Journal of Colloid And Interface Science (v.289, #1)

In this paper we present results that describe the speciation of Cu sorbed to the clay mineral vermiculite, with special attention to the effects of reduction on Cu sorption complexes. Sorption complexes were studied using powdered extended X-ray absorption fine structure (EXAFS) spectroscopy, polarized EXAFS spectroscopy, and X-ray diffraction (XRD). Ionic strength (I), background ion, and clay reduction (structural iron) were varied in the sorption samples. At low I EXAFS results indicate that Cu was surrounded by up to six water molecules sorbed in the interlayer of reduced vermiculite. EXAFS results from Cu-equilibrated reduced vermiculite with high I Ca background electrolyte revealed that Cu was surrounded by 4 O atoms at 1.95 Å and 8 second shell O atoms at 3.14 Å. Angular dependence of the second shell O atoms interpreted from the polarized-EXAFS spectra indicated that the atoms are out of plane from the basal plane of the vermiculite (inclined ∼25° from the ab plane). The local atomic environment and angular dependence of the EXAFS spectra suggest that the Cu atoms are adsorbed above the hexagonal cavities of the reduced clay mineral and form a Cu dimer in the interlayer. This adsorption mechanism was not observed in the non-reduced vermiculite under identical equilibration conditions. Our results provide molecular level evidence that Cu sorption mechanisms on vermiculite are dependent on solution conditions, such as redox potential and background electrolyte. These results can be used to develop better models of Cu sorption mechanisms on clay mineral surfaces.
Keywords: Polarized-XAFS; Vermiculite; Copper sorption; Reduced clay;

Using solution strontium and pH as independent variables, the metastable equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 °C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 °C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca10–n Sr n (PO4)6(OH)2 ( n = 0 – 10 )) and the calcium/hydroxide deficient hydroxyapatite template (Ca9–n Sr n (HPO4)(PO4)5OH ( n = 0 – 9 )). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
Keywords: Carbonated apatites; Solubility; Surface complex; Strontium;

Structure, stability, and orientation of BSA adsorbed to silica by Helén Larsericsdotter; Sven Oscarsson; Jos Buijs (26-35).
In this investigation, the structure, stability, and orientation of bovine serum albumin (BSA) adsorbed onto silica particles were studied using differential scanning calorimetry (DSC) and limited proteolysis in combination with mass spectrometry (MS). DSC gave information on the overall structural stability of BSA while limited proteolysis was used to probe the accessibility of enzymatic cleavage sites, thereby yielding information on the orientation and structure of BSA adsorbed to silica surfaces. Thermal investigation of BSA in various buffers, both free in solution and in the adsorbed state, showed that solutes that surround the protein played an important role with respect to the overall structural stability and the structural heterogeneity of BSA. Limited proteolysis with trypsin and chymotrypsin indicated that BSA in the adsorbed state is oriented with domain 2 facing the silica surface. Also, upon adsorption, no additional cleavage sites were exposed. The combination of the results presented in this study implied that BSA molecules adsorbed onto silica particles were significantly reduced in their structural stability, but not to an extent that internal residues within the native structure became fully exposed to the solution.
Keywords: Adsorption; BSA; DSC; Protein; Proteolysis; Mass spectrometry; Silica;

PM2.5 thermophoretic deposition efficiency in turbulent gas flowing over tube surfaces is analyzed using the new concept of potential capacity and potential capacity variation. According to Romay's model, a thermophoretic deposition efficiency model in turbulent gas flowing over tube surfaces is built up based on the potential capacity. Through computing and analyzing thermophoretic deposition efficiency, PM2.5 thermophoretic deposition efficiency in turbulent gas flowing over tube surfaces in direct ratio to the potential capacity variation and in inverse ratio to the temperature ratio of tube wall to entrance gas–particle mixture. And the imprecise notion of thermophoretic deposition efficiency direct ratio to the temperature difference between tube wall and entrance gas–particle mixture is reviewed. There are credible foundations to be provided for improving and researching thermophoretic deposition efficiency in theory and experiment.
Keywords: PM2.5; Thermophoretic deposition efficiency; Potential capacity; Turbulent flow;

Interaction of Ag(I), Hg(II), and Cu(II) with 1,2-ethanedithiol immobilized on chitosan: Thermochemical data from isothermal calorimetry by Eunice F.S. Vieira; Antonio R. Cestari; Elias de B. Santos; Francisco S. Dias (42-47).
The nature of interactions between metal ions Ag(I), Hg(II), Cu(II) and chitosan derivative of 1,2-ethanedithiol, QTDT, was investigated by isothermal calorimetry using the membrane breaking technique. Simultaneous determination of thermal effects, Q int , and amount of cation that interacts, n int , are described. The experimental data have been interpreted in terms of the Langmuir equation to determine the maximum adsorption capacity to form a monolayer, N mon , and the energy of interaction for a saturated monolayer per gram of QTDT, Q mon . With N mon and Q mon , the molar enthalpy of interaction for formation of a monolayer of anchored cations per gram of QTDT, Δ mon H m , was determined. The Δ mon H m values for Ag(I), Hg(II), and Cu(II) were −60.56, −58.05, and −84.36 kJ mol−1, respectively. Negative values of ΔG show the spontaneity of the interaction processes. The least entropically favourable processes, i.e., those which present more negative ΔS values, seem to be compensated by the more favourable enthalpic parameter.
Keywords: Isothermal calorimetry; Interaction of metal ions; Adsorption; Chitosan;

