The rate constants of the dissolution process were evaluated using the model of Gibbs free energy of cluster formation, which considers the size effect, among other things. As a result, we could show that the observed maxima in the concentration profiles are due to a size effect of the starting material having a primary particle radius of 10.1 nm.Nanodispersed γ-alumina is technically produced to a great extent and used for different applications. The dissolution kinetics of this special type of alumina shows clear evidence for a size effect leading to increased solubility and a distinct concentration maximum.
Keywords: γ; -Alumina; Dissolution behavior; Gibbs free energy of cluster formation; Aluminum hydroxide; Bayerite; X-ray powder diffraction; Nitrogen adsorption; IR/UV-Raman spectroscopy
Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry
by Jörg Kleimann; Guy Lecoultre; Georg Papastavrou; Stéphane Jeanneret; Paolo Galletto; Ger J.M. Koper; Michal Borkovec (pp. 460-471).
Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.Surface coverage of positive particles on cellulose substrates are found to be substantially lower than on silica, indicating the important influence of the substrate on the deposition process.
Keywords: Stagnation point flow cell; Island film theory; Atomic force microscopy; AFM; Ellipsometry; Random sequential absorption model; RSA model
Synthesis of a red iron oxide/montmorillonite pigment in a CO2-rich brine solution
by G. Montes-Hernandez; J. Pironon; F. Villieras (pp. 472-476).
The homoionic calcium-montmorillonite was used to synthesize a red iron oxide/clay pigment in a CO2-rich brine solution (0.5 M of NaCl) by using an agitated batch-reactor (engineer autoclave). The operating conditions were 15 days of reaction, 200 bars of pressure and 150 °C of temperature. SEM/EDS, STEM/EDS, XRD and Infrared Spectrometry were performed to characterize before and after reaction the solid phase. The results showed the precipitation of spherical nanoparticles (50–500 nm) of iron oxide (Fe2O3) dispersed and/or coagulated in the clay-matrix. Evidently, this oxide produced red coloration in the final product. For this case, the Fe3+ cation was provided to the aqueous solution by the dissolution of Ca-montmorillonite, particularly, the dissolution of most fine particles contained in the starting clay material. The cation exchange process and precipitation of polymorph silica were also observed.
Keywords: Red pigment; Ca-montmorillonite; CO; 2; -rich brine solution; Iron oxide; Synthesis
Effect of quaternary ammonium cations on dye sorption to fly ash from aqueous media
by Shashwat S. Banerjee; Milind V. Joshi; Radha V. Jayaram (pp. 477-483).
The efficacy of the surface modification of fly ash by quarternary ammonium cations in the removal of dyes from aqueous solution is demonstrated. A series of organo-fly ash materials were synthesized by treating fly ash with quarternary ammonium cations such as tetraethylammonium, hexadecyltrimethylammonium, and benzyldimethyltetradecylammonium (TEA, HDTMA, and BDTDA). Two types of dyes were used for the investigation, disperse and anionic dyes. The effects of initial dye concentration, contact time, temperature, and the mechanism of dye sorption were investigated. The sorption was found to be affected by the structure and size of the quaternary ammonium cations as well as that of the dyes. Sorption of dyes was considerably enhanced by the surface modification. Thermodynamic parameters such as free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) for the sorption process were also calculated.A series of organo-fly ash materials were synthesized by treating fly ash with quarternary ammonium cations to remove dyes from aqueous solution.
Keywords: Organo-fly ash; Dyes; Quaternary ammonium cations; Thermodynamic parameters
Preparation and characterization of N-methylene phosphonic and quaternized chitosan composite membranes for electrolyte separations
by Arunima Saxena; Arvind Kumar; Vinod K. Shahi (pp. 484-493).
