Journal of Colloid And Interface Science (v.321, #2)

Recognition of melting of nanoparticle catalysts with cubically shaped Co3O4 nanoparticles by Yongquan Qu; Daniel J. Masiel; Neal N. Cheng; Alexander M. Sutherland; Joshua D. Carter; Nigel D. Browning; Ting Guo (251-255).
Cubically shaped cobalt oxide nanoparticle catalysts were used for the first time to investigate the melting of the nanoparticle catalysts responsible for the synthesis of silica nanocoils at 1050 °C and straight nanowires at 1100 °C. Cobalt nanoparticles remained morphologically highly anisotropic after the growth of nanocoils at 1050 °C, whereas they became predominately spherical after straight nanowires were made at 1100 °C. These results strongly indicated that cobalt nanoparticles responsible for the synthesis of straight nanowires were completely molten and that melting occurred to these nanoparticles between 1050 and 1100 °C.Transformation of cubically shaped cobalt nanoparticle catalysts from room temperature to 1100 °C: The reduction occurs at below 600 °C, and total melting occurs between 1050 and 1100 °C.
Keywords: Nanoparticles; Catalysis; Nanocoils; Nanowires; Morphology; Silica nanocoils;

The influence of ionic strength, nutrients and pH on bacterial adhesion to metals by Xiaoxia Sheng; Yen Peng Ting; Simo Olavi Pehkonen (256-264).
Bacteria–metal interactions in aqueous solutions are important in biofilm formation, biofouling and biocorrosion problems in the natural environment and engineered systems. In this study, the adhesion forces of two anaerobes (Desulfovibrio desulfuricans and Desulfovibrio singaporenus) and an aerobe (Pseudomonas sp.) to stainless steel 316 in various aqueous systems were quantified using atomic force microscopy (AFM) with a cell probe. Results show that the nutrient and ionic strength of the solutions influence the bacteria–metal interactions. The bacteria–metal adhesion force was reduced in the presence of the nutrients in the solution, because a trace organic film was formed and thus decreased the metal surface wettability. Stronger ionic strength in the solution results in a larger bacteria–metal adhesion force, which is due to the stronger electrostatic attraction force between the positively charged metal surface and negatively charged bacterial surface. Solution pH also influences the interaction between the bacterial cells and the metal surface; the bacteria–metal adhesion force reached its highest value when the pH of the solution was near the isoelectric point of the bacteria, i.e. at the zero point charge. The adhesion forces at pH 9 were higher than at pH 7 due to the increase in the attraction between Fe ions and negative carboxylate groups.The force of bacterial adhesion to the metal surface is influenced by the nutrients and the ionic strength of the solution.
Keywords: Adhesion force; Atomic force microscopy; Sulfate-reducing bacteria; Cell probe; Desulfovibrio desulfuricans; Desulfovibrio singaporenus; Pseudomonas sp.;

Preparation and magnetic properties of Zn―Cu―Cr―La ferrite and its nanocomposites with polyaniline by Liangchao Li; Hui Liu; Yuping Wang; Jing Jiang; Feng Xu (265-271).
Nanosized Zn0.6Cu0.4Cr0.5Fe1.5−x La x O4 ( x = 0 ∼ 0.06 ) ferrites doped with La are synthesized by a rheological phase reaction method. Polyaniline (PANI)/ferrite nanocomposites are prepared by in situ polymerization method. The structure, morphology and ferromagnetic property of ferrite powders and nanocomposites are characterized by X-ray powder diffractometer (XRD), transmission electron microscope (TEM), Fourier transform infrared spectra (FTIR), UV–visible spectroscopy (UV), thermogravimetric analysis (TGA) and vibrating sample magnetometer (VSM). The results indicate that the PANI and nanosized ferrite powders can be combined effectively. The polymers can reduce the agglomeration of nanosized ferrite particles to some extent, which is good for the dispersedness and stabilization of nanoparticles. The PANI/ferrite nanocomposites under applied magnetic field exhibit the hysteretic loops of the ferromagnetic nature. The magnetic properties of nanocomposites are tailored by controlling the ferrite content.The saturation magnetization of PANI/Zn0.6Cu0.4Cr0.5Fe1.46La0.04O4 composites decrease with ferrite content ( a = 25 , b = 15 , c = 5   wt% ), which depends mainly on the volume fraction of ferrite particles.
Keywords: Ferrites; Rheological phase method; In situ polymerization; Polyaniline; Magnetic;