The effect of cesium, potassium, sodium, and lithium cations on the adsorption of natural guar gum onto quartz was investigated. The role of these ions was analyzed in terms of their water structure-making or -breaking capabilities. In the presence of structure makers (Na+, Li+) the polymer adsorption density did not change compared to the adsorption levels observed in distilled water. However, in dilute solutions (0.01 N) of structure-breaking cations (Cs+, K+) the adsorption density of guar gum significantly increased, with potassium and cesium producing the same adsorption densities of the polymer. The resulting colloidal aggregation/dispersion equilibria in the quartz–guar gum system were discussed and mechanisms of guar gum–quartz interactions were also suggested. Assuming hydrogen bonding to be the driving adsorption mechanism, it was proposed that guar gum molecules compete with water for silanol surface sites. Structure-breaking cations disturb the interfacial water structure around the quartz particles thus allowing the polymer to more closely approach the quartz surface and interact with the surface groups.
Keywords: Guar gum; Quartz; Polymer adsorption; Ionic strength; Water structure;

An effective and novel approach to obtaining electrorheological particles with high performance through the formation of host–guest complexes has been achieved. The significant preponderance of the host–guest complex formation is that the host structure can be controlled easily by adding different guests. Based on this point, six supramolecular complexes of β-cyclodextrin cross-linking polymer with salicylic acid (β-CDP-1), 5-chlorosalicylic acid (β-CDP-2), 3,5-dichlorosalicylic acid (β-CDP-3), 5-nitrosalicylic acid (β-CDP-4), 3,5-dinitrosalicylic acid (β-CDP-5), or 3-hydroxy-2-naphthoic acid (β-CDP-6) particles were synthesized. The electrorheological yield stresses of the suspensions of these particles in silicone oil have been investigated under DC electric fields. It was found that the yield stress of the typical β-CDP-1 ER fluid was 5.6 kPa in 4 kV/mm, which is much higher than that of pure β-cyclodextrin polymer (β-CDP), that of pure salicylic acid as well as that of the mixture of the host with the guest. It is clearly indicated that the formation of supramolecular complexes between β-CDP and salicylic acid can enhance the ER properties of the host. The similar results for other supramolecular complexes with different guests have also been obtained under the same DC electric fields. The yield stress of supramolecular complexes is strongly affected by the structure of guests. Among the six investigated guests, 3-hydroxy-2-naphthoic acid gave the highest ER property having a yield stress of 9.8 kPa under 4 kV/mm DC while cross-linked with β-CDP to form β-CDP-6. The yield stress of β-CDP-6 was significantly increased by 72% in comparison with that of the pure β-CDP. However, the yield stress of β-CDP-15 slightly increased by 34–41% as compared with that of the pure β-CDP. The achieved results indicate that the ER effect of host–guest complexes can be greatly affected by the changes of the tremendous guest structure, whereas the slight guest structural transposition, such as altering different groups of a guest, can only obtain the adjacent electrorheological behavior. The dielectric properties of these host–guest complexes also proved that the ER effect can be affected by the properties of guest.
Keywords: Electrorheological fluids; β-Cyclodextrin; Supramolecular complex;

Structural and magnetic properties of Fe and Co nanoparticles embedded in powdered Al2O3 by Otávio Santini; Adriano R. de Moraes; Dante H. Mosca; Paulo E.N. de Souza; Adilson J.A. de Oliveira; Rafael Marangoni; Fernando Wypych (63-70).
The structural and magnetic properties of powdered composites consisting of nanosized Fe and Co particles embedded in alumina grains have been investigated. The composites were synthesized by a novel and simple method using co-precipitation from a hybrid gel solution containing layered double hydroxide and aluminium hydroxide. After a vacuum annealing procedure, the Fe composites have a negligible number of Fe+3 ions and a high concentration of crystalline α-Fe nanoparticles having truncated polyhedron shapes with an average diameter of 20 nm that are physically well separated from each other. Magnetization measurements as a function of temperature revealed a superparamagnetic-like behavior characteristic of an assembly of fine particles. A spurious ferromagnetism related to surface interaction between magnetic particles from different powder grains and the formation mechanism of the composites are also discussed.
Keywords: Layered materials; Nanostructures; Alumina; Iron nanoparticles; Magnetic properties;