Chitosan was functionalized either by introducing a phosphonic acid group or by quaternization of existing primary ammonium groups in order to make it a water-soluble material. Functionalized chitosans and poly(vinyl alcohol) (PVA)-based nanoporous charged membranes were prepared in aqueous media and gelated in methanol at 10 °C to tailor their pore structure. These membranes were extensively characterized for their physicochemical, electrochemical, and permeation characteristics using FTIR, TGA, DSC, water content, ion-exchange capacity, ionic transport properties, and membrane permeability studies. N-Methylene phosphonic chitosan (NMPC)/PVA-based membranes exhibited mild cation selectivity and quaternized chitosan (QC)/PVA composite membranes had mild anion selectivity, while a blend of NMPC-QC/PVA membranes exhibited weak cation selectivity because of formation of zwitterionic structure. Viscosity measurements and interaction studies for individual and mixed solutions of NMPC and QC were carried out for the prediction of charge interactions betweenPO3H2 andN+(CH3)3 groups and effect on molecular weight due to functionalization. Elaborate electrochemical and permeation experiments were conducted in order to predict suitability of these membranes for the separation of mono- and bivalent electrolytes based on their hydrated ionic radius, and it was found that among all the synthesized membranes, PC/QC-30 had the highest relative permeability, which may extend its suitability for electrolyte separations. Observations were correlated with equivalent pore radius of the different membranes as estimated by membrane permeability measurements.Functionalized chitosans and poly(vinyl alcohol) (PVA)-based nanoporous charged membranes were prepared in aqueous media and gelated in methanol at 10 °C to tailor their pore structure. Based on different studies, it was concluded that N-methylene phosphonic chitosan and quaternized chitosan composite membrane (PC/QC-30) membrane could be efficiently used for the separation of Na+ from Ca2+ and Mg2+.
Keywords: Chitosan-based membranes; N; -Methylene phosphonic chitosan; Quaternized chitosan; Membrane conductivity; Membrane potential; Membrane permeability; Equivalent pore radius
Mesopore control of high surface area NaOH-activated carbon
by Ru-Ling Tseng (pp. 494-502).
Activated carbon with BET surface areas in a narrow range from 2318 to 2474 m2/g was made by soaking the char made from corncob in a concentrated NaOH solution at NaOH/char ratios from 3 to 6; the mesopore volumes of the activated carbon were significantly changed from 21 to 58%. The relationships between pore properties (Sp,Vpore,Vmicro/Vpore,Dp) and NaOH dosage were investigated. Comparisons between the methods of NaOH and KOH activation revealed that NaOH activation can suitably control the mesopore specific volume of the activated carbon. Elemental analysis revealed that the H/C and O/C values of the activated carbons of NaOH/char ratios from 3 to 6 were significantly lower. SEM observation of surface hole variation of the activated carbon ascertained that the reaction process was inner pore etching. Based on the above three measurements and experimental investigations, the assumption made by previous researchers, namely that NaOH and KOH produce similar results, was challenged. Furthermore, the adsorption kinetics was used to investigate the adsorption rate of an Elovich equation to determine the relationships between the adsorption behavior on larger molecules (dyes) and smaller molecules (phenols) and the pore structure of the activated carbon.
Keywords: Activated carbon; NaOH activation; Adsorption; High surface area; Mesopore
On the dynamics of liquid lenses
by R.V. Craster; O.K. Matar (pp. 503-516).
The spreading of a lens of one liquid on the surface of another liquid is examined. Lubrication theory is used to derive a coupled system of equations for the air–liquid and liquid–liquid interfaces. In the case of highly viscous lenses, extensional stresses are promoted and an additional equation for the lens velocity is derived. The potential singularity at the three-phase line is relieved by a microscopic precursor layer of the spreading fluid assumed to be present ahead of the macroscopic lens. This layer is stabilised via the inclusion of disjoining pressure effects in the lens. The results of our full parametric study show that, for weak gravitational forces, the shape of the lens at equilibrium depends solely on the surface tension ratio for sufficiently deep substrate thicknesses. For thin substrates, the underlying liquid film deforms severely near the point of deposition exhibiting flattening and dimpling.Schematic of the equilibrium configuration of a two-dimensional lens.
Keywords: Lenses; Thin films; Lubrication; Surface tension; Capillarity
Low voltage electrowetting using thin fluoroploymer films
by Shaun Berry; Jakub Kedzierski; Behrouz Abedian (pp. 517-524).
