Journal of Colloid And Interface Science (v.286, #2)

A thermodynamic model of gas/solid adsorption has been constructed from two elements. The first is the original Gibbs equation. The second consists of functions ψ ( Θ ) or ψ ( P ) that are calculable from measured isotherms. Based on this model the characteristic adsorption functions (CAFs) were defined and calculated. The CAFs, which concentrate into one function all measured isotherms having the same change in relative free energy of the surface, are very sensitive to the structure of the adsorbents. This statement was tested with nitrogen isotherms measured at 77 K on well-characterized chemically/physically treated activated carbons prepared from poly(ethylene terephthalate). Changes in the surface structure were followed by small angle X-ray scattering (SAXS). This experimental approach made it possible to observe the correspondence between structural changes and the CAF.
Keywords: Characteristic adsorption functions (CAFs); Sensitivity of CAFs to structure of adsorbents; Nitrogen isotherms measured on virgin and treated activated carbons; Surface structure measured by SAXS; Correlations between CAF and SAXS measurements;

Mechanisms and kinetics of trisodium 2-hydroxy-1,1′-azonaphthalene-3,4′,6-trisulfonate adsorption onto chitosan by Tapan Kumar Saha; Subarna Karmaker; Hideki Ichikawa; Yoshinobu Fukumori (433-439).
Chitosan, a naturally abundant biopolymer, has widely been studied for metal adsorption from various solutions, but the extension of chitosan as an adsorbent to remove organic substances from water and wastewater has seldom been explored. In this study, the adsorption of an azo dye, trisodium 2-hydroxy-1,1′-azonaphthalene-3,4′,6-trisulfonate (1), from aqueous solution onto the various degrees of deacetylated chitosan has been investigated. Equilibrium studies have been carried out to determine the capacity of chitosan for dye. The experimental data were analyzed using two isotherm correlations, namely, Langmuir and Freundlich equations. The linear correlation coefficients were determined for each isotherm and the Langmuir provided the best fit. The experimental adsorption isotherms were perfectly reproduced in the simulated data obtained from numerical analysis on the basis of the Langmuir model and the isotherm constants. Adsorption of (1) onto the chitosan flakes was found to be strongly depending on degrees of deacetylation in chitosan and temperatures. Significant amounts of (1) were adsorbed by chitosan 8B (higher degree of deacetylated chitosan), but the adsorption capacity was reduced remarkably with increasing solution temperatures. Thermodynamic parameters such as change in free energy ( Δ G ) , enthalpy ( Δ H ) , and entropy ( Δ S ) were also determined. In addition, kinetic study indicated that the adsorption process mechanisms were both transport- and attachment-limited.
Keywords: Chitosan; Dye adsorption; Mechanisms; Kinetics; Thermodynamics;

Kinetic and thermodynamic studies of boron removal by Siral 5, Siral 40, and Siral 80 by Mürüvvet Yurdakoç; Yoldaş Seki; Senem Karahan; Kadir Yurdakoç (440-446).
In this study, adsorption of boron onto Siral 5, Siral 40, and Siral 80 samples was studied in a batch system with a function of temperature and contact time. The results of adsorption studies indicate that adsorption process was well described with the Freundlich equation and Dubinin–Radushkevich (DR) equation. The Langmuir model does not appear to fit the adsorption better than Freundlich or DR. It has also been found that the magnitudes of free energies are less than 8 kJ mol−1 and within the energy range of physical adsorption. The intraparticle diffusion model and pseudo-first-order and pseudo-second-order kinetic model were tested to find out rate constants of adsorption. From kinetic experiments, it is realized that boron adsorption onto Siral samples obeyed the pseudo-second-order kinetic model. The thermodynamic studies showed that adsorption process was not spontaneous nature and endothermic. It could also be added that the randomness increased during the adsorption.
Keywords: Boron; Kinetic; Thermodynamic parameters; FTIR;

Effect on uptake of heavy metal ions by phosphate grafting of allophane by Kiyoshi Okada; Koji Nishimuta; Yoshikazu Kameshima; Akira Nakajima (447-454).
The effect of phosphate grafting on the uptake of various heavy metal ions was investigated using allophane, a clay mineral having a chemical composition of 1–2SiO2⋅Al2O3⋅5–6H2O with hollow nanoparticles 3.5–5 nm in size. Phosphate grafting was performed on allophane with an atomic ratio Si/Al = 0.85 using the orthophosphates (NH4)2HPO4, Na2HPO4 and CaHPO4 and the triphosphate Na5P3O10. The cations in the phosphates were partially retained in the samples, along with the grafted phosphate. Uptake experiments were performed with various heavy metal ions (Cu2+, Zn2+, Ni2+, Co2+, and Mn2+) and alkaline earth ions (Mg2+ and Ca2+) using a batch method. With increasing amounts of grafted phosphate, the Cu2+ uptake capacity increases to about double of that of ungrafted allophane. Although part of the grafted phosphate is released during the uptake experiments, this can be largely suppressed by heat-treating the samples at 200–500 °C. The Cu2+ uptake ability of the (NH4)2HPO4-grafted sample showed a steep decrease with higher heating temperature according to the thermal decomposition of NH+ 4. The ratios of cations released from the samples to cations removed from solution (2Na/Cu and Ca/Cu) are close to unity. From these results, the replacement reaction is thought to be the main uptake mechanism of heavy metal ions in the present samples. The uptake abilities of the present samples for various cations are explained with respect to the solubilities of the corresponding metal phosphates.
Keywords: Allophane; Phosphate grafting; Heavy metal ions; Uptake mechanism; Aluminosilicate hydrate;

The adsorption of methylene blue and crystal violet on pumice powder samples of varying compositions was investigated using a batch adsorption technique. The effects of various experimental parameters, such as adsorbent dosage, initial dye concentration, and contact time, were also investigated. The extent of dye removal increased with decreased initial concentration of the dye and also increased with increased contact time and amount of adsorbent used. Adsorption data were modeled using the Freundlich adsorption isotherm. The adsorption kinetic of methylene blue and crystal violet could be described by the pseudo-second-order reaction model.
Keywords: Adsorption; Pumice powder; Color removal; Adsorption isotherm;

Analysis of application of Langmuir isotherm to heterogeneous systems: High-pressure conditions by Cristian Nunes Cordeiro; Milton Silva da Rocha; Antônio C. Faleiros; Koshun Iha (459-461).
In this paper, an adsorption model that is based on the statistical mechanics approach was applied to study the saturation phenomena in adsorption to calculate the minimum pressure needed to attain the complete surface coverage on a physical adsorption. The fundamental integral equation, Θ T ( Q ) = ∫ N ( Q ) ⋅ Θ ( Q ) d Q , for the calculation of the coverage degree of the surface was developed for an exponential distribution function, N ( Q ) = ( m / R T ) exp ( − m Q / R T ) , and the representation of local adsorption sites is given by the Langmuir expression, Θ ( Q ) = b 0 C exp ( Q / R T ) / ( 1 + b 0 C exp ( Q / R T ) ) . At high values of the pressure C, a solution of the fundamental integral equation was obtained by imposing the condition b 0 C > 1 . The expression for the saturation condition, b 0 C > m / ( m + 1 ) , was obtained; that is, the saturation phenomenon is dependent on two parameters, correlated with the energetic heterogeneity and adsorption energy of the system. The pressure in the analysis of the cited expression shows that, for low m values (more heterogeneous systems), saturation is attained for b 0 C ≫ m , while for m ≅ 1 (more homogeneous systems), the saturation is attained for b 0 C ≫ 0.5 .
Keywords: Langmuir model; Energetic heterogeneity; Extended Freundlich model;