Polyacrylonitrile nanoparticles in sizes ranging from approximately 35 to 270 nm were prepared by dispersion/emulsion polymerization of acrylonitrile in a continuous aqueous phase in the presence of potassium persulfate as initiator and various alkyl-sulfate and sulfonate surfactants. The influence of various polymerization parameters (e.g., concentration of monomer and initiator, type and concentration of surfactant, temperature and time of polymerization, ionic strength, pH and co-solvent concentration) on the properties (e.g., size and size distribution, yield, stability, etc.) of the particles has been investigated. The polymerization of acrylonitrile may occur in two major locations: in the aqueous continuous phase (dispersion polymerization) and/or within the surfactant micelles (emulsion polymerization). A discussion concerning the role of these two mechanisms under different conditions, including comparison with previous literature, is also presented. Surface and bulk characterizations of the particles were performed by methods such as transmission and scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, zeta potential, and gravimetric measurements.
Keywords: Nanoparticles; Polyacrylonitrile nanoparticles; Dispersion polymerization; Emulsion polymerization;

Aqueous Laponite dispersions containing a sodium polyacrylate were analyzed, at fixed ionic strength and pH, by rheometric and electroacoustic (for zeta-potential determinations) techniques at 7 days after their preparation. The rheological behavior of these dispersions was determined by oscillatory and flow experiments. Addition of sodium polyacrylate modifies the interactions between Laponite particles and therefore the physical state of the dispersion. The phase diagram of Laponite dispersion as a function of sodium polyacrylate concentration shows different sol–gel transitions for a specific Laponite concentration as a function of the polyacrylate concentration. Under equilibrium flow conditions the Laponite dispersions fit the pseudoplastic Oswald–de Waele power law model. At the same time, these dispersions show thixotropy, which was analyzed using a second-order kinetic equation. The kinetic processes were characterized by breakdown and build-up parameters, which were found to depend on shear rate. This kinetic equation was modified by a power law exponent of viscosity with shear rate that takes into account the viscosity variations when the shear rates are suddenly changed, in order to fit the hysteresis loops.
Keywords: Rheology; Viscoelasticity; Phase diagram; Thixotropy; Kinetic model; Laponite; Sodium polyacrylate;

Photophoresis of an aerosol sphere normal to a plane wall by Huan J. Keh; Fu C. Hsu (94-103).
A combined analytical–numerical study is presented for the quasisteady photophoretic motion of a spherical aerosol particle of arbitrary thermal conductivity and surface properties exposed to a radiative flux perpendicular to a large plane wall. The Knudsen number is assumed to be so small that the fluid flow is described by a continuum model with a temperature jump, a thermal slip, and a frictional slip at the surface of the radiation-absorbing particle. In the limit of small Peclet and Reynolds numbers, the appropriate equations of conservation of energy and momentum for the system are solved using a boundary collocation method and numerical results for the photophoretic velocity of the particle are obtained for various cases. The presence of the neighboring wall causes two basic effects on the particle velocity: first, the local temperature gradient on the particle surface is enhanced or reduced by the wall, thereby speeding up or slowing down the particle; second, the wall increases viscous retardation of the moving particle. The net effect of the wall can decrease or increase the particle velocity, depending upon the relative conductivity and surface properties of the particle as well as the relative particle–wall separation distance. In general, the boundary effect of a plane wall on the photophoresis of an aerosol particle can be quite significant in some situations. In most aerosol systems, the boundary effect on photophoresis is weaker than that on the motion driven by a gravitational field.
Keywords: Photophoresis; Aerosol sphere; Thin Knudsen layer; Boundary effect;

Assessment of the surface areas of silica and clay in acid-leached clay materials using concepts of adsorption on heterogeneous surfaces by J.P. Nguetnkam; R. Kamga; F. Villiéras; G.E. Ekodeck; A. Razafitianamaharavo; J. Yvon (104-115).
Two clays of the areas of Kaélé and Kousseri (extreme North Cameroon) containing mainly smectites and minor amounts of kaolinite were activated with sulfuric acid (1 to 8 N). Crystal-chemical properties were studied using X-ray diffraction, Fourier transform infrared spectroscopy, and chemical analysis, while textural properties were analyzed by step-by-step nitrogen adsorption at 77 K and low-pressure quasi-equilibrium argon adsorption at 77 K. As is generally observed, smectite is more sensitive to acid leaching than kaolinite. As a result of smectite decomposition, amorphous Al-containing silica forms, leading to an increase in the specific surface area of the leached materials. The content of the clay minerals and amorphous silica can be estimated on the basis of changes in the chemical composition of the samples upon acid leaching. As far as adsorption energy distributions derived from low-pressure argon derivative adsorption isotherms are concerned, the main modifications occur when 1 N sulfuric acid is used, due to the replacement of calcium and sodium compensating cations by protons. When higher acid concentrations are used, variations in adsorption energy distribution can be assigned to the presence of amorphous silica. It was possible to model experimental adsorption energy distributions as weighted sums of argon adsorption energy distributions obtained on (i) 1 N samples representing protonated clays and (ii) a silica gel used as a reference aluminous silica. Using such an approach, increasing acid concentration results in an increase in the surface area of silica, whereas the surface area of the remaining clay minerals remains roughly constant.
Keywords: Clay minerals; Silica; Acid leaching; Specific surface area; Surface heterogeneity;