This paper investigates the nonideal electrowetting behavior of thin fluoroploymer films. Results are presented for a three phase system consisting of: (1) an aqueous water droplet containing sodium dodecyl sulfate (SDS), (2) phosphorous-doped silicon topped with SiO2 and an amorphous fluoroploymer (aFP) insulating top layer on which the droplet is situated, and (3) a dodecane oil that surrounds the droplet. The presented measurements indicate that the electrowetting equation is valid down to a 6 nm thick aFP film on a 11 nm thick SiO2. At this dielectric thickness, a remarkable contact angle change of over 100° can be achieved with an applied voltage less than 3 V across the system. The data also shows that for this water/surfactant/oil system, contact angle saturation is independent of the electric field, and is reached when the surface energy of the solid–water interface approaches zero.The figure illustrates an example of electrowetting behavior. It shows a sessile drop,∼1mm diameter, at 0 V and when 5 V is applied.
Keywords: Electrowetting; Electrocapillary; Saturation; Surface energy; Contact angle; Sodium dodecyl sulfate; Dodecane; Amorphous fluoroploymer; Dielectric; Microfluidics
Photopatterning in poly-l-lysine thin films using UV-enhanced hydrophilicity
by J.M. Taguenang; A. Kassu; A. Sharma (pp. 525-531).
Deep UV lithography on poly-l-lysine thin films was used to generate microarrays with enhanced hydrophilicity. This was manifested as adsorption of ambient humidity from air by areas exposed to UV fluence around 5 J/cm2 and was made visible by phase-contrast microscopy. Kinetics of adsorption was investigated by a novel technique involving fabrication of submicrometer hydrophilicity grating by two-beam UV interferometry. In an aqueous colloidal medium, gold and polystyrene microspheres preferentially attach to areas that are relatively less hydrophilic, i.e., those areas not exposed to UV light. This observation provides a method for fabricating micro- and nanoporous arrays with controlled porosity. The technique is demonstrated with microspheres of sizes between 250 nm and 10 μm.Deep UV lithography on poly-l-lysine films creates microarrays with enhanced hydrophilicity as well as micro- and nanoporous arrays with controlled porosity.
Keywords: Poly-L-lysine thin films; UV photopatterning; UV-enhanced hydrophilicity; Nano- and microporous arrays; Adsorption
Rupture of thin films with resonant substrate patterning
by Justin C.-T. Kao; Alexander A. Golovin; Stephen H. Davis (pp. 532-545).
We study the stability and rupture of thin liquid films on patterned substrates. It is shown that striped patterning on a length scale comparable to that of the spinodal instability leads to a resonance effect and an imperfect bifurcation of equilibrium film shapes. Weakly nonlinear analysis gives predictions for film shapes, stability, growth rates, and rupture times, which are confirmed by numerical solution of the thin-film equation. Film behavior is qualitatively different in the resonant patterning regime, but with sufficiently large domains rupture occurs on a spinodal length scale regardless of patterning. Instabilities transverse to the patterning are examined and shown to behave similarly as disturbances to films on uniform substrates. We explain some previously reported effects in terms of the imperfect bifurcation.The stability and rupture of thin liquid films on patterned substrates show a resonance effect and an imperfect bifurcation of equilibrium film shapes.
Keywords: Thin liquid films; Rupture; Dewetting; Patterning; Templating; Imperfect bifurcation; Nonlinear dynamics
The influence of electric field-induced orientation on the retraction of a liquid crystal droplet immersed in a flexible polymer matrix
by Youjun Wu; Wei Yu; Chixing Zhou (pp. 546-551).
We measured the apparent interfacial tension between a liquid crystal and a flexible polymer by deformed droplet retraction method. An external electric field is applied to change the director orientation in liquid crystal droplet. The deformation and recovery of a single liquid crystal droplet dispersed in a polydimethylsiloxane (PDMS) matrix were realized by a transient shear flow and observed by polarized optical microscope. In order to control the director orientation in LC droplet, the electric field is applied perpendicular and parallel to the flow field, respectively. The different orientation induced by electric field in liquid crystal droplet has different behavior during droplet retraction and affect the apparent interfacial tension between liquid crystal and flexible polymer.The different orientation induced by electric field in liquid crystal droplet has different behavior during droplet retraction and affects the apparent interfacial tension between liquid crystal and flexible polymer.