Spontaneous volume transition of polyampholyte nanocomposite hydrogels based on pure electrostatic interaction by Kun Xu; Jihua Wang; Qiang Chen; Yumei Yue; Wende Zhang; Pixin Wang (272-278).
A circular system is employed in this paper to investigate the swelling behaviors of polyampholyte hydrogels; this circular system can effectively eliminate the disturbance of various factors and keep the surrounding environment constant. It is found that there exists a spontaneous volume transition to the collapsed state of polyampholyte hydrogels, which is attributed to the overshooting effect, and the transition can occur repeatedly under certain conditions. 13C NMR is employed to investigate the swelling behavior of polyampholyte hydrogels. The swelling kinetics of polyampholyte hydrogels under various circular media and various circular runs are also investigated in this paper. All the results suggest that the spontaneous volume transition to the collapsed state of polyampholyte hydrogels is dominated by pure electrostatic interaction between different charges in polymer chains.A polyampholyte nanocomposite hydrogel shows a spontaneous volume transition to the collapsed state based on a pure electrostatic interaction.
Keywords: Polyampholyte hydrogel; Volume transition; Swelling behavior;

Effects of anionic surfactants on ligand-promoted dissolution of iron and aluminum hydroxides by Naraya Carrasco; Ruben Kretzschmar; Marie-Laure Pesch; Stephan M. Kraemer (279-287).
We investigated the influence of the surfactants sodium dodecyl sulfate (SDS) and rhamnolipid (RhL) on ligand-promoted dissolution of goethite (α-FeOOH) and boehmite (γ-AlOOH) at pH 6. The siderophore desferrioxamine B (DFOB), its derivate desferrioxamine D (DFOD), ethylenediaminetetraacetic acid (EDTA), and 8-hydroxyquinoline-5-sulfonic acid (HQS) were used as ligands. The rates of ligand-promoted dissolution of goethite were significantly increased in the presence of low concentrations of anionic surfactants (<80 μM SDS; <6 mg/L RhL). At higher surfactant concentrations, however, the effects of surfactants were negligible. The dissolution rates in the presence of surfactants were not correlated with adsorbed amounts of ligands. Three possible factors contributing to these observations were further investigated and discussed: (i) adsorbed surfactants may influence ligand adsorption by changes in the ligand's surface speciation, (ii) re-adsorption of Fe–DFOB or Fe–DFOD complexes may lead to an underestimation of siderophore-promoted dissolution rates at high surfactant concentrations, and (iii) co-adsorption of protons to goethite with SDS may influence the dissolution rates. However, our results show that none of these three factors can satisfactorily explain the observed effects of anionic surfactants on ligand-promoted dissolution rates of iron and aluminum hydroxides.Low concentrations of surfactants enhance ligand-promoted dissolution of oxides, while higher surfactant concentrations have only a small effect. Possible mechanisms were investigated and discussed.
Keywords: Siderophore; Surfactant; Goethite; Oxide; Dissolution; Kinetics; Adsorption;

Interaction of DNA bases with silver nanoparticles: Assembly quantified through SPRS and SERS by Soumen Basu; Subhra Jana; Surojit Pande; Tarasankar Pal (288-293).
Colloidal silver nanoparticles were prepared by reducing silver nitrate with sodium borohydride. The synthesized silver particles show an intense surface plasmon band in the visible region. The work reported here describes the interaction between nanoscale silver particles and various DNA bases (adenine, guanine, cytosine, and thymine), which are used as molecular linkers because of their biological significance. In colloidal solutions, the color of silver nanoparticles may range from red to purple to orange to blue, depending on the degree of aggregation as well as the orientation of the individual particles within the aggregates. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and absorption spectroscopy were used to characterize the assemblies. DNA base-induced differential silver nanoparticle aggregation was quantified from the peak separation (relates to color) of surface plasmon resonance spectroscopy (SPRS) and the signal intensity of surface-enhanced Raman scattering (SERS), which rationalize the extent of silver–nucleobase interactions.DNA base-induced differential silver nanoparticle aggregation has been quantified from surface plasmon resonance spectroscopy (SPRS) and surface-enhanced Raman scattering (SERS), which rationalize the extent of silver–nucleobase interactions.
Keywords: Nucleobase; Surface plasmon; Aggregation; Bathochromic shift;

