Journal of Colloid And Interface Science (v.282, #2)

The paper is an attempt to explain the mechanism of water adsorption on carbonaceous adsorbents with high-energy centers (surface or cation-exchange groups). The equations formulated previously by Dubinin et al. and Barton et al. are analyzed. Thus, four types of empirical factors defining the decrease in adsorption site concentration are taken into account. The isotherms of water adsorption measured on two microporous activated carbons containing various densities of hydrophilic sites are described by the above-mentioned theoretical isotherm equations. It is shown that none of these models provides an adequate description of the experimental data, especially at low relative pressures. This leads to the conclusion that with regard to the Dubinin and Serpinsky theory, the chemisorption of water at very high-energy adsorption sites needs to be taken into account. In view of the above, a new theoretical relationship is proposed which includes a correct description of the decreasing number of active sites. As a consequence, a good correlation between the new theoretical model and experimental data is observed over the whole range of relative pressures.
Keywords: Carbon; Water adsorption; Chemisorption; Microporosity; Hydrophilic centers; Primary adsorption sites; Dubinin–Serpinsky theory;

The process of concentrating amphetamine (1-phenyl-2-propanamine, C6H5CH2CH(NH2)CH3) and its N-alkyl substituted derivatives C6H5CH2CH(NHR)CH3 and C6H5CH2CH(N(CH3)R)CH3 (R=(CH2) n CH3 at n=0, 1, 2, and 3) from diluted aqueous solution was investigated using six adsorbents having different textures and chemical compositions. Three chemically modified carbon adsorbents prepared from plum stones and routinely used SPE cartridges packed with graphitized adsorbents such as Hypercarb and Envicarb and polymeric LiChrolut EN were applied. Recovery rates of amphetamines increase nearly linearly with growing free energy of solvation due to better adsorption of amphetamines with larger side groups from polar solution. Reduction of a carbon surface leads to a decrease in the recovery rate. Its minimal values are observed for the adsorption of amphetamines on graphitized carbons due to both lower adsorption and worse desorption (elution) in comparison with those for activated carbons.
Keywords: Amphetamines; Free energy of solvation; Activated carbons; Graphitized carbons; Structural characteristics; Solid phase extraction;

Competitive sorption of protons and metal cations onto kaolinite: experiments and modeling by Ilona Heidmann; Iso Christl; Christian Leu; Ruben Kretzschmar (270-282).
Competitive sorption of protons, Cu, and Pb onto kaolinite (KGa-2) was investigated over wide concentration ranges and quantitatively described using three different models based on surface complexation and cation exchange reactions. In all models, two types of binding sites were assumed for kaolinite: edge sites (SOH0.5−) with pH-dependent charge and face sites (X) with permanent negative charge. In a first step, proton sorption was measured by potentiometric acid–base titrations of kaolinite dispersed in 0.01, 0.03, and 0.1 M NaNO3 electrolyte solutions. The acid–base titration data were fitted to obtain site densities and protonation constants for the edge and face sites, respectively. In a second step, the sorption of Cu and Pb onto kaolinite was investigated at fixed pH values by metal titration using ion-selective electrodes for Cu2+ and Pb2+, respectively, and by independent batch sorption experiments. Our metal sorption data cover a range of pH 4–8 for Cu and pH 4–6 for Pb, three different ionic strengths (0.01, 0.03, and 0.1 M NaNO3), and up to eight orders of magnitude in free metal ion activity. An additional experiment was conducted to explore the sorption competition between Cu and Pb. In all three models, sorption of protons and metal cations to the edge sites of kaolinite was described with a 1-pK basic Stern (BS) approach. The three models differed only in the description of cation sorption to the face sites. In the first model (BS/GT), we used a Gaines–Thomas (GT) cation exchange equation for the face sites. This model yielded a satisfactory description of Cu sorption, but failed to describe Pb sorption isotherms at pH 4, 5, and 6. In the second model (BS/BS), we replaced the Gaines–Thomas equation by a basic Stern surface complexation formulation, thereby introducing electrostatic terms for sorption to face sites and allowing for free binding sites X. This did not improve the fits of Cu or Pb sorption to kaolinite, however. In the third model (BS/BSext), we extended the BS/BS-model by introducing additional monodentate sorption complexes at face sites (X―Cu+ and X―PbNO3). This model described both Cu and Pb sorption very well over the entire range in metal concentrations and pH. It also correctly predicted the competitive effect of Pb on sorption of Cu. Model calculations with all three models suggested that Cu and Pb were sorbed mainly to face sites at low pH, while sorption to edge sites dominated at high pH values.
Keywords: Adsorption; Kaolinite; Copper; Lead; Surface complexation; Cation exchange; Modeling; Titration; Ion-selective electrodes;

