Journal of Colloid And Interface Science (v.281, #2)

Biosorption of chromium(VI) and arsenic(V) onto methylated yeast biomass by Hideshi Seki; Akira Suzuki; Hideo Maruyama (261-266).
Yeast biomass was methylated in a 0.1 M HCl methyl alcohol solution at room temperature and the methylated yeast (MeYE) was applied to the adsorptive separation of Cr(VI) and As(V) anions from aqueous solutions. At near-neutral pH, while Cr(VI) and As(V) anions were scarcely adsorbed onto unmethylated yeast biomass, the amounts adsorbed increased with increasing methylation degree. The amount of Cr(VI) adsorbed onto MeYE was almost constant at pH 4–6 and decreased with increasing pH above pH 6. The amount of As(V) adsorbed onto MeYE was rather lower than that of Cr(VI) and it had a peak at about pH 7. A metal-binding model was used to describe the adsorption characteristics of Cr(VI) and As(V) on MeYE. The results showed that MeYE has two different types of adsorption sites. The saturated amount of Cr(VI) and As(V) adsorbed onto MeYE having methylation degree 0.94 was 0.55 mmol g−1.
Keywords: Biosorption; Chromium; Arsenic; Methylated yeast biomass;

The NICA (nonideal competitive adsorption)–Donnan model is employed to describe the interactions between Cu2+, Pb2+, Cd2+, Mn2+, and Fe3+ ions and the lignins extracted from wheat bran (lignocellulosic substrate, LS) and from kraft pulp (residual kraft lignin, RKL), and between Cu2+, Mn2+, and Fe3+ ions and wood fibers from kraft pulps. The charge of the LS and the fiber charge need to be obtained from potentiometric titration data for the LS, and by use of Donnan equilibrium, mass balance, and electroneutrality equations for the kraft fiber. The proton binding parameters for the LS and the kraft fiber, the total site densities ( Q max , 1 and Q max , 2 ), the median protonation constants ( K 1 and K 2 ), and nonideality–generic heterogeneity parameters ( m 1 and m 2 ) (subscripts 1 and 2 refer to the carboxyl and phenolic functional groups) are obtained by fitting these charge data. With the above proton parameters, the interactions between metal ions and the lignins (LS and RKL)/kraft fibers are calculated, and the metal binding parameters are obtained. These parameters are the binding constants of metal i ( K i , 1 and K i , 2 ), ion-specific nonideality parameters ( n i , 1 and n i , 2 ), and intrinsic heterogeneity parameters ( p 1 and p 2 ). p 1 and p 2 are the same for all metal ions binding to a specific sorbent. Here, p 1 and p 2 values obtained by fitting the binding data of a specific metal ion are used directly in binding calculations for other metal ions, and do not need to be fitted. By use of the above parameters for single metal ion binding, the binding relationship between a mixed metal ion and lignocellulosic substrate/kraft fiber can be predicted.
Keywords: NICA–Donnan model; Ion binding; Sorption; Lignin; Wood fiber; Metals;

Adsorption and precipitation of an aminoalkylphosphonate onto calcite by Amy T. Kan; Gongmin Fu; Mason B. Tomson (275-284).
The mechanism of nitrilotris(methylenephosphonic acid) (H6NTMP)/calcite reaction was studied with a large number of batch experiments where phosphonic acid was neutralized with 0 to 5 equivalents of NaOH per phosphonic acid and the concentration ranged from about 10 nmol/L to 1 mol/L. It is proposed that the phosphonate/calcite reactions are characterized in three steps. At low phosphonate concentration ( < 1  μmol / L NTMP concentration), the phosphonate/calcite reaction can be characterized as a Langmuir isotherm. At saturation, only ∼ 7 % of the calcite surface is covered with phosphonate; presumably these are the kinks, step edges, or other imperfect sites. At higher phosphonate concentrations, the attachment is characterized by calcium phosphonate crystal growth to a maximum of four to five surface layer thick, with solid phase stoichiometry of Ca2.5HNTMP and a constant solubility product of 10−24.11. After multiple layers of phosphonate are formed on the calcite surface, the solution is no longer at equilibrium with calcite. Further phosphonate retention is probably due to mixed calcium phosphonate solid phase formation at lower pH and depleted solution phase Ca conditions. The proposed mechanism is consistent with phosphate/calcite reaction and can be used to explain the fate of phosphonate in brines from oil producing wells and the results are compared with two oil wells.
Keywords: Phosphonates; Calcite; Adsorption; Precipitation; Scale inhibition;