Shear-thickening flow of nanoparticle suspensions flocculated by polymer bridging by Masashi Kamibayashi; Hironao Ogura; Yasufumi Otsubo (294-301).
The steady-shear viscosity, dynamic viscoelasticity, and stress relaxation behavior were measured for suspensions of silica nanoparticles dispersed in aqueous solutions of poly(ethylene oxide) (PEO). The suspensions of silica with diameters of 8–25 nm show striking shear-thickening profiles in steady shear and highly elastic responses under large strains in oscillatory shear. Since the silica particles are much smaller than the polymer coils, one molecule can extend through several particles by intrachain bridging. Each polymer coil may remain isolated as a floc unit and the silica particles hardly connect two flocs. Therefore, the flow of suspensions is Newtonian with low viscosity at low shear rates. When the polymer coils containing several nanoparticles are subjected to high shear fields, three-dimensional network is developed over the system. The shear-thickening flow may arise from the elastic forces of extended bridges. But, the polymer chain is easily detached from particle surface by thermal energy because of large curvature of particles. As a result, the network structures are reversibly broken down in a quiescent state and the suspensions behaves as viscoelastic fluids with the zero-shear viscosity.Photographs and structural models of shear-thickening suspensions in a quiescent state (a) and right after shaking (b). The suspensions are converted from fluids with low viscosity to gel-like pastes on the application of violent shear.
Keywords: Bridging flocculation; Shear-thickening; Silica nanoparticle; Suspension;

Edge-modification of LAPONITE® with dimethyl-octylmethoxysilane by Lisa M. Daniel; Ray L. Frost; Huai Yong Zhu (302-309).
This study examines the edge modification of LAPONITE®, with a monoalkoxy silane, dimethyl-octylmethoxysilane. The influence of ultrasonics, aging time and silane concentration on the resultant materials is examined. The silylated clays are characterized by XRD, IES, TGA, and Si NMR. The amount of grafted silane was increased by increasing the ratio of silane to clay, sonication of the reaction mixture and ageing the reaction mixture for no more than 24 h to avoid removal of grafted silane due to equilibrium effects.
Keywords: LAPONITE®; Silane; Silylation; Edge-modification; IES;

Spherical Ru(bpy)2+ 3-doped silica (RuSi) nanoparticles were prepared via a water-in-oil microemulsion approach. The electrochemical and electrochemiluminescent properties of the RuSi nanoparticles immobilized on an indium tin oxide (ITO) electrode were investigated. Further, electrochemiluminescence (ECL) of the RuSi nanoparticles with covalently coated biomacromolecules was studied. By covalent cross-linking with glutaraldehyde, γ-(aminopropyl) triethoxysilane (APTES)-pretreated RuSi nanoparticles were coupled with different concentrations of bovine serum albumin (BSA), hemoglobin, and myoglobin, respectively. ECL from these biomacromolecule-coated RuSi nanoparticles decreased with the increase of the loading of biomacromolecules. Moreover, the ECL of coreactants with different sizes was studied. The ECL decrease could be assigned to the steric hindrance and limited diffusion of coreactant molecules into the RuSi nanoparticles after biomacromolecule conjugation. Since tens of thousands of Ru(bpy)2+ 3 molecules are contained in the silica particles and the RuSi nanoparticle surface modification could improve their biocompatibility, the biomacromolecule-coated RuSi nanoparticles could be readily used as efficient and stable ECL tag materials in the future.Electrochemiluminescent properties of spherical Ru(bpy)2+ 3-doped silica (RuSi) nanoparticles with covalently grafted biomacromolecules were studied. ECL from these RuSi nanoparticles decreased with the increase of the loading of biomacromolecules.
Keywords: Electrochemiluminescence; Silica nanoparticles; Biomacromolecules; ECL tag;