Two-dimensional dynamic Monte Carlo simulations are applied to the protein-like HP chain model to investigate the influence of lateral confinement of the adsorbed chain on adsorption thermodynamics and the ensemble of accessible chain conformations. The structure of the model makes it possible to enumerate all possible chain conformations and thereby define with precision the relation between adsorption thermodynamics and changes in accessible chain conformations resulting from the adsorption process. Lateral confinement of the adsorbed chain is shown to dramatically reduce the number of accessible energy states and unique chain conformations such that, under certain conditions, adsorption is predicted to actually stabilize the chain against denaturation. Lateral confinement preferentially eliminates expanded conformations of the adsorbed chain, shifting the equilibrium from the unfolded state toward the native state. As a result, the conformational entropy of the adsorbed chain is predicted to be lower than that of the chain free in solution. The protein-like HP chain responds to an increase in the hydrophobicity of the sorbent surface by strongly favoring those conformations that minimize the overall internal energy of the system. As a result, adsorption severely destabilizes the native-state conformation. The ability of our simulation results to provide insights into underlying mechanisms for nonspecific protein adsorption is illustrated through qualitative comparison with activity data for hen egg-white lysozyme adsorbed on silica at different surface concentrations.
Keywords: Protein adsorption; Thermodynamics; Conformational entropy; Dynamic Monte Carlo simulations;

An EXAFS and TRLFS investigation on uranium(VI) sorption to pristine and leached albite surfaces by Marcus Walter; Thuro Arnold; Gerhard Geipel; Andreas Scheinost; Gert Bernhard (293-305).
Uranium(VI) was sorbed to freshly ground and leached albite in batch and flow-through systems in the pH range 5.0–6.4. The uranium(VI) surface complexes were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The EXAFS analysis of uranium(VI) sorbed on albite at pH 5.8 and 5 × 10−6 M U(VI) showed one silicon atom at a U―Si distance of 3.09 Å, which is indicative of the formation of an inner-sphere, mononuclear, bidentate uranium(VI) surface complex, =Si(O)2 =UO2, on the silicate tetrahedra of albite. Two additional uranium(VI) sorption complexes were detected by TRLFS at higher initial aqueous U(VI) concentrations. However, the structure of these surface complexes could not be derived from EXAFS, since the measured EXAFS spectra represent the average of two surface complex structures. In order to simulate U(VI) sorption onto weathered feldspar surfaces, albite was leached with 0.01 M HClO4, resulting in surface material similar to amorphous silica gel. EXAFS showed that the equatorial oxygen shell of uranium(VI) sorbed on this material at pH 5.0 and 5.8 was split in two distances of 2.23 and 2.44 Å. This indicates the formation of an inner-sphere surface complex.
Keywords: Uranium(VI); Feldspar; Albite; Leached layer; Sorption; EXAFS; TRLFS;

A robust approach to studying the adsorption of Pluronic F108 on nonporous membranes by S. Govender; E.P. Jacobs; M.W. Bredenkamp; P. Swart (306-313).
A method for poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) desorption from synthetic nonporous polymeric membranes, using hexane:isopropanol treatment and subsequent colorimetric quantification, is described. The polymers polysulfone, poly(vinyldiene fluoride), and poly(ether imide) were used to fabricate solid adsorption matrices. The desorbed Pluronic F108 forms a color complex with ammonium ferrothiocyanate (NH4FeSCN) and is based on partitioning of a chromophore present in NH4FeSCN from an aqueous phase to a chloroform phase in the presence of Pluronic. The protocols for Pluronic desorption and detection are simple, sensitive, inexpensive, rapid, and reproducible over a wide range of Pluronic coating concentrations and membrane surface chemistries. A linear response over the concentration range from 3 to 130 μg ml−1 is obtained. The adsorption isotherms for flat sheet membranes are also described and the Langmuir equation provides the best fit for the adsorption data obtained within the concentration range studied. The absence of any significant interference from certain proteins, vitamins, carbohydrates, plasma, and halogenated derivatives makes the assay equally suitable for the estimation of Pluronic F108 in the attendant Pluronic conjugates or in biomedical applications. Using nonporous hollow fine fibers and capillary membranes as model curved substrates we were also able to correlate an increase in the radius of curvature with a corresponding increase in the surface interfacial adsorption of Pluronic F108.
Keywords: Pluronic F108; Nonporous membranes; Adsorption isotherms; Colorimetric detection; Interfacial curvature;