Light extinction at agglomerates of spheres—A practical test on the submicroscale by Uwe Kätzel; Frederic Gruy; Frank Babick; Wolfgang Klöden (116-124).
Today's theories applied to the inversion of measurement data from optical measurement devices are restricted to single spherical particles. However, particles formed in industrial processes such as precipitation and crystallization are often nonspherical or agglomerates. Theoretical approaches to describe the optical behavior of such particle systems have already been proposed. The verification of these theories has mostly been done using microwave scattering experiments with agglomerates in the millimeter range. This paper provides a first but surely not all-embracing practical test for a general extension of the Mie theory to agglomerates of submicroscale spheres. For the sake of simplicity and from practical viewpoints of online-sensor development only light extinction of an agglomerated suspension has been examined. The required rigid agglomerates have been produced using a spray-drying method that generates particles with a much higher mechanical stability than can be obtained by the usual procedures. Subsequent fractionation of the suspension delivers systems with only a limited number of agglomerate configurations. Extinction measurements at multiple wavelengths using dynamic extinction spectroscopy have been conducted to determine the extinction cross section of the agglomerated dispersions. These data are compared with computations of agglomerates scattering.
Keywords: Light extinction; Mie theory; Agglomerates; Dynamic extinction spectroscopy;

A novel method for synthesis of silica nanoparticles by Kota Sreenivasa Rao; Khalil El-Hami; Tsutomu Kodaki; Kazumi Matsushige; Keisuke Makino (125-131).
A sequential method has been used, for the first time, to prepare monodisperse and uniform-size silica nanoparticles using ultrasonication by sol–gel process. The silica particles were obtained by hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol medium and a detailed study was carried out on the effect of different reagents on particle sizes. Various-sized particles in the range 20–460 nm were synthesized. The reagents ammonia (2.8–28 mol L−1), ethanol (1–8 mol L−1), water (3–14 mol L−1), and TEOS (0.012–0.12 mol L−1) were used and particle size was examined under scanning electron microscopy and transmission electron microscopy. In addition to the above observations, the effect of temperature on particle size was studied. The results obtained in the present study are in agreement with the results observed for the electronic absorption behavior of silica particles, which was measured by UV–vis spectrophotometry.
Keywords: Silica nanoparticles; Scanning electron microscopy; UV–vis spectrophotometry;

Identification of superactive centers in thermally treated formamide-intercalated kaolinite by Erzsébet Horváth; János Kristóf; Ray L. Frost; Emma Jakab; Éva Makó; Veronika Vágvölgyi (132-138).
The thermal behavior of a formamide-intercalated mechanochemically activated (dry-ground) kaolinite was investigated by thermogravimetry–mass spectrometry (TG-MS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). After the removal of adsorbed and intercalated formamide, a third type of bonded reagent was identified in the temperature range 230–350 °C decomposing in situ to CO and NH3. The presence of formamide decomposition products, as well as CO2 and various carbonates identified by DRIFT spectroscopy, indicates the formation of superactive centers as a result of mechanochemical activation and heat treatment (thermal deintercalation). The structural variance of surface species decreases with the increase of grinding time. The unground mineral contains a small amount of weakly acidic and basic centers. After 3 h of grinding, the number of acidic centers increases significantly, while on further grinding the superactive centers show increased basicity. With the increase of grinding time and treatment temperature the number of bicarbonate- and bidentate-type structures decreases in favor of the carboxylate- and monodentate-type ones.
Keywords: Kaolinite; Intercalation; Mechanochemical activation; Thermogravimetry–mass spectrometry (TG-MS); Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT);

Measuring and modeling the surface charge of clays, and more especially smectites, has become an important issue in the use of bentonites as a waste confinement material aimed at retarding migration of water and solutes. Therefore, many studies of the acid–base properties of montmorillonite have appeared recently in the literature, following older studies principally devoted to cation exchange. It is striking that beyond the consensus about the complex nature of the surface charge of clays, there are many discrepancies, especially concerning the dissociable charge, that prevents intercomparison among the published data. However, a general trend is observed regarding the absence of common intersection point on raw titration curves at different ionic strengths. Analysis of the literature shows that these discrepancies originate from the experimental procedures for the preparation of the clays and for the quantification of their surface charge. The present work is an attempt to understand how these procedures can impact the final results. Three critical operations can be identified as having significant effects on the surface properties of the studied clays. The first one is the preparation of purified clay from the raw material: the use of acid or chelation treatments, and the repeated washings in deionized water result in partial dissolution of the clays. Then storage of the purified clay in dry or wet conditions strongly influences the equilibria in the subsequent experiments respectively by precipitation or enhanced dissolution. The third critical operation is the quantification of the surface charge by potentiometric titration, which requires the use of strong acids and bases. As a consequence, besides dissociation of surface sites, many secondary titrant consuming reactions were described in the literature, such as cation exchange, dissolution, hydrolysis, or precipitation. The cumulated effects make it difficult to derive proper dissociation constants, and to build adequate models. The inadequation of the classical surface complexation models to describe the acid–base behavior of clays is illustrated by the electrokinetic behavior of smectites, which is independent from the pH and the ionic strength. Therefore, there is still a need on one hand for accurate data recorded in controlled conditions, and on the other hand for new models taking into account the complex nature of the charge of clays.
Keywords: Clay; Montmorillonite; Potentiometric titration; Surface charge; Electrokinetics;