This study investigates the effects of added Ca2+ on the properties of poly(Bd/MAA) dispersions (1,3-butadiene and methacrylic acid) and considers the effect of particle composition on the p K a . Four latex dispersions are considered in detail. These include poly(Bd/6MAA) and poly(Bd/20MAA) which contain, 6 and 20 wt% MAA, respectively, based on the total monomer mass used for dispersion preparation. Two model systems are also used for comparison. These are poly(Bd) and poly(EA/33MAA/BDDA) (EA and BDDA are ethyl acrylate and butanediol diacrylate). The latter is a well-studied model pH-responsive microgel. The apparent p K a of the poly(Bd/MAA) dispersions was determined from potentiometric titrations and found to increase with Bd content. The pH-dependence of the particle size was studied using photon correlation spectroscopy. Electrophoretic mobility measurements were also used. The hydrodynamic diameters and mobilities exhibited major changes as the pH approached the p K a for the particles. The critical coagulation concentrations were also measured. The results indicate that Ca2+ caused pronounced dispersion instability at low pH. Moreover, Ca2+ prevents swelling of the poly(Bd/MAA) particles at high pH. It was found that efficient ionic binding of all of the RCOO groups within the poly(Bd/20MAA) particles occurred when the mole ratio of RCOO to Ca2+ was less than or equal to 2.0. Consideration of all the data leads to the suggestion that poly(Bd/MAA) particles have a core–shell structure. It is suggested that the particle core contains mostly poly(Bd) and that the shell is comprised of lightly crosslinked poly(Bd-co-MAA) copolymer.We investigate the effect of pH and added Ca2+ on the particle and dispersion properties of poly(Bd/MAA) latexes and use the data to propose a morphology for the particles.
Keywords: pH-responsive particles; Polybutadiene latex; Methacrylic acid; Calcium binding; Electrophoretic mobility;

Site-selective time-resolved laser fluorescence spectroscopy of Eu3+ in calcite by M. Marques Fernandes; M. Schmidt; T. Stumpf; C. Walther; D. Bosbach; R. Klenze; Th. Fanghänel (323-331).
Three samples of calcite homogeneously doped with Eu3+ were synthesized in a mixed-flow reactor. By means of selective excitation of the D 0 5 → F 0 7 transition at low temperatures ( T < 20 K ), three different Eu3+ species (species A, B, and C, respectively) could be discriminated. For each one, the emission spectrum and lifetime were obtained after selective excitation of the single species. On the basis of these data, species C could be identified as Eu3+ incorporated into the calcite lattice on the (nearly) octahedral Ca2+ site. Species B was also identified as Eu3+ incorporated into the calcite lattice, but the ligand field shows a much weaker symmetry. Species A, however, is not incorporated into the crystal's bulk, having 1–2 H2O ligands left in its first coordination sphere and showing very little symmetry, and is considered as Eu3+ adsorbed onto the calcite surface. The emission spectra of species C for Eu:calcite grown in the presence of Na+ were found to differ from those of Eu:calcite synthesized in the presence of K+. The latter revealed a strong distortion in site symmetry, which was not observed in the samples grown in Na+ solutions. This finding provides spectroscopic evidence in favor of an incorporation mechanism based on the charge–balanced coupled substitution of Na + + Eu 3 + ↔ 2 Ca 2 + .
Keywords: Lanthanides/actinides; TRLFS; Incorporation; Calcite; Charge compensation;

Enhancement of the reductive transformation of pentachlorophenol by polycarboxylic acids at the iron oxide–water interface by Fangbai Li; Xugang Wang; Yongtao Li; Chengshuai Liu; Fang Zeng; Lijia Zhang; Mingde Hao; Huada Ruan (332-341).
The enhancement effect of polycarboxylic acids on reductive dechlorination transformation of pentachlorophenol (PCP) reacting with iron oxides was studied in anoxic suspension. Batch experiments were performed with three species of iron oxides (goethite, lepidocrocite and hematite) and four species of polycarboxylic acids (oxalate, citrate, succinate, and tartrate) through anoxic abiotic reactors. The chemical analyses and morphological observation from scanning and transmission electron microscopy showed that different combinations between polycarboxylic acids and iron oxides produced distinct contents of Fe(II)–polycarboxylic ligand complexes, which significantly enhanced PCP transformation. Generation of the surface-bound Fe(II) depended on concentration of polycarboxylic acids. The optimal concentration for the enhancement was 2.0 mM oxalic acid. The dechlorination mechanism was further demonstrated by generation of chloride ions. The results suggest that surface-bound Fe(II) formed on the iron oxides surface appears to be a key factor in enhancing PCP transformation, and the mole ratio of oxalate to surface-bound Fe(II) (oxalate/Fe(II)) acted as an indicator of the enhancement effect. The enhancement mechanism attributes to strong nucleophilic ability and low reductive potential of the Fe(II)–polycarboxylate complexes. Therefore, the enhancement effects might be helpful for understanding the natural attenuation of reducible organic pollutants at the interface of contaminated soil in anoxic condition.The enhancement effect of carboxylic acids on pentachlorophenol reductive transformation depends on the interaction of iron oxide and carboxylic acid.
Keywords: Polycarboxylic acids; Reductive dechlorination; Abiotic transformation; Pentachlorophenol; Iron oxide–water interface;