Adsorption characteristics of carboxylic acids, amines, an octapeptide composed of four l-alanine and four l-aspartic acid residues (Peptide-A4D4), and β-lactoglobulin (β-Lg) on tantalum (Ta), titanium (Ti), and zirconium (Zr) particles were examined at 30 °C and in some case, were compared with their adsorption onto SUS316L stainless steel particles (S6L). The adsorption isotherms on the Ta, Ti, and Zr particles could usually be expressed either by a Langmuir-type equation for reversible adsorption or by a modified Langmuir-type adsorption equation including terms for both reversible and irreversible adsorption. The adsorption equilibrium of benzoic acid, benzylamine, and m-xylylenediamine on all the metal surfaces followed a Langmuir-type equation, while those of phthalic acid, mellitic acid, and Peptide-A4D4 could be fitted to the modified Langmuir-type adsorption equation. The adsorption characteristics of different adsorbates on the different surfaces were discussed particularly with reference to the pH dependencies of the q irrev, q rev, and K values and the electrostatic properties of the oxidized surface of the metal particles. Fourier transform infrared spectroscopic analyses using a reflection/absorption technique (RA-IR) indicated that phthalic acid and mellitic acid are adsorbed in similar adsorption states irrespective of the type of metal. β-Lg was adsorbed onto the surfaces principally in an irreversible manner. The desorption behavior of β-Lg from Ta, Ti, and S6L surfaces was examined, in order to evaluate the extent of interaction between β-Lg and the metals.
Keywords: Adsorption equilibrium; Carboxylic acid; Amine; Protein; Tantalum; Titanium; Zirconium; FTIR;

In this study, the sorption behavior of Zn2+ on calcite, kaolinite, and clinoptilolite, in addition to mixtures of calcite with kaolinite and clinoptilolite, was investigated at various loadings and mixture compositions using atomic absorption spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray powder diffraction, and Fourier transform infrared techniques. According to the obtained results, within the experimental operating conditions, the sorption capacity was enhanced with increasing amount of calcite in both types of mixtures. Under neutral–alkaline pH conditions and high loadings, the order of Zn2+ retention was observed as calcite > clinoptilolite > kaolinite. The experiments on the retention of Zn2+ by pure calcite under conditions of oversaturation showed that the uptake process proceeds via an initial adsorption mechanism (possibly ion-exchange type) followed by a slower mechanism that leads to the overgrowth of the hydrozincite phase, Zn5(OH)6(CO3)2.
Keywords: Zinc; Kaolinite; Clinoptilolite; Calcite; Hydrozincite;

We have investigated the temperature dependence of the intermolecular force between poly(N-isopropylacrylamide) (PNiPAM) grafted surface and bovine serum albumin (BSA) in phosphate buffer (pH 7.4) using atomic force microscopy at the nanonewton scale. These observations show that the interaction force is nearly zero below the phase transition temperature of PNiPAM and that it increases steeply during the phase transition. Since the PNiPAM chains are grafted onto the aminosilane (γ-aminopropyltriethoxysilane)-treated silicon wafer, we measured the force–distance curve of BSA-immobilized tips for the bare and the aminosilane-treated silicon wafer. These surfaces show no temperature dependence and their values are different from those of the PNiPAM-grafted surfaces at 30 °C. The results indicate that the measured adhesion force is between the PNiPAM-grafted surface and the BSA-immobilized tip. Our studies on the intermolecular force between other surfaces (CH3- and COOH-terminated self-assembled monolayers) and the BSA-immobilized tip indicate that the variation in the intermolecular force between the PNiPAM surface and BSA with temperature can be attributed to the changes in the properties of the PNiPAM chains. From consideration of the PNiPAM phase transition mechanism, it is speculated that the intermolecular force between the PNiPAM-grafted surface and BSA would be affected by changes in the arrangement of the bound water molecules around the PNiPAM chain and by changes in the conformation (i.e., in the chain mobility) of the PNiPAM chain during the phase transition.
Keywords: Poly(N-isopropylacrylamide); Bovine serum albumin; AFM; Intermolecular force; Temperature dependence;

In order to clarify the role of picocyanobacteria in aquatic biogeochemical processes (e.g., calcite precipitation), cell surface properties need to be investigated. An experimental study of the cell surface characteristics of two Synechococcus-type unicellular autotrophic picocyanobacterial strains was carried out. One strain was isolated from Lake Plön and contained phycocyanin, the other strain came from Lago Maggiore and was rich in phycoerythrin. Potentiometric titrations were conducted to determine the different types of sites present on the bacteria cell walls. Infrared spectroscopy allowed characterization of the various functional groups (R―NH2, R―COOH, R―OH, R―PO2) and investigations of zeta potential provided insight into the isoelectrical points of the strains. Titrations reveal three distinct sites on the bacterial surfaces of phycocyanin- and phycoerythrin-rich strains with pK values of 4.8 ± 0.3/5.0 ± 0.2, 6.6 ± 0.2/6.7 ± 0.4, and 8.8 ± 0.1/8.7 ± 0.2, corresponding to carboxyl, phosphate, and amine groups with surface densities of 2.6 ± 0.4/7.4 ± 1.6 × 10−4, 1.9 ± 0.5/4.4 ± 0.8 × 10−4, and 2.5 ± 0.4/4.8 ± 0.7 × 10−4 mol/g of dry bacteria. The deprotonation constants are similar to those of bacterial strains and site densities are also within an order of magnitude of other strains. The phycoerythrin-rich strain had a higher number of binding sites than the phycocyanin-rich strain. The results showed that picocyanobacteria may adsorb either calcium cations or carbonate anions and therefore strongly influence the biogeochemical cycling of calcite in pelagic systems.
Keywords: Cyanobacteria; Cell surface; Synechococcus;

BSA adsorption on bimodal PEO brushes by W.T.E. Bosker; P.A. Iakovlev; W. Norde; M.A. Cohen Stuart (496-503).
BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long ( N = 770 ) and short ( N = 48 ) PEO chains and were prepared on PS surfaces, applying mixtures of PS29–PEO48 and PS37–PEO770 block copolymers and using the Langmuir–Blodgett technique. ΠA isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS29–PEO48 show hysteresis between compression and expansion cycles, indicating aggregation of the PS29–PEO48 upon compression. Mixtures of PS29–PEO48 and PS37–PEO770 demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS37–PEO770 to PS29–PEO48 is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO48 and PEO770 brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO770 monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO770 can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible.
Keywords: Bimodal brush; Protein adsorption; PEO; Interaction; Adsorption mechanism; Interfacial pressure isotherm; Block copolymer;

Interactions between dialkyldimethylammonium bromides (DXDAB) and sterols—a monolayer study by Katarzyna Hąc-Wydro; Paweł Wydro; Patrycja Dynarowicz-Łątka (504-510).
Langmuir monolayers of cholesterol/ergosterol and dialkyldimethylammonium bromides (DXDABs) differing in alkyl chain length—14 (DTDAB), 16 (DHDAB), and 18 (DODAB)—spread at the air/water interface are examined. All the systems investigated are found to be nonideal and miscible. Negative values of the total free energy of mixing, proving film stability in the whole range of compositions and surface pressures, are observed for all the studied mixtures except for DTDAB/cholesterol. The strength of interactions, quantified with Δ G Exc values, was found to be of the same order for mixtures of cholesterol/ergosterol and DHDAB or DODAB. Differences occurring for the mixtures of DTDAB with sterols indicate the affinity of DTDAB to ergosterol in contrast to cholesterol.
Keywords: Mixed monolayers; Cholesterol; Ergosterol; Dialkyldimethylammonium bromide; Interactions; Air/water interface;