The effects of experimental procedures on the acid–base consumption titration curves of montmorillonite suspension were studied using continuous potentiometric titration. For that purpose, the hysteresis amplitudes between the acid and base branches were found to be useful to systematically evaluate the impacts of storage conditions (wet or dried), the atmosphere in titration reactor, the solid–liquid ratio, the time interval between successive increments, and the ionic strength. In the case of storage conditions, the increase of the hysteresis was significantly higher for longer storage of clay in suspension and drying procedures compared to “fresh” clay suspension. The titration carried out under air demonstrated carbonate contamination that could only be cancelled by performing experiments under inert gas. Interestingly, the increase of the time intervals between successive increments of titrant strongly emphasized the amplitude of hysteresis, which could be correlated with the slow kinetic process specifically observed for acid addition in acid media. Thus, such kinetic behavior is probably associated with dissolution processes of clay particles. However, the resulting curves recorded at different ionic strengths under optimized conditions did not show the common intersection point required to define point of zero charge. Nevertheless, the ionic strength dependence of the point of zero net proton charge suggested that the point of zero charge of sodic montmorillonite could be estimated as lower than 5.
Keywords: Clay; Montmorillonite; Potentiometric titration; Acid–base properties; Point of zero charge;

Composition and structure of iron oxidation surface layers produced in weak acidic solutions by Gonzalo Montes Atenas; Ela Mielczarski; Jerzy A. Mielczarski (157-170).
Although oxidation/passivation of iron in basic solution has been extensively investigated, there is very little information on iron corrosion in weak acidic solutions. In this work, iron surface composition and structure, produced in aerobic aqueous solutions ranging from pH 2 to 5, were determined in detail by the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The most striking observation is that at pH 2 and 3 almost all oxidized iron is dissolved in solution, whereas at pH 4 and 5 the product of iron oxidation is deposited on the iron surface in the form of lepidocrocite, γ-FeOOH. Detailed iron surface and solution analyses allow the proposition of the following overall oxidation reactions: Fe 0 + 2 H + + 1 2 O 2 + 5H 2 O → Fe ( H 2 O ) 6 ( aq ) 2 + , for pH 2 and 3 , Fe 0 + 3 4 O 2 + 1 2 H 2 O → FeOOH ( surf ) , for pH 4 and 5 . At pH 2 and 3, only a very thin surface layer consisting of FeO and Fe(OH)2 with polymeric structure is observed on the iron surface. The amounts of these surface species remain almost constant (2–5 nm) from the first minutes to a few hours of reaction, if pH is kept constant. Nevertheless, with time the akaganeite-like, β-FeOOH structure is also detected. At pH 4 and 5, the amount of lepidocrocite deposited on the iron surface increases with reaction time. Detailed quantitative evaluation of the lepidocrocite produced at pH 5 and its surface distribution on iron was performed based on the comparison of infrared spectroscopic data with spectral simulation results of assumed surface structures. At pH 4 and 5 and a temperature of 40–50 °C, in addition to a very large amount of lepidocrocite other oxy-hydroxide surface species such as goethite (α-FeOOH) and feroxyhite (δ-FeOOH), were identified. Addition of Cl ions to solution at 10−3 M concentration at pH 5 increases the oxidation rate of iron by about 50%, and lepidocrocite remains the only oxidation product. Similarly, an addition of Fe2+ ions to solution at pH 5 very strongly enhances lepidocrocite formation as well as its conductivity. The latter finding is important for the possible application of metallic iron as a catalyst in redox reactions, for example, for decomposition of difficult-to-biodegrade water pollutants.
Keywords: Iron surface oxidation; Spectroscopic studies; Reaction pathways; Surface composition and structure; Acidic pH;

