Journal of Colloid And Interface Science (v.301, #1)

The double layer structure of metal (hydr)oxides is discussed. Charge separation may exist between the minimum distance of approach of electrolyte ions and the DDL domain. The corresponding capacitance value of the outer Stern layer is similar to the capacitance value of the inner Stern layer. The extended Stern model implicitly supports a hydration structure at the near-surface with some discrete layering of water and electrolyte ions. The significance of dipole orientation is analyzed theoretically. Dipole theory in combination with a calculated ion charge distribution is compared with the experimental overall charge distribution. Ion charge distribution for various oxyanions has been calculated applying the Brown bond valence concept to the geometry of surface complexes that have been optimized with MO/DFT calculations. The comparison is done in detail for silicic acid adsorption on goethite. In addition, results are discussed for arsenite, carbonate, sulfate, and phosphate, using the same approach. The dipole correction depends on the charge introduced in a neutral surface by ion adsorption, which differs for the various ions studied. The fractional correction factor ϕ derived for the experimental data agrees with the theoretical value ϕ m = 0.17 ± 0.02 . On an absolute scale, the dipole corrections are usually limited to the range about 0–0.15 v.u. The CD values calculated with MO/DFT are not particularly sensitive (∼0.03 v.u.) to the precise Fe-octahedral geometry, which suggests that a calculated CD is a reasonable approximation in ion adsorption modeling for ill-defined Fe-oxides like HFO and natural Fe oxide materials of soils.Introduction of ion charge results in structuring of near-surface water, which induces a dipole feedback that in combination with a quantum-chemically calculated ion charge distribution determines the overall CD coefficients.
Keywords: Hematite; HFO; Anatase; Rutile; Magnetite; Alumina; Lithium; Rubidium; Cesium; Cadmium; Mercury; Copper; Arsenite; Arsenate; Silicic acid; Phosphate; Sulfate; Carbonate; Surface complex; CD model; MUSIC; EXAFS; XRS; ATR-FTIR;

Layered double hydroxides (LDHs) have shown great promise as anion getters. In this paper, we demonstrate that the sorption capability of a LDH for a specific oxyanion can be greatly increased by appropriately manipulating material composition and structure. We have synthesized a large set of LDH materials with various combinations of metal cations, interlayer anions, and molar ratios of divalent cation M(II) to trivalent cation M(III). The synthesized materials have then been tested systematically for their sorption capabilities for pertechnetate (TcO 4). It is discovered that for a given interlayer anion (either CO2− 3 or NO 3) the Ni–Al LDH with a Ni/Al ratio of 3:1 exhibits the highest sorption capability among all the materials tested. The sorption of TcO 4 on M(II)–M(III)–CO3 LDHs may be dominated by the edge sites of LDH layers and correlated with the basal spacing d 003 of the materials, which increases with the decreasing radii of both divalent and trivalent cations. The sorption reaches its maximum when the layer spacing is just large enough for a pertechnetate anion to fit into a cage space among three adjacent octahedra of metal hydroxides at the edge. Furthermore, the sorption is found to increase with the crystallinity of the materials. For a given combination of metal cations and an interlayer anion, the best crystalline LDH material is obtained generally with a M(II)/M(III) ratio of 3:1. Synthesis with readily exchangeable nitrate as an interlayer anion greatly increases the sorption capability of a LDH material for pertechnetate. The work reported here will help to establish a general structure–property relationship for the related layered materials.Pertechnetate sorption on layered double hydroxides increases with basal spacing d 003 . Further expansion of d 003 by replacing carbonate with sulfate results in LDHs with no sorption capabilities, suggesting a possible cage effect on pertechnetate sorption.
Keywords: Layered double hydroxide; Pertechnetane anion; Surface adsorption; Edge site adsorption; Structure–property relationship; Cage effect; Oxyanion; Waste disposal;

In order to prevent engine failure, the oil must be changed before it loses its protective properties. It is necessary to monitor the actual physical and chemical condition of the oil to reliably determine the optimum oil-change interval. Our study focuses on the condition of the lubricating oil in an operated car engine. Shear stress curves and viscosity curves as a function of the shear rate for fresh and used lubricating oil were examined. Metal nitrate was detected in the lubricating oil from the operated car engine through the use of a chelating self-assembled monolayer.This article focuses on trace of metal ions in used lubricating oil from a passenger car by chelating self-assembled monolayer fabricated on the surface of the silicon wafer.
Keywords: Self-assembled monolayer; Lubricating oil; Metal ion; XPS; Detection;

The adsorption of hyperbranched polymers on silicon oxide surfaces by Sedat Ondaral; Lars Wågberg; Lars-Erik Enarsson (32-39).
The electrostatic interaction between quarternised hyperbranched polymers (polyesteramides) and a silicon oxide (SiO2) surface has been studied via adsorption studies with quartz crystal microbalance instrument with dissipation (QCM-D). Frequency shift (Δf) results show that the increase in both pH and salt concentration positively affect the adsorbed amount of these polymers, calculated by Sauerbrey equation, on the QCM crystal. The adsorbed amount of HA1 (with lower molecular weight and higher charge density) was lower than that of HA2 (with higher molecular weight and lower charge density) in all experiments. It was also observed that there are no significant changes in the dissipation after adsorption of these polymers. This indicated that both hyperbranched polymers formed rigid adsorbed layers on the negative SiO2 surface. Additionally, the results were compared with adsorption of poly-DADMAC and the results showed that the hyperbranched polymers formed thicker and more rigid layers as compared with the poly-DADMAC.Charge density of the hyperbranched polymers as a function of pH. The charge densities of polymers were determined by polyelectrolyte titration and pH was adjusted by addition of HCl and NaOH.
Keywords: Hyperbranched polymers; Quartz crystal microbalance; Adsorption; Sauerbrey equation; Layer thickness; Silicon oxide;

Comparison of isotherms for the ion exchange of Pb(II) from aqueous solution onto homoionic clinoptilolite by M.S. Berber-Mendoza; R. Leyva-Ramos; P. Alonso-Davila; L. Fuentes-Rubio; R.M. Guerrero-Coronado (40-45).
The ion exchange equilibrium of Pb(II) on clinoptilolite modified with NH4Cl and NaCl can be represented by two types of isotherms. The first one is the ion exchange isotherm based upon the constant of thermodynamic equilibrium for the ion exchange reaction; however, the fitting procedure for this isotherm can be very tedious due to all the calculations involved and additional thermodynamic data. The second one is the Langmuir isotherm. The use of the Langmuir isotherm to represent ion exchange equilibrium has increased in recent last years since it adequately fits the equilibrium data and, furthermore, its calculation is much simpler. A comparison between the two isotherms showed that they fitted the experimental data reasonably well, but the Langmuir isotherm is much simpler and easier to use.The ion exchange isotherm based on the thermodynamic equilibrium constant and the Langmuir isotherm fitted the ion exchange equilibrium data of Pb(II) from aqueous solution on homoionic clinoptilolite reasonably well.
Keywords: Clinoptilolite; Ion exchange; Isotherm; Lead; Zeolite;

