Journal of Colloid And Interface Science (v.288, #2)

The electrochemistry of dithiophosphinate (DTPI) adsorption on pyrite was investigated by cyclic voltammetry (CV) and diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy at oxidizing potentials under acid, neutral, and alkaline conditions. CV study showed that the mineral surface was covered by porous ferric oxyhydroxide species. Irreversible DTPI adsorption on the pyrite surface and inhibition of electron transfer to some extent were also observed from electrochemical work. Major collector species were determined from DRIFT spectroscopy study as adsorbed collector compound, DTPI°, and dimer of dithiophosphinate, (DTPI)2. Maximum adsorption density of DTPI was obtained in acid solution. It decreased sharply under neutral and alkaline conditions due to heavy surface oxidation at higher pH.
Keywords: Pyrite; Cyclic voltammetry; DRIFT spectroscopy; Redox potential; Dithiophosphinate; Electrochemistry;

Asphaltenic aggregates are polydisperse oblate cylinders by Keith L. Gawrys; Peter K. Kilpatrick (325-334).
Small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and morphologies of asphaltenic aggregates in solution. A wide variety of intra-particle structure factors have previously been applied to SANS scattering spectra, but the studies often provided limited information concerning the quality of the fits and the Q range over which the models were applied. Selection of an appropriate form factor that closely approximates the structure of asphaltenic aggregates is important for determining the properties of asphaltenic aggregates, such as the radius of gyration ( R G ), molar mass, and apparent fractal dimension. This study evaluates various mono- and polydisperse intra-particle structure factor models as applied to four asphaltene scattering spectra. Agreement of the model fit parameters ( I 0 and R G ) with those obtained from Guinier analyses suggests that such a form factor model is physically reasonable. Reduced χ 2 values for each non-linear least squares fit indicates how well a given model fits to the entire Q range studied for the scattering intensity distribution. In the polydispersity analyses, an analytical function is introduced to model the scattering behavior of oblate cylinders with a Schultz distribution of radii. Results indicate that the polydisperse radius oblate cylinder model best approximates the shape of asphaltenic aggregates.
Keywords: Asphaltenes; Small-angle neutron scattering; Polydispersity;

A study of the static and dynamic adsorption of Zn(II) ions on carbon materials from aqueous solutions by Miguel A. Álvarez-Merino; Victoria López-Ramón; Carlos Moreno-Castilla (335-341).
The effect of surface oxidation, solution pH, and ionic strength on the adsorption of Zn(II) ions from aqueous solution under static conditions was studied using commercial activated carbons in the form of grains and cloth. In addition, the effects of surface oxidation and the presence of dissolved natural organic matter (tannic acid) were studied under dynamic conditions using activated carbon cloth column beds. Under static conditions, surface oxidation largely increased Zn2+ uptake and two H+ ions were displaced from the oxidized carbon surface per Zn(II) ion adsorbed. It is proposed that adsorption of Zn(II) on the as-received basic carbons was due to C π –cation interactions. An increase in solution pH in the range 3–6 increased Zn(II) uptake, whereas an increase in ionic strength decreased Zn(II) uptake because of the screening effect of the added salt. In the experiments carried out with carbon column beds, the oxidized activated carbon cloth was also more effective than the as-received carbon to remove Zn(II) ions. In this case, the presence of tannic acid decreased the efficiency of the oxidized activated carbon cloth bed to remove Zn(II) ions. An increase in the tannic acid initial concentration had a greater effect on the removal of tannic acid than on the removal of Zn(II) by the column bed. This may be a consequence of the greater size of tannic acid molecules and their low affinity for oxidized carbon surfaces.
Keywords: Zn(II) ion adsorption; Carbon materials; Surface oxygen complexes; Acid and basic carbon surfaces; Tannic acid adsorption;

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant by Md. Mufazzal Hossain; Toshiyuki Suzuki; Teiji Kato (342-349).
We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate (n-TDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure ( π ) –time ( t ) adsorption isotherm is found to occur from 2 to 20 °C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 °C and it is considered as a gas (G)–liquid condensed (LC) phase transition. Above 6.5 °C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the πt curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 °C, the former transition is classified as a first-order G–liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE–LC phase transition. The critical surface pressure (π c) necessary for the G–LC and G–LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE–LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 °C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air–water interface.
Keywords: Gibbs monolayers; Phase transition; Phase diagram; Surface pressure; Brewster angle microscopy; n-Tetradecyl phosphate;

Divalent Cd and Pb uptake on calcite { 10 1 ¯ 4 } cleavage faces: An XPS and AFM study by Venkata Gopal Reddy Chada; Douglas B. Hausner; Daniel R. Strongin; Ashaki A. Rouff; Richard J. Reeder (350-360).
The interaction of divalent Cd and Pb with the { 10 1 ¯ 4 } cleavage faces of calcite has been investigated with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Analysis of the { 10 1 ¯ 4 } cleavage planes of calcite was carried out with X-ray photoelectron spectroscopy (XPS) after exposure to divalent metal-bearing solutions in the 0.1–100 μM concentration range for times ranging from 1 to 24 h. The uptake of Cd2+ by calcite was determined to be greater than that of Pb2+ under similar experimental conditions (1 μM, pH 8.2, 24 h exposure time). In both cases, the majority of the divalent metal was postulated to exist in a surface precipitate. AFM results showed that the exposure of calcite to a 1 μM Pb2+ solution resulted in ellipsoidal surface growths that were attributed to the nucleation of a PbCO3 bulk phase. In the Cd circumstance, AFM showed comparatively flat growth features forming on the calcite surface even at concentrations down to 0.1 μM, where the solution would be expected to be undersaturated with respect to Cd bulk phases. These features were attributed to a (Ca,Cd)CO3 solid solution. The individual exposure of these Cd/CaCO3 and Pb/CaCO3 samples to water pre-equilibrated with calcite (metal free) for 1 h led to the removal of no more than 20% of the divalent metal, suggesting that if there was an adsorbed Pb or Cd complex initially on the calcite surface, it was an minority species compared to the precipitate phase. Exposure of calcite to 100 μM Cd and Pb resulted in the accumulation of precipitate on the calcite surface presumably due to the divalent metal initial solution concentrations exceeding the solubility products of CdCO3 and PbCO3, respectively.
Keywords: Calcite uptake; Divalent metal; AFM of calcite; Cadmium; Lead; XPS; Solid solution;