Although the decomposition of water pollutants in the presence of metallic iron is known, the reaction pathways and mechanisms of the decomposition of azo-dyes have been meagerly investigated. The interface phenomena taking place during orange I decomposition have been investigated with the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The studies presented in this paper establish that there are close relationships between the composition and structure of the iron surface oxidized layers and the kinetics and reaction pathway of orange decomposition. The influence of the molecular structure of azo-dye on the produced intermediates was also studied. There are remarkable differences in orange I decomposition between pH 3 and pH 5 at 30 °C. Decomposition at pH 3 is very fast with pseudo-first-order kinetics, whereas at pH 5 the reaction is slower with pseudo-zero-order kinetics. At pH 3, only one amine, namely 1-amino-4-naphthol, was identified as an intermediate that undergoes future decomposition. Sulfanilic acid, the second harmful reduction product, was not found in our studies. At pH 3, the iron surface is covered only by a very thin layer of polymeric Fe(OH)2 mixed with FeO that ensures orange reduction by a combination of an electron transfer reaction and a catalytic hydrogenation reaction. At pH 5, the iron surface is covered up to a few micrometers thick, with a very spongy and porous layer of lepidocrocite enriched in Fe2+ ions, which slows the electron transfer process. The fastest decomposition reaction was found at a potential near −300 mV (standard hydrogen electrode). An addition of Fe2+ ions to solution, iron preoxidation in water, or an increase of temperature all result in an increasing decomposition rate. There is no single surface product that would inhibit the decomposition of orange. This information is crucial to perform efficient, clean and low cost waste water treatment. The findings presented here make the treatment of wastewater in the presence of metallic iron a very promising solution.
Keywords: Water pollutants; Azo-dye; Decomposition pathways; Kinetics; Surface reactions; Acidic solutions;

The inhibiting action of a nonionic surfactant of Tween-20 on the corrosion of cold rolled steel (CRS) in 0.5–7.0 M sulfuric acid (H2SO4) was studied by weight loss and potentiodynamic polarization methods. Atomic force microscope (AFM) provided the surface conditions. The results show that inhibition efficiency increases with the inhibitor concentration, while it decreases with the sulfuric acid concentration. The adsorption of inhibitor on the cold rolled steel surface obeys the Langmuir adsorption isotherm equation. Effect of immersion time was studied and discussed. The effect of temperature on the corrosion behavior of cold rolled steel was also studied at four temperatures ranging from 30 to 60 °C, the thermodynamic parameters such as adsorption heat, adsorption free energy, and adsorption entropy were calculated. The results revealed that the adsorption was physisorption mechanism. A kinetic study of cold rolled steel in uninhibited and inhibited acid was also discussed. The kinetic parameters such as apparent activation energy, pre-exponential factor, rate constant, and reaction constant were calculated for the reactions of corrosion. The inhibition effect is satisfactorily explained by both thermodynamic and kinetic models. Polarization curves show that Tween-20 is a cathodic-type inhibitor in sulfuric acid. The results obtained from weight loss and potentiodynamic polarization are in good agreement, and the Tween-20 inhibition action could also be evidenced by surface AFM images.
Keywords: Tween-20; Corrosion inhibitor; Cold rolled steel; AFM; Sulfuric acid; Adsorption;

Vibration-induced mobilization of trapped oil ganglia in porous media: Experimental validation of a capillary-physics mechanism by Wenqing Li; R. Dennis Vigil; Igor A. Beresnev; Pavel Iassonov; Robert Ewing (193-199).
The development of methods for mobilizing residual organic liquids trapped in porous media is becoming increasingly important as world demand for oil increases and because of the need to remediate aquifers degraded by slow-dissolving organic contaminants. Low-frequency elastic wave stimulation is one such technique, but until recently the lack of a mechanistic understanding of the effects of vibration on mobilization of oil ganglia has prevented the method from being applied predictably in the field. Recently, a simple capillary-physics mechanism has been developed to explain vibration-induced mobilization of a trapped non-wetting organic phase in porous media. Specific predictions that follow from this hypothesized mechanism are that vibrations will be most effective in mobilizing trapped oil when the acceleration amplitude is within an optimal range of values (that depend on the magnitudes of the capillary forces trapping the ganglia and the imposed static pressure gradients) and for sufficiently low vibration frequencies. In this paper we describe two-dimensional glass micromodel experiments that support these predictions.
Keywords: Elastic wave stimulation; Multiphase flow; Porous media; Acoustic stimulation; Enhanced oil recovery; Non-aqueous phase liquids;

Amphiphilic phthalocyanines with one crown ether and three alkyl chain substitutions can form stable monolayers on a water surface. This monolayer can be transferred to a substrate by a vertical dipping method. The arrangement of phthalocyanine molecules in LB films was affected by the length of alkyl chains and the coordination of alkali ions in crown ether. Davydov splitting was observed in the absorption spectra of the LB films of phthalocyanine with the shortest alkyl chain substitutions, and this splitting was affected by the alkali ions in the subphase.
Keywords: Phthalocyanine; Langmuir–Blodgett; Crown ether; Davydov splitting; Polarized UV–vis;