The removal of textile dyes by diatomite earth by Emin Erdem; Gülay Çölgeçen; Ramazan Donat (314-319).
The adsorption of some textile dyes by diatomite was investigated using Sıf Blau BRF (SB), Everzol Brill Red 3BS (EBR), and Int Yellow 5GF (IY). Adsorption of these textile dyes onto diatomite earth samples was studied by batch adsorption techniques at 30 °C. The adsorption behavior of textile dyes on diatomite samples was investigated using a UV–vis spectrophotometric technique. The effect of particle size of diatomite, diatomite concentration, the effect of initial dye concentrations, and shaking time on adsorption was investigated. Adsorption coverage over the surface of diatomite was studied using two well-known isotherm models: Langmuir's and Freundlich's. These results suggest that the dye uptake process mediated by diatomite has a potential for large-scale treatment of textile mill discharges. According to the equilibrium studies, the selectivity sequence can be given as IY > SB > EBR. Values of the removal efficiency of the dyes ranged from 28.60 to 99.23%. These results show that natural diatomite holds great potential to remove textile dyes from wastewater.
Keywords: Adsorption; Textile dyes; Diatomite; Clay; Isotherm;

Adsorption of metal ions onto Moroccan stevensite: kinetic and isotherm studies by A. Benhammou; A. Yaacoubi; L. Nibou; B. Tanouti (320-326).
The aim of this paper is to study the adsorption of the heavy metals (Cd(II), Cu(II), Mn(II), Pb(II), and Zn(II)) from aqueous solutions by a natural Moroccan stevensite called locally rhassoul. We carried out, first, a mineralogical and physicochemical characterization of stevensite. The surface area is 134 m2/g and the cation exchange capacity (CEC) is 76.5 meq/100 g. The chemical formula of stevensite is Si3.78Al0.22Mg2.92Fe0.09Na0.08K0.08O10(OH)2⋅4H2O. Adsorption tests of Cd(II), Cu(II), Mn(II), Pb(II), and Zn(II) in batch reactors were carried out at ambient temperature and at constant pH. Two simplified models including pseudo-first-order and pseudo-second- order were used to test the adsorption kinetics. The equilibrium time and adsorption rate of adsorption were determined. The increasing order of the adsorption rates follows the sequence Mn(II) > Pb(II) > Zn(II) > Cu(II) > Cd(II). The Dubinin–Radushkevich (D–R), Langmuir, and Redlich–Peterson (R–P) models were adopted to describe the adsorption isotherms. The maximal adsorption capacities at pH 4.0 determined from the D–R and Langmuir models vary in the following order: Cu(II) > Mn(II) > Cd(II) > Zn(II) > Pb(II). The equilibrium data fitted well with the three-parameter Redlich–Peterson model. The values of mean energy of adsorption show mainly an ion-exchange mechanism. Also, the influence of solution pH on the adsorption onto stevensite was studied in the pH range 1.5–7.0.
Keywords: Adsorption; Heavy metals; Isotherm; Kinetic; Stevensite;

Calculation of immersion enthalpy data from adsorption isotherms by Grit Kalies; Rico Rockmann; Renaud Denoyel (327-334).
The thermodynamic equations for the calculation of binary and ternary immersion data in excess formalism are presented. Immersion enthalpies and entropies of the n-hexane/n-octane, n-octane/n-tetradecane and n-hexane/n-tetradecane binary mixtures as well as the n-hexane/n-octane/n-tetradecane ternary mixture on activated carbon are calculated from the temperature dependence of adsorption isotherms. In order to evaluate the quality of the calculations, the calculated immersion enthalpies of the binary mixtures on activated carbon are compared with those that were measured calorimetrically. It is shown that phenomenological thermodynamics can be used successfully to predict calorimetric data on the basis of adsorption excess isotherms.
Keywords: Excess formalism; Thermodynamic calculation; Calorimetric quantities; Binary and ternary liquid mixtures; Activated carbon;

We present two approaches to description of the adsorption equilibrium in adsorbate–adsorbent systems, the treatment where the adsorbate is considered as a two-dimensional gas, and the treatment where the adsorbate and adsorbent form a solution where the adsorbate is the solute and vacancies play the role of the solvent. In the first case the application of different equations describing the state of a two-dimensional gas leads to some fundamental adsorption isotherm equations, for example, Henry, Volmer, Hill–deBoer, Fowler–Guggenheim, and Langmuir. In contrast, the application of different equations (existing in the theory of solutions) describing the activity of the solvent (e.g., the Wilson model or the model of Flory and Huggins) leads to some new adsorption equations and the assumption of the ideality of the solvent leads to the Langmuir adsorption isotherm. We present some new adsorption equations basing on the second method described above and on the Redlich–Kister and Wohl expansions. Assuming that the activity coefficients are given by the one-constant Margules equation, the two-constant Margules equation, the van Laar equation, the Wilson equation, and, finally, the Flory–Huggins equation, we derive the respective adsorption isotherm formulas. We also present the assumptions leading to the best-known adsorption equations derived applying the first method. The equation developed by Cochran et al. is also corrected, taking into account the fundamental assumptions of the Flory–Huggins theory.
Keywords: Adsorption; Thermodynamics of adsorption; Theory of adsorption; Osmotic theory of adsorption; Vacancy solution model;