The surface adsorption of n-dodecyl phosphocholine (C12PC) has been characterised by a combined measurement of surface tension and neutron reflectivity. The critical micellar concentration (CMC) was found to be 0.91 mM at 25 °C in pure water. At the CMC, the limiting area per molecule ( A cmc ) was found to be 52 ± 3   Å 2 and the surface tension ( γ cmc ) to be ca. 40.0 ± 0.5   mN / m . The parallel study of chain isomer n-hexadecyl phosphocholine (C16PC) showed a decrease of the CMC to 0.012 mM and a drop of γ cmc to 38.1 ± 0.5   mN / m . However, A cmc for C16PC was found to be 54 ± 3   Å 2 , showing that increase in alkyl chain length by four methylene groups has little effect on A cmc . The almost constant A cmc suggested that the limiting area per molecule was determined by the bulky PC head group. It was further found that the surface tension and related key physical parameters did not vary much with temperature, salt addition, solution pH or any combination of these, thus showing that surface adsorption and solution aggregation from PC surfactants is largely similar to the zwitterionic betaine surfactants and is distinctly different from ionic and non-ionic surfactants. The thickness of the adsorbed monolayers measured from both dC12hPC and dC16hPC was found to be 20–22 Å at the CMC from neutron reflectivity. Neither A cmc nor layer thickness varied with alkyl chain length, indicating that as the alkyl chain length became longer it was further tilted away from the surface normal direction and the layer packing density increased. It was also observed that the thickness of the layer varied little with surfactant concentration, indicating that the average conformational orientation of the alkyl chain remained unchanged against varying surface coverage.
Keywords: Surfactant adsorption; Phosphocholine surfactant; Zwitterionic surfactant; Neutron reflection; Biosurfactant;

Removal of dyes from colored textile wastewater by orange peel adsorbent: Equilibrium and kinetic studies by Mokhtar Arami; Nargess Yousefi Limaee; Niyaz Mohammad Mahmoodi; Nooshin Salman Tabrizi (371-376).
The use of low-cost and ecofriendly adsorbents has been investigated as an ideal alternative to the current expensive methods of removing dyes from wastewater. Orange peel was collected from the fields of orange trees in the north of Iran and converted into a low-cost adsorbent. This paper deals with the removal of textile dyes from aqueous solutions by orange peel. Direct Red 23 (DR23) and Direct Red 80 (DR80) were used as model compounds. The adsorption capacity Q 0 was 10.72 and 21.05 mg/g at initial pH 2. The effects of initial dye concentration (50, 75, 100, 125 mg/l), pH, mixing rate, contact time, and quantity of orange peel have been studied at 25 °C. The Langmuir and Freundlich models were used for this study. It was found that the experimental results show that the Langmuir equation fit better than the Freundlich equation. The results indicate that acidic pH supported the adsorption of both dyes on the adsorbent. Orange peel with concentrations of 8 and 4 g/l has shown adsorption efficiencies of about 92 and 91% for DR23 and DR80, respectively. Furthermore, adsorption kinetics of both dyes was studied and the rates of sorption were found to conform to pseudo-second-order kinetics with a good correlation ( R ⩾ 0.998 ). Maximum desorption of 97.7% for DR23 and 93% for DR80 were achieved in aqueous solution at pH 2. Finally, the effect of adsorbent surface was analyzed by scanning electron microscope (SEM). SEM images showed reasonable agreement with adsorption measurements.
Keywords: Orange peel; Adsorption; Desorption; Isotherms; SEM;

Catalityc effects of ruthenium and osmium spontaneous deposition on platinum surfaces toward methanol oxidation by S.H. Bonilla; C.F. Zinola; J. Rodríguez; V. Díaz; M. Ohanian; S. Martínez; B.F. Giannetti (377-386).
The influence of ruthenium and osmium spontaneous deposition on polycrystalline platinum in sulfuric acid was studied by conventional electrochemical techniques. The inhibition of the hydrogen adatom voltammetric profile by the foreign adatoms was used to calculate the degree of surface coverage of ruthenium, osmium, and a mixture of both metal ions from solutions of different composition. Methanol adsorption and oxidation were compared on bare platinum, platinum/ruthenium, platinum/osmium, and ternary compounds, considering the efficiency of methanol oxidation per hydrogen adatom displaced by the foreign metal on platinum.
Keywords: Ruthenium; Osmium; Platinum; Electrocatalysis; Voltammetry;

The mean-field, lattice-based model of polymer melt intercalation in organically-modified layered silicates (OLS) originally developed by Vaia and Giannelis was applied for different polymers such as poly(methyl methacrylate) (PMMA), polypropylene (PP), and poly(ethylene oxide) (PEO). The nature of each polymer controls significantly the intercalation of the system. The internal energy change caused by the interaction of polymer, surfactant and clay is the strongest factor in determining the equilibrium structure of the nanocomposite system.
Keywords: Polymer nanocomposites; Organo-silicates; Intercalation; Model; Clay;