AFM study of mineral wettability with reservoir oils by K. Kumar; E. Dao; K.K. Mohanty (206-217).
Wettability plays a key role in determining fluid distributions and consequently the multiphase flow and transport in petroleum reservoirs. Many crude oils have polar organic components that collect at oil–water interfaces and can adsorb onto the mineral surface if the brine film breaks, rendering the medium oil-wet or mixed-wet. Mica and silica surfaces have been aged with brine and crude oils to induce oil component adsorption. Bulk oil is eventually replaced by water in these experiments by washing with common solvents without ever drying the mineral surface. The organic deposit on the mineral surface is studied by atomic force microscopy in the tapping mode under water. Drying the surface during the removal of bulk oil induces artifacts; it is essential to keep the surface wet at all times before atomic force microscopy or contact angle measurement. As the mean thickness of the organic deposit increases, the oil–water contact angle increases. The organic deposits left behind after extraction of oil by common aromatic solvents used in core studies, such as toluene and decalin, are thinner than those left behind by non-aromatic solvents, such as cyclohexane. The force of adhesion with a probe sphere for minerals aged with just the asphaltene fraction is similar to that of the whole oil. The force of adhesion for the minerals aged with just the resin fraction is the highest of all SARA (saturates, aromatics, resins, and asphaltenes) fractions.
Keywords: Wettability; Atomic force microscopy; Contact angle; Asphaltenes; Oil-wet;

A method to calculate and predict the surface tension of n-alkanes including heptane, eicosane, docosane, tetracosane, and their asymmetric mixtures has been proposed. Reduced coordinates, σ ∗ and T ∗ , where σ ∗ is the reduced surface tension and T ∗ is the reduced temperature, are introduced for the prediction of surface tension. According to the phenomenological scaling and considering the law of corresponding states correlation, these reduced coordinates result in a single curve for the surface tension as a function of temperature. In the correlation the melting temperature, T m , is applied as the corresponding temperature for these substances and their mixtures. The relationship between σ ∗ and T ∗ has a linear form and is expressed by σ ∗ = a + b T ∗ , where a and b are temperature-independent constants. With this relationship the predicted values of surface tension of these substances and their mixtures are in good agreement with experimental ones. %AAD, percent average absolute deviation, for these four pure n-alkanes is 1.02% and for their four asymmetric mixtures is less than 0.70%.
Keywords: Corresponding states correlation; Surface tension; Reduced surface tension; Reduced temperature; n-Alkanes; Asymmetric mixtures;

Electrorheological Kelvin–Helmholtz instability of a fluid sheet by Yusry O. El-Dib; R.T. Matoog (223-241).
The present work deals with the gravitational stability of an electrified Maxwellian fluid sheet shearing under the influence of a vertical periodic electric field. The field produces surface charges on the interfaces of the fluid sheet. Due to the rather complicated nature of the problem a mathematical simplification is considered where the weak effects of viscoelastic fluids are taken into account. The solutions of the linearized equations of motion and boundary conditions lead to two simultaneous Mathieu equations with damping terms and having complex coefficients. Stability criteria are discussed through the assumption of symmetric and anti-symmetric deformations. The disappearance of surface charges from the interfaces obeys a certain relation derived in the marginal state. Furthermore, the case dealing with general deformation is discussed through marginal state analysis. The stability behavior in resonant and nonresonant cases are studied. In addition, the stability picture in the case of absence of the field frequency is studied. The numerical examination for stability showed that the relaxation time ratio plays a destabilizing influence in the case of anti-symmetric deformation or in the general deformation. The stabilizing effect for the relaxation time ratio is saved in the case of general deformation in the presence of the field frequency. In the later case the viscosity, the velocity, and the thickness parameter play a stabilizing influence. A dual role is readied for these parameters in the absence of the field frequency or in the anti-symmetric deformation. The field frequency still plays a destabilizing role in both cases.
Keywords: Electrorheological fluids; Maxwell fluids; Kelvin–Helmholtz instability; Multiple scale method;

Extended time range modeling of myelin growth by Rajiv Taribagil; M.A. Arunagirinathan; C. Manohar; Jayesh R. Bellare (242-248).
When some poorly water-soluble solid surfactants are contacted with water, several microstructures are observed as part of the dissolution process of the surfactants in water. One such microstructure called “myelin,” which is observed when a surfactant like phosphatidylcholine is contacted with water, is the subject of this paper. In this study we have used video microscopy to investigate myelin growth over a wide time range, namely 0.25–700 s, and found that existing models do not correctly express myelin growth over extended time ranges. When studied over a wide time range, the myelin growth was found to evolve over three distinct regimes, namely ballistic, diffusional, and subdiffusional regimes. The underlying growth models are physically explained and mathematically expressed. A relationship is derived between the width of myelin and the growth rate at long times. The estimated width of myelin is consistent with experiments.
Keywords: Myelin; Phosphatidylcholine; Modeling; Ballistic; Diffusional; Subdiffusional;

Silane adsorption onto cellulose fibers: Hydrolysis and condensation reactions by Marie-Christine Brochier Salon; Makki Abdelmouleh; Sami Boufi; Mohamed Naceur Belgacem; Alessandro Gandini (249-261).
The hydrolysis of three alkoxysilane coupling agents, γ-methacryloxypropyltrimethoxysilane (MPS), γ-aminopropyltriethoxysilane (APS), and γ-diethylenetriaminopropyltrimethoxysilane (TAS), was carried out in an ethanol/water (80/20) solution and followed by 1H, 13C, and 29Si NMR spectroscopy, which showed that its rate increased in the order MPS < APS < TAS. The formation of the silanol groups was followed by their self-condensation to generate oligomeric structure. APS and MPS only gave soluble products, whereas colloidal particles precipitated in the medium when TAS was hydrolyzed. Pristine and hydrolyzed MPS were then adsorbed onto a cellulose substrate and thereafter a thermal treatment at 110–120 °C under reduced pressure was applied to the modified fibers to create permanent bonding of the coupling agent at their surface.