To develop the polymeric adsorbent that possess anionic exchangeable function, PP-g-AA-Am fibers were prepared by photoinduced grafting of acrylic acid (AA) onto polypropylene (PP) nonwoven fibers and subsequent conversion of carboxyl group in grafted AA to an amine (Am) group by reaction with diethylene triamine (DETA). The amination of grafted AA increased with increase in the degree of grafting, the reaction time and temperature of the chemical modification process. Catalytic effect of metal chlorides such as AlCl3 and FeCl3 on the amination of grafted AA was significant but not essential to lead the amination. FT-IR and solid 13C NMR data indicate that amine group was introduced into PP-g-AA fiber through amide linkage between grafted AA and DETA. The anion exchange capacity of PP-g-AA-Am fiber increased with increase in the degree of amination, but reached maximum value at about 60% amination of 150% grafted AA. PP-g-AA-Am fiber showed much higher maximum capacity for PO4–P and a similar capacity for NO3–N compared to commercial anion resins. Furthermore, the PP-g-AA-Am fiber also has adsorption ability for cations because of unaminated residual carboxyl group.
Keywords: Anion exchanger; Photoinduced grafting; Amination; Polypropylene nonwoven fabric; Acrylic acid; DETA;

Removal of methylene blue dye from an aqueous media using superabsorbent hydrogel supported on modified polysaccharide by Alexandre T. Paulino; Marcos R. Guilherme; Adriano V. Reis; Gilsinei M. Campese; Edvani C. Muniz; Jorge Nozaki (55-62).
The removal of methylene blue (MB) in water with the superabsorbent hydrogel (SH) formed by modified gum arabic, polyacrylate, and polyacrylamide was investigated. The SH exhibited excellent performance in MB absorption. The maximum absorption capacity was 48 mg of the dye per g of SH, representing 98% of the MB removed. Experimental parameters were used as follows: pH 8, hydrogel mass 50 mg, and initial concentration of MB 50 mg L−1. In a procedure with an individual solution of orange II, an opposite effect related to the MB was observed: the hydrogel only absorbed water, resulting in an orange II-richer solution. The orange II concentration in solution increased about 50 times (relative to the initial concentration). In another experiment using an aqueous mixture of orange II and MB, the SH absorbed the MB exclusively. Compared to the MB, the orange II is separated from water by SH selectivity-absorption through an inverse process. This effect was attributed to the formation of a ionic complex between the imine groups of MB and the ionized carboxylic groups of SH.The hydrogel has an excellent performance of methylene blue absorption (ca. 98%). This effect has assigned to the formation of an ionic complex between the imines groups of methylene blue and the ionized carboxylic groups of hydrogel.
Keywords: Textile wastewater; Color removal; Superabsorbent hydrogel; Gum arabic;

The interaction in aqueous solution between either the normal block copolymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide): Pluronic PE6200 [(EO)11–(PO)28–(EO)11], or the reverse block copolymer poly(propylene oxide)–poly(ethylene oxide)–poly(propylene oxide): Pluronic 25R4 [(PO)19–(EO)33–(PO)19] and the surfactants sodium decylsulfate, C10OS, decyltrimethyl ammonium bromide, C10TAB, and pentaethylene glycol monodecyl ether, C10E5, was investigated and the aggregation behavior of these surfactants with Pluronics was compared. Surface tension measurements show that Pluronics in their non-aggregated state better interact with the anionic surfactant C10OS than with cationic and non-ionic ones. The presence of the two Pluronics induces the same lowering of the aggregation number of C10OS as shown by fluorescence quenching measurements. The number of polymer chains necessary to bind each C10OS aggregate has been estimated to be ∼6 for PE6200 and ∼2 for 25R4. Furthermore, this surfactant also induces the same increment in the gyration radius of the polymers as revealed by viscosimetry. Calorimetric results have been reasonably reproduced by applying a simple equilibrium model to the aggregation processes.
Keywords: Pluronic; Surfactant; Interaction; Surface tension; Fluorimetry; Calorimetry;

ZnO/PS core–shell hybrid microspheres prepared with miniemulsion polymerization by JianJun Zhang; Ge Gao; Ming Zhang; Dan Zhang; ChunLei Wang; DaCheng Zhao; FengQi Liu (78-84).
The hybrid microspheres of ZnO/PS with different core–shell structures were prepared in miniemulsion polymerization. 3-(trimethoxysilyl)propyl methacrylate (MPS) was used as a functional comonomer to enhance the surface polarity of ZnO nanoparticles and to prevent water from quenching the luminescent properties of ZnO. The morphology of hybrid particles was examined with a transmission electron microscope. The luminescence spectra were measured using a Shimadzu RF-5301 PC spectrofluorimeter (Xe source) at room temperature. The crystallization structure of samples was characterized with a Rigaku wide-angle X-ray diffractometer. The chemical composition and structure of the ZnO colloids, MPS-modified ZnO colloids, and ZnO/PS hybrid microspheres were analyzed with IR.ZnO/PS hybrid microspheres with different core–shell structures were prepared in miniemulsion polymerization. The morphology, luminescence spectra and the crystallization structure of hybrid particles were examined with TEM, spectrofluorimeter and XRD, respectively.
Keywords: ZnO colloids; Miniemulsion polymerization; Hybrid microspheres;

Double-responsive polymer brushes on the surface of colloid particles by Mingming Zhang; Li Liu; Hanying Zhao; Yi Yang; Guoqi Fu; Binglin He (85-91).
Well-defined poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were synthesized on the surface of polystyrene latex particles by atom transfer radical polymerization (ATRP). It was found that the surface-initiated polymerization of DMAEMA catalyzed by CuCl/CuCl2/bpy was under good control in the solvent of acetone/water at ambient temperature (35 °C). High-density PDMAEMA brushes with low polydispersity (PDI 1.21) were obtained. TEM results demonstrate that the PDMAEMA-grafted particles have core–shell structure. Dynamic light scattering studies indicate that the particles with PDMAEMA brushes are both pH and temperature responsive.pH/Temperature-responsive polymer brushes were synthesized by ATRP of 2-(dimethylamino)ethyl methacrylate from the surface of functionalized polystyrene latex. The polymerization catalyzed by CuCl/CuCl2/bpy was under good control in acetone/H2O.
Keywords: Atom transfer radical polymerization (ATRP); Polymer brush; Stimuli-sensitive polymers;

We show that ionically self-assembled polyelectrolyte/surfactant complexes allow a facile route to tailor the electrical surface resistance of paper sheets for antistatic dissipative regime. We use anionic polyelectrolyte carboxymethyl cellulose (CMC) where cationic alkyltrimethylammonium chloride surfactants ( C n TAC ) with the alkyl chain lengths n = 12 , 14 or 16 methyl units are ionically complexed by precipitation from aqueous solutions. Such alkyl chains are sufficiently long to allow self-assembly in solid films after solvent evaporation. Short chain lengths, e.g., n = 8 , did not lead to precipitation. Small angle X-ray scattering indicates cylindrical self-assembly in bulk samples. Upon exposing bulk samples under humidity of 50% RH for 18 h, conductivity of ca. 10 −5   S / cm at room temperature is achieved based on AC-impedance analysis. Flexographic printing and spray coating were selected to conceptually test the feasibility as paper coatings and surface sheet resistances of ca. 10 9   Ω are reached. The results indicate that self-assembled polyelectrolyte/surfactant complexes can allow sufficient conductivity levels for antistatic paper coatings potentially due to protonic conductivity and suggest to develop processes and materials for realistic applications.