Resistance of zwitterionic telomers accumulated on metal surfaces against nonspecific adsorption of proteins by Hiromi Kitano; Akio Kawasaki; Hideaki Kawasaki; Shinta Morokoshi (340-348).
Telomers of N,N-dimethyl-N-(3-sulfopropyl)- 3 ′ -methacryloylaminopropanaminium inner salt (SPB), 2-methacryloyloxyethyl phosphorylcholine (MPC), and N,N-dimethyl-N-(1-carboxymethyl)- 2 ′ -methacryloyloxylethanaminium inner salt (CMB) were prepared by UV irradiation in the presence of N,N, N ′ , N ′ -tetraethylthiuram disulfide and a disulfide-group-carrying iniferter (a compound which pursues initiation, chain transfer, and termination), Cys-BDC, which had been prepared by coupling N,N-diethyldithiocarbamoylmethylbenzoic acid succinimidyl ester with cystamine dihydrochloride. The telomers formed a self-assembled monolayer (SAM) on a gold electrode and a monolayer of colloidal gold deposited on a glass plate, as confirmed by the increase in potential difference ( Δ E p ) of the voltammogram for hydroquinone as a probe using cyclic voltammetry (CV) and the increase in absorbance using localized surface plasmon resonance (LSPR) absorption spectroscopy, respectively. Nonspecific adsorption of various proteins onto the surfaces of various telomer SAMs was examined from the decrease in peak current (ΔI) using CV and the absorption increase at 550 nm using LSPR absorption spectroscopy. The zwitterionic telomer SAM in general did not adsorb proteins significantly, suggesting the usability of zwitterionic polymer SAMs and brushes to coat various materials used in biomedical fields. The correlation between the structure of water in the vicinity of zwitterionic telomers and the resistance of the zwitterionic telomer SAMs against the nonspecific adsorption of proteins was discussed.
Keywords: Cyclic voltammetry; Iniferter; Nonspecific adsorption; Protein adsorption; Self-assembled monolayer; Surface plasmon resonance; Telomerization; Zwitterionic telomer;

Binary polymer brushes grafted to Si wafers were prepared from hydrophilic and hydrophobic polymer components, which allow switching the surface properties (as revealed by contact angles) by exposure to varying solvents. The hydrophilic component was poly-2-vinylpyridine; the hydrophobic component was a copolymer of styrene and 2-(4-vinylphenyl)indene containing the photodimerizable phenylindene chromophore. The brushes were prepared via thermal and via photochemical grafting-to methods, which led to distinct layer thicknesses. Structural patterns differing in surface properties were produced and fixed via crosslinking the hydrophobic component by 2 π + 2 π photodimerization of phenylindene moieties. The patterns were visually observable.
Keywords: Polymer brushes; Photochemical grafting-to; Photocrosslinking; Photodimerization; Surface microstructures; Switchable surface properties;

The influence of four calcium complexing substances, i.e., citric acid (CIT), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and pyromellitic acid (PMA), on the crystal growth rate of the calcium carbonate polymorphs aragonite and calcite has been studied. Using a seeded constant supersaturation method supersaturation was maintained at 4 by keeping a constant pH of 8.5 through addition of sodium carbonate and calcium chloride solutions. The unique composition of each solution was calculated using chemical speciation. The growth rate was interpreted in terms of an overall growth rate. For both calcite and aragonite, the crystal growth rate is significantly reduced in the presence of the calcium complexing substances. The growth retarding effect depends on both the concentration and the polymorph. The relative crystal growth rate was correlated to the total complexing agent concentration using a Langmuir adsorption approach. Aragonite appeared fully covered for lower total concentrations than calcite. Furthermore, CIT very efficiently blocked aragonite growth contrary to what was observed for calcite. This is thought to be related to certain distinct features of the dominant aragonite crystal faces compared to the dominant calcite faces.
Keywords: Crystal growth; Calcium carbonate; Precipitation;