Adsorption of NH3 onto activated carbon prepared from palm shells impregnated with H2SO4 by Jia Guo; Wang Sheng Xu; Yan Lin Chen; Aik Chong Lua (285-290).
Adsorption of ammonia (NH3) onto activated carbons prepared from palm shells impregnated with sulfuric acid (H2SO4) was investigated. The effects of activation temperature and acid concentration on pore surface area development were studied. The relatively large micropore surface areas of the palm-shell activated carbons prepared by H2SO4 activation suggest their potential applications in gas adsorption. Adsorption experiments at a fixed temperature showed that the amounts of NH3 adsorbed onto the chemically activated carbons, unlike those prepared by CO2 thermal activation, were not solely dependent on the specific pore surface areas of the adsorbents. Further adsorption tests for a wide range of temperatures suggested combined physisorption and chemisorption of NH3. Desorption tests at the same temperature as adsorption and at an elevated temperature were carried out to confirm the occurrence of chemisorption due to the interaction between NH3 and some oxygen functional groups via hydrogen bonding. The surface functional groups on the adsorbent surface were detected by Fourier transform infrared spectroscopy. The amounts of NH3 adsorbed by chemisorption were correlated with the contents of elemental oxygen present in the adsorbents. Mechanisms for chemical activation and adsorption processes are proposed based on the observed phenomena.
Keywords: NH3 adsorption; Activated carbon; H2SO4 activation; Chemisorption; Oxygen functional groups; Reaction mechanism;

Systematic studies on adsorption of lead on sea nodule residues by Archana Agrawal; K.K. Sahu; B.D. Pandey (291-298).
Lead adsorption from aqueous solutions was studied with the aim of detoxifying industrial effluents before their safe disposal onto land or into river waters. Sea nodule residue (SNR), a waste material containing oxides and oxyhydroxides of manganese, iron, silicon, etc., was used as an effective adsorbent for lead in this study. The effect of various parameters such as contact time, initial lead concentration, pulp density, particle size of the adsorbent, pH, and temperature was studied to optimize the conditions for maximum adsorption. Adsorption followed first-order kinetics and 99% of lead adsorption was achieved at a solid:liquid ratio of 1:330, in the pH range 5.5–6.0 at a particle size of − 150  μm in 8 h for solution containing 200 ppm lead. The adsorption capacity was found to be 99.0 mg of lead per gram of SNR and the adsorption isotherms followed the Langmuir and Freundlich adsorption models. The mechanism of adsorption of lead onto the sea nodule residue was also investigated. It was possible to reduce the lead level from 25–200 ppm to acceptable levels (0.1 ppm) by adsorption over this solid waste.
Keywords: Adsorption; Sea nodule residue; Lead; Adsorption isotherm;

Crystallization and prevention of supercooling of microencapsulated n-alkanes by Xing-xiang Zhang; Yao-feng Fan; Xiao-ming Tao; Kit-lun Yick (299-306).
Microencapsulated n-alkanes (n-octadecane, n-nonadecane, and n-eicosane) were synthesized by in situ polymerization using urea–melamine–formaldehyde polymer as shells. Microcapsules 5.0 and 10.0 wt% of 1-tetradecanol, paraffin, and 1-octadecanol were used as nucleating agents. The fabrication was characterized using Fourier transform infrared, light microscopy, and scanning electron microscopy. The crystallization and prevention of supercooling of the microcapsules are studied using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction. The crystal system of the microencapsulated n-alkane is the same as that of the bulk. The enthalpies of the microcapsules containing 70 wt% n-alkanes are approximately 160 J/g. The melting temperature of the n-alkanes in the microcapsule is the same as that in the bulk. There are multiple peaks on the DSC cooling curves that are attributed to liquid–rotator, rotator–crystal, and liquid–crystal transitions. The DSC cooling behavior of microencapsulated n-octadecane is affected by the average diameters. The measured maximum degree of supercooling of the microencapsulated n-octadecane is approximately 26.0 °C at a heating and cooling rate of 10.0 °C/min. The degree of supercooling of microencapsulated n-octadecane is decreased by adding 10.0 wt% of 1-octadecanol as a nucleating agent.
Keywords: Microcapsules; n-Alkanes; Crystallization; Supercooling; Nucleating agents;