Keywords: Liquid crystal; Anisotropy; Interfacial tension; Electric field
How does β-cyclodextrin affect oxygen solubility in aqueous solutions of sodium perfluoroheptanoate?
by A.M.A. Dias; C. Andrade-Dias; S. Lima; J.A.P. Coutinho; J.J.C. Teixeira-Dias; I.M. Marrucho (pp. 552-556).
The solubility of oxygen in aqueous solutions of sodium perfluoroheptanoate (NaPFHept) at different concentrations was measured at 310.15 K with an apparatus based on the saturation method. The effect of adding β-cyclodextrin ( βCD) on the solubility of oxygen was also studied. Conductimetry measurements showed that the presence of βCD in aqueous solutions of NaPFHept increases its critical micellar concentration (CMC). In the presence of βCD (15 mM), the characteristic minimum of oxygen solubility observed at the CMC is shifted from 83 to 114 mM, and the curvature at the minimum is reduced to 64% of the value in the absence of βCD. Chemical shift changes for the H5 protons of βCD, recorded as functions of the initial concentration of NaPFHept, point to the formation of a relatively strong 1:1 inclusion in βCD of the perfluoroheptanoate anion. Hence, it is suggest that the effect of adding βCD on the solubility of oxygen cannot be accounted for only by the perfluoroheptanoate anion inclusion in βCD, but has to be ascribed to the direct influence of this inclusion complex on disrupting the aggregation process reducing the increase of oxygen solubility after the CMC value.The figure represents the different phenomena/interactions that are expected to occur during the solubilization of oxygen into aqueous solutions of sodium perfluoroheptanoate with β-cyclodextrin.
Keywords: Sodium perfluoroheptanoate; β; -Cyclodextrin; Oxygen solubility; Aqueous solution; Self-aggregation; Inclusion; NMR chemical shifts
Formation and stability of paraffin oil-in-water nano-emulsions prepared by the emulsion inversion point method
by Weirong Liu; Dejun Sun; Caifu Li; Qian Liu; Jian Xu (pp. 557-563).
Paraffin oil-in-water nano-emulsions stabilized by Tween 80/Span 80 were prepared using the emulsion inversion point method at different emulsification temperatures. Nano-emulsions with droplet size below 200 nm were formed above a critical surfactant-to-oil ratio of 0.20 at 50 °C. The main destabilization mechanism of the systems was found to be Ostwald ripening. An interesting phenomenon was that the Ostwald ripening rate declined as the surfactant concentration rose. Furthermore, flocculation was also found to contribute to the instability of the nano-emulsions, especially for those with low surfactant concentrations. Study on the electrophoretic properties of emulsion droplets revealed a negative value of the zeta potential, which was strongly dependent on the pH of the systems.From the figure we can conclude that: (1) the main driving force for instability is Ostwald ripening, (2) the Ostwald ripening rate (the slope of the lines) decreases with the increasing surfactant concentrations.
Keywords: Nano-emulsions; Emulsion inversion point; HLB; Long-term stability; Ostwald ripening; Zeta potential
β-Lactoglobulin–dextran Maillard conjugates: Their effect on interfacial thickness and emulsion stability
by Tim J. Wooster; Mary Ann Augustin (pp. 564-572).
The influence of dextran molecular weight on the steric layer thickness and oil-in-water (O/W) emulsion flocculation stability of β-lactoglobulin–dextran Maillard conjugates was investigated. Maillard conjugates were formed by reacting β-lactoglobulin with various molecular weight dextrans (Mw=18.5–440 kDa) under mild conditions (60 °C, 76% RH). Purified Maillard conjugates or β-lactoglobulin were adsorbed onto latex spheres and the thickness of the adsorbed layer measured using photon correlation spectroscopy. The adsorbed layer thickness was 3 nm for β-lactoglobulin alone. Attachment of dextran increased adsorbed layer thickness to 5 nm for the conjugate with low molecular weight dextran (Mw=18.5 kDa) and 20 nm for that with high molecular weight dextran (Mw=440 kDa). Enzymatic digestion of the adsorbed layers with dextranase reduced the layer to a thickness corresponding to that of β-lactoglobulin alone. This suggests that the protein segment of the Maillard conjugate anchors the emulsifier to the interface. Attachment of dextran, irrespective of its molecular weight (18.5–440 kDa), increased the stability of emulsions against calcium induced flocculation, demonstrating that a low molecular weight dextran is sufficient for imparting high steric stability. The observation that the steric layer size was controlled by the dextran molecular weight, suggests that the results of layer thickness and emulsion stability should be universal across all globular proteins.Dextranase digestion of β-lactoglobulin–dextran Maillard conjugate layers, adsorbed onto latex spheres, demonstrated that the large increase in interfacial layer thickness was due to dextran attachment.