The influence of four calcium complexing additives, i.e., citric acid (CIT), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and pyromellitic acid (PMA), and their concentration on the induction time of calcium carbonate nucleation has been studied. The experiments were performed by rapidly mixing a sodium carbonate solution and a calcium chloride solution of various concentrations. The induction time was obtained by recording the white light absorption of the solution. Chemical speciation was used to estimate the initial thermodynamic driving force of each experiment. The induction time was found to increase with additive concentration. The effect varies from one additive to another. CIT causes the greatest increase in induction time and PMA the least. Using classical nucleation theory the experimental data were evaluated in terms of the interfacial energy. In pure water a value of 37.8 mJ m−2 was obtained, showing good agreement with other works. CIT, DTPA and EDTA caused a notable increase of the interfacial energy at a concentration of 0.5 mmol l−1. PMA does not appear to have any effect at all on the interfacial energy. Different mechanisms for the influence of the additives on the measured induction time and on the estimated interfacial energy are discussed.
Keywords: Nucleation; Calcium carbonate; Precipitation; Interfacial energy;

CFD simulation of aggregation and breakage processes in laminar Taylor–Couette flow by L. Wang; D.L. Marchisio; R.D. Vigil; R.O. Fox (380-396).
An experimental and computational investigation of the effects of local fluid shear rate on the aggregation and breakage of ∼ 10  μm latex spheres suspended in an aqueous solution undergoing laminar Taylor–Couette flow was carried out according to the following program. First, computational fluid dynamics (CFD) simulations were performed and the flow field predictions were validated with data from particle image velocimetry experiments. Subsequently, the quadrature method of moments (QMOM) was implemented into the CFD code to obtain predictions for mean particle size that account for the effects of local shear rate on the aggregation and breakage. These predictions were then compared with experimental data for latex sphere aggregates (using an in situ optical imaging method) and with predictions using spatial average shear rates. The mean particle size evolution predicted by CFD and QMOM using appropriate kinetic expressions that incorporate information concerning the particle morphology (fractal dimension) and the local fluid viscous effects on aggregation collision efficiency match well with the experimental data.
Keywords: Computational fluid dynamics; Aggregation; Breakage; Taylor–Couette flow;

The mechanism of rhythmic pattern formation in reaction–diffusion systems is investigated theoretically by introducing a new concept. The boundary that separates the two reacting species virtually migrates as the diffusion proceeds into the gelatinous medium. Based on this boundary migration scenario, all the well-established relations on Liesegang patterns could be proved, in a rather modified way. The idea of formation of intermediate colloidal haze prior to patterning along with the moving boundary model proved to be efficient in predicting the concentration dependence of the width of the spatiotemporal patterns. The experimental observations support the width law relation developed.
Keywords: Pattern formation; Reaction–diffusion systems; Colloids; Liesegang rings; Moving boundary;

The effect of surface properties on visible luminescence of nanosized colloidal ZnO membranes by Z.Y. Xiao; Y.C. Liu; L. Dong; C.L. Shao; J.Y. Zhang; Y.M. Lu; D.Z. Zhen; X.W. Fan (403-407).
Luminescence properties of nanosized zinc oxide (ZnO) colloids depend greatly on their surface properties, which are in turn largely determined by the method of preparation. ZnO nanoparticles in the size range from 3 to 9 nm were prepared by addition of tetramethylammonium hydroxide ((CH3)4NOH) to an ethanolic zinc acetate solution. X-ray diffraction (XRD) indicates nanocrystalline ZnO membranes with polycrystalline hexagonal wurtzite structure. The ZnO membranes have a strong visible-emission intensity and the intensity depends upon hydrolysis time. The infrared spectra imply a variety of forms of zinc acetate complexes present on the surface of ZnO particles. The effect of the ZnO membrane surface properties on photoluminescence is discussed.
Keywords: Photoluminescence; Zinc acetate complexes; Sol–gel; Hydrolysis time;

Studies on the Merrill–Crowe process as applied to silver recovery have shown that one half of the used zinc powder is wasted in water reduction at high cyanide concentrations, while the other half reduces silver ions from the cyanide solution. However, the cementation mechanisms as an electrochemical process taking place on the zinc surface do not explain the split of the electric current resulting from the anodic dissolution of zinc into two equal values. This study demonstrates that the mechanism for silver precipitation at high and low cyanide concentrations differs considerably. At low cyanide concentrations cementation is essentially an electrochemically-controlled process following a shrinking-core behavior. At high cyanide concentrations, the process seems not to be electrochemically controlled. The areas for zinc dissolution and silver deposition are not connected by an electrical-conducting medium and reduction of silver–cyano complex ions takes place by hydrogen adsorbed onto silver growing outward from the cementing zinc particles. The results are based on scanning electron microscopy of solids recovered from cementations in stirred reactors and in situ observations by optical microscopy of the cementation process on the edge of thin zinc disks in cyanide solutions.
Keywords: Silver; Metal cementation; Cyanide leaching; Zinc; Mineral processing;