Surface characterization of cotton coated by a thin film of polystyrene with and without a cross-linking agent by Thirawudh Pongprayoon; Nantaya Yanumet; Edgar A. O'Rear; Walter E. Alvarez; Daniel E. Resasco (307-315).
Thin polystyrene film coated on cotton was successfully formed by admicellar polymerization. Divinylbenzene (DVB) was used as a cross-linking agent to form networked polystyrene to improve film coverage. A wettability test and XPS analysis were used to characterize the coated surface. The optimum amount of DVB was around 1%. At this amount, the film coverage was most complete, as judged by the reduction of the O1s signal in XPS analysis.
Keywords: Cotton; Polystyrene film; Cross-linking agent; Divinylbenzene; XPS; Admicellar polymerization; Wettability;

Aggregation of hydrophobically modified polysaccharides in solution and at the air–water interface by Widad Henni; Michel Deyme; Michel Stchakovsky; Didier LeCerf; Luc Picton; Véronique Rosilio (316-324).
The present article focuses on the comparative study of physicochemical properties of two ionic pullulan derivatives modified by 10 or 35 C8 chains per 100 anhydroglucose units, named CMP10C8 and CMP35C8, respectively. In aqueous solutions, these derivatives exhibited an associative behavior as evidenced by pyrene fluorescence spectroscopy. This phenomenon, which stems from intra- and/or intermolecular interactions between the hydrophobic groups grafted on the polymer backbone, results in the formation of more or less condensed aggregates depending on the C8 ratio and the ionic strength of the media. The hydrophobically modified pullulans also displayed surface properties. Their adsorption at the air–solution interface was assessed from surface tension measurements. The results showed that both hydrophobized polymers adsorb in a coil conformation occupying a large interfacial molecular area. The comparison of these molecular areas indicated that CMP35C8 adopts a more shrunken conformation at the interface than CMP10C8, due to stronger intramolecular interactions. The stability of the adsorbed monolayer under bulk dilution was investigated by ellipsometric measurements. Whereas bulk dilution had no effect on the stability of the adsorbed CMP35C8 film, it provoked significant changes in the adsorbed CMP10C8 monolayer. The stability of the CMP35C8 monolayers was attributed to the existence of intermolecular associations between the adsorbed coils.
Keywords: Hydrophobized carboxymethylpullulan; Monolayers; Pyrene fluorescence; Surface tension; Ellipsometry;

The adhesive and rheological properties of model acrylic pressure-sensitive adhesive (PSA) films prepared from high solid emulsions with different particle sizes and distributions have been investigated with a customized probe tack apparatus. For each emulsion, the monomer composition and gel content were kept constant but different average particle sizes and distributions were used. Adhesive films 100 μm thick were then prepared from these emulsions and their rheological properties in the linear regime and adhesive properties were systematically characterized. Surprisingly, both the rheological and adhesive properties were found to be very dependent on the initial latex particle size distribution. A series of experiments were carried out to assess the adhesive properties of films made from blends of small- and large-particle-size latexes. Using the probe tack test, a maximum in adhesion energy of the dry films was found for 60% of small particles in the blend, a composition clearly different from that giving a minimum viscosity of the latex implying that optimizing for properties may not be equivalent to optimizing for processing in these adhesive applications. Finally, the adhesive properties of two multimodal latexes with different particle size distributions were investigated. Both gave significantly higher adhesion energies and clear evidence of a fibrillar detachment process. This important result suggests that the spatial distribution of gel domains in the dry film and the molecular connectivity between those gel domains also play an important role in controlling its adhesive properties.
Keywords: Adhesion; Pressure-sensitive adhesive; Tack; Acrylic; Particle size distribution; Gel; Adhesive; Rheology;

Nanosilica particles with different surface properties were designed and prepared using colloidal silica particles and four different qualitative silane coupling agents (SCA), namely methyltriethoxysilane (MTES), octyltriethoxysilane (OTES), vinyltriethoxysilane (VTES) and methacryloxypropyltrimethoxysilane (MATMS), and further applied in acrylic resins and corresponding polyurethane coats by in situ polymerization. It was found that different qualitative SCA molecules had very different impacts on the redispersibility of nanosilica particles, the adsorbed acrylic polyol molecules, the viscosities of acrylic polyol/silica hybrid resins, and the properties of acrylic-based polyurethane/silica composites.
Keywords: Colloidal silica; Silane coupling agent; Acrylic-based polyurethane; Coats;