Physicochemical and catalytic properties of grafted vanadium species on different mesoporous silicas by Tanya Tsoncheva; Ljubomira Ivanova; Rayna Dimitrova; Jessica Rosenholm (342-349).
Vanadium modified mesoporous silicas type MCM-41, MCM-48, and SBA-15 are obtained by a solid state method. The samples are characterized and compared at the different steps of their preparation by X-ray diffraction, N2-physisorption, FTIR, UV–vis, XPS, and TG-TPR. Samples catalytic activity is tested in ethylacetate oxidation. Formation of various vanadium species, mainly isolated and small oligomeric ones, grafted to the support surface silanol groups, is observed. It is found that the state and the properties of the vanadium species depend on the porous characteristics of the silica host matrix. The nature of the catalytic active center in the ethylacetate oxidation is discussed.Isolated and small oligomeric vanadium species are prepared by a solid state reduction technique from V2O5 and mesoporous silica of types MCM-41, MCM-48, and SBA-15. The reduction usually leads to increase in the catalytic activity and selectivity to partial ethylacetate oxidation. It was hypothesis on the key role of the oligomeric catalytic active centers, containing vanadium species in a mixed valence, which formation is facilitated for the more opened 3-D structure of MCM-48.
Keywords: Vanadium modified silica; MCM-41; MCM-48; SBA-15; Ethylacetate oxidation;

This study reports on the application of surface complexation modeling to interpret observed kinetic trends for FeII redox reactions with model nitroaromatic (4-chloronitrobenzene) and oxime carbamate (oxamyl) contaminants in aqueous TiO2(s) suspensions. Pseudo-first-order rate constants for reduction of the two probe contaminants (k red, s−1) vary by several orders of magnitude with changing conditions (100–500 μM FeII, 0–15 g L−1 TiO2(s), pH 2–9), but the relationship between reaction rates and FeII speciation differs considerably for the two contaminants. For oxamyl, k red measurements are most strongly correlated with the volumetric total adsorbed FeII concentration (moles FeII adsorbed per liter of TiO2(s) suspension), whereas k red measurements for 4-chloronitrobenzene are proportional to the concentration of the hydrolyzed FeII surface complex (TiOFeIIOH0). The differing trends demonstrate that FeII redox reactivity at the aqueous/TiO2(s) interface is influenced, in part, by specific molecular interactions with the target oxidant. Results are also geochemically relevant in that they demonstrate unambiguously that mononuclear FeII–metal (hydr)oxide surface complexes are sufficiently reactive species to reduce nitroaromatic contaminants, an issue that remained open following earlier studies in FeIII (hydr)oxide suspensions because structural FeII species are simultaneously present in such systems because of interfacial FeII-to-FeIII electron transfer processes that occur on FeII adsorption.Observed rate constants for reduction of 4-chloronitrobenzene and oxamyl (k red) in iron(II)-amended aqueous TiO2(s) suspensions are correlated with iron(II) adsorption and speciation.
Keywords: Ferrous iron; Surface complexation; Titanium dioxide; Nitroaromatic; Carbamate; Abiotic reduction;

An AFM relative stiffness technique was applied to reconstituted Beta vulgaris L. wax films. Consecutive force arrays ( n = 100 ) made on the waxy surface at the same locations showed that there was no relative change in surface elasticity and this information was used as a reference to further experimental measurements. A surfactant solution was subsequently dropped on the waxy surface and the same array of indents was made at the same location as the reference test. The plant wax surface showed a reduction in its surface elasticity properties. The study has demonstrated that the AFM technique could be used to undertake a systematic assessment of the plasticising effects of agrochemicals on native and reconstituted plant wax films.AFM force plots on the wax film coated on silica wafer before and after non-ionic surfactant treatment, indicating drastic changes in surface stiffness after surfactant exposure.
Keywords: AFM; Relative stiffness; Plasticization; Surfactant;