The growth kinetics of colloidal Al(III)-containing particles ( diameter < 1000  nm ), nucleated in optically clear, supersaturated sodium aluminate solutions as a precursor to Al(OH)3 crystals, has been studied using dynamic light scattering. Two series of solutions were examined at 22 °C to determine the influence of Al(III) supersaturation and NaOH concentration on the initial particle growth behavior. One solution series consisted of solutions with constant Al(III) absolute supersaturation (ΔC) of 1.48 M and [NaOH] range 1.83–4.00 M ( [NaOH]/[Al(III)] = 1.13 – 2.15 ) and Al(III) relative supersaturation ( σ ) = 3.86 – 10.36 . The other solution series had a constant σ of 7.55 and [NaOH] range of 1.50–4.27 M ( [NaOH]/[Al(III)] = 1.18 – 1.54 ) and Δ C = 0.86 – 3.19 . The correlation between the initial particle growth rates and supersaturation (ΔC or σ) revealed marked anomalies over the entire supersaturation range studied. The growth rate remained substantially constant in the ΔC range 0.86–2.55 M (for the constant σ solution series), before increasing sharply upon a further increase of ΔC beyond 2.55 M. The variation of the growth rate with σ in the range 3.86–9.00 (for the constant ΔC solution series) was remarkably weak, contrary to expectation. At higher σ (>9.00), however, a marked increase in growth rate with increasing σ was displayed. At constant ΔC or σ, the growth rate showed a strong variation with NaOH concentration, indicating that Na+ and OH species play a pivotal role in the Al(OH) precursor particles (nuclei) growth process. Furthermore, the kinetics of growth displayed by these nanosized particles are an order of magnitude slower than those observed for macroscopic gibbsite (γ-Al(OH)3) crystals at similar supersaturations and temperature. The difference may be rationalized in terms of particle size and Al(OH)3 dimorphic phase dependent solubility effects. An empirically adequate growth kinetics modeling was achieved when the growth rates were correlated with the Al(III) supersaturation (ΔC or σ) and the excess (free) NaOH concentration, rather than the former alone, as is commonly the case. A critical [NaOH]/[Al(III)] molar ratio of 1.27–1.35, below which the particle growth rate increased markedly and above which the rate was significantly reduced, was observed. This behavior is believed to be linked to solution speciation change that occurs at certain Al(III) and NaOH compositions.
Keywords: Aluminum hydroxide; Crystallization; Nucleation; Growth rate; Supersaturation; Dynamic light scattering;

The effects of water content on the growth rate and the final particle size of AgCl nanoparticles in a reversed micelle (RM) system of polyoxyethylene (6) nonylphenyl ether (NP-6)/water/cyclohexane were investigated using a double-jet technique, in which RM solutions of AgNO3 and KCl were added concurrently to a RM solution containing the excess concentration of chloride ion. As a result, the particle growth rate and the final particle size at a constant Rw ( ≡ [water]/[surfactant]) below 5 were found to be in excellent agreement with our theoretical prediction based on a dynamic Ostwald ripening mechanism governed by the overall solubility of the solid and the diffusivity of the reversed micelles, whereas the final particle size was far beyond the size of the water pool of a reversed micelle. Thus, the dramatic reduction of the particle size in the RM system can be explained by the drastic reduction of the overall solubility of the solid and the small diffusivity of the bulky reversed micelles as a carrier of silver ion, and not by the size of the water pool of a reversed micelle as conventionally explained. Some additional contribution of a coagulation process was also suggested in a high Rw range above 5. Significant coagulation of AgCl particles was observed in a RM system with AOT in place of NP-6 even under the standard conditions for the NP-6 system.
Keywords: Reversed micelles; Microemulsions; Water content; Particle growth; Particle synthesis; Nanoparticles; Monodispersed particles; Silver chloride;

Colloid stability of synthetic titania and the influence of surface roughness by David R.E. Snoswell; Jinming Duan; Daniel Fornasiero; John Ralston (526-535).
The colloid stability of synthetic titania particles was studied as a function of KCl concentration at pH values of 6.3, 6.7, and 8.4, using static light scattering to obtain stability ratios. Standard DLVO theory was then used to calculate the stability ratios as a function of salt concentration. Reasonable agreement between theory and experiment could only be obtained if an effective interaction radius, corresponding to surface asperities on the titania particles, was used in the calculation. High-resolution TEM images suggest that the effective interaction radius corresponds to the size of surface crystallites formed during synthesis.
Keywords: Titania; Surface roughness; Colloid stability; Zeta potential;

To obtain large-sized, monodispersed spherical particles of silica by sol precipitation, a seed particle growth method was attempted. The formation of secondary particles during seed particle growth causing a multimodal distribution of particle size was suppressed via fine adjustment of the reaction conditions, such as TEOS, ammonia, and water concentrations, as well as operational conditions such as feeding time and agitation speed. Among the reaction conditions, an increase of TEOS concentration promoted secondary particle formation, resulting in bimodal particle distribution. However, secondary particle formation was depressed with increasing ammonia and water concentrations. In addition, long feeding time (low feed flow rate) and rigorous agitation significantly reduced secondary particle formation because they contributed to the slow generation of supersaturation and rapid seed particle growth, respectively.
Keywords: Sol precipitation; Silica particles; Seed growth; Secondary particles;

Ceramic nanomaterials from aqueous and 1,2-ethanediol supersaturated solutions at high temperature by Michelangelo Moroni; Daniele Borrini; Luca Calamai; Luigi Dei (543-550).
The aim of this study was the synthesis and physicochemical characterization of some nanosized hydroxides/oxides interesting in the field of ceramic materials with the aim of developing new materials for technological applications in the field of high-performance coatings. In particular, attention was focused on ZrO2nH2O, ZrO2, Zn(OH)2, ZnO, Al2O3nH2O, and Al2O3. The synthesis was carried out in 1,2-ethanediol at 150–160 °C or in water at 90 °C at a high degree of supersaturation in order to achieved nucleation rate much greater than the growth rate. The obtained ZrO2nH2O, Zn(OH)2, and Al2O3nH2O particles were peptized to eliminate possible agglomeration and then calcinated at a suitable temperature to allow the formation of the corresponding anhydrous oxides. The synthesized particles were characterized by scanning and transmission electron microscopy, X-ray diffractometry, and differential thermal analysis coupled with thermogravimetry and Fourier transform infrared spectroscopy. The study showed that all the products can be obtained in the nanostructured crystalline form; ZrO2 presented the interesting feature of different tetragonal/monoclinic ratios depending on the solvent used for the synthesis. Preliminary results on the potentialities of these ZrO2 particles as agents for ultrahard coatings on ceramic surfaces were encouraging and very promising.
Keywords: Ceramic nanomaterials; Polymorphism; Zirconium; Zinc and aluminium oxides; Surface coating;