Ionic liquid ion sources: characterization of externally wetted emitters by Paulo Lozano; Manuel Martínez-Sánchez (415-421).
The feasibility of electrostatically extracting and accelerating ions from room temperature ionic liquids in a high vacuum environment is investigated using externally wetted emitters similar to those manufactured for liquid metal ion sources, made out of tungsten wire and electrochemically treated to produce a sharp tip and to increase surface wettability. The ionic liquid EMI-BF4 is used as a prototypical example. The temperature dependence on emission current suggests that liquid flow over the metallic surface is limited by viscosity. Time-of-flight spectrometry indicates that the beam is composed of EMI + and (EMI-BF4)EMI+ ions in the positive polarity and BF 4 − and (EMI-BF4) BF 4 − ions in the negative polarity, and that these ions are emitted with energies very close to their applied potentials. Angular distribution measurements in positive and negative polarities show that ions travel near the propagation axis, diverging by not more than 18° from the centerline. Thanks to the extraordinary variety of ionic liquids it should be possible to generate a correspondingly large number of bipolar nonmetallic ion beams each with unique properties and applicability in fields as diverse as ion lithography, analytical equipment and space propulsion.
Keywords: Ion source; Ionic liquid; Electrochemical window; Double layer; Electrospray; Colloid thruster; Micropropulsion; Wetting; Ion lithography;

The structure and dynamics of methane in hydrated potassium montmorillonite clay have been studied under conditions encountered in sedimentary basin and compared to those of hydrated sodium montmorillonite clay using computer simulation techniques. The simulated systems contain two molecular layers of water and followed gradients of 150 bar km−1 and 30 K km−1 up to a maximum burial depth of 6 km. Methane particle is coordinated to about 19 oxygen atoms, with 6 of these coming from the clay surface oxygen. Potassium ions tend to move away from the center towards the clay surface, in contrast to the behavior observed with the hydrated sodium form. The clay surface affinity for methane was found to be higher in the hydrated K-form. Methane diffusion in the two-layer hydrated K-montmorillonite increases from 0.39 × 10 −9  m 2 s −1 at 280 K to 3.27 × 10 −9  m 2 s −1 at 460 K compared to 0.36 × 10 −9  m 2 s −1 at 280 K to 4.26 × 10 −9  m 2 s −1 at 460 K in Na-montmorillonite hydrate. The distributions of the potassium ions were found to vary in the hydrates when compared to those of sodium form. Water molecules were also found to be very mobile in the potassium clay hydrates compared to sodium clay hydrates.
Keywords: Methane; K; Na; Montmorillonite; Surface complex; Clay; Simulations;

The surface tension, σ, of dilute solutions of cyclohexanol and cyclopentanol in ethylene glycol was measured in the temperature range between 293.15 and 323.15 K by means of the ring detachment method. The surface entropies and enthalpies were calculated. The surface excess values were obtained using the Gibbs equation. The surface tension data were analyzed using the extended Langmuir (EL) model and the surface composition were obtained from this model. It was shown that clathrate-like solvates (hydrophobic-like solvation) are forming in the dilute solution of cyclohexanol in ethylene glycol.
Keywords: Surface tension; Surface thermodynamics; Extended Langmuir model; Cycloalkanols; Ethylene glycol; Hydrophobic solvation;

We studied wetting phenomena on heterogeneous surfaces by a mean-field free-energy lattice Boltzmann method recently proposed [Phys. Rev. E 69 (2004) 32,602]. Our results suggest that the Cassie equation in macroscopic contact angle measurements is in general not valid. It was found that the Cassie equation is valid only when the patch size is on the same order of the liquid–vapor interfacial thickness. We also demonstrated that contact angle manifests itself from local surface properties near the contact point and does not result from the specific solid–liquid interactions across the contact area.
Keywords: Contact angles; Cassie's equation; Heterogeneous surface; Lattice Boltzmann method; Solid–fluid interaction;

Adsorption of different amphiphilic molecules onto polystyrene latices by A.B. Jódar-Reyes; J.L. Ortega-Vinuesa; A. Martín-Rodríguez (439-447).
In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain–surface and chain–chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.
Keywords: Interfaces and colloids; Amphiphilic molecules; Surfactant and peptide adsorption; Latex particles;