In emulsion polymerization, complete entry of an initiator-derived, surface-active radical may involve its adsorption onto latex particles/water interfaces and subsequently its propagation with one more monomer molecule therein. However, all publications to date have defined this propagation step as a three-dimensional bulk reaction between a surface-active entry radical and a monomer molecule. This is incorrect conceptually. It is proposed that the rate of the propagation of surface-active entry radicals with monomer at latex particles/water interfaces be expressed as − d [ M Z ⋅ ] w / l / d t = k I A [ M ] P [ M Z ⋅ ] aq . In this equation, A is the interfacial area between water and latex particles; [ M ] P and [ M Z ⋅ ] aq are the mean concentrations of monomer in the particle phase and entry radicals in the aqueous phase, respectively; k I is the radical propagation constant at the interfaces, and may be estimated via transition state theory. For seeded styrene polymerization by Hawkett et al. (J. Chem. Soc. Faraday Trans. 1 76 (1980) 1323), k I ≈ ∼ 4.2 × 10 −9 k p   ( mol −1 dm 4 s −1 ) is estimated. Here k p is the propagation rate coefficient in bulk polymerization. This alternative approach should be useful for one to simulate radical entry rate in emulsion polymerization where the propagation step may be rate-determining, such as under monomer-starved conditions.
Keywords: Emulsion polymerization; Radical entry; Interfacial reaction; Rate coefficient;

Preparation of nanosized gold particles in a biopolymer using UV photoactivation by Anjali Pal; Kunio Esumi; Tarasankar Pal (396-401).
Gold nanoparticles have been prepared by UV photoactivation in the presence of a biopolymer, sodium alginate. The particles are characterized by UV–vis spectra and TEM studies. Both particle size and the UV–visible absorption peak are dependent on the sodium alginate concentration. The effects of various other parameters such as change of light source, cell material of the reaction chamber, heating effect, irradiation time, and HAuCl4 concentration are studied. The particles are spherical and in situ stabilized by the biopolymer. The method is very simple and reproducible.
Keywords: Gold nanoparticles; Sodium alginate; UV photoactivation;

Surface properties of aqueous amino acid solutions by Norihiro Matubayasi; Shohei Matsuyama; Ryosuke Akizuki (402-406).
To understand the distinction between the effects of zwitterionic, anionic, and cationic l-leucine upon adsorption and lateral interactions at air/water surface, the surface tensions of aqueous solutions of l-leucine–l-leucine hydrochloride and l-leucine–sodium l-leucinate mixtures were measured as a function of concentration and composition at 25 °C. The surface activity decreases in the order l-leucine >l-leucine hydrochloride > sodium l-leucinate. Both l-leucine hydrochloride and sodium l-leucinate form gaseous adsorbed films through the experimentally accessible concentration range, while the adsorbed film of zwitterionic l-leucine shows a transition between gaseous and expanded film.
Keywords: Surface tension; Adsorbed films; Amino acids; Leucine;

Calcium oxalate (CaOx) particles with different morphologies and phase structures were prepared by a facile precipitation reaction of sodium oxalate with calcium chloride in the absence and presence of poly(sodium 4-styrene-sulfonate) (PSSS) at different temperatures. The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. The influence of experimental conditions including pH, temperature, and concentration of PSSS and CaC2O4 on the morphologies and phase structures of the prepared calcium oxalate particles were investigated. It was found that variations in the concentration of PSSS and CaC2O4, temperature, and pH significantly influenced the crystal structure, morphology, and particle size of the samples. Various crystal morphologies of calcium oxalate, such as plate, leaf-shaped, bipyramid, and cylinder could be fabricated, depending on the experimental conditions. Higher PSSS concentration and reaction temperature favored the formation of metastable calcium oxalate dihydrate (COD) crystals and stable calcium oxalate monohydrate (COM), respectively. Especially, cylinder-shaped CaC2O4 particles were obtained at 80 °C in the presence of PSSS for the first time. This research may provide new insight into understanding and potentially regulating the formation of kidney stones and the control of morphology and phase structures of calcium oxalate particles.
Keywords: Calcium oxalate; PSSS (poly(sodium 4-styrene-sulfonate)); Morphology; Phase structure;

Foaming and interfacial properties of hydrolyzed β-lactoglobulin by J.P. Davis; D. Doucet; E.A. Foegeding (412-422).
β-lactoglobulin (β-lg) was hydrolyzed with three different proteases and subsequently evaluated for its foaming potential. Foam yield stress ( τ 0 ) was the primary variable of interest. Two heat treatments designed to inactivate the enzymes, 75 °C/30 min and 90 °C/15 min, were also investigated for their effects on foam τ 0 . Adsorption rates and dilatational rheological tests at a model air/water interface aided data interpretation. All unheated hydrolysates improved foam τ 0 as compared to unhydrolyzed β-lg, with those of pepsin and Alcalase 2.4L® being superior to trypsin. Heat inactivation negatively impacted foam τ 0 , although heating at 75 °C/30 min better preserved this parameter than heating at 90 °C/15 min. All hydrolysates adsorbed more rapidly at the air/water interface than unhydrolyzed β-lg, as evidenced by their capacity to lower the interfacial tension. A previously observed relationship between interfacial dilatational elasticity ( E ′ ) and τ 0 was generally confirmed for these hydrolysates. Additionally, the three hydrolysates imparting the highest τ 0 not only had high values of E ′ (approximately twice that of unhydrolyzed β-lg), they also had very low phase angles (essentially zero). This highly elastic interfacial state is presumed to improve foam τ 0 indirectly by improving foam stability and directly by imparting resistance to interfacial deformation.
Keywords: Dilatational modulus; Dilatational elasticity; Adsorption; Interfacial rheology; β-lactoglobulin; Peptide; Hydrolysate; Foam; Yield stress; Whey protein;

Formation of individually dispersed nanoparticles or compactly aggregated nanoparticles from sols via a spray-drying route at low pressure was investigated experimentally. Silica sol was used as a sample material. Effects of operating temperature, colloid size, sol concentration, pressure, pH and zeta potential of sols on the morphology of product particles were investigated. From the experimental results, it was shown that dispersed nanoparticles could be obtained at a relatively low pressure (20 Torr) and low temperature (200 °C). The experiment also showed that dispersed nanoparticles could be achieved by careful control of the interfacial energy (pH value) of the colloidal precursor. A possible mechanism of sol-to-dry-particle formation in the spray-drying process at low pressure is suggested, based on the experimental results and the available theories. This mechanism was able to explain the experimental results well.
Keywords: Aerosol; Droplet-to-particle formation; Interfacial energy; Zeta potential; Silica sol;