A simple bimodal particle dynamics model was proposed to describe a particle system undergoing simultaneous particle formation, coagulation, coalescence, and surface growth. We extended our previous bimodal model, which used two discrete modes, to account for the surface growth of nonspherical particles. Surface reaction from TiCl4 was considered and its effects on the formation and growth of TiO2 particles were investigated. Simulation results of the present bimodal model considering surface growth showed very good agreement with those of the moving sectional model for the growth of spherical TiO2 in the literature. Surface growth of TiO2 aggregates was also simulated by using this bimodal model and a new design diagram indicating the significance of surface reaction of TiCl4 was proposed.
Keywords: Particle dynamics; Bimodal distribution; Coalescence; Aggregate; Surface growth;

Centrifugation equilibrium for spheres and spherocylinders by L.S.F. Martins; F.W. Tavares; R.P. Peçanha; M. Castier (360-367).
The centrifugation equilibrium problem is formulated and solved using a new procedure in which the specified variables are the temperature, system volume, particle dimensions and concentrations, angular speed, cell length, and cell distance from the rotation axis. As a result, we obtain the concentration profiles for all types of particles present in the system, which are considered to be immersed in a fluid. The particles are modeled as hard nonattractive spherocylinders using an equation of state, but the procedure is not restricted to any geometrical shape, and can be used with any equation of state available. The fluid is treated as a continuous medium, responsible for centrifugal buoyancy. We make calculations for colloidal suspensions of silica, often used for separations in biotechnology. Results are in good agreement with experiments and show excellent agreement in comparison with Monte Carlo simulations. Our calculations also predict focusing and shifting phenomena that have been experimentally observed in separations of fine particles.
Keywords: Centrifugation; Equilibrium; Equations of state; Spheres; Spherocylinders; Colloidal suspensions;

Magnetic birefringence of minerals by Barry R. Jennings; Stephen R. Wilson; Peter J. Ridler (368-376).
The earliest reports of magnetically induced optical birefringence included data for liquids, magnetic fluids and colloidal suspensions. Recent work has shown that with relatively straightforward apparatus, when carefully designed and aligned, measurable effects can be recorded even for suspensions of relatively weak diamagnetic materials, including mineral particles. By recording the magnitude of the birefringence induced in magnetic fields of up to two Tesla, a method for the analysis of the magnetic and optical characteristics of these diamagnetic colloids is evidenced. The principles, apparatus and methodology involved are described and novel data reported for the minerals attapulgite, bentonite, hectorite, kaolinite, montmorillonite and vermiculite. Preliminary experiments using pulsed fields on vermiculite sols show that, in favourable circumstances, estimates of particle size can be made by analysing signal response rates.
Keywords: Clay minerals; Magnetic characterisation; Magneto-optics; Cotton–Mouton effect; Diamagnetic colloids; Instrumentation;

One of the most debated effects the magnetic fields exert on aqueous solutions and dispersions is their influence on the crystal structure of the main scale component, CaCO3. This study presents the results of an experimental program performed to quantitatively evaluate influence of the key magnetic treatment parameters—magnetic induction, exposure time, and fluid velocity—on the polymorph composition of CaCO3, precipitated from carbonized aqueous solutions. The results show that magnetic treatment favored the precipitation of aragonite. The key treatment parameters affecting the aragonite content were the magnetic induction and the exposure time, while the fluid velocity exerted no significant influence. The magnetic field has no significant influence on the zeta potential of the precipitated particles in any stage of the treatment. These experimental findings indicate that the magnetic field influence on the crystal structure of CaCO3 cannot be attributed to the magnetohydrodynamic influence on the charge distribution within the electrical double layer of the forming crystallites. The results rather suggest that the magnetic fields influence the CaCO3 polymorph phase equilibrium either by influencing the CO2/water interface or through the hydration of CO 3 2 − ions prior to the formation of stable crystal nuclei in the solution.
Keywords: Crystal structure; Crystal morphology; Nucleation; Zeta potential; Magnetic fields; Calcium carbonate;