The copolymer of β-casein-graft-dextran was prepared using the Maillard reaction and the acidic solution properties of the copolymer were studied with dynamic light scattering. At pH range 4–5 where is close to the isoelectric point of β-casein, the copolymer forms micelles which are spherical verified by atomic force microscopy imaging. The size and existent time of the micelles depend on the graft degree and the length of dextran side chains of the copolymers. During storage at pH 4.6, the micelles formed by the copolymers with short side chains and low graft degree tend to precipitate, while the micelles formed by the copolymers with long side chains and high graft degree tend to dissociate. The micellization of the copolymers can be suppressed by adding NaCl. Optical microscopy and turbidity studies show that the copolymers dissolved in molecular state and with higher hydrophilicity have better emulsifying ability.
Keywords: β-Casein; Dextran; β-Casein-graft-dextran copolymer; Emulsification; Maillard reaction; Micelle; Solubility;

Hydrolysis of fluid supported membrane islands by phospholipase A2: Time-lapse imaging and kinetic analysis by Adam Cohen Simonsen; Uffe Bernchou Jensen; Per Lyngs Hansen (107-115).
The activity of phospholipase A2 (PLA2) which catalyzes the hydrolysis of phospholipids into free fatty acids and lysolipids, depends on the structure and thermodynamic state of the membrane. To further understand how the substrate conformation correlates with enzyme activity, model systems that are based on time-resolved membrane microscopy are needed. We demonstrate a methodology for preparing and investigating the dynamics of fluid supported phospholipid membranes hydrolyzed by snake venom PLA2. The method uses quantitative analysis of time-lapse fluorescence images recording the evolution of fluid bilayer islands during hydrolysis. In order to minimize interactions with the support surface, we use double bilayer islands situated on top of a complete primary supported membrane prepared by hydration of spincoated lipid films. Our minimal kinetic analysis describes adsorption of enzyme to the membrane in terms of the Langmuir isotherm as well as enzyme kinetics. We use two related models assuming hydrolysis to occur either at the perimeter or at the surface of the membrane island. We find that the adsorption constant is similar for the two cases, while the estimated turnover rate is markedly different. The PLA2 concentration series is measured in the absence and presence of β-cyclodextrin which forms water soluble complexes with the reaction products. The results demonstrate the versatility of double bilayer islands as a membrane model system and introduces a new method for quantifying the kinetics of lipase activity on membranes by directly monitoring the evolution in substrate morphology.The shape of fluid phospholipid bilayer islands is recorded during hydrolysis by phospholipase A2. Image data are analyzed quantitatively in terms of enzyme binding and catalytic turnover.

Synthesis and characterization of ZnS-montmorillonite nanocomposites and their application for degrading eosin B by Shiding Miao; Zhimin Liu; Buxing Han; Haowen Yang; Zhenjiang Miao; Zhenyu Sun (116-122).
Nanocomposites of zinc sulfide (ZnS) and montmorillonite (MMT) were prepared via a hydrothermal route. In this method, the MMT treated with hexadecyltrimethyl ammonium bromide (HTAB) aqueous solution was dispersed in the aqueous solution of thiourea and Zn(OOCHCH3)2⋅2H2O, and heated at 170 °C for about 4 h, resulting in ZnS-MMT composites. The as-prepared nanocomposites were characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption analysis. It was demonstrated that the interlayer space of MMT was enlarged from 0.98 to 3.77 nm after the treatment with HTAB aqueous solution, and the ZnS nanoparticles were deposited on the layers of MMT. Nitrogen sorption analysis demonstrated that the specific surface area of the samples decreased from 39.2 m2/g of the pristine MMT to 5.9 m2/g of the final ZnS-MMT composites. The resulting ZnS-MMT nanocomposites (50.0 mg) could degrade eosin B completely in aqueous solution (75 ml, 3.2 × 10 −5   M ) within 20 min under UV irradiation.ZnS/montmorillonite (MMT) nanocomposites prepared in this work exhibited structure with ZnS nanocrystals on the surface and intercalated into the interlayers of MMT, which showed highly efficient catalytic activity to degrade eosin B.
Keywords: Montmorillonite; ZnS; Nanocomposites; Eosin B; Degradation;

Dependence of fragmentation behavior of colloidal aggregates on their fractal structure by S. Harada; R. Tanaka; H. Nogami; M. Sawada (123-129).
The fragmentation dynamics of aggregate of non-Brownian particles in shear flow is investigated numerically. The breakup behaviors of aggregates having the same connectivity but the different space-filling properties are examined. The Lagrangian particle simulation in a linear flow field is performed. The effect of surrounding fluid on the motion of multiple particles is estimated by Stokesian dynamics approach. The inter-particle force is calculated from the retarded van der Waals potential based on the Lifshitz theory. The results obtained in this work indicate that the fragmentation behavior of colloidal aggregates depends on their fractal structure. However, if the resultant aggregate size is smaller than the critical one, the fragmentation behavior shows the universality regardless of their original structure. Furthermore, the restructuring of aggregate in shear flow and its effect on the fragmentation process are also discussed.The fragmentation dynamics of fractal and non-fractal aggregates in shear flow is investigated numerically. The fragmentation of aggregates is influenced by both the strain and the rotation effects of shear flow.
Keywords: Aggregate; Breakup; Shear flow; Numerical analysis; Restructuring; Stokesian dynamics;