Thickening properties and emulsification mechanisms of new derivatives of polysaccharides in aqueous solution by Eri Akiyama; Akio Kashimoto; Keiichi Fukuda; Hajime Hotta; Toshiyuki Suzuki; Tomohito Kitsuki (448-457).
The thickening properties of aqueous solutions of HHM-HEC (hydrophobically–hydrophilically modified hydroxyethylcellulose) and the emulsification mechanisms of HHM-HEC/water/oil systems were investigated. A dramatic increase in viscosity was observed with increased HHM-HEC concentration in water, caused by aggregation of hydrophobic alkyl chains. At higher concentrations of HHM-HEC (above 0.6 wt%) in water, it forms an elastic gel, which has good thixotropic properties and a high yield value. O/W (oil-in-water) type emulsions were obtained using HHM-HEC, which can emulsify various kinds of oil, including hydrocarbon, silicone, and perfluoropolymethylisopropyl ether. The viscosity of these emulsions depends only upon the oil volume fraction, not on the kind of oil. In addition, the oil particle size in the emulsions remained constant after a certain period because HHM-HEC formed a strong gel network structure and a protective layer, which prevented the emulsion from coalescing. Measurements of interfacial tension revealed that the alkyl chains in HHM-HEC did not significantly lower the interfacial tension at the water/oil interface when 0.5 wt% of HHM-HEC was added to water. Steady flow and oscillatory experimental results show that the rheological behavior of HHM-HEC/water/oil emulsions was similar to that of aqueous solutions of HHM-HEC. In the HHM-HEC/water/oil emulsion system, oil droplets were dispersed and kept stable in the strong gel structure of HHM-HEC. The aqueous solution of HHM-HEC showed salt resistance. It is thought to be due to sulfonic acid groups in HHM-HEC. The stability of the emulsion using HHM-HEC is based on both protective colloidal effects and associative thickening caused by alkyl chains in HHM-HEC.
Keywords: HHM-HEC; Water-soluble polymer; Oil-in-water emulsion; Rheology;

Liquid drainage through aqueous foam: study of the flow on the bubble scale by O. Pitois; C. Fritz; M. Vignes-Adler (458-465).
Macroscopic properties of foams are highly dependent on the liquid volume fraction, which has motivated many studies on foam drainage in the last decade. Theoretical developments and recent experimental results have suggested that two macroscopic drainage regimes could be expected, in relation with flow transitions occurring at the microscopic level, essentially in the Plateau border channels. We have constructed a setup, the Plateau border apparatus, to study the hydrodynamics of a single Plateau border channel, focusing on the surface properties of the foaming solution. Experimental results have shown that the actual theoretical models only partially predict the dissipation of liquid flow through a Plateau border channel. The major discrepancies can be explained considering additional dissipation processes related to the properties of the interface, and to the liquid flows induced in adjoining films as the liquid flows in the channel. Evidence of the hydrodynamic coupling between the channel and the adjoining films is given in the paper.
Keywords: Drainage; Plateau border; Surface shear viscosity; Foam;

The interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), P103 and F108, EO n PO m EO n , m = 56 and n = 17 and 132, respectively, and m–s–m type gemini surfactants, m=8, 10, 12, and 18, and s = 3, 6, 12, and 16, have been studied in aqueous solution using isothermal titration calorimetry and dynamic light scattering techniques. The enthalpograms of F108 as a function of surfactant concentration show one broad peak at polymer concentrations C p ⩽ 0.50  wt% , below the cmc of the copolymer at 25 °C. It is attributed to interactions between the surfactant and the triblock copolymer monomer. DLS results show hydrodynamic radii ( R h ) initially consistent with copolymer monomers that change to values consistent with gemini surfactant micelles as the surfactant concentration is increased. In P103 solutions at C p ⩾ 0.05  wt% , two peaks appear in the enthalpograms, and they are attributed to the interactions between the gemini surfactant and the micelle or monomer forms of the copolymer. An origin-based nonlinear fitting program was employed to deconvolute the two peaks and to obtain estimates of peak properties. An estimate of the fraction of copolymer in aggregated form was also obtained. The enthalpy change due to interactions between the surfactants and P103 aggregates is very large compared to values obtained for traditional surfactants. This suggests that extensive reorganization of copolymer aggregates and surrounding solvent occurs during the interaction. DLS results for the P103 systems containing C p ⩾ 0.05 % show evidence of very large aggregates in solution, likely P103 micelle clusters. The transitions observed in the hydrodynamic radii are consistent with a breakdown of micelle clusters with addition of gemini surfactant, followed by mixed micelle formation and/or deaggregation into monomer P103. This is followed by interactions similar to those typically observed in surfactant–nonionic polymer systems. Mechanisms for the interaction and the observed structural changes are discussed.
Keywords: Surfactant–polymer interaction; Gemini surfactants; m–s–m; Pluronic block copolymers; F108; P103; Isothermal titration calorimetry; Dynamic light scattering; Polymer hydration;