The rheological behavior of concentrated alumina-coated TiO2 slurries has been investigated in connection with the type of surface counterions (monovalent cations: X = Li+, Na+, TMA+) in the absence and in the presence of polyacrylic acid (PAA). The study has been conducted in a pH range of 4–10 and with ionic strengths lower than 0.01 M. The pH and ionic strength were adjusted with XOH and XCl, respectively. The surface properties have been investigated by titration of surface counterions and the apparent yield stress has been measured using a dynamic stress rheometer. It appears from the results that the pH at the maximum yield stress and the magnitude of the yield stress depend on the nature of the counterion. The yield stress measurements were also conducted in the presence of PAA (0.5 segment/nm2) adsorbed on the particle surface. In that case, the mineral surface and adsorbed polymer were neutralized with XOH. The results show that the dispersion efficiency depends on the polymer counterion. In general, it is found that the maximum yield stress and the corresponding counterion surface density both follow the sequence TMA+ < Na+ < Li+. The adsorption of PAA apparently amplifies the effects observed with the corresponding cation. An electrostriction effect of the hydration layer at the interface is suggested in order to explain the increasing yield stress as the surface density of Li+ increases. The so-called structure-making/structure-breaking model explains the yield stress reduction with the TMA+ surface density.

The early events pertaining to gelatin adsorption and desorption onto quartz surfaces were studied, employing an electrode-separated piezoelectric sensor (ESPS). The adsorption of gelatin on a quartz crystal surface corresponds to a mass increase, which can be monitored in real time by the changes in the impedance parameters of the ESPS. It was shown that the adsorption of gelatin on a quartz surface is partly irreversible with respect to the dilution of the bulk phase. The observed adsorption kinetics is compatible with a mechanism that involves adsorption, desorption, and transformation from a reversible adsorption state to irreversible one. A progressive approach method was established to simulate the adsorption process. The adsorption densities and kinetic parameters in the early adsorption process were obtained from the responses of the ESPS in the adsorption process. The influence of pH and ionic strength was tested. A comparison with the Langmuir adsorption model was made.
Keywords: Adsorption kinetics; Gelatin; Piezoelectric sensor; Electrode-separated;

The room temperature reaction of 4-(triethoxysilyl)butyronitrile, 4-TBN ((C2H5O)3 ―Si―(CH2)3 ―CN), on weakly hydrated silica samples pretreated at 393 K has been studied by desorption experiments and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy at different aging times under various water partial pressures. The reaction is demonstrated by the decrease of desorption of 4-TBN with time and the simultaneous disappearance of the 2980 and 1394 cm−1 signals in the DRIFT spectra, assigned to the CH3 moiety of the ethoxy functions. Water partial pressure is shown to have a crucial effect on the rate and efficiency of the process as, after 6 days, for samples kept at room temperature under vacuum, ca. 50% of the silane has reacted, while for those kept in a water-saturated atmosphere the silane reaction reaches 96%. Although the silane appears to be irreversibly bonded to the surface, no definite conclusion may be drawn from these preliminary results as to the nature of the bonding (grafting or coating). These samples are compared to modified silicas prepared according to conventional methods. The same extent of silane reaction (50%) is achieved for preadsorbed samples kept under vacuum and either cured at 473 K for 30 h or kept at room temperature for 6 days. A mild and controlled modification of silica by triethoxysilanes can thus be achieved by first physisorbing known amounts of the modifying silanes from an organic solvent on pretreated silica and then letting the samples mature for a few days at room temperature in a water-saturated atmosphere.
Keywords: Modified silica; Alkoxysilanes; Grafting; Coating; DRIFT;

First insight into catalytic activity of anionic iron porphyrins immobilized on exfoliated layered double hydroxides by Shirley Nakagaki; Matilte Halma; Alesandro Bail; Gregório Guadalupe Carbajal Arízaga; Fernando Wypych (417-423).
Mg–Al layered double hydroxide (LDH) intercalated with glycinate anions was synthesized through co-precipitation and exfoliated in formamide and the single-layer suspension was reacted with aqueous iron porphyrin solutions (Fe(TDFSPP) and Fe(TCFSPP)). The obtained materials were characterized by X-ray powder diffraction, UV–vis, and electron paramagnetic resonance and investigated in the oxidation reaction of cyclooctene and cyclohexane using iodosylbenzene as oxidant. The iron porphyrin seems to be immobilized at the surface of the glycinate intercalated LDH. The catalytic activities obtained in heterogeneous media for iron porphyrin, Fe(TDFSPP), was superior to the results obtained under homogeneous conditions, but the opposite effect was observed on the Fe(TCFSPP), indicating that, instead of the structural similarity of both iron porphyrins (second-generation porphyrins), the immobilization of each one produced different catalysts. The best catalytic activity of the Fe(TDFSPP)/Gly-LDH, compared to Fe(TCFSPP)/Gly-LDH, can be explained by the easy access of the oxidant and the substrate to the catalytic sites in the former, probably located at the surface of the layered double hydroxide pillared with glycinate anions. A model for the immobilization and a mechanism for the oxidation reaction will be discussed.
Keywords: Iron porphyrin; Supported catalysts; Layered double hydroxide (LDH); Exfoliation; Oxidation; Catalysis;