An oil-in-water emulsion (5 wt% corn oil, 0.5 wt% β-lactoglobulin (β-Lg), 0.1 wt% ι-carrageenan, 5 mM phosphate buffer, pH 6.0) containing anionic droplets stabilized by interfacial membranes comprising of β-lactoglobulin and ι-carrageenan was produced using a two-stage process. A primary emulsion containing anionic β-Lg coated droplets was prepared by homogenizing oil and emulsifier solution together using a high-pressure valve homogenizer. A secondary emulsion containing β-Lg–ι-carrageenan coated droplets was formed by mixing the primary emulsion with an aqueous ι-carrageenan solution. The stability of primary and secondary emulsions to sodium chloride (0–500 mM), calcium chloride (0–12 mM), and thermal processing (30–90 °C) were analyzed using ζ-potential, particle size and creaming stability measurements. The secondary emulsion had better stability to droplet aggregation than the primary emulsion at NaCl ⩽ 500 mM, CaCl2  ⩽ 2 mM, and holding temperatures ⩽60 °C for 20 min. The interfacial engineering technology used in the study could therefore lead to the creation of food emulsions with improved stability to environmental stresses.
Keywords: Emulsion; ι-Carrageenan; β-Lactoglobulin; Flocculation; Creaming;

The adsorptive bubble separation of zinc and cadmium cations from solution in the presence of ferric and aluminum hydroxides was carried out by means of Tween 80 (nonionic surfactant), and sodium laurate and stearate (anionic surfactants). The mechanism of metal removal is different depending on the nature of the surfactant used. The removal of zinc cations by adsorbing colloid flotation is higher than that of cadmium cations. It increases with increases in the amount of hydroxide precipitate and the concentration of Tween 80. The removal of zinc cations by ion flotation is lower than that of cadmium cations. It does not change with increases in the hydroxide amount. It increases, however, with increased sodium laurate or stearate concentration. Both separation methods turned out to be helpful for studying both the solution's structure and the interactions at the solution–solid interface.
Keywords: Adsorbing colloid flotation; Ion flotation; Zn2+; Cd2+; Tween 80; Sodium soaps; Ferric hydroxide; Aluminum hydroxide;

Flocculation, deflocculation, and ions migration in latex suspensions by D.Z. Gunes; J.P. Munch; M. Dorget; A. Knaebel; F. Lequeux (564-572).
We have studied the dynamics of the flocculation of poly(styrene–butadiene–acrylic acid) latex suspensions. These suspensions were flocculated by the addition of Ca2+ ions at high concentrations of latex particles. Using diffusing wave spectroscopy and dynamic single light scattering after dilution, we have observed—depending on the pH and on the Ca2+ concentration—several scenarios for flocculation including successive flocculation and deflocculation. This complex behavior reveals that the Ca2+ migration within the shell of the latex is slow in acidic solvent but fast in basic solvent.
Keywords: Latex; Suspension; Concentrated; Flocculation; Ions migration; Dynamic diagram; Diffusing-wave spectroscopy; Core–shell;

A simple self-assembly method for colloidal photonic crystals with a large area by Jianguo Deng; Xiaoming Tao; Pei Li; Pu Xue; Yihe Zhang; Xiaohong Sun; Kai Cheong Kwan (573-578).
Colloidal photonic crystals were fabricated using polystyrene particles (180 nm) and PMMA particles (450 nm), respectively, with a new and simple self-assembly method without special equipment. SEM images indicate that the prepared samples have ordered structures with few defects. The position of the stop-band scale nicely agrees with the particles' size. The sintering process of the PS photonic crystal film was studied with AFM heating system.
Keywords: Photonic crystal; Stop band; Colloid particle;

Electrical conductivity and stability of concentrated aqueous alumina suspensions by Robinson C.D. Cruz; Jörg Reinshagen; Rainer Oberacker; Ana M. Segadães; Michael J. Hoffmann (579-588).
This work describes the effect of solids load and ionic strength on the electrical conductivity ( K S ) of concentrated aqueous suspensions of commercial α-alumina (1–35 vol% solids). The results obtained show that the dependency of the electrical conductivity of the suspending liquid ( K L ) on the volume fraction of solids is well described by Maxwell's model. The change in the conductivity of the suspensions relative to that of the suspending liquid ( K S / K L ) was found to be inversely proportional to the solids content, as predicted by Maxwell's model. The relative conductivity rate, ΔK, could be interpreted in terms of the DLVO theory and the particles double layer parameter, κa, and used as a stability criterion. As κa changes, in response to the changes in ionic strength, so does the conducting to insulating character of the particles and, as such, their contribution to the overall suspension conductivity (expressed by ΔK). When the particles become insulating, the suspension conductivity decreases when the solids load increases. The turning point in this particle behaviour corresponds to a critical concentration of ions in the solution that destabilises the suspension and is associated with the critical coagulation concentration (ccc). It is the electrical double layer that ultimately determines the conducting or insulating character of the particles, and that character can be made to change, as required for suspension stability, and accessed by the relative conductivity rate.
Keywords: Stability of suspensions; Nondestructive evaluation; Electrical conductivity; Alumina;

Fabrication of photosensitive multilayer films based on polyoxometalate and diazoresin by Yuhua Feng; Jun Peng; Zhangang Han; Huiyuan Ma (589-595).
A novel photosensitive organic–inorganic composite film incorporating polyoxometalate, K7[SiW11O39Co(H3P2O7)] (SiW11CoPP), and diazoresin (DR) has been prepared via layer-by-layer (LBL) self-assembly. Under UV irradiation, followed the decomposition of diazonium in DR, the ionic bonds between the adjacent interfaces of the multilayer film convert to covalent bonds. The LBL multilayers were characterized by UV–vis spectroscopy, X-ray photoelectron spectra (XPS), atomic force microscopy (AFM), FTIR spectrum, cyclic voltammograms (CV), and electron spin resonance (ESR) measurements. UV spectroscopy shows that the deposition process is regular and highly reproducible from layer to layer. XPS spectra confirm the incorporation of DR and SiW11CoPP into the films. Atomic force microscopy image indicates that the film surface is uniform and smooth. Solvent etching experiment proves that the film has significant stability towards polar solvent. Electrochemical behavior of the multilayers is investigated.
Keywords: Layer-by-layer self-assembly; Ultrathin multilayer films; Polyoxometalate; Diazoresin; Photosensitive;

Adsorption of several metal ions onto a model soil sample: Equilibrium and EPR studies by Karine Flogeac; Emmanuel Guillon; Michel Aplincourt (596-601).
Soils play an important role in the control of metallic cations in the environment. Therefore, knowledge of the adsorption properties of soil is crucial in understanding and solving pollution problems. Adsorption isotherms provide a macroscopic view of the retention phenomena. The aim of this paper is to study iron, manganese, and chromium adsorption onto a soil sample as a function of the reaction time, pH, and metal concentration. The adsorption isotherms allow the determination of the affinity order of metals for the surface of the soil sample as such: Fe3+  > Cr3+  > Mn2+. The equilibrium data fit well with the Langmuir and Freundlich models and confirm the affinity order of the soil sample for these metals. These adsorption data are combined with EPR spectroscopy to obtain structural information about the surface complexes formed. Iron is held in inner-sphere complexes. Manganese is simultaneously held in outer- and inner-sphere complexes. Due to poor resolution, chromium was not detected by EPR and thus it is impossible to infer coordination sphere and coordination number. Iron and manganese are in an octahedral environment.
Keywords: Soil; Iron; Manganese; Chromium; Adsorption isotherms; EPR;