A non-contact adhesion measurement technique based on acoustic base excitation and laser interferometery has been introduced and demonstrated. The vibrational motion of 21.4-μm polystyrene latex particles (PSL) microspheres on surfaces were excited in the frequency range of 0–3.5 MHz, and their axial displacement responses were measured by an interferometer. It is shown that the rolling motion is dominant compared to the axial displacement of the bond. Using a formula for the rotational moment resistance of the particle–surface adhesion bond and the equation of rotational motion, the natural frequency of the rotational motion is related to the work of adhesion of the particle and substrate materials. The substrate materials used in the experiments include copper, aluminum, tantalum, and silicon. Measured work of adhesion values are compared to the data reported in the literature and good agreement is found.
Keywords: Rolling resistance; Work of adhesion; Microspheres; Adhesion measurements;

The role of poly(ethylene glycol) in the formation of silver nanoparticles by Chengcai Luo; Yuhong Zhang; Xiaowei Zeng; Yuewu Zeng; Yanguang Wang (444-448).
Silver nanoparticles were prepared under mild conditions by exploiting poly(ethylene glycol) (PEG) in the absence of other chemicals. The chain length of PEG was discovered to play the key role in the formation of silver nanoparticles. Ethylene glycol, which was used as a reducing agent for the preparation of metal particles at high temperatures (>170 °C), was inactive for the reducing of Ag+ at 80 °C. Surprisingly, Ag+ can be smoothly reduced to silver nanoparticles at the same condition in PEG 2000. Further studies demonstrated that the reducing rate of Ag+ to nano-Ag was remarkably enhanced with the increase of polymer chain length of PEG. The particle size depended upon the reaction temperature and concentration of the precursor and an increase in temperature effected a shift of particle size range from ca. 10 to 80 nm. In addition, a noticeable change of sphere to polyprism nanosilver was observed when the reaction temperature was elevated to 120 °C.
Keywords: Silver nanoparticles; Reduction; Poly(ethylene glycol);

Formation of calcium crystallites in dry natural rubber particles by Márcia M. Rippel; Carlos A.P. Leite; Lay-Theng Lee; Fernando Galembeck (449-456).
In this paper, the effects of drying and aging of natural rubber particles of Hevea brasiliensis are studied. The evolutions of the particle morphology and of the elemental distribution are investigated using electron-energy-loss spectroscopy imaging in a low-energy transmission electron microscope (ESI-TEM). It is found that when the sample is aged, calcium salt crystallites are formed around the particles. Fusion of these crystallites with time to form larger crystals shows evidence of ion mobility in the dry rubber matrix. Electron diffraction patterns and elemental mapping analysis indicate the crystals to be calcium sulfate. These crystallites are closely associated with membrane materials of the rubber particle and are surprisingly compatible with the hydrocarbon matrix of the rubber particle. It is proposed that polar sites on the membrane materials provide nucleation sites for the crystallization of calcium and sulfate ions from the latex serum.
Keywords: Natural rubber particles; Calcium crystallites; Elemental mapping;

A novel method of preparing AgCl nanoparticles by mixing AgCl powder and a microemulsion consisting of dioctyldimethylammonium chloride/n-decanol/water/isooctane is introduced. This new method was discovered during the preparation of AgCl nanoparticles in single microemulsions by direct reaction with the dioctyldimethylammonium chloride surfactant counterion. The effect of the following variables on the concentration of the colloidal AgCl nanoparticles (the nanoparticle uptake) and the particle size were studied: (1) operating variables, including mixing and temperature; and (2) microemulsion variables, including surfactant and cosurfactant concentration, and water to surfactant mole ratio. Manipulating these variables provides an insight into the role of the surfactant surface layer rigidity on the phenomenon. The results were explained by the effect of these variables on reaction rates and the colloidal nanoparticle stability. Mixing had a significant effect on the nanoparticle uptake. At 300 rpm an equilibrium AgCl nanoparticle uptake was achieved in about 1 h, while without mixing only 5% of the equilibrium value was reached even after 24 h. An optimum temperature was found for which a maximum nanoparticle uptake was obtained. At higher temperatures, the nanoparticle uptake declined. The nanoparticle uptake increased linearly with the surfactant concentration, and the particle size increased as well. A monotonic decrease in the nanoparticle uptake accompanied by an increase in the particle size was observed when increasing n-decanol concentration or the water to surfactant mole ratio.
Keywords: Nanoparticles; Quantum dots; Silver chloride; Photosensitive materials; Microemulsions; Surfactants;

Preparation and characterization of carbonated barium–calcium hydroxyapatite solid solutions by Akemi Yasukawa; Eiichi Ueda; Kazuhiko Kandori; Tatsuo Ishikawa (468-474).
Particles of carbonated barium–calcium hydroxyapatite solid solutions (BaCaHap) with different Ba/(Ba + Ca) ( X Ba ) atomic ratios were prepared by a wet method at 100 °C and characterized by various means. The crystal phases and structures of the products strongly depended on the composition of the starting solution, that is, the Ba/(Ba + Ca) atomic ratio ( [ X Ba ] ) and H3PO4 concentration ([H3PO4]) in the solution. BaCaHap with X Ba ⩽ 0.43 could be prepared at [ X Ba ] ⩽ 0.7 by changing [H3PO4], but could never be obtained at [ X Ba ] = 0.8 – 0.95 regardless of [H3PO4]. The carbonated calcium hydroxyapatite particles prepared at [ X Ba ] = 0 were fine and short rod-shaped particles (ca. 14 × 84 nm). With increasing [ X Ba ] from 0 to 0.8, the particles obtained became large spherical agglomerates. The carbonated barium hydroxyapatite particles formed at [ X Ba ] = 1 were long rod-shaped agglomerates (ca. 0.2 × 2 μm) of fine primary particles. The amount of CO2 adsorbed irreversibly on a series of BaCaHaps showed a minimum at (Ba + Ca)/(P + C) atomic ratio of around 1.56, which agreed well with the minimum cation/P ratio obtained for the other hydroxyapatites, as already reported.
Keywords: Barium hydroxyapatite; Calcium hydroxyapatite; Solid solution; Carbonate ion; Molecular adsorption;