Removal of toxic metals from wastewater by Brazilian natural scolecite by Sandra Maria Dal Bosco; Ricardo Sarti Jimenez; Wagner Alves Carvalho (424-431).
The cation-exchange capacity of Brazilian natural zeolite, identified as scolecite, was studied with the aim of evaluating its applications in wastewater control. We investigated the process of sorption of chromium(III), nickel(II), cadmium(II), and manganese(II) in synthetic aqueous effluents, including sorption isotherms of single-metal solutions at 298, 313, and 333 K, by batch experiments, and the influence of pH on the process. The results have demonstrated that removal of metals from specific metal solutions is best described by a Freundlich isotherm, in which the values obtained for the K f constants were in the following order: Cr > Mn > Cd > Ni. A Lagergren pseudo-second-order was the model that best described the sorption mechanism. The retention of metals was shown to be a function of the pH; the maximum binding capacity occurring at pH values around 6.0. Thermodynamic data indicate the spontaneity of the endothermic cation-exchange process. The values of Δ G 0 suggest the following selectivity series at 298 K: Ni > Cr > Cd > Mn. The desorption process reaches equilibrium during the first 60 min of binding, suggesting that the mechanism involves specific sites located in the external surface of the scolecite.
Keywords: Natural zeolites; Cation exchange; Scolecite; Toxic metals; Sorption isotherm; Wastewater; Metal removal; Brazilian zeolites;

Preparation of mesoporous Fe3O4 (magnetite) hollow spheres has been reported using hydrothermal synthesis and calcinations. The carboxyl-functionalized PS spheres were used as the templates coated by Fe3O4 particles and ethylene glycol (EG) as an organic structure directing agent. PS and EG were removed by calcinations method. The surface area after calcination at 500 °C is found to be 74 m2 g−1. The hollow spheres exhibited the weak ferromagnetism.
Keywords: Mesoporous; Magnetite; Hollow spheres; Calcinations; Ferromagnetism;

An activated carbon with high specific surface area was prepared from polyurethane foam by chemical activation with K2CO3 and the influences of carbonization temperature and impregnation ratio on the pore structure of the prepared activated carbon were investigated. It was found that the specific surface area of the activated carbon was at a maximum value (about 2800 m2/g) at a carbonization temperature of 1073 K and at an impregnation ratio of 1.0. It was concluded that the polyurethane foam structure was modified during impregnation by K2CO3, K2CO3 promoted charring during carbonization, and then the weight loss behavior was changed below 700 and above 1000 K, carbon in the char was consumed by K2CO3 reduction, and this led to the high specific surface area. The prepared activated carbon had a very sharp micropore size distribution, compared with the commercial activated carbon having high specific surface area. The amounts of three organic vapors (benzene, acetone, and octane) adsorbed on the prepared activated carbons was much larger than those on the traditional coconut shell AC and the same as those on the commercial activated carbon except for octane. We surmised that the high specific surface area was due to the modification of the carbonization behavior of polyurethane foam by K2CO3.
Keywords: Activated carbon; Polyurethane foam; K2CO3 activation; Porosity; VOC;

This paper presents a new form of the Cahn–Hilliard equation. The new equation is substantially more robust to simulating intrinsic instabilities of the spinodal dewetting process compared to the standard one.
Keywords: Film; Dewetting; Spinodal; Nonlinearity; Instability;

In this paper, an adhesion model for spherical noncontact is proposed based on the Maugis–Dugdale (MD) adhesive contact model. The proposed noncontact model is combined with the MD contact model, thus providing a full range adhesion model with analytical transition from noncontacting to contacting asperity geometry. The proposed model is favorably compared with the full range improved DMT model for low surface energy values. The transition process from noncontact to contact and the adhesion instability that occurs during this transition are also investigated. It is found that jump-off points are different for displacement control and force control. Moreover, under displacement control, jump-on and jump-off points are different when the adhesion parameter λ is greater than 0.95, whereas they are identical for λ < 0.95 . By curve fitting a relationship between the critical approach under displacement and force control separately and the adhesion parameter λ, approach prediction equations for jump-on and jump-off under different adhesion levels were obtained.
Keywords: Maugis–Dugdale; Adhesive instability; Head/disk interface;