Gold–silver binary nanoparticles, which feed atomic ratios of gold to silver were 3:1, 1:1, and 1:3, were prepared. These particles were stabilized by amine-terminated (generation (G) 3.0 and 5.0) and carboxyl-terminated (G 3.5 and 5.5) poly(amidoamine) (PAMAM) dendrimers in water. UV–vis spectra indicate that the particles are not mere physical mixtures of monometallic particles or core/shell type but alloy. According to transmission electron microscope (TEM) observation, the mean diameters of the particles were 7–10 nm for silver particles and 3–4 nm for both gold and alloy particles, respectively. Catalytic activities for reduction of p-nitrophenol were investigated by monitoring the absorbance at 400 nm during the reaction. They were proportional to the feed ratio of gold in the particles and showed a maximum at the ratio of Au:Ag = 3:1.
Keywords: PAMAM dendrimers; Hydrolyzed PAMAM dendrimers; Dendrimer gold–silver alloy nanocomposite; Catalytic reduction of p-nitrophenol;

Surface potential at the hematite–water interface by N. Kallay; Z. Dojnović; A. Čop (610-614).
Construction of a metal oxide electrode enabling measurement of surface potential is described. The electrode was made using a hematite monocrystal, which avoids the problems arising from the possible porosity of the oxide layer. The potential of this electrode was measured as a function of pH. The hematite electrode provides reproducible results, especially in the acidic region. Surface potentials were calculated from electrode potentials using the electrokinetic isoelectric point. The slope of surface potential with respect to pH was found to be lower than the Nernstian, especially in the basic region. The effect was more pronounced at higher ionic strengths. It was shown how the measurement of surface potential can help to interpret the equilibrium data and evaluate the choice of a theoretical model describing the interfacial equilibrium at the metal oxide–water interface.
Keywords: Surface potential; Hematite electrode; Hematite–water interface;

Influence of relative humidity on electrical properties of α-Al2O3 powders: Resistivity and electrochemical impedance spectroscopy by François Favre; Frédéric Villieras; Yann Duval; Edward McRae; Christophe Rapin (615-620).
The influence of humidity on the electrical properties of α-Al2O3 powders has been investigated using adsorption isotherms, DC resistivity, and electrochemical impedance spectroscopy. Samples of two α-Al2O3 particle sizes were examined, both individually and mixed together. The results show that the grain-bed resistivity decreases with humidity, whereas the grain capacitance is almost constant. The resistivity difference between the two particle sizes is of several orders of magnitude, while the capacitance values are not very different. These results are interpreted in terms of the layer-by-layer growth of water adsorbed on the grain surfaces. The first, more tightly bound adsorbed layer does not provoke the same effects as those layers adsorbed at higher relative humidity.
Keywords: α-Al2O3; Impedance spectroscopy; Electrical resistivity; Porous media;

Photocatalytic degradation and adsorption of 2-naphthol on suspended TiO2 surface in a dynamic reactor by Samir Qourzal; Malika Tamimi; Ali Assabbane; Yhya Ait-Ichou (621-626).
The photocatalytic oxidation of 2-naphthol has been investigated at room temperature in a dynamic photoreactor with system UV/O2 (air) and aqueous suspension of titanium dioxide TiO2 irradiated under a variety of conditions. The kinetics of disappearance of pollutant were affected by several operating parameters such as TiO2 mass, concentration of the substrate and reaction pH. The experiments were measured by high performance liquid chromatography. A Langmuir–Hinshelwood model was found to be accurate for photocatalytic degradation and indicates that adsorption of the solute on the surface of semiconductor particles plays a role in photocatalytic reaction.
Keywords: Dynamic photoreactor; Photocatalysis; Titanium dioxide; 2-Naphthol; Adsorption;

Ordered hexagonal and cubic mesoporous tin oxides were synthesized for the first time in the presence of mixed cationic and neutral surfactants (a mixture of cetyltrimethylammonium bromide cationic surfactant and dodecylamine neutral surfactant) with different alkali and simple inorganic precursors at room temperature. In the synthesis systems, the dodecylamine neutral surfactant may function as a polar organic cosolvent and cosurfactant. The formation of the tin oxide mesostructured material was proposed to be due to the presence of hydrogen-bonding interactions between the supramolecular template and inorganic precursors Sn4+ and OH, which were assumed to self-assemble around the cationic surfactant molecules. The materials are characterized by X-ray powder diffraction, transmission electron microscopy, thermogravimetric analysis, and N2 adsorption/desorption isotherm. The surface areas of materials evaluated from the N2 sorption isotherms are about 248 m2/g for hexagonal mesoporous tin oxide (Sn―H) and 281 m2/g for cubic mesoporous tin oxide (Sn–C) for calcination at 350 °C.
Keywords: Mesoporous tin oxide; Mixed surfactants; Hexagonal; Cubic; Inorganic precursor; Chemical self-assembly;

Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM by K. Boussu; B. Van der Bruggen; A. Volodin; J. Snauwaert; C. Van Haesendonck; C. Vandecasteele (632-638).
Determination of the surface roughness by AFM is crucial to the study of particle fouling in nanofiltration. It is, however, very difficult to compare the different roughness values reported in the literature because of a lack in uniformity in the methods applied to determine surface roughness. AFM is used in both noncontact mode and tapping mode; moreover, the size of the scan area is highly variable. This study compares, for six different nanofiltration membranes (UTC-20, N30F, Desal 51HL, Desal 5DL, NTR7450, NF-PES-10), noncontact mode AFM with tapping mode AFM for several sizes of the scan area. Although the absolute roughness values are different for noncontact AFM and tapping mode AFM, no difference is found between the two modes of AFM in ranking the nanofiltration membranes with respect to their surface roughness. NTR 7450 and NF-PES-10 are the smoothest membranes, while the roughest surface can be found with Desal 51HL and Desal 5DL. UTC-20 and N30F are characterized by an intermediate roughness value. An increase in roughness with increasing scan area is observed for both AFM modes. Larger differences between the roughnesses of the membranes are obtained with tapping mode AFM because of the tapping of the tip on the surface. Phase imaging is an extension of tapping mode AFM, measuring the phase shift between the cantilever oscillation and the oscillation of the piezo driver. This phase shift reflects the interaction between the cantilever and the membrane surface. A comparison with contact angle measurements proves that a small phase shift corresponds to a large contact angle, representing a hydrophobic membrane surface.
Keywords: Nanofiltration; Tapping mode AFM; Noncontact AFM; Phase imaging; Contact angle; Surface roughness;

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The objectives of the present work were to investigate the effect of the composition in calcium and carbonate of a model solution to be treated by conventional electrodialysis on their migration kinetics and the composition in cations of the membrane fouling. In the absence of sodium carbonate in the solution, no fouling was visually observed on anion-exchange membranes (AEM) and fouling was observed only at 1600 mg/L CaCl2 on cation-exchange membrane (CEM), while at only 800 mg/L CaCl2 with sodium carbonate, a deposit was observed on both membranes. This difference could be explained by the fact that carbonate has a high buffer capacity, and the time to reach pH 4.0 was then longer than the one without carbonate. Consequently, the migration of the ionic species was carried out over a longer period of time during ED treatment with sodium carbonate addition and in extent the demineralization rates were higher: 43 vs 86%. For treatment with sodium carbonate and 1600 mg/L CaCl2, the higher migration during ED treatment, increased the concentration of calcium, from 14.24 to 93.38 mg/g dry membrane and from 0.74 to 10.27 mg/g dry membrane for CEM and AEM, respectively. Due to the basic pH on the side of the membrane in contact with the NaCl solution, the calcium would precipitate to form calcium hydroxide on CEM while the calcium migrated through the CEM was blocked by the AEM where it formed another fouling.
Keywords: Electrodialysis; Ionic membrane; Fouling; Calcium; Cation kinetic;