Keywords: Maillard reaction; β; -Lactoglobulin; Dextran; Glycation
Synthesis and electrorheological behavior of sterically stabilized polypyrrole–silica–methylcellulose nanocomposite suspension
by Dae Jeon Yoon; Young Dae Kim (pp. 573-578).
Various polypyrrole (PPy)–silica–methylcellulose nanocomposite particles were synthesized by suspension polymerization in the presence of silica nanoparticles controlling the ratio of pyrrole, silica, and methylcellulose during the polymerization. The electrorheological (ER) and dielectric properties of the sterically stabilized PPy–silica–methylcellulose nanocomposite suspensions were investigated. The ER response increases with the increase in the silica/pyrrole ratio. The ER behavior also depends on the methylcellulose amount during the polymerization. The yield stress initially increases with the methylcellulose amount, passes through a maximum, and then decreases with the methylcellulose amount. The dielectric constants and dc conductivities of the PPy–silica–methylcellulose nanocomposite particles and the dielectric properties of their suspensions indicate that the increased ER response arises from the enhanced interfacial and particle polarization which depends on the silica/pyrrole ratio and the methylcellulose amount during the polymerization.The yield stress increases with the increase in the silica/pyrrole ratio and electric field strength. The ER response increase with the silica/pyrrole ratio may arise from the different degrees of polarization of the PPy–silica–methylcellulose nanocomposite particles of different silica/pyrrole ratios.
Keywords: Electrorheology; Sterically stabilized nanocomposite particle; Conducting polymer; Nanocomposite; Suspension
Electrophoretic motion of a spherical particle in a converging–diverging nanotube
by Shizhi Qian; Aihua Wang; Juan K. Afonien (pp. 579-592).
A charged spherical particle is concentrically positioned in a converging–diverging nanotube filled with an electrolyte solution, resulting in an electric double layer (EDL) forming around the particle's surface. In the presence of an axially applied electric field, the particle electrophoretically migrates along the axis of the nanotube due to the electrostatic and hydrodynamic forces acting on the particle. In contrast to a cylindrical nanotube with a constant cross-sectional area in which the electric field is almost uniform, the presence of a converging–diverging section in a nanotube alters the electric field, perturbs the charge distribution, and induces a pressure gradient and a recirculating flow that affect the electrostatic and hydrodynamic forces acting on both the particle and the fluid. Depending on the magnitude of the surface charge density along the nanotube's wall, the particle's electrophoretic motion may be significantly accelerated as the particle transverses the converging–diverging section. A continuum model consisting of the Nernst–Planck, Poisson, and Navier–Stokes equations for the ionic concentrations, electric potential, and flow field is implemented to compute the particle's velocity as a function of the particle's size, the nanotube's geometry, surface charges, electric field intensity, bulk electrolyte concentration, and the particle's location. When the particle is negatively charged and the wall of the nanotube is uncharged, the particle migrates in the direction opposite to that of the applied electric field and the presence of the converging–diverging section significantly accelerates the particle's motion. This, however, is not always true when the nanotube's wall is charged with the same sign as that of the particle. Once the ratio of the surface charge density of the nanotube's wall to that of the particle exceeds a certain value, the negatively charged particle will not translocate through the tube toward the anode and does not attain the maximum velocity at the throat of the converging–diverging section. One can envision such a device to be a nanofilter that allows molecules with surface charge densities much higher than that of the wall to go through the nanofilter, while preventing molecules with surface charge densities lower than that of the wall from passing through the nanofilter. The induced recirculating flow may be used to enhance mixing and to stretch, fold, and trap molecules in nanofluidic detectors and reactors.A converging–diverging section in a nanotube intensifies the electric field, polarizes the EDLs, and induces a pressure gradient and recirculating flow which affects the fluid's motion and particle's electrophoretic motion.