In situ synthesis of sulfide-coated polystyrene composites for the fabrication of photonic crystals by Yi Jin; Yihua Zhu; Xiaoling Yang; Haibo Jiang; Chunzhong Li (130-136).
Sulfide (CdS, ZnS, and SnS2)-coated polystyrene (PS) nanocomposites with a diameter of about 160 nm were synthesized by an in situ synthesis method. The PS spheres adsorbed polyelectrolytes, which were coordinated with Cd2+ and Zn2+ reacted with sulfions released through the hydrolysis of thioacetamide in an aqueous bath. As to the SnS2-coated PS composite, we introduced a deposition method in which a thin layer of SnS2 was deposited on the surface of PS spheres. The PS spheres assembled on the patterned substrate of porous aluminum oxide (PAO) were more regular than those on the nonpatterned microslide, so the PS spheres coated with CdS and ZnS composites were assembled into ordered arrays on the PAO substrate, respectively. And the obtained λ max of CdS-coated PS colloid crystal array red-shifted 262 nm compared with that of the pure PS colloid crystal array. All the particles and colloid crystal arrays were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Energy-dispersive X-ray (EDX) was undertaken to determine the elements Cd, S, and Sn.PS sphere-adsorbed polyelectrolytes coordinated with metal ions in situ and reacted with sulfions released from the hydrolysis of thioacetamide in an aqueous bath.
Keywords: Photonic crystals; In situ; Sulfides; Semiconductors;

Concentration effects on irreversible colloid cluster aggregation and gelation of silica dispersions by Anna Schantz Zackrisson; Anna Martinelli; Aleksandar Matic; Johan Bergenholtz (137-144).
Effects of particle concentration on the irreversible aggregation of colloidal silica are studied using in situ destabilization via the ionic strength increase derived from the enzymatic hydrolysis of urea by urease. Aggregation is monitored by time-resolved optical density and dynamic light scattering measurements. It terminates at a gel boundary, signaled by a prominent increase of the optical density and incipient non-ergodicity. Raman scattering is used to demonstrate that the enzymatic reaction continues, well beyond gelation for the compositions studied here, until the urea is consumed. Calibration of the ionic conductivity permits for constructing stability diagrams in terms of particle and salt concentration. As with reversible gelation, the process exhibits a collective character in that lower ionic strengths are required for gelation of concentrated dispersions and vice versa. However, light scattering demonstrates that the gel boundary is preceded here by a line marking the transition from reversible to irreversible cluster formation, with the two transition boundaries tracking each other. Comparisons are made with dispersions destabilized by direct addition of salt solutions, which gel under very different conditions.
Keywords: Colloidal dispersions; Silica particles; Aggregation; Sol–gel transition; Optical density; Raman spectroscopy; Light scattering;

This paper describes the preparation of Au core–Au–Ag shell nanoparticles (NPs) in different morphologies by controlling both the pH and the glycine concentration. Using a seed-growth method, we prepared high-quality Au core–Au–Ag shell NPs from a glycine solution under alkaline conditions (pH > 8.5). By controlling both the pH and the glycine concentration, we prepared dumbbell-shaped and peanut-shaped Au core–Au–Ag shell NPs readily by depositing gold and silver, reduced by ascorbate, onto the gold nanorods. We have found that the glycine concentration that is optimal for preparing high-quality Au core–Au–Ag shell NPs differs at the various values of pH. At pH < 8.5, the glycine concentration is not important, but, when preparing dumbbell- and peanut-shaped Au core–Au–Ag shell NPs, it should be greater than 50 mM and greater than 20 mM at pH 9.5 and 10.5, respectively. Glycine plays a number of roles during the synthesis of the Au core–Au–Ag shell NPs by controlling the solution pH, altering the reduction potentials of gold and silver ions through forming complexes with metal ions (Au+ and Ag+), minimizing the formation of Ag2O, AgCl, and AgBr precipitates, and stabilizing the thus-prepared NPs. At pH 9.7, we observed the changes in the morphologies of the Au core–Au–Ag shell NPs—from regular (rectangular) to peanut- and dumbbell-shaped, and finally to jewel-, diamond-, and/or sphere-shaped—that occurred during the course of a 60-min reaction. In addition, we were able to affect the shapes and sizes of the Au core–Au–Ag shell NPs by controlling the reaction time.The preparation of Au core–Au–Ag shell nanoparticles in different morphologies in 0.1 M glycine solution at pH 9.7 by controlling the reaction time.
Keywords: Gold nanorod; Au core–Au–Ag shell nanoparticles; Glycine;

Simulation of the hydrodynamic drag of aggregated particles by Christian Binder; Christian Feichtinger; Hans-Joachim Schmid; Nils Thürey; Wolfgang Peukert; Ulrich Rüde (155-167).
The drag force on aggregates and partially sintered agglomerates is assessed using the lattice Boltzmann method (LBM) and accelerated Stokesian dynamics (ASD). Both methods have been compared in terms of accuracy and computational effort. It is shown that they give comparable results if all numerical parameters are controlled carefully. LBM requires a much higher computational effort, however, in contrast to ASD it is able to simulate partially sintered agglomerates as well. The results show that even a very small amount of sintering leads to a significant reduction in the drag force. The analysis of the drag force on agglomerates as well as on aggregates shows that there is no simple geometric quantity which is uniquely related to the drag force. Moreover, there is a significant variation in drag force for single aggregates at different orientations or for the orientation averaged drag force of different aggregates of the same size. This is explained by the structural effects which may lead to a variation in the drag force up to ± 20 % .Effect of sintering and structure on the drag force. Even a small amount of sintering leads to a significant reduction of the drag force.
Keywords: Simulation; Drag force; Agglomerates; Aggregates; Fractal clusters; Lattice Boltzmann method; Stokesian dynamics;

Influence of very small bubbles on particle/bubble heterocoagulation by N. Mishchuk; J. Ralston; D. Fornasiero (168-175).
Very small bubbles which partially coat the surface of particles influence whether or not heterocoagulation between a particle and a bubble occurs. The electrostatic and van der Waals forces of interaction between particles and bubbles were calculated as a function of electrolyte concentration, particle size, and the size and distributions of these very small bubbles present on the particle surface. The height of the surface force barrier was compared with the hydrodynamic pressing force under conditions of flotation. The presence of these very small bubbles has a profound effect on the interaction between particles and bubbles and, in particular, strongly decreases the critical particle radius for heterocoagulation.
Keywords: Bubble–particle interaction; Electrostatic force; Van der Waals force; Nanobubble;

Finite thickness and charge relaxation in double-layer interactions by Aldemar Torres; René van Roij; Gabriel Téllez (176-183).
We extend the classical Gouy–Chapman model of two planar parallel interacting double layers, which is used as a first approximation to describe the force between colloidal particles, by considering the finite thickness of the colloids. The formation of two additional double layers due to this finite thickness modifies the interaction force compared to the Gouy–Chapman case, in which the colloids are semi-infinite objects. In this paper we calculate this interaction force and some other size-dependent properties using a mean-field level of description, based on the Poisson–Boltzmann (PB) equation. We show that in the case of finite-size colloids, this equation can be set in a closed form depending on the geometrical parameters and on their surface charge. The corresponding linear (Debye–Hückel) theory and the well-known results for semi-infinite colloids are recovered from this formal solution after appropriate limits are taken. We use a density functional corresponding to the PB level of description to show how in the case where the total colloidal charge is fixed, it redistributes itself on their surfaces to minimize the energy of the system depending on the aforementioned parameters. We study how this charge relaxation affects the colloidal interactions.
Keywords: Electrical double layer; Finite-thickness colloids; Poisson–Boltzmann theory;