A finite element based algorithm for determining interfacial tension (γ) from pendant drop profiles by Nicole M. Dingle; Kristianto Tjiptowidjojo; Osman A. Basaran; Michael T. Harris (647-660).
This paper introduces a robust algorithm to determine the interfacial tension (γ) from pendant drop profiles using the Galerkin finite element method (γ-PD-FEM) to solve the axisymmetric form of the Young–Laplace (YL) equation. In this algorithm, the theoretical profiles are generated by solving the spherical coordinate form of the YL equation. γ-PD-FEM also solves for the parameter estimates by minimizing the difference between the theoretical and experimental surface functions, f ( θ ) . This technique is compared to the widely used method of converting the YL equation to the three arc length-based (ALB) first-order ODEs developed by Bashforth and Adams (BA) in 1883, or as denoted in this paper, the γ-PD-BA method. The drop apex is the initial condition for the γ-PD-BA algorithm and the integration is terminated at a specified location along the drop profile. In contrast to techniques based on the BA approach, computation of the theoretical drop profile in γ-PD-FEM is obtained from a second-order ordinary differential equation and requires boundary conditions at the drop apex and at the contact line of the drop to the nozzle. By incorporating both boundary conditions into the problem formulation, the algorithm can also determine if the drop shape is at static equilibrium. Results to be presented include an outline of the computer algorithm, and comparison of γ values obtained from the γ-PD-FEM and the traditional γ-PD-BA method using simulated and experimental drop profile data sets.
Keywords: Surface tension; Interfacial tension; Pendant drop; Finite element method;

Impact of droplets onto inclined surfaces by Š. Šikalo; C. Tropea; E.N. Ganić (661-669).
Drop impacts onto dry walls and liquid films at low impact angles and low normal Weber numbers are experimentally investigated. Measurements were performed using a high spatial resolution CCD camera and short exposure times, yielding both qualitative and quantitative information about the impact. Whereas a droplet generally deposits on the surface for high impact angles, a rebound can occur at lower angles and for smooth or wetted surfaces. No rebound is observed for rough surfaces. A low viscous liquid (water) will either rebound or deposit on smooth or wetted surfaces. A high viscous liquid (glycerin) may also disjoin into two droplets, depending on the impact angle. A correlation is presented for the size of the secondary droplet. A further correlation quantifies the critical impact angle at which rebounding first occurs in terms of the normal Weber number.
Keywords: Drop impact; Droplet rebound; Partial rebound;

The pendant and sessile drop profile analysis using the finite element method (PSDA-FEM) is an algorithm which allows simultaneous determination of the interfacial tension (γ) and contact angle ( θ c ) from sessile drop profiles. The PSDA-FEM algorithm solves the nonlinear second-order spherical coordinate form of the Young–Laplace equation. Thus, the boundary conditions at the drop apex and contact position of the drop with the substrate are required to solve for the drop profile coordinates. The boundary condition at the position where the drop contacts the substrate may be specified as a fixed contact line or fixed contact angle. This paper will focus on the fixed contact angle boundary condition for sessile drops on a substrate and how this boundary condition is used in the PSDA-FEM curve-fitting algorithm. The PSDA-FEM algorithm has been tested using simulated drop shapes with and without the addition of random error to the drop profile coordinates. The random error is varied to simulate the effect of camera resolution on the estimates of γ and θ c values obtained from the curve-fitting algorithm. The error in the experimental values for γ from sessile drops of water on acrylic and Mazola corn oil on acrylic falls within the predicted range of errors obtained for γ values from simulated sessile drop profiles with randomized errors that are comparable in magnitude to the resolution of the experimental setup.
Keywords: Sessile drop; Interfacial tension; Contact angle; Finite element method;

Microscopic treatment of a barrel drop on fibers and nanofibers by Gersh O. Berim; Eli Ruckenstein (681-695).
The microscopic approach of Berim and Ruckenstein (J. Phys. Chem. B 108 (2004) 19330, 19339) regarding the shape and stability of a liquid drop on a planar bare solid surface is extended to a liquid barrel drop on the bare surface of a solid cylinder (fiber) of arbitrary radius. Assuming the interaction potentials of the liquid molecules between themselves and with the molecules of the solid of the London–van der Waals form, the potential energy of a liquid molecule with an infinitely long fiber was calculated analytically. A differential equation for the drop profile was derived by the variational minimization of the total potential energy of the drop by taking into account the structuring of the liquid near the fiber. This equation was solved in quadrature and the shape and stability of the barrel drop were analyzed as functions of the radius of the fiber and the microscopic contact angle θ 0 which the drop profile makes with the surface of the fiber. The latter angle is dependent on the fiber radius and on the microscopic parameters of the model (strength of the intermolecular interactions, densities of the liquid and solid phases, hard core radii, etc.). Expressions for the evaluation of the microcontact angle from experimentally measurable characteristics of the drop profile (height, length, volume, location of inflection point) are obtained. All drop characteristics, such as stability, shape, are functions of θ 0 and a certain parameter a which depends on the model parameters. In particular, the range of drop stability consists of three domains in the plane θ 0 –a, separated by two critical curves a = a c ( θ 0 ) and a = a c 1 ( θ 0 ) [ a c ( θ 0 ) < a c 1 ( θ 0 ) ]. In the first domain, below the curve a = a c ( θ 0 ) , the drop can have any height at its apex. In the second and third domains the values of h m are limited; in the second domain (above the curve a = a c 1 ( θ 0 ) ) there is an upper limit of h, h m 1 , and the drops with h m > h m 1 cannot exist, whereas in the third domain (between those curves) the drop can have values of h m either smaller than h m 1 or larger than h m 2 , where h m 2 > h m 1 is a second critical height. For sufficiently large fiber radii, R f ⩾ 1   μm , the critical curves almost coincide and only two domains, the first and the second, remain. The smaller the radius, the larger is the difference between the critical curves and the larger is the second domain of drop stability. The shape of the drop depends on whether the point ( θ 0 , a ) on the θ 0 –a plane is far from the critical curve or near it. In the first case the drop profile has generally a large circular part, while in the second case the shape is either almost planar or contains a long manchon that is similar to a film on the fiber.
Keywords: Contact angle; Barrel drop; Fiber; Microscopic theory;