Anisotropy in the wetting of rough surfaces by Yong Chen; Bo He; Junghoon Lee; Neelesh A. Patankar (458-464).
Surface roughness amplifies the water-repellency of hydrophobic materials. If the roughness geometry is, on average, isotropic then the shape of a sessile drop is almost spherical and the apparent contact angle of the drop on the rough surface is nearly uniform along the contact line. If the roughness geometry is not isotropic, e.g., parallel grooves, then the apparent contact angle is no longer uniform along the contact line. The apparent contact angles observed perpendicular and parallel to the direction of the grooves are different. A better understanding of this problem is critical in designing rough superhydrophobic surfaces. The primary objective of this work is to determine the mechanism of anisotropic wetting and to propose a methodology to quantify the apparent contact angles and the drop shape. We report a theoretical and an experimental study of wetting of surfaces with parallel groove geometry.

Advancing contact-angle ( θ ) measurements were carried out with aqueous solutions of propanol and four series of aqueous solutions of dodecyl sulfate (SDDS) and propanol mixtures at constant dodecyl sulfate concentrations equal to 1 × 10 −5 , 6 × 10 −4 , 1 × 10 −3 , and 1 × 10 −2 M , respectively. The obtained results indicate that in the range of propanol concentrations studied there were considerable contact-angle changes, with exception of the solution series at a constant concentration value of SDDS higher than its critical micelle concentration. From the results of contact-angle measurements and application of the Gibbs and Young equations the ratio of the excess concentration of surfactant and propanol at the solid–aqueous solution interface to the excess of their concentration at the aqueous solution–air interface was calculated. From the calculations it appears that there is a straight linear dependence between the adhesion tension and surface tension of aqueous solutions of SDDS and propanol mixtures, and the slope of the line is equal to −1, which suggests that the surface excess of the SDDS and propanol mixture at the polytetrafluoroethylene–solution interface is the same as the at the solution–air interface. The extrapolation of the straight line to the point corresponding to the surface tension of the aqueous solution, which completely spreads over the polytetrafluoroethylene surface, gives a critical surface tension of wetting equal to 23.7 mN/m. On the basis of the critical surface tension and the Young and modified Szyszkowski equations it was found that in a polytetrafluoroethylene–aqueous solution of the SDDS and propanol mixture, the interface tension can be predicted by the modified Szyszkowski equation.
Keywords: Polytetrafluoroethylene; Sodium dodecyl sulfate; Propanol; Wettability; Critical surface tension; Modified Szyszkowski equation;

Effect of the spacer length on the association and adsorption behavior of dissymmetric gemini surfactants by M. Sikirić; I. Primožič; Y. Talmon; N. Filipović-Vinceković (473-481).
A series of dissymmetric gemini surfactants with the general formula [C12H25(CH3)2N(CH2) s N(CH3)2C14H29]Br2 designed as 12–s–14, where s = 2 , 6, and 10, were synthesized and their physicochemical properties investigated. The effect of spacer length on Krafft temperature, adsorption at the air/solution interface, and association in aqueous solution was studied by tensiometry, conductometry, and cryo-transmission electron microscopy. The Krafft temperature was found to increase linearly with spacer length. In the submicellar concentration range the dissymmetric 12–s–14 surfactants display ion pairing and premicellar association. Adsorption at air/solution interfaces and micellization in aqueous solution are similar to the behavior of their symmetric counterparts and depend strongly on spacer length.
Keywords: Adsorption; Association; Dissymmetric gemini surfactants; Krafft point; Spacer length;

Examination of the pseudophase model of monomer-micelle interconversion in cetylpyridinium chloride by Santiago J. Yunes; Nicholas D. Gillitt; Clifford A. Bunton (482-487).
The 35Cl NMR chemical shift and line width and the 1H chemical shifts of cetylpyridinium chloride, CPyCl, change abruptly at the critical micelle concentration, indicating conversion of monomeric surfactant into micelles within a very small range of concentration. The simple pseudophase treatment fits these results up to 0.05 M CPyCl, but there then appears to be a modest change in micellar structure. Premicelles of single chain surfactants, detected kinetically or photochemically, are probably formed by interactions between reactant(s) and surfactant.
Keywords: Micellization; Premicelles; Pseudophase treatment; NMR spectroscopy;