We have investigated aggregation phenomena in a polydisperse colloidal dispersion of ferromagnetic particles simulated by employing the cluster-moving Monte Carlo method in an applied magnetic field. The influence of both particle–particle and particle–field interactions on the aggregate structures is analyzed in terms of a pair correlation function. The results obtained in this study are summarized as follows: Under a strong magnetic field, chainlike clusters are formed along the magnetic field direction, and they become thickly clustered with an increase in the strength of the external magnetic field. Moreover, the thickly clustered chains are formed for a polydisperse system that has a large standard deviation of particle diameters. In contrast, for a very weak magnetic field, the strong interaction between the larger particles gives rise to the formation of various shapes in the chainlike clusters, including bending, looping, and branching. With an increase in the external magnetic field, these structures reorganize to form straight chainlike clusters. Furthermore, the thickness of the chainlike clusters for the polydisperse system is found to depend on the standard deviation of the particle-size distribution but is found to be independent of the magnetic field strength.
Keywords: Magnetic fluid; Ferromagnetic colloidal dispersion; Particle-size distribution; Chain-like cluster; Monte Carlo simulation;

Uniform, well-dispersed silver particles of various morphologies have been prepared by reducing highly acidic silver nitrate solutions with ascorbic acid in the presence of a sodium naphthalene sulfonate–formaldehyde copolymer as dispersing agent. By varying the temperature of the reaction, the free acid content, the addition rate of the reductant, and the aging time, both isometric and anisotropic silver particles could be obtained. It was found that the latter were formed by aggregation of nanosize subunits, which were identified by electron microscopy and X-ray diffractometry.
Keywords: Colloidal silver; Metal nanoparticles; Nanoparticle structure; Tabular silver particles; Silver particles;

The influence of a surfactant over water on the polymorphism and crystal size of calcium carbonate produced by reaction crystallization in microemulsion systems was investigated in a mixing tank reactor. The crystallization was induced by the reaction between two aqueous micelle solutions (Na2CO3–CaCl2) stabilized by anionic surfactants, SDS (sodium dodecyl sulfate) or AOT (sodium bis(2-ethylhexyl) sulfosuccinate). With increasing surfactant ratio to water, the water-in-oil microemulsion was stably developed and the morphology of the calcium carbonate crystallized in the micelles sharply transformed from calcite to vaterite. The influence of SDS on the polymorphism and crystal size of calcium carbonate was much clearer than that of AOT. In addition, with AOT, certain step changes in the morphology and crystal size occurred around a surfactant ratio to water ( R = [ H 2 O ] / [ surfactant ] ) of 15 due to a two-phase separation of the microemulsion.
Keywords: Calcium carbonate; Structural control; Calcite; Vaterite; Microemulsion;

Patterned magnetic rings fabricated by dewetting of polymer-coated magnetite nanoparticles solution by Lijuan An; Wei Li; Yaru Nie; Bin Xie; Zhaoqiang Li; Junhu Zhang; Bai Yang (503-507).
Here we present a simple and controlled method for direct fabrication of ordered 2D arrays of magnetic rings. This method utilizes polystyrene-coated magnetite nanoparticles as a solution, and the magnetic rings are fabricated on patterned self-assembled monolayers by dewetting of the solution. Polystyrene-coated magnetite nanoparticles were synthesized by atom-transfer radical polymerization, which promoted the dispersibility and stability of magnetite nanoparticles in chloroform. Magnetic rings were studied using optical photograph, SEM, and magnetic force microscopy. This approach offers a new way for patterning nanoparticulate rings with deliberate control over feature composition, size, as well as interfeature distance.
Keywords: Magnetic ring; Magnetite nanoparticle; Pattern; Dewetting; ATRP;

A simple optical device for measuring free surface deformations of nontransparent liquids by E.G. Megalios; N. Kapsalis; J. Paschalidis; A.G. Papathanasiou; A.G. Boudouvis (508-512).
In this paper, a novel optical device for measuring the deformation of liquid free surfaces is presented. The device employs a laser beam, which can be focused on any chosen location on the free surface. The key measurement is of the intensity of the beam reflected from a location on the free surface where the deformation exhibits a local extremum. The optics of the device is so designed as to measure a maximum intensity when the distance between the focusing lens and the selected point on the free surface is equal to the focal length, thus enabling a height measurement. The device is tested in ferrofluid pools where the height of the spikes of the normal field instability is measured. The simplicity of the suggested technique enables the fabrication of a quite cheap device for measuring surface deformation of nontransparent liquids, which provides good accuracy and reproducibility.
Keywords: Nontransparent liquid; Free surface; Optical device; Ferrofluid; Laser beam;

CdS nanoparticles, prepared in reverse micellar system, were immobilized onto thiol-modified aluminosilicate particles (AS―SH) by a simple operation: addition of AS―SH in the micellar solution and mild stirring. The resulting CdS nanoparticles–aluminosilicate composites (AS―CdS) were used as photocatalysts for H2 generation from 2-propanol aqueous solution. The chemical properties of the aluminosilicate, such as affinity for water and other reactants, were found to affect the photocatalytic property of the CdS nanoparticles immobilized. Zeolite particles, having affinity for water and 2-propanol, gave a good AS―CdS photocatalyst with respect to H2 generation.
Keywords: CdS nanoparticles; Aluminosilicate support; Photocatalyst; Hydrogen generation;