Surface enhanced Raman scattering (SERS) spectra of C60 (C70) were obtained in nonaqueous colloidal systems with newly developed methods. The enhancement factor was estimated to be over 105. This paper aims at the investigation of a fine influence mechanism for SERS in the nonliquid phase, which may help bring forth the perfect SERS mechanism. The detailed investigation is based on abundant comparative experiments where we found that the SERS effect is sensitive not only to the character of colloidal particles, dielectric constants, and polarizability, but also to the substrates, the solvent intermediate, and even the coating techniques. These detailed comparisons enrich the proofs of the SERS mechanism and provide a new way to optimize SERS systems.
Keywords: SERS; C60/C70; Nanoparticles;

Synthesis and characterization of catalytic iridium nanoparticles in imidazolium ionic liquids by Gledison S. Fonseca; Giovanna Machado; Sergio R. Teixeira; Gerhard H. Fecher; Jonder Morais; Maria C.M. Alves; Jairton Dupont (193-204).
The reduction of [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) dissolved in 1-n-butyl-3-methyl tetrafluoroborate, hexafluorophosphate and trifluoromethane sulphonate ionic liquids in the presence of 1-decene by molecular hydrogen produces Ir(0) nanoparticles. The formation of these nanoparticles follows the two-step [A → B, A + B → 2B ( k 1 , k 2 )] autocatalytic mechanism. The same mean diameter values of around 2–3 nm were estimated from in situ TEM and SAXS analyses of the Ir(0) nanoparticles dispersed in the ionic liquids and by XRD of the isolated material. XPS and EXAFS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the iridium nanoparticles. SAXS analysis indicated the formation of an ionic liquid layer surrounding the metal particles with an extended molecular length of around 2.8–4.0 nm depending on the type of the anion.
Keywords: Ionic liquids; Hydrogenation; Nanoparticles; Iridium; EXAFS; SAXS; XPS; TEM; XRD;

Surface characterization by XPS, contact angle measurements and ToF-SIMS of cellulose fibers partially esterified with fatty acids by Carmen S.R. Freire; Armando J.D. Silvestre; Carlos Pascoal Neto; Alessandro Gandini; Pedro Fardim; Bjarne Holmbom (205-209).
The topochemistry of the controlled heterogeneous esterification of cellulose fibers with fatty acid chlorides of different chain length, both in swelling and non-swelling media, was assessed by X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and contact angle measurements. On the one hand, the results provided by the combined use of these three powerful techniques showed unambiguously the occurrence of the reaction at the fibers' surface and, on the other hand, the XPS results showed that the surface coverage with the fatty acid moieties increased with their chain length, but was only modestly affected by the degree of substitution (DS), suggesting that when the esterification yield was increased (higher DS values), an in-depth reaction also occurred, particularly when DMF was used as a cellulose swelling medium, involving the OH groups buried below the fibers' surface.
Keywords: Cellulose fibers; Fatty acids; Surface and in-depth esterification; Surface characterization; XPS; Contact angle; ToF-SIMS;

Thinning of wetting films formed from aqueous solutions of non-ionic surfactant by O.V. Eliseeva; R.G. Fokkink; N.A.M. Besseling; L.K. Koopal; M.A. Cohen Stuart (210-216).
We investigated the thinning of wetting films formed from aqueous solution of non-ionic triblock copolymer Pluronic F127 on the surface of silica using a home-made thin film balance and time-resolved ellipsometry. Imaging ellipsometry was used to visualize the film structures at subsequent stages of their development. The results unambiguously show that the time required for the formation of steady films strongly depends on the electrolyte concentration. When increasing the latter from 10−4 to 0.1 M, this time typically increases with several orders of magnitude, from a few minutes to several hours. Moreover, for sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified. After initial quick thinning, further thinning slows down enormously. These typical kinetic regimes are thought to result from the coupled dependencies of the bulk and interfacial properties of F127 on salt concentration. Possible explanations of the phenomenon are discussed.
Keywords: Wetting film; Relaxation; Triblock copolymers; Non-ionic polymeric surfactant;

Identification of the elastic properties of an artificial capsule membrane with the compression test: Effect of thickness by Mohamed Rachik; Dominique Barthes-Biesel; Muriel Carin; Florence Edwards-Levy (217-226).
The mechanical properties of a capsule membrane are evaluated by means of a compression experiment between two parallel plates. Since large deformations of the membrane are involved, the choice of the wall material constitutive law is essential. In this paper, we explore different classical laws to describe the behavior of the membrane and evaluate also the limit of application of the thin shell approximation. A numerical study of the compression process is performed using Abaqus software and an inverse method is used to identify the material constants of the constitutive laws. The comparison between the model predictions and experimental measurements on capsules with serum albumin–alginate membranes, indicates that the thin shell approximation is valid only for thickness to radius ratios up to 5% and that thick membranes obey non linear elastomer type constitutive laws. The Young modulus of the membrane material is found to increase non-linearly with membrane thickness, thus indicating that fabrication of thicker serum albumin–alginate walls results in capsules stiffer than expected.A finite element mechanical analysis of a compression experiment on a capsule allows to compute the membrane elastic modulus for different initial thicknesses h 0 .
Keywords: Artificial cells; Capsules; Serum albumin–alginate membrane; Compression experiment; Inverse identification;

This paper numerically evaluates the hydrodynamic drag force exerted on two highly porous spheres moving steadily along their centerline through a quiescent Newtonian fluid over a Reynolds number ranging from 0.1 to 40. At creeping-flow limit, the drag forces exerted on both spheres were approximately identical. At higher Reynolds numbers the drag force on the leading sphere (sphere #1) was higher than the following sphere (sphere #2), revealing the shading effects produced by sphere #1 on sphere #2. At dimensionless diameter β < 2 ( β = d f / 2 k 0.5 , d f and k are sphere diameter and interior permeability, respectively), the spheres can be regarded as “no-spheres” limit. At increasing β for both spheres, the drag force on sphere #2 was increased because of the more difficult advective flow through its interior, and at the same time the drag was reduced owing to the stronger wake flow produced by the denser sphere #1. The competition between these two effects leads to complicated dependence of drag force on sphere #2 on β value. These effects were minimal when β became low.Information on the hydrodynamic drag force exerted on a highly porous object is necessary when considering its motion, such as sedimentation or centrifugation of sludge flocs. This paper numerically evaluates the hydrodynamic drag force exerted on two highly porous spheres moving steadily along their centerline through a quiescent Newtonian fluid over a Reynolds number ranging from 0.1 to 40. By 3D simulation, the interaction between two nonhomogeneous porous spheres could be discussed.
Keywords: Porous; Drag force; Centerline; Simulation; 3D;