Co-solvent effects on drag reduction, rheological properties and micelle microstructures of cationic surfactants by Ying Zhang; Judith Schmidt; Yeshayahu Talmon; Jacques L. Zakin (696-709).
Some quaternary cationic surfactants, when mixed with a counterion, are known to self-assemble into threadlike micelles in water. Such behavior causes drastic changes in rheological properties of even very dilute solutions, allowing them to be used as drag reducing agents (DRA) in turbulent pipe flow circulating systems, such as district cooling/heating systems. Surfactant self-assembly is a physicochemical phenomenon whose character depends on surfactant nature and concentration, nature of the solvent, temperature and type and concentration of counterions. This study investigates drag reduction (DR) and rheological properties of two cationic surfactants, Ethoquad O/12 (oleyl bis(hydroxyethyl)methylammonium chloride) and Ethoquad O/13 (oleyl tris(hydroxyethyl) ammonium acetate), with excess salicylate counterion (NaSal), in mixed solvents containing 0 to 28 wt% ethylene glycol (EG) and water. The addition of EG to the solvent had greater effects on solutions' DR ability, shear viscosity, apparent extensional viscosity and viscoelasticity at 25 °C than at ∼0 °C. Cryo-TEM images show threadlike micelle in these systems. DR at low temperatures in solutions containing moderate amount of EG can be utilized in a new approach to energy saving in district cooling systems using EG–water based mixtures as the cooling fluids.
Keywords: Drag reduction; Co-solvent; Cationic surfactants; Rheological properties; Threadlike micelles; Cryo-TEM;

Quantitative comparison of foam films and the corresponding foams is very demanding. One problem is the fact that investigations of foam films are usually performed at constant capillary pressures P, whereas in foams P is a function of the height of the foam column. A way out of this dilemma is to examine films and foams at the same P. The method of choice for the foam films is the thin film pressure balance (TFPB), whereas the corresponding investigation of foams is based on the foam pressure drop technique (FPDT). An extensive TFPB study on foam films stabilized by the cationic alkyltrimethylammonium bromides C n TAB with n = 10 , 12, 14, and 16 was performed by Bergeron. For this series a steep increase of the foam film stability was observed when the chain length was increased from n = 12 to n = 14 . Moreover, the influence of impurities was found to be limited to the films stabilized by C12TAB. In order to study the correlation between the properties of films and foams, the present study deals with the respective foam properties investigated with the FPDT. It was found that both the steep increase in the film stability and the influence of impurities are also reflected in the properties of the foam.
Keywords: Foams; Foam stability; Foam films; Foam pressure drop technique; Disjoining pressure; Capillary pressure;

Marangoni effects in aqueous polypropylene glycol foams by Su Nee Tan; Daniel Fornasiero; Rossen Sedev; John Ralston (719-729).
The foam behavior of three polypropylene glycols covering the molecular weight range between 192 and 725 g/mol has been examined. Static and dynamic surface tension data, as well as bubble size distribution and retention time in the foam, were incorporated into a simple model of foam stability. The latter clearly indicates that surface tension differences between the plateau border and lamellar region adjacent to the bubble surface are the dominant factor in controlling foamability, causing liquid flow in the direction opposite to liquid drainage, a process termed the Marangoni effect.
Keywords: Foams; Dynamic surface tension; Foamability; Marangoni effect; Bubble size; Surface tension difference;

Oil coating of hydrophobic surfaces from aqueous media: Formation and kinetic study by Wafa Essafi; Kenneth Wong; Jérôme Bibette; Philippe Poulin (730-738).
We perform oil coating of hydrophobic solid surfaces via aqueous media, from emulsions, and under the presence of a shear flow. The principle of such coating is based on the use of a system at the limit of aggregation to give rise to adhesion, with asymmetrical interfaces (oil droplet/water and solid surface/water) in order to favor the oil/surface adhesion in comparison to the oil/oil adhesion. This way, droplets stick to the solid substrate, whereas they are stable and homogeneously dispersed in the bulk. We have realized coatings from two systems of emulsions made of a mixture of hydroxy-terminated silicone oil and classical silicone oil and a mixture of sunflower oil and mineral oil. The kinetics of the coating is described by a Langmuir model where the adhesion between the oil particle and the surface is modeled as a first-order reaction. The resulting coatings are formed of oil droplets uniformly covering the solid surface. The coating density can vary with the nature of the experimental systems.
Keywords: Emulsion; Coating; Hydrophobic surface; Flow;

The mixed micelles of cationic gemini surfactants C12C S C12Br2 ( S = 3 , 6, and 12) with the nonionic surfactant Triton X-100 (TX100) have been studied by steady-state fluorescence, time-resolved fluorescence quenching, electrophoretic light scattering, and electron spin resonance. Both the surfactant composition and the spacer length are found to influence the properties of mixed micelles markedly. The total aggregation number of alkyl chains per micelle ( N T ) goes through a minimum at X TX 100 = 0.8 . Meanwhile, the micropolarity of the mixed micelles decreases with increasing X TX 100 , while the microviscosity increases. The presence of minimum in N T is explained in terms of the competition of the reduction of electrostatic repulsion between headgroups of cationic gemini surfactant with the enhancement of steric repulsion between hydrophilic headgroups of TX100 caused by the addition of TX100. The variations of micropolarity and microviscosity indicate that the incorporation of TX100 to the gemini surfactants leads to a more compact and hydrophobic micellar structure. Moreover, for the C12C3C12Br2/TX100 mixed micelle containing C12C3C12Br2 with a shorter spacer, the more pronounced decrease of N T at X TX 100 lower than 0.8 may be attributed to the larger steric repulsion between headgroups of TX100. Meanwhile, the increase of microviscosity and the decrease of micropolarity are more marked for the C12C12C12Br2/TX100 mixed micelle, owing to the looped conformation of the longer spacer of C12C12C12Br2.
Keywords: Mixed micelles; Gemini surfactants; TX100; Surfactant composition; Spacer length;

Effect of glycerol on micelle formation by ionic and nonionic surfactants at 25 °C by Gerardino D'Errico; Donato Ciccarelli; Ornella Ortona (747-754).
The effect of glycerol on the micellization of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and of the ethoxylated nonionic surfactant Brij 58 has been investigated by various experimental techniques. For both surfactants the critical micellar concentration (cmc), determined by surface tension measurements, is almost unaffected by the presence of glycerol in the mixture; only at high glycerol concentrations ( ⩾ 20 %  w/w ) does the cmc significantly increase. The area per surfactant molecule at the air–solution interface, A, increases with increasing glycerol weight percentage, w g . Fluorescence quenching measurements indicate that the presence of glycerol induces a lowering of the aggregation number of both surfactants. The glycerol intradiffusion coefficient has been measured by the pulsed-gradient spin–echo NMR technique as a function of glycerol content at constant surfactant concentration. It is almost unaffected by the presence of the surfactants, indicating that no direct glycerol–surfactant interaction occurs in the mixture. The surfactant intradiffusion coefficient has been also measured. In the case of CTAB, it increases with increasing glycerol concentration, a reflection of the decreased aggregation number. For Brij 58, in spite of the lowering of the aggregation number, the surfactant intradiffusion coefficient decreases with increasing glycerol concentration, suggesting an increase of the intermicellar interaction. The experimental evidence shows that for both surfactants the micellization is affected by the presence of glycerol through an indirect, solvent-mediated mechanism. In the case of CTAB, the main effect of glycerol is a lowering of the medium dielectric constant, which enhances the electrostatic interactions in solution. In the case of Brij 58, the results can be interpreted in terms of a salting-out effect according to which glycerol competes with the surfactant for water molecules, causing a dehydration of the surfactant ethoxylic headgroup.
Keywords: Micellization; Glycerol; Surface tension; Fluorescence quenching; Intradiffusion;

Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution by Nan Jiang; Peixun Li; Yilin Wang; Jinben Wang; Haike Yan; Robert K. Thomas (755-760).
Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F, Cl, Br, NO 3, and ½SO2− 4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization ( α ) , and enthalpy of micellization ( Δ H mic ) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO2− 4  < NO 3  < Br  < Cl  < F, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. Δ H mic becomes less negative through the sequence NO 3  < Br  < Cl  < F  < SO2− 4, and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization ( Δ G mic ) and entropies of micellization ( Δ S mic ) have been calculated from the values of Δ H mic , CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in Δ H mic and Δ S mic have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.
Keywords: Hexadecyltrimethylammonium; Micelles; Counterion effect;

Electrokinetic phenomena at grafted polyelectrolyte layers by Stanislav S. Dukhin; Ralf Zimmermann; Carsten Werner (761-773).
During the last decades the electrokinetic theory of Smoluchowski (Z. Phys. Chem. 92 (1918) 129) was extended to be applicable for soft surfaces (grafted polyelectrolyte layers (PL), biological and artificial membranes, etc.) by either using the Debye approximation or numerical solutions. In the theory of Ohshima (Colloids Surf. A 103 (1995) 249) the nonlinearized Poisson–Boltzmann (PB) equation for thick and uniform PL is solved analytically and a general hydrodynamic equation is derived in an integral form. These advantages in the theory of Ohshima provided a base for the further development of a generalized electrokinetic theory for soft surfaces. In his theory the final equation for the electroosmotic (electrophoretic) velocity is specified for the case of the complete dissociation of ionic sites within PL. Accordingly, the equation may be used only if the difference between pK and pH is very large. However, it turned out that an analytical solution of the nonlinearized PB equation for thick PL is possible for any degree of dissociation. This was achieved using the approximation of excluded coions if the absolute value of the reduced Donnan potential is larger than 2 and due to the simplification in the case of weak dissociation, when the absolute value of the reduced Donnan potential is less than 2. Combining this generalized double layer (DL) theory for PL and the theory of Ohshima enables to obtain an analytical equation for electroosmosis for the general case of any degree of dissociation. This equation creates for the first time a theoretical base for the interpretation of electrokinetic fingerprinting (EF) for the characterization of soft surfaces.
Keywords: Soft surfaces; Electrokinetics; Grafted polyelectrolyte layers; Apparent zeta potential; Donnan potential; Surface potential; Fractional polyelectrolyte charge; Electrokinetic fingerprint;

The diffusiophoretic and electrophoretic motions of a dielectric spherical particle in an electrolyte solution located between two infinite parallel plane walls are studied theoretically. The imposed electrolyte concentration gradient or electric field is constant and parallel to the two plates, which may be either impermeable to the ions/charges or prescribed with the far-field concentration/potential distribution. The electrical double layer at the particle surface is assumed to be thin relative to the particle radius and to the particle–wall gap widths, but the polarization effect of the mobile ions in the diffuse layer is incorporated. The presence of the neighboring walls causes two basic effects on the particle velocity: first, the local electrolyte concentration gradient or electric field on the particle surface is enhanced or reduced by the walls, thereby speeding up or slowing down the particle; second, the walls increase the viscous retardation of the moving particle. To solve the conservative equations, the general solution is constructed from the fundamental solutions in both rectangular and spherical coordinates. The boundary conditions are enforced first at the plane walls by the Fourier transforms and then on the particle surface by a collocation technique. Numerical results for the diffusiophoretic and electrophoretic velocities of the particle relative to those of a particle under identical conditions in an unbounded solution are presented for various values of the relevant parameters including the relative separation distances between the particle and the two plates. For the special case of motions of a spherical particle parallel to a single plate and in the central plane of a slit, the collocation results agree well with the approximate analytical solutions obtained by using a method of reflections. The presence of the lateral walls can reduce or enhance the particle velocity, depending on the properties of the particle–solution system, the relative particle–wall separation distances, and the electrochemical boundary condition at the walls. In general, the boundary effects on diffusiophoresis and electrophoresis are quite significant and complicated, and they no longer vary monotonically with the separation distances for some situations.
Keywords: Diffusiophoresis; Electrophoresis; Boundary effects; Thin but polarized double layer;

There is a concentration-polarization (CP) force acting on a particle submerged in an electrolyte solution with a concentration (conductivity) gradient under an externally applied DC electric field. This force originates from the two mechanisms: (i) gradient of electrohydrodynamic pressure around the particle developed by the Coulombic force acting on induced free charges by the concentration polarization, and (ii) dielectric force due to nonuniform electric field induced by the conductivity gradient. A perturbation analysis is performed for the electric field, the concentration field, and the hydrodynamic field, under the assumptions of creeping flow and small concentration gradient. The leading order component of this force acting on a dielectric spherical particle is obtained by integrating the Maxwell and the hydrodynamic stress tensors. The analytical results are validated by comparing the surface pressure and the skin friction to those of a numerical analysis. The CP force is proportional to square of the applied electric field, effective for electrically neutral particles, and always directs towards the region of higher ionic concentration. The magnitude of the CP force is compared to that of the electrophoretic and the conventional dielectrophoretic forces.
Keywords: Concentration-polarization force; Electric field; Concentration gradient; Particle; Electrohydrodynamics; EHD; Dielectrophoresis; Microfluidics; Microchannel;

On the adsorption and diffusion of Methylene Blue in glass fibers by Sampa Chakrabarti; Binay K. Dutta (807-811).
The adsorption and diffusion phenomena of Methylene Blue dye on glass fiber have been explored. Both isotherm and kinetics have been studied. Effect of process parameters such as pH, adsorbent loading, and initial dye concentration was investigated. Temperature had a little effect on adsorption. A diffusion model was developed to estimate the diffusivity of the dye in glass fiber. The diffusivity was found to be varying exponentially with the dye concentration.
Keywords: Adsorption; Glass fiber; Dye; Methylene Blue; Diffusivity;

Synthesis of porphyrin-introduced silica gels by sol–gel process by Hidekazu Tanaka; Toshiyuki Yamada; Shinichiro Sugiyama; Hideo Shiratori; Ryozi Hino (812-815).
Using a hydroxyl group appended free base porphyrin derivative (HP), porphyrin-introduced silica gels were synthesized by a sol–gel process. The HP content in the materials linearly increased with increasing the HP concentration. Meanwhile, free base tetraphenylporphyrin (TPP) with no hydroxyl groups were almost not incorporated into the silica gels. These facts suggested that the interaction between hydroxyl groups of the HP molecules and silica network is considerably strong. The UV–vis characters of HP-introduced materials were almost the same as pure HP molecules. The Beer's plot indicated that the HP molecules in the materials are dispersed.
Keywords: Silica gel; Porphyrin; Sol–gel process; UV–vis spectrum;

Photoinduced dissolution and redeposition of Au nanoparticles supported on TiO2 by Tetsuro Kawahara; Tetsuro Soejima; Tomohiro Mitsui; Tomokazu Kiyonaga; Hiroaki Tada; Seishiro Ito (816-819).
Au particles (mean size ca. 3 nm) supported on TiO2 particles were irradiated by UV light (>300 nm) in aqueous solutions at 278 K. Photo-induced dissolution of Au nanoparticles followed by redeposition occurred in aqueous solutions containing halogen ions. The dissolution of Au nanoparticles yielded a Au(III) complex with a halogen ion; subsequent reduction of the Au(III) complex caused precipitation of larger Au particles on TiO2.
Keywords: Gold; Nanoparticles; TiO2; Photoinduced dissolution; Redeposition; Photocatalysis;

by Arthur Hubbard (820).