Keywords: Electrophoresis; Electroosmotic flow; Nanofluidics; Nanopore; Converging–diverging nanochannel; Recirculating flow
Electroosmosis in porous solids for high zeta potentials
by A.K. Gupta; D. Coelho; P.M. Adler (pp. 593-603).
When surface potentials (or the surface charges) are high, the exponential term on the right-hand side of the Poisson–Boltzmann equation cannot be linearized. This nonlinear regime is systematically studied for various porous media and various physicochemical conditions. As in the linear regime, the numerical data for the electroosmotic coefficient when made dimensionless are shown to follow the semianalytical solution derived for a plane Poiseuille flow. Therefore, this coefficient can be deduced either from the specific surface or from the permeability and the formation factor.The electroosmotic coefficient which was calculated for various porous media is shown to follow the semianalytical solution derived for a plane Poiseuille flow (solid lines).
Keywords: Porous media; Electrokinetics; Zeta potential; Electroosmosis; Nonlinearity
Templated synthesis of silver nanowires based on the layer-by-layer assembly of silver with dithiodipropionic acid molecules as spacers
by Sheela Berchmans; R.G. Nirmal; G. Prabaharan; S. Madhu; V. Yegnaraman (pp. 604-610).
The layer-by-layer assembly of silver nanoclusters with 3,3′-dithiodipropionic acid (DTDPA) as spacers was prepared through self-assembly on a gold foil and has been characterized by cyclic voltammetric and AFM techniques. The DTDPA molecules acting as spacers between the layers of silver serve as molecular interconnects for the four layers prepared in this work. The organization of layers was found to decrease with an increase in the number of layers. The layer-by-layer assembly of silver clusters motivated us to prepare silver nanowires stabilized by the bifunctional molecules DTDPA through template synthesis using cellulose nitrate membranes. The nanostructures formed by this method were characterized by SEM, TEM, AFM, FTIR, CV, and photoluminescence studies. It is observed that the DTDPA molecules, instead of forming molecular interconnects, protect the structures by self-assembling themselves along the edges of the nanostructures. The concept of self-assembly protecting the nanostructures is demonstrated in this work.The concept of self-assembly protecting silver nanostructures is demonstrated on this work.
Keywords: Silver nanowires; Self-assembly; Dithiodipropionic acid; Molecular interconnects; Gold surface
Synthesis and dispersion of isolated high aspect ratio gold nanowires
by Bin Wu; John J. Boland (pp. 611-616).
We report the assembly properties of high density and high aspect ratio metal nanowire arrays (Au, Cu and Ag with diameters ranging from 40 to 250 nm) after release from the anodic alumina oxide (AAO) templates. Individual Ag and Cu nanowires were observed following release from the template, however, in the case of gold nanowires, the dispersion was dependent on size and aspect ratio. 40–100 nm gold wires aggregated to form bundles or disordered mats. We show that a simple cyanide-mediated release from the AAO template, results in isolated dispersion of wires even for the smallest wire diameters. Possible stabilising mechanisms for observed tendency of nanowires dispersion are discussed.We describe a new cyanide-mediated method to release high density, high aspect ratio Au nanowires from the anodic alumina oxide templates in which they are electrochemically grown.
Keywords: Anodic alumina oxide templates; Electrodeposition; Au nanowires; Assembly properties
Determination of the surface energy of kaolinite and serpentine using PACHA formalism—Comparison with immersion experiments
by Fabrice Salles; Marc Henry; Jean-Marc Douillard (pp. 617-626).