Three-dimensionally ordered macroporous (3 DOM) M/Ti (M = Zr or Ta) mixed oxides were prepared by cohydrolysis of a mixture of Zr(n-OC4H9)4/TTIP or TaCl5/TTIP (TTIP = titanium isopropoxide) combined with a polystyrene (PS) latex sphere templating technique. The resulting products exhibited homogeneous wall compositions, namely, Zr or Ta was uniformly dispersed into the TiO2 framework with the loading levels of 5, 10, and 20 mol% for Zr and 2.5, 5.0, and 7.5 mol% for Ta, respectively. The estimated macropore diameter, wall thickness, and particle size of the products ranged from 280 to 290 nm, from 30 to 50 nm, and from 10 to 12 nm, respectively. The products showed only anatase phase structure although their starting solitary metal oxides exhibited suitable crystalline structures under the same preparation conditions. Raman scattering spectroscopy showed that the crystal structure of titania had a slight interference due to the incorporation of Zr or Ta, and UV–vis diffuse reflectance spectroscopy (DRS) showed the narrower band gap of the products compared with that of pure anatase TiO2. The products exhibited mesoporous wall structures, and their BET surface areas were higher than those of the corresponding pure 3 DOM metal oxides. The UV-light photocatalytic activity of the products was assessed by monitoring the photodegradation of two organic molecules including 4-nitrophenol (4-NP) and rhodamine B (RB). Both the photocatalytic reactions confirmed that the presence of the second metal oxide in the titania framework resulted in enhanced photocatalytic activity compared with the pure titania framework.Three-dimensionally ordered macroporous TiO2/Ta2O5 (J) and TiO2/ZrO2 (H) mixed oxides were prepared via sol–gel and polystyrene (PS) latex sphere templating technique. The photocatalytic reactions confirmed that mixed oxides showed enhanced photocatalytic photocatalytic activity relative to the pure titania framework.
Keywords: Photocatalytic; Macroporous; Titania; Sol–gel; Composites;

New blocking law models for dead-end constant flux microfiltration of colloids forming cakes that compressed in a linear and power law manner were derived. Constant pressure and constant flux experiments were performed using bacteria, colloidal silica, and treated natural waters to validate these new models and quantitatively verify blocking law predictions on the role of cake compressibility in microfilter fouling. Statistically invariant values of cake specific resistance and compressibility were obtained for constant flux and constant pressure operation for each feed suspension. This suggests that colloids formed cakes whose hydraulic resistance is dominated by a morphology that did not depend on their mode of deposition, confirming that the cake permeability was determined by the instantaneous pressure. Additionally, an inverse relationship between extracellular polymeric substances (EPS) secreted by bacteria and hydrodynamic flux restoration procedures was obtained demonstrating the importance of linking EPS to backwashing frequency when bacteria are present in the feed water.Quantitative description and experimental validation of new blocking laws for compressible cake microfiltration of bacteria and flocculated natural colloids are presented.
Keywords: Microfiltration; Compressible cakes; Biofouling; Water and wastewater treatment; EPS; Biofilms;

The influence of subphase temperature on miltefosine–cholesterol mixed monolayers by J. Miñones; I. Rey Gómez-Serranillos; O. Conde; P. Dynarowicz-Łątka; J. Miñones Trillo (258-266).
Effects of the subphase temperature on the surface pressure (π)–area (A) isotherms of mixed monolayers of miltefosine (hexadecylphosphocholine), a potential anticancer drug, and cholesterol were investigated at the air/water interface, which were supplemented with Brewster angle microscopy (BAM) observations. Comparison of the collapse pressure values, mean molecular areas, excess areas and excess free energy of mixing between the mixed monolayer at various molar ratios and the pure component monolayers showed that, regardless of the subphase temperature, the investigated miltefosine–cholesterol system is much more stable than that the pure component monolayers, suggesting strong attractive interactions between miltefosine and cholesterol in mixed monolayers. As a consequence, it was postulated that stable “complexes” of the two components could form at the interface, for which stoichiometry may vary with the subphase temperature. Such “surface complexes” should be responsible for the contraction of the mean molecular area and thus the high stability of the mixed monolayer.
Keywords: Miltefosine–cholesterol mixed monolayers; Langmuir mixed monolayers; Air/water interface;

A novel gemini surfactant with two hydrocarbon chains and two different hydrophilic parts containing a quaternary ammonium salt and a sulfobetaine moiety, N , N -dimethyl-N-{2-[ N ′ -methyl- N ′ -(3-sulfopropyl)-alkylammonium]ethyl}-1-alkylammonium bromides ( 2 C n AmSb , where n represents hydrocarbon chain lengths of 8, 10, 12, and 14), was synthesized by reacting N , N , N ′ -trimethylethylenediamine with n-alkyl bromide, followed by reacting with 1,3-propane sultone. The adsorption and aggregation properties of 2 C n AmSb in the absence and presence of NaCl were characterized by the measurement of surface tension, steady-state fluorescence, and dynamic light scattering. The critical micelle concentration (cmc) and surface tension at the cmc of 2 C n AmSb were compared with those for the corresponding monomeric surfactants, their mixtures, and gemini surfactants. The poor solubility in water shown by 2 C n AmSb was improved by the addition of NaCl. The cmc of 2 C n AmSb in the presence of NaCl is lower than that in the absence of NaCl, and the surface tension is almost identical for both systems. The radius of the aggregates formed by 2 C n AmSb in a solution containing NaCl is found to be 0.70 ± 0.3 , 2.5 ± 0.7 , 5.6 ± 1.4 , and 10.7 ± 2.4 nm for n = 8 , 10, 12, and 14, respectively. The particle size increases with an increase in hydrocarbon chain length; in particular, relatively large aggregates are formed for n = 12 and 14.Heterogemini surfactant with nonidentical headgroups exhibits lower cmc and lower surface tension than the corresponding monomerics and their mixture.
Keywords: Gemini surfactant; Heterogemini; Nonidentical headgroups; cmc; Surface tension; Micellization; Salt effect;

Aqueous foam stabilized by dispersed surfactant solid and lamellar liquid crystalline phase by Lok Kumar Shrestha; Durga P. Acharya; Suraj Chandra Sharma; Kenji Aramaki; Hiroshi Asaoka; Keiichi Ihara; Takeshi Tsunehiro; Hironobu Kunieda (274-281).
Foaming properties of dilute aqueous solutions of pentaglycerol monostearate (C18G5) and pentaglycerol monooleate (C18:1G5) have been studied at 25 °C. The aqueous C18G5 system formed highly persistent foams, which did not rupture for several days. Foamability and foam stability were increased on increasing the surfactant concentration in both C18G5 and C18:1G5 systems. The C18:1G5/water system showed lower foam stability compared to the C18G5/water system. Aqueous phase behavior of the C18G5 and C18:1G5 systems showed the dispersion of α-solid and L α phase respectively in water rich region at 25 °C. Stable foam in the C18G5/water system was mainly due to the finely dispersed small surfactant solid particles. The average particles diameter of α-solid and L α dispersion is found less than 1 μm and it decreases with increasing surfactant concentration. There is no appreciable difference in the particle size of the α-solid and L α dispersion; however, the foam stability differs largely. Foam stabilized by lamellar liquid crystal dispersion in C18:1G5/water system, is less stable compared to the foam stabilized by the surfactant solid dispersion in C18G5/water system. The foamability and foam stability of the surfactant systems show poor correlation with the dynamic surface tension properties.
Keywords: Foamability; Foam stability; Polyglycerol fatty acid esters; α-Solid; Lamellar liquid crystal;