The influence of decyl alcohol on the potentiometric response of three para-alkylbenzenesulfonate (p-RBS) electrodes is analyzed. The results are clearly dependent on the membrane surface polarity due to the presence of the alcohol. The ionophore was the complex trioctylmethylammonium–p-RBS, (TOMA+)–p-RBS, with R = H , CH3, and C2H5. The nature of the complex plays a fundamental role on the potentiometric behavior of the electrode showing that the more hydrophobic the complex, the better the potentiometric responses. Moreover, the electrodes selectivities for several hydrophilic and hydrophobic interfering anions were determined. The potentiometric results with interfering anions were coherent with the Pearson's hard and soft acid–base character of these anions.
Keywords: Ion-selective electrodes; Membrane polarity; Interfering agents; Pearson soft and hard acids and bases;

The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface.
Keywords: Interfaces; Intermetallic compounds; Electron microscopy; Diffusion; Microstructure;

A new way of modeling imbibition is proposed in this paper. It combines two elements. One is a physically consistent, dynamic criterion for the imbibition of an individual pore originally suggested by Melrose (SPEJ (November 1965) 259–271). The other is the use of a simple but physically representative model of porous media: a dense random packing of spheres that is geometrically predetermined. This approach allows truly a priori predictions of imbibition curves (saturation vs capillary pressure) for different values of contact angle, different initial conditions (e.g., different drainage endpoints), and different macroscopic sample geometries (the ratio of external to internal pores). It also provides a mechanistic basis for understanding the influence of pore-scale phenomena such as “snap-off” of nonwetting phase in the pore throats due to the coalescence of pendular rings. The simulations show that the capillary pressure curve for this unconsolidated packing is very sensitive to the wettability parameters (such as contact angle), whereas the influence of different initial conditions and snap-off is almost negligible. Predicted capillary pressure curves are compared to experimental data presented in the literature, and are consistent with them.
Keywords: Porous media; Imbibition; Finney packing; Snap-off; Meniscus; Residual saturation;

Simultaneous electroosmotic and permeation flows through a Nafion membrane by V.M. Barragán; C. Ruiz-Bauzá; J.P.G. Villaluenga; B. Seoane (540-547).
The volume flow of methanol–water potassium chloride solutions through a Nafion membrane originated by the simultaneous action of electric potential and pressure gradients has been measured at different percentages of methanol. Measurements were conducted when both gradients act in the same and in the opposite directions under different experimental conditions. The results indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the individual electroosmotic and permeation flows due to each force acting separately. The application of the irreversible thermodynamics theory, which includes second-order terms, allowed the study of the influence of the composition of the solutions on the determination of the different phenomenological coefficients.
Keywords: Electroosmotic flow; Permeation; Nafion membrane; Phenomenological coefficients; Irreversible thermodynamics; Methanol–water solutions;

We study interfacial properties of rigid-rod-like poly(n-hexyl isocyanate) (PHIC), flexible poly(vinyl acetate) (PVAc), and mixed films of PHIC and PVAc spread at the air–water interface as a function of the molar fraction of PHIC by surface pressure measurements and fluorescence microscopy. From the plots of the experimental mean area of the mixed polymer films at a constant surface pressure as a function of the molar fraction of PHIC in the mixed films, the binary mixtures of PHIC/PVAc were concluded to be compatible at the air–water interface. This means that the hydrophobic hexyl group of PHIC takes a horizontal orientation to the air–water interface rather than a perpendicular one, leading to PHIC and PVAc having the same interfacial orientation. Compatibility of the binary mixtures of PHIC/PVAc at the air–water interface is also confirmed by their fluorescence microscopic images, since PHIC proves to be inhomogeneous and PVAc is homogeneous with the aid of a fluorescence probe, respectively.
Keywords: Mixed films; Poly(n-hexyl isocyanate); Poly(vinyl acetate); Air–water interface; Surface pressure measurements; Fluorescence microscopy; Compatibility;

The capillary instability of a liquid crystalline cylinder in magnetic field is considered using an energy approach. The boundary problem is solved in the linear approximation of the anisotropy χ a of the magnetic susceptibility χ. The effect of anisotropy, in the region 1 ≫ | χ | > | χ a | ≫ χ 2 , can be strong enough to counteract and even reverse the tendency of the field to enhance stabilization by enlarging the cutoff wave number k s beyond the conventional one set by Rayleigh.
Keywords: Plateau instability; Nematic liquid crystal; Magnetic field; Anisotropy of susceptibility;

Simultaneous imbibition–heat convection process in a non-Darcian porous medium by M. Sánchez; E. Luna; A. Medina; F. Méndez (562-569).
In the present work, the nonisothermal imbibition process in a porous medium was numerically analyzed using a non-Darcian model for the momentum equation and energy equations for the wetting and dry zones. In order to show the thermal character of the problem, we assume initially that the porous medium is found at a uniform temperature T 0 and suddenly begins the imbibition process into the porous medium with a penetrating fluid at temperature T 1 . The physical influence of nondimensional parameters such as Peclet number, Pe, effective heat capacity number, β w , porous Reynolds number, Re p , and the inertial coefficient of the porous medium, F, serve us to evaluate the position and velocity of the imbibition front as well as temperature profiles in both zones. In particular, for values of Re p F / β w ≫ 1 , we recover a type of nonisothermal Washburn law. The numerical predictions show that the imbibition front and the temperature fields strongly depend on the above nondimensional parameters, revealing a clear deviation of the simple Washburn law.
Keywords: Nonisothermal imbibition; Porous medium; Heat convective transport; Non-Darcian flow; Imbibition front;