Continuous process for singlet oxygenation of hydrophobic substrates in microemulsion using a pervaporation membrane by Laurent Caron; Véronique Nardello; José Mugge; Erik Hoving; Paul L. Alsters; Jean-Marie Aubry (478-485).
Chemically generated singlet oxygen (1O2, Δ g 1 ) is able to oxidize a great deal of hydrophobic substrates from molybdate-catalyzed hydrogen peroxide decomposition, provided a suitable reaction medium such as a microemulsion system is used. However, high substrate concentrations or poorly reactive organics require large amounts of H2O2 that generate high amounts of water and thus destabilize the system. We report results obtained on combining dark singlet oxygenation of hydrophobic substrates in microemulsions with a pervaporation membrane process. To avoid composition alterations after addition of H2O2 during the peroxidation, the reaction mixture circulates through a ceramic membrane module that enables a partial and selective dewatering of the microemulsion. Optimization phase diagrams of sodium molybdate/water/alcohol/anionic surfactant/organic solvent have been elaborated to maximize the catalyst concentration and therefore the reaction rate. The membrane selectivity towards the mixture constituents has been investigated showing that a high retention is observed for the catalyst, for organic solvents and hydrophobic substrates, but not for n-propanol (cosurfactant) and water. The efficiency of such a process is illustrated with the peroxidation of a poorly reactive substrate, viz., β-pinene.
Keywords: Microemulsion; Oxidation; Singlet oxygen; Pervaporation; Ceramic membrane; Dewatering;

Electrophoresis of a concentrated aqueous dispersion of non-Newtonian drops by Eric Lee; Chia-Jeng Chang; Jyh-Ping Hsu (486-492).
The electrophoresis of a concentrated dispersion of non-Newtonian drops in an aqueous medium, which has not been investigated theoretically in the literature, is analyzed under conditions of low zeta potential and weak applied electric field. The results obtained provide a theoretical basis for the characterization of the nature of an emulsion and a microemulsion system. A Carreau fluid, which has wide applications in practice, is chosen for the non-Newtonian drops, and the unit cell model of Kuwabara is adopted to simulate a dispersion. The effects of the key parameters of a dispersion, including its concentration, the shear-thinning nature of the drop fluid, and the thickness of the double layer, on the electrophoretic behavior of a drop are discussed. In general, the more significant the shear-thinning nature of the drop fluid is, the larger the mobility is, and this effect is pronounced as the thickness of the double layer decreases. However, if the double layer is sufficiently thick, this effect becomes negligible. In general, the higher the concentration of drops is, the smaller the mobility is; however, if the double layer is either sufficiently thin or sufficiently thick, this effect becomes unimportant.
Keywords: Electrophoresis; Concentrated dispersion; Non-Newtonian drops; Carreau fluid; Cell model; Pseudo-spectral method;

In this paper, we present a newly developed rare earth modified amorphous barium strontium titanate (Ba x Sr1−x TiO3) gel/silicone oil electrorheological (ER) fluid. The ER behaviors of suspensions of pure and rare earth modified amorphous Ba x Sr1−x TiO3 particles in silicone oil have been investigated under a dc electric field. The shear yield stress of the rare earth modified amorphous BaTiO3 gel/silicone oil ER fluid could reach 14.9 kPa at E = 3.5  kV / mm while the leaking current density was very low, about 7.64 μA/cm2. The ER fluids with a higher volume fraction had a higher current density and a higher shear yield stress under the same electric field. The ER fluid has a long-term stability against sedimentation. The problem of caking was not serious and the agglomerated particles could be easily broken up by strongly stirring.
Keywords: Electrorheological fluid; Sol–gel; Infrared (IR) spectroscopy;

Stepwise thickening in aqueous foam films stabilized by sodium naphthenates by Shawn D. Taylor; Jan Czarnecki; Jacob Masliyah (499-502).
During drainage of a foam film formed from an aqueous sodium naphthenate solution, a transient, local, stepwise thickening process was observed. Film stratification is related to the stepwise thinning drainage process where individual layers of material are drained from a film. The process typically involves the appearance of a sequence of small, uniformly thick spots that eventually expand to the size of the film. The appearance and growth of each spot represents a discrete decrease in the thickness of the film. The size of each decrease or step typically corresponds to the size of one or more lamella layers of the stratified film. Stepwise thinning was observed in a foam film formed from an aqueous sodium naphthenate solution, as frequently reported for a variety of systems. However, during the drainage process, a transient stepwise thickening process was also observed. Bright spots began to appear and grow, indicate a discrete increase in the thickness of a portion of the film. This local, stepwise thickening process appeared to be an alternate and temporary drainage process directly related to the expansion of the stepwise thinning spots.
Keywords: Foam film; Stratification; Stepwise thickening; Sodium naphthenates;