Rheological properties of aqueous Pluronic–alginate systems containing liposomes by G. Grassi; A. Crevatin; R. Farra; G. Guarnieri; A. Pascotto; B. Rehimers; R. Lapasin; M. Grassi (282-290).
Rheological and erosion studies regarding a liposome-containing polymeric blend that is propaedeutic to its use in paving techniques in tubular organs, such as blood vessels, are reported. Attention is focused on an aqueous polymeric blend composed of Pluronic (PF127) and alginate (Protanal LF 10/60) because both polymers, when dissolved in water at a sufficiently high concentration, are subjected to different structural mechanisms, which are driven by temperature increase and addition of bivalent cations, respectively, and both result in marked viscoelastic and plastic properties. After proving the compatibility between PF127 and alginate, we show that the structural transition temperature of the blend, T ST , can be properly modulated. In particular, we found that T ST for an aqueous solution of pure Pluronic 20% w/w is about 21 °C and that even slight reductions in polymer concentration result in considerable T ST decrease. The addition of salts or alginate (provided as Na-alginate) provokes a substantial decrease of T ST and thus the alginate concentration in the blend should not exceed 1% w/w. In addition, liposomes slow down the structural transition but do not substantially affect the rheological properties of the system in the final state at higher temperatures, thus showing that they can be added to the polymeric blend without significant effects. Finally, erosion tests show that after contact with a source of bivalent cations, the polymeric blend containing PF127 and alginate shows an erosion resistance neatly improved with respect to the simple structured Pluronic system having the same polymer concentration. As a whole, all these results constitute the basis for future potential applications of the considered polymeric blend in tubular organs such as blood vessels.The addition of salts or Na-alginate to Pluronic F127 aqueous solution provokes a decrease of the structural transition temperature T ST and, thus, alginate concentration in the blend should not exceed 1% w/w.
Keywords: Pluronic; Alginate; Rheology; Erosion;

Electroviscous sphere–wall interactions by S.M. Tabatabaei; T.G.M. van de Ven; A.D. Rey (291-301).
A theoretical analysis is presented to determine the forces of interaction between an electrically charged spherical particle and a charged plane wall when the particle translates parallel to the wall and rotates around its axis in a symmetric electrolyte solution at rest. The electroviscous effects, arising from the coupling between the electrical and hydrodynamic equations, are determined as a solution of three partial differential equations, derived from Cox's general theory [R.G. Cox, J. Fluid Mech. 338 (1997) 1], for electroviscous ion concentration, electroviscous potential and electroviscous flow field. It is a priori assumed that the double layer thickness surrounding each charged surfaces is much smaller than the particle size. Using the matched asymptotic expansion technique, the electroviscous forces experienced by the sphere are explicitly determined analytically for small particle–wall distances, but low and intermediate Peclet numbers.The electroviscous drag and normal force acting on a charged sphere moving and rotating near a charged wall have been calculated by solving the standard set of electrokinetic and hydrodynamic equations, using a matched asymptotic expansion technique.
Keywords: Electrokinetic lift; Electroviscous drag interactions; Particle–wall interactions;

Determination of the effective charge of individual colloidal particles by Filip Strubbe; Filip Beunis; Kristiaan Neyts (302-309).
An optical method is presented that allows simultaneous determination of the diffusion constant and electrophoretic mobility of individual charged particles with radius down to 0.2 μm. By this method the size dependency of the effective charges and zeta potentials of individual particles can be investigated, as well as interparticle interactions and Brownian motion in confined geometries. The diffusion constant and mobility are determined from the power spectrum of the particle speed in a sinusoidal electrical field. The accuracy of the method was tested on PMMA spheres of known size in water. Experiments have been carried out on charged pigment particles with low concentration in a nonaqueous medium containing a charging agent. The mobility is found to be independent of the particle size.The motion of a charged colloidal particle in a sinusoidal field can be used to simultaneously determine its mobility and diffusion constant. The size dependency of the effective charge of pigment particles was investigated in a nonaqueous medium containing charged inverse micelles.
Keywords: Colloids; Electrokinetics; Zeta potential; Electrophoretic ink; Pigments; Surfactants;

Numerical values of the electrokinetic potentials of anatase at high concentration of NaI by Marek Kosmulski; Björn Granqvist; Jarl B. Rosenholm (310-314).
The electrokinetic potential of anatase (titanium dioxide) in 0.3, 0.5, and 1 mol dm−3 NaI was studied by means of AcoustoSizer 1, AcoustoSizer 2 and DT-1200. Different stirring modes were studied for DT-1200. The increase in the electrolyte concentration produced a shift in the IEP of anatase to higher pH, and for 1 mol dm−3 NaI there was no IEP at all, and the electrokinetic potential was positive over the entire pH range. The shifts in the IEP observed by means of different instruments were qualitatively consistent. On the other hand, the numerical values of the electrokinetic potentials of anatase at high concentration of NaI at low pH obtained by means of Acoustosizer 2 were substantially higher than those obtained by means of DT-1200. The discrepancies in the numerical values of the electrokinetic potentials obtained by means of different instruments increase as the ionic strength increases. The procedure of correction for the electroacoustic signal of the electrolyte is probably the main source of the discrepancies between DT-1200 and Acoustosizer at high ionic strengths. The difference in the efficiency of mixing in different stirring modes plays rather insignificant role in the observed discrepancies.