Clays play an important role in a wide variety of industrial processes. Indeed, they present interesting surface properties. For these reasons, we study the surface energy of models of clays by solid state calculations using electronegativities equalization. In this article, we focus on kaolinite and serpentine, two clays characterized by a simple structure of the TO type. We describe the clays and their structures and we develop a simple model from solid state calculations used to determine the surface energy. The results are in agreement with a recent interpretation of the immersion of kaolinite in water. This article must be related to some others focusing on solid surface energy, especially some treating talc and chlorites, and montmorillonites saturated by alkaline cations, to be published.In this paper, we focus our studies about the surface energy of clays on kaolinite and serpentine. Heats of immersion and calculations of solid surface tension are linked in view of comparison of results. Data and calculations are in correct agreement for kaolinite and allow the prediction of heat of immersion for serpentine to be made.
Keywords: Kaolinite; Serpentine; Clays; Surface energy; Lattice energy; Electronegativities equalization; Immersion
Probing colloidal forces between a Si3N4 AFM tip and single nanoparticles of silica and alumina
by J. Drelich; J. Long; Z. Xu; J. Masliyah; C.L. White (pp. 627-638).
The atomic force microscope (AFM) has been used to measure surface forces between silicon nitride AFM tips and individual nanoparticles deposited on substrates in 10−4 and10−2M KCl solutions. Silica nanoparticles (10 nm diameter) were deposited on an alumina substrate and alumina particles (5 to 80 nm diameter) were deposited on a mica substrate using aqueous suspensions. Ionic concentrations and pH were used to manage attractive substrate–particle electrostatic forces. The AFM tip was located on deposited nanoparticles using an operator controlled offset to achieve stepwise tip movements. Nanoparticles were found to have a negligible effect on long-range tip–substrate interactions, however, the forces between the tip and nanoparticle were detectable at small separations. Exponentially increasing short-range repulsive forces, attributed to the hydration forces, were observed for silica nanoparticles. The effective range of hydration forces was found to be 2–3 nm with the decay length of 0.8–1.3 nm. These parameters are in a good agreement with the results reported for macroscopic surfaces of silica obtained using the surface force apparatus suggesting that hydration forces for the silica nanoparticles are similar to those for flat silica surfaces. Hydration forces were not observed for either alumina substrates or alumina nanoparticles in both10−4M KCl solution at pH 6.5 and10−2M KCl at pH 10.2. Instead, strong attractive forces between the silicon nitride tip and the alumina (nanoparticles and substrate) were observed.
Keywords: Colloidal forces; Hydration force; Nanoparticles
Investigations into the kinetics and thermodynamics of Sb(III) adsorption on goethite ( α-FeOOH)
by Rhyadd Watkins; Dominik Weiss; William Dubbin; Kate Peel; Barry Coles; Tim Arnold (pp. 639-646).
This study reports thermodynamic and kinetic data of Sb(III) adsorption from single metal solutions onto synthetic aqueous goethite ( α-FeOOH). Batch equilibrium sorption experiments were carried out at 25 °C over a Sb:Fe molar range of 0.005–0.05 and using a goethite concentration of 0.44 g Fe/L. Experimental data were successfully modelled using Langmuir(R2⩾0.891) and Freundlich(R2⩾0.990) isotherms and the following parameters were derived from triplicate experiments:Kf=1.903±0.030mg/g and1/n=0.728±0.019 for the Freundlich model andb=0.021±0.003L/mg andQmax=61±8mg/g for the Langmuir model. The thermodynamic parameters determined were the equilibrium constant,Keq=1.323±0.045, and the Gibb's free energy,ΔG0=−0.692±0.083kJ/mol. The sorption process is very fast. At a Sb:Fe molar ratio of 0.05, 40–50% of the added Sb is adsorbed within 15 min and a steady state is achieved. The experimental data also suggest that desorption can occur within 24 h of reaction due to the oxidation of Sb(III) on the goethite surface. Finally, calculated pH of the aqueous solution using MINTEQ2 agrees well with the measured pH (3.9±0.7;n=30). At pH 4, the dominant Sb species in solution are Sb(OH)3 and HSbO2 which both likely adsorb as inner sphere complexes to the positively charged goethite surface.
Keywords: Adsorption; Isotherms; Thermodynamics; Speciation; Kinetics; Sb(III); Sb(V); Goethite
by Arthur Hubbard (pp. 647-647).