We solve the problem of the creeping motion of a uniaxial extensional flow past two touching spherical drops when the line of centers is parallel to the axis of symmetry of the flow, using tangent sphere coordinates. We apply this solution to the case of two equal size drops. It provides an exact result for the equal and opposite force acting on each drop along the line of centers. We also use it to determine the magnitude of the internal recirculating flow in the vicinity of the rear stagnation point, which can be used to evaluate the importance of this flow on the film drainage process for two (nearly) touching drops in a coalescence process for the limiting case, Ca ≪ 1 .
Keywords: Particle fluid mechanics; Force calculation; Extensional flow;

Experiments on the role of gas height in the Rayleigh–Marangoni instability problem by O. Ozen; E. Theisen; D.T. Johnson; P.C. Dauby; R. Narayanan (271-275).
Experimental evidence is provided to show the effect of gas phase dynamics on the onset of thermal convection and on the accompanying patterns in a silicone oil–air convecting bilayer. Very good agreement with three-dimensional calculations for linearized stability is obtained mostly for small and large gas heights. Reasons for this agreement as well as the results at intermediate gas heights are qualitatively explained from the perspective of well-established nonlinear analysis.
Keywords: Interfacial instability; Convection; Marangoni; Bilayers;

We have theoretically investigated the particle orientational distribution and viscosity of a dense colloidal dispersion composed of ferromagnetic spherocylinder particles under an applied magnetic field. The mean field approximation has been applied to take into account the magnetic interactions of the particle of interest with the other ones that belong to the neighboring clusters, besides those that belong to its own cluster. The basic equation of the orientational distribution function, which is an integrodifferential equation, has approximately been solved by Galerkin's method and the method of successive approximation. Some of the main results obtained here are summarized as follows. Even when the magnetic interaction between particles is of the order of the thermal energy, the effect of particle–particle interactions on the orientational distribution comes to appear more significant with increasing volumetric fraction of particles; the orientational distribution function exhibits a sharper peak in the direction nearer to the magnetic field one as the volumetric fraction increases. Such a significant inclination of the particle in the field direction induces the large increase in viscosity in the range of larger values of the volumetric fraction. The above-mentioned characteristics of the orientational distribution and viscosity come to appear more significantly when the influence of the applied magnetic field is not so strong compared with that of magnetic particle–particle interactions.
Keywords: Ferromagnetic colloidal dispersion; Mean field approximation; Simple shear flow; Spherocylinder; Orientational distribution function; Viscosity; Galerkin's method;

Near-wall electrophoretic motion of spherical particles in cylindrical capillaries by Xiangchun Xuan; Chunzhen Ye; Dongqing Li (286-290).
In this paper, near-wall electrophoretic motion of spherical particles in cylindrical capillaries was experimentally investigated. It is demonstrated that same-sized particles move faster in smaller capillaries. This observation verifies the theoretical prediction of the wall enhancing effect in our recent paper (C. Ye, X. Xuan, D. Li, Microfluid. Nanofluid. 1 (2005)). Our experiments show that this wall enhancing effect becomes more significant when the size difference between particle and capillary becomes smaller and when a dilute electrolyte solution (i.e., thicker electric double layer) is used.
Keywords: Electrophoretic motion; Wall effect; Spherical particle; Cylindrical capillary;

General solutions are developed for direct current (DC) and alternating current (AC) electroosmotic flows in microfluidic channels with arbitrary cross-sectional geometry and arbitrary distribution of wall charge (zeta potential). The applied AC electric field can also be of arbitrary waveform. By proposing a nondimensional time scale ϖ defined as the ratio of the diffusion time of momentum across the electric double-layer thickness to the period of the applied electric field, we demonstrate analytically that the Helmholtz–Smoluchowski electroosmotic velocity is an appropriate slip condition for AC electroosmotic flows in typical microfluidic applications. With this slip condition approach, electroosmotic flows in rectangular and asymmetric trapezoidal microchannels with nonuniform wall charge, as examples, are investigated. The unknown constants in the proposed general solutions are numerically determined with a least-squares method through matching the boundary conditions. We find that the wall charge affects significantly the electroosmotic flow while the channel geometry does not. Moreover, the flow feature is characterized by another nondimensional time scale Ω defined as the ratio of the diffusion time of momentum across the channel hydraulic radius to the period of the applied electric field. The onset of phase shift between AC electroosmotic velocity and applied electric field is also examined analytically.
Keywords: Electroosmotic flow; Microchannel; Direct current; Alternating current;