A novel viscoelastic system from a cationic surfactant and a hydrophobic counterion by R. Abdel-Rahem; M. Gradzielski; H. Hoffmann (570-582).
The phase behavior of 2-hydroxy-1-naphthoic acid (2,1-HNC) mixed with cetyltrimethylammonium hydroxide (CTAOH) is reported. This novel system is compared with the published one of 3-hydroxy-2-naphthoic acid (3,2-HNC) mixed with CTAOH. We investigated the phase behavior and properties of the phases in aqueous solutions of 100 mM CTAOH with 2,1-HNC. In both systems a multilamellar vesicle phase is formed when the naphthoate/surfactant ratio (r) reaches unity. When an increasing amount of 2,1-HNC is mixed with a micellar solution of 100 mM CTAOH, an isotropic low-viscous micellar solution, a viscoelastic gel (consisting of rodlike micelles), a turbid region (two-phase region), and a viscoelastic liquid crystalline gel (consisting of multilamellar vesicles, MLV) were formed. The vesicular phase is highly viscoelastic and has a yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/2,1-HNC over a two-phase region, where micelles and vesicles coexist. Also it was noticed that 2,1-HNC is dissolved in 100 mM CTAOH until the naphthoate/surfactant ratio reaches ∼1.5, and the liquid crystalline phases were found to change their color systematically when they were viewed between two crossed polarizers. The vesicles have been characterized by differential interference contrast microscopy, freeze-fracture electron microscopy, and cryo-electron microscopy (cryo-TEM). The vesicles were polydisperse and their diameter ranged from 100 to 1000 nm. The interlamellar spacing between the bilayers was determined with small angle neutron scattering and agrees with the results from different microscopical methods. The complex viscosity rises by six orders of magnitude when rodlike micelles are formed. The complex viscosity decreases again in the turbid region, and then rises approximately six orders of magnitude above the water viscosity. This second rising is due to the formation of the liquid crystalline MLV phase.
Keywords: Cationic surfactant; Hydrophobic counterion; FF-TEM; Cryo-TEM; Multilamellar vesicles; Viscoelastic; Rheology;

Self-assembled Gemini surfactant film-mediated dispersion stability by Y.I. Rabinovich; J.R. Kanicky; S. Pandey; H. Oskarsson; K. Holmberg; B.M. Moudgil; D.O. Shah (583-590).
The force–distance curves of 12-2-12 and 12-4-12 Gemini quaternary ammonium bromide surfactants on mica and silica surfaces obtained by atomic force microscopy (AFM) were correlated with the structure of the adsorption layer. The critical micelle concentration was measured in the presence or absence of electrolyte. The electrolyte effect (the decrease of CMC) is significantly more pronounced for Gemini than for single-chain surfactants. The maximum compressive force, F max , of the adsorbed surfactant aggregates was determined. On the mica surface in the presence of 0.1 M NaCl, the Gemini micelles and strong repulsive barrier appear at surfactant concentrations 0.02–0.05 mM, which is significantly lower than that for the single C12TAB (5–10 mM). This difference between single and Gemini surfactants can be explained by a stronger adsorption energy of Gemini surfactants. The low concentration of Gemini at which this surfactant forms the strong micellar layer on the solid/solution interface proves that Gemini aggregates (micelles) potentially act as dispersing agent in processes such as chemical mechanical polishing or collector in flotation. The AFM force–distance results obtained for the Gemini surfactants were used along with turbidity measurements to determine how adsorption of Gemini surfactants affects dispersion stability. It has been shown that Gemini (or two-chain) surfactants are more effective dispersing agents, and that in the presence of electrolyte, the silica dispersion stability at pH 4.0 can also be achieved at very low surfactant concentrations (∼0.02 mM).
Keywords: Atomic force microscopy; Surface forces; Micelles; Gemini surfactants; Surface tension; Suspension stability;

The dissociation of tris-(2,2′-bipyridyl) iron(II) ([Fe(bipy)3]2+) has been studied in the Triton X-100/hexanol/cyclohexane reverse micellar medium. The reaction obeys simple first-order kinetics with no evidence of autoinhibition. The first-order rate constant ( k 1 ) has been determined at different values of W ([H2O]/[Triton X-100]). The rate ( k 1 ) decreases with increasing value of W. k 1 also increases with increase in Triton X-100 concentration at constant values of W, showing that the reaction takes place at greater speed at the micellar interface. The kinetic results can be interpreted by the monomolecular pseudo-phase model. The effect of W on rate ( k 1 ) is more pronounced in the range of W from 1.55 to 4.2 but less pronounced at higher W. The reaction is further accelerated by Cl and SCN ions and the kinetic results provide evidence for the formation of ion pairs between the cation [Fe(bipy)3]2+ and each of these anions. The formation of such ion pairs has not been observed in aqueous medium but has been reported earlier in aqueous–alcohol mixtures. This result therefore provides evidence for the lower micropolarity of solubilized water compared to ordinary water.
Keywords: Reverse micelles; Triton X-100; Tris-(2,2′-bipyridyl) iron(II); Micropolarity; Water pool; Ion pair formation;

Interaction of surfactants with thickeners used in waterborne paints: A rheological study by Alicia Maestro; Carmen González; José M. Gutiérrez (597-605).
Studies of the phase diagram and linear viscoelasticity of aqueous solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC), a thickener used in water-based paints, and SDS reveal that SDS–HMHEC mixed micelles are formed that increase the number of hydrophobic junctions and enhance interpolymer association up to an [SDS]/[HMHEC] ratio. This fact produces a strong increase of viscoelasticity or a phase separation, depending on the [HMHEC]. At higher ratios the excess of micelles with predominant SDS isolates hydrophobes and disrupts the micellar network. Then, viscoelastic functions decrease and HMHEC behaves as a nonassociative polymer. TTAB and Brij30 also interact with HMHEC, but in a different way. No phase separation is observed with these surfactants. TTAB forms mixed micelles and new junction points in the same way as SDS. However, this surfactant does not stabilize the micelles as SDS does, presumably due to the different interaction between the ―OH from the cellulose and the charged groups. Results seem to indicate that Brij30 enters into the hydrophobic aggregates of HMHEC and stabilizes them, increasing relaxation time, but it does not form new junction points, since it forms quite big micelles.
Keywords: HMHEC; Associative thickener; Surfactant; Viscoelasticity; Rheology;