Semifluorinated alcohols in Langmuir monolayers—A comparative study by Marcin Broniatowski; Patrycja Dynarowicz-Łątka (315-322).
A series of semifluorinated alcohols differing in the proportion of the perfluorinated to hydrogenated chains length was synthesized and investigated in Langmuir monolayers using surface pressure and surface potential measurements. All the investigated semifluorinated alcohols were found to be capable of stable floating monolayer formation. The stability of monolayers was found to be higher upon increasing the length of the perfluorinated segment. A lower stability of the monolayers from alcohols having shorter perfluorinated fragment was attributed to the aggregation process, which was visualized with Brewster angle microscopy (BAM). Most condensed monolayers were formed by compounds with longer perfluorinated moiety, whereas monolayers composed by molecules with an iso-branched perfluorinated segment were found to be more expanded. The change of electric surface potential was negative along the whole compression. The maximum absolute values of ΔV varies, depending on the number of CF2 groups, from ca. − 400   mV for F6H10OH to ca. − 700   mV for F10H10OH. The dipole moments of free molecules were calculated with Hyperchem, and the obtained values were approximately the same (within the experimental error), i.e., 2.8D for all the investigated molecules, independently on the perfluorinated fragment length. The dipole moment vector was found to be virtually aligned to the main molecular axis for the studied compounds. Therefore, the observed differences in the measured values of ΔV can result from a different dielectric permittivity of a particular monolayer.Schematic representation of semifluorinated alcohol molecules in the Langmuir monolayer.
Keywords: Semifluorinated alcohols; Langmuir monolayers; Air/water interface; Dipole moments;

The imbibition of aqueous solutions of Triton X-100 in calcium fluoride columns has been studied in order to determine the influence of the interfacial adsorption of the surfactant in the capillary rise of the solutions. This system has been chosen because this surfactant behaves as non-adsorbable at the surface of this solid when it is in aqueous solution. The experiments have consisted of the measurement of the increase in the weight of the porous columns caused by the capillary rise of the solutions. The analysis of the results has been made through a modified expression of Washburn's equation that takes into account that the experimental increase in the weight is caused by the imbibition as well as by the development of a liquid meniscus around the bottom base of the columns. From this analysis, it has been deduced that the surfactant concentration does not influence on the imbibition rate, it being equal to the observed for water. However, it has been also proved that the contact angle depends on the surfactant concentration, taking decreasing values as the surface tension of the solutions decreases. In order to justify these findings, a study about the influence of the interfacial adsorption on the imbibition has been carried out. By means of them, it has been proved that the absence of adsorption at the solid–liquid interface is the reason that explains both the independence of the imbibition rate from the surfactant concentration and the decrease of the contact angle. Moreover, this fact indicates that the depletion of the surfactant molecules from the advancing meniscus, which has been normally adduced as the phenomenon causing the observed behaviour, has to be ruled out as the physical cause that justifies the behaviour found from the analysis of the imbibition experiments. As a corollary, it has been also stated that only if the adsorption at the solid interfaces happened, the imbibition of aqueous solution of surfactant in hydrophilic media could be influenced by the surfactant concentration.The results indicate the imbibition is not affected by the surfactant concentration whereas the contact angle is. The explanation for this atypical situation is the lack of adsorption of the surfactant employed in the experiments at the solid–liquid interface.
Keywords: Imbibition; Surfactant; Porous media; Adsorption; Free energy; Contact angle;

Our understanding of metal ion adsorption to clay minerals has progressed significantly over the past several decades, and theories have been promulgated to describe and predict the impacts of pH, ionic strength, and background solution composition on the extent of adsorption. Studies evaluating the effects of ionic strength on adsorption typically employ a broad range of background electrolyte concentrations. Measurement of pH in these systems can be inaccurate when pH values are measured with liquid junction pH probes calibrated with standard buffers due to changes in the liquid junction potential between standard, low ionic strength (0.05 M) buffers and high ionic strength solutions (>0.1 M). The objective of this research is to determine the extent of the error in pH values measured at high ionic strength, and to develop an approach for accurately measuring pH over a range of ionic strengths using a combined pH electrode. To achieve this objective, the adsorption of cobalt (10−5 M) onto gibbsite (10 g/L) from various electrolyte solutions (0.01–1 M) was studied. The pH measurements were determined from calibrations with standard buffers and ionic strength corrected buffer calibrations. The results show a significant effect of the aqueous solution background electrolyte anion and ionic strength on pH measurement. The 0.5 and 1 M ionic strength metal ion adsorption edges shifted to lower pH with increasing ionic strength when pH was calibrated with standard buffers whereas no shift in the adsorption edges was observed when calibrated with ionic strength corrected buffers. Therefore, to obtain an accurate pH measurement, pH calibration should contain the same electrolyte and ionic strength as the samples.Adsorption edge data shifts to lower pH (the left) when pH measurements are calibrated with standard buffers (□, ○). When calibrated with ionic strength corrected buffers (■, ●) they align with low ionic strength data (▴, ).
Keywords: Adsorption; Surface complexation; Ionic strength; pH electrode calibration; Cobalt; Gibbsite;

Reversible hydrophobization and lipophobization of cellulose fibers via trifluoroacetylation by Ana G. Cunha; Carmen S.R. Freire; Armando J.D. Silvestre; Carlos Pascoal Neto; Alessandro Gandini (333-336).
The surface modification of cellulose fibers with trifluoroacetic anhydride (TFAA) was studied using the heterogeneous cellulose/TFAA/pyridine/toluene system. The degree of substitution (DS) of the ensuing trifluoroacetylated fibers ranged from 0.04 to 0.30. This treatment conferred a high degree of both hydrophobicity and lipophobicity on the fibers' surface, even at low DS values. Both the dispersive and the polar contributions to the surface energy were drastically reduced. However, the original cellulose hydrophilicity could be readily restored through hydrolysis, by treating the modified fibers with neutral water.
Keywords: Cellulose fibers; Surface modification; Trifluoroacetic anhydride; Esterification; Reversible hydrophobization and lipophobization; Neutral hydrolysis;

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH3O(CH2CH2O)17NHCO(CH2)2SH (PEG17) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG17 SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted “soft” and “hard” due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG17, the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG17 SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG17 SAM.
Keywords: Self-assembled monolayer; SAM; Poly(ethylene glycol); PEG; Surface effects; Flame-annealing; Atomic force microscopy; AFM; Electron beam lithography; EBL; Silicide;

Nanoparticle adsorption and stabilisation of surfactant-free emulsions by Catherine P. Whitby; Alex M. Djerdjev; James K. Beattie; Gregory G. Warr (342-345).
The formation of particle-stabilised emulsions by adding partially hydrophobised silica particles to surfactant-free oil-in-water emulsions (average drop diameter ∼700 nm) stabilised by hydroxide ions adsorbed at the oil–water interface has been investigated. Nanoparticles (average particle diameter 18 nm) adsorbed onto the drops under alkaline conditions to produce particle-stabilised emulsions with the same drop size distribution as the surfactant-free emulsions. Unlike the surfactant-free emulsions, the particle-stabilised emulsions were stable even in acidic conditions. Strongly flocculated nanoparticles (average particle diameter 150 nm) adsorbed onto the drop surfaces under acidic conditions where the emulsions were destabilised, forming coarser particle-stabilised emulsions with micron-sized drops.The surfactant-free emulsions were unstable under acidic conditions and the average drop size increased with time. The particle-stabilised emulsions, in contrast, were stable even in acidic conditions.
Keywords: Particle-stabilised emulsion; Flocculation; Interface;