In this work, we present an experimental methodology to investigate the dynamics under shear flow of a drop that is gelling as a consequence of a temperature quench. The experiments were carried out on the system water/gellan/κ-carrageenan in the biphasic region of the phase diagram, the gellan–rich phase being used as the dispersed phase. Gelation was brought about by lowering the temperature during flow after steady state drop deformation had been reached. Simple shear flow was applied by using a parallel plate apparatus equipped with optical microscopy and image analysis, which made it possible to monitor drop shape evolution before, during, and after gelation. The onset of gelation trapped drop deformation, thus producing anisotropic particles. The fingerprint of gelation was the simultaneous tumbling of the drops, which rotated as rigid ellipsoids under the action of shear flow. Interfacial tension between the two equilibrium phases was determined at different times during the temperature quench by analyzing drop retraction upon cessation of flow. Up to gelation, no significant change was observed in the measured values.
Keywords: Gellan; κ-Carrageenan; Gelation; Drop; Shear flow;

Entropic interfaces in hard-core model amphiphilic mixtures by Joseph M. Brader; Matthias Schmidt (495-502).
We investigate bulk and interfacial properties of a recently proposed hard-body model for a ternary mixture of amphiphilic particles, spheres and needles using density functional theory. The simple model amphiphiles are formed by bonding a vanishingly thin needle tail radially to a hard-sphere head group. Such particles provide a natural amphiphile when added to a binary mixture of spheres and needles. As all interactions are hard, we seek to find whether amphiphilic effects can be driven by entropy without the need to invoke attractive interactions. In order to assess the amphiphilic character of the model we first examine the spatial and orientational distribution of the amphiphiles at the free interface between demixed needle-rich and amphiphile-rich fluid phases of the binary amphiphile–needle subsystem. We then consider the free interface between sphere-rich and needle-rich phases upon adding amphiphiles with low concentration to the demixed system. In both cases the orientational distribution of the particles in the interface provides strong evidence that amphiphilic properties can arise purely from geometrical packing effects.

Determination of stagnant layer conductivity in polystyrene suspensions: temperature effects by M.L. Jiménez; F.J. Arroyo; F. Carrique; U. Kaatze; A.V. Delgado (503-509).
In the classical theory of electrokinetic phenomena, it is admitted that the whole electrokinetic behavior of any colloidal system is fully determined by the zeta potential, ζ, of the interface. However, both experimental data and theoretical models have shown that this is an incomplete picture, as ions in the stagnant layer (the region between the solid surface and the slip plane—the plane where the equilibrium potential equals ζ) may respond to the field. In this paper, we aim at the evaluation of this contribution by the estimation of both K SL σ (the surface conductivity of the stagnant layer) and K d σ (the conductivity associated with the diffuse layer). This will be done by measuring the high-frequency dielectric dispersion (HFDD) in polystyrene suspensions; here “high-frequency” means the frequency interval where Maxwell–Wagner–O'Konski relaxation takes place (typically at MHz frequencies). Prior to any conclusions, a treatment of electrode polarization effects in the measurements was needed: we used two methods, and both led to similar results. Simulating the existence of surface conductivity by bulk conductivity, we reached the conclusion that no consistent explanation can be given for our HFDD and additional electrophoresis data based on the existence of diffuse-layer conductivity alone. We thus show how K SL σ can be estimated and demonstrate that it can be explained by an ionic mobility very close to that characteristic of ions in the bulk solution. Such mobility, and hence also the values of K SL σ , increases with temperature as expected on simple physical grounds.
Keywords: Dielectric dispersion; Polystyrene; Stagnant-layer conductivity; Surface conductivity; Zeta potential;

Low-temperature synthesis of anatase–brookite composite nanocrystals: the junction effect on photocatalytic activity by Toshiaki Ozawa; Mitsunobu Iwasaki; Hiroaki Tada; Tomoki Akita; Koji Tanaka; Seishiro Ito (510-513).
Anatase–brookite composite nanocrystals have successfully been synthesized at 50 °C using a simple liquid-phase process. The photocatalytic activity of the sample for the gas-phase oxidation of CH3CHO is 5.4 times greater than that of a single-phase anatase sample with comparable crystallite size and surface area. Electron energy loss spectra suggest that this high activity results from junction between anatase and brookite crystals.
Keywords: Fine particles; Photocatalyst; TiO2; Anatase; Brookite;