Generating stable microscale concentration gradients is key to numerous biological and chemical analyses. Microfluidic systems offer the ability to maintain laminar fluid diffusion interfaces ideal for the production of temporally stable concentration gradients. Previous efforts have focused on pressure driven flows and have relied on networks of branching channels to create streams of varying concentrations which can subsequently be combined to form the desired gradients. In this study, we numerically and experimentally demonstrate a novel electrokinetic technique which utilizes applied voltages and surface charge heterogeneity in simpler channel geometries to control and manipulate microscale concentration gradients without the need for parallel lamination. Flow rates ranged from 30 to 460 nl min−1 for Péclet numbers between 70 and 1100. Spatial stability of 0.6 mm or greater was obtained for a wide range of gradient shapes and magnitudes over lateral dimensions of 400–450 μm. Sensitivity analysis determined that this technique is largely independent of channel depth and species electrophoretic mobility, however channel width and the diffusion coefficient of the analyte are critical. It was concluded that by adjusting applied voltages and/or channel width, this approach to concentration gradient generation can be adapted to a wide range of applications.
Keywords: Electrokinetics; Microfluidics; Concentration gradient;

A boundary element (BE) procedure is developed to numerically calculate the electrophoretic mobility of highly charged, rigid model macroions in the thin double layer regime based on the continuum primitive model. The procedure is based on that of O'Brien (R.W. O'Brien, J. Colloid Interface Sci. 92 (1983) 204). The advantage of the present procedure over existing BE methodologies that are applicable to rigid model macroions in general (S. Allison, Macromolecules 29 (1996) 7391) is that computationally time consuming integrations over a large number of volume elements that surround the model particle are completely avoided. The procedure is tested by comparing the mobilities derived from it with independent theory of the mobility of spheres of radius a in a salt solution with Debye–Hückel screening parameter, κ. The procedure is shown to yield accurate mobilities provided κa exceeds approximately 50. The methodology is most relevant to model macroions of mean linear dimension, L, with 1000 > κ L > 100 and reduced absolute zeta potential ( q | ζ | / k B T ) greater than 1.0. The procedure is then applied to the compact form of high molecular weight, duplex DNA that is formed in the presence of the trivalent counterion, spermidine, under low salt conditions. For T4 DNA (166,000 base pairs), the compact form is modeled as a sphere (diameter = 600 nm) and as a toroid (largest linear dimension = 600 nm). In order to reconcile experimental and model mobilities, approximately 95% of the DNA phosphates must be neutralized by bound counterions. This interpretation, based on electrokinetics, is consistent with independent studies.
Keywords: Electrokinetic transport; Thin double layer theory; Electrophoretic mobility; Compact DNA; Boundary element method;

Surfactant intercalated α-hydroxides of cobalt and nickel and their delamination-restacking behavior in organic media by C. Nethravathi; G. Harichandran; C. Shivakumara; N. Ravishankar; Michael Rajamathi (629-633).
Dodecyl sulfate and dodecylbenzene sulfonate intercalated α-hydroxides of nickel and cobalt were synthesized by ammonia precipitation. These solids delaminate to give a colloidal dispersion of layers in organic solvents such as 1-butanol. The dispersed layers could be reassembled either by evaporation of the colloid or by coagulation by the addition of a polar solvent.
Keywords: α-Nickel hydroxide; α-Cobalt hydroxide; Intercalation; Delamination; Colloidal dispersion;

Facile formation of water-insoluble cyclodextrin/Nafion composite films on solid surfaces by Kou Nakata; Minoru Sakamoto; Satoshi Taguchi; Noboru Yoshida; Katsuaki Shimazu (634-637).
The water-insoluble poly-β-cyclodextrin (poly-CD)/Nafion composite film was easily prepared by casting a mixed solution of poly-CD and Nafion onto substrate plates. FT-IR measurements showed that the 50/50 wt% poly-CD/Nafion composite film remained stable on the glass and quartz substrates after immersion in water for more than 3 h, while a pure poly-CD film was almost completely dissolved by immersion within 1 h. The film stability was also evaluated from the amount of p-nitrophenol (p-NP) inclusion in the film, which was determined from the decrease in UV–vis absorbance of the p-NP solution into which the film was immersed. The composition dependence of the inclusion amount showed that the film was stable up to 50 wt% CD, but became less stable with further increase in the CD concentration in the film. From the isotherms for the inclusion of p-NP and 1-naphthoate (1-Naph) into the film, the inclusion (stability) constants were determined to be 3.7 × 10 3  M −1 and 1.9 × 10 2  M −1 , respectively. These results show that the selective inclusion of CD is retained after preparation of the composite film.
Keywords: β-Cyclodextrin; Nafion; Composite film; Stability; p-Nitrophenol; 1-Naphthoate; Inclusion constant;

The effect of sucrose on the structure of molecular assemblies formed in an MO/H2O mixture has been studied using a small-angle X-ray scattering method. It was found that the phase transition Ia3dPn3m → HII occurs with increased sucrose concentration in the mixture with the composition 70 wt% MO at 20 °C. This structural change induced by sucrose addition would be ascribed to a cosmotropic property of sucrose, which leads to the dehydration of the MO head group and hence causes a decrease in the effective area occupied by the MO molecule at the polar/apolar interface, which facilitates the formation of molecular assemblies with a high curvature for the reversed liquid-crystal mesophase.
Keywords: Cubic phase; Hexagonal phase; Monoolein; Phase transition; SAXS;