Synthesis of carboxyl-modified rod-like SBA-15 by rapid co-condensation by Shoucang Shen; Pui Shan Chow; Sanggu Kim; Kewu Zhu; Reginald B.H. Tan (365-372).
Carboxyl-modified SBA-15 rod-like mesoporous materials have been synthesized by a facile rapid co-condensation of tetraethylorthosilicate (TEOS) and 2-cyanoethyltriethoxysilane (CTES), followed by hydrolysis of cyanide groups in sulfuric acid. The concentration of carboxylic groups was varied by changing the silica source ratio of CTES/TEOS from 0.05 to 0.3. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the uniform ordered mesoporous structure and rod-like morphology of SBA-15 have been preserved even at the high concentration of carboxylic groups employed. Characterization by Fourier transformed infrared spectroscopy (FTIR), solid-state NMR investigation indicated that carboxylic groups have been successfully grafted onto the surface of SBA-15 through siloxane bonds [(O3)Si―CH2 ―CH2 ―COOH. The negative charges of the modified SBA-15 materials were enhanced by the presence of the carboxylic groups on the surface. The capacity of lysozyme adsorption of the modified SBA-15 materials were found to be significantly improved as compared with pure silica SBA-15. The maximum amount of lysozyme adsorption on carboxyl-modified was increased with the pH of solution increased from 5.5 to 9.0.Carboxyl-modified rod-like SBA-15 was synthesized by rapid co-condensation. The modification strengthened the negative charge of SBA-15 and improved the capacity for lysozyme adsorption.
Keywords: SBA-15; Rod-like; Rapid co-condensation; Carboxyl-modified; Lysozyme adsorption; Negative charge; Zeta-potential;

Solvent effect on the film formation and the stability of the surface properties of poly(methyl methacrylate) end-capped with fluorinated units by Xiaofang Wang; Huagang Ni; Dongwu Xue; Xinping Wang; Ran-ran Feng; Hong-fei Wang (373-383).
The surface structure and stability (the resistance to surface reconstruction) of end-capped poly(methyl methacrylate) films were greatly affected by the solvents used for film preparation. Films of end-capped PMMA with about four 2-perfluorooctylethyl methacrylate units cast with benzotrifluoride solution exhibited excellent stability and resistance to polar environments compared with those cast with cyclohexanone and toluene solutions. The observed difference in stability between these fluorinated surfaces is attributed to their surface microstructures formed during the film formation processes, which are closely related to the associative behavior of the end-capped PMMA in the solution. A relatively perfect close-packed and well-ordered structure of the perfluoroalkyl side chains at the surface of the PMMA857-ec-FMA3.3 film was formed when the film was cast with benzotrifluoride solution, in which only unimers existed. This study indicates that such a solvent effect may be used to promote the formation of a well-ordered packing structure of the fluorinated moieties at the film surface. The ordering of the packing structure is to a certain extent more important than the content of the fluorinated moieties at the surface for improving the surface stability.Solvent effects in film formation on the surface structure and the stability of the surface properties of end-capped poly(methyl methacrylate) with 0.38 mol% 2-perfluorooctylethyl methacrylate films.
Keywords: Hydrophobic surface; Perfluoroalkyl side chain; Surface structure; End-capped poly(methyl methacrylate); Solvent effect;

The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic–anionic surfactants at the air/water interface were investigated by measurements of surface pressure–area per alkyl chain ( π – A ) and surface potential–area per alkyl chain ( Δ V – A ) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C12 to C16 were used to form mixed surfactant monolayers on the water subphase at 21 °C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the Δ V – A isotherms were generally more sensitive than the π – A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic–anionic surfactants were closely related to the alkyl chain lengths of the components.Alkyl chain length-dependent morphology of mixed cationic–anionic surfactant monolayers at the air/water interface.
Keywords: Air/water interface; Mixed cationic–anionic surfactant monolayer; Mixed monolayer; Monolayer morphology; Monolayer phase behavior;

Constitutive modeling of contact angle hysteresis by Srikanth Vedantam; Mahesh V. Panchagnula (393-400).
We introduce a phase field model of wetting of surfaces by sessile drops. The theory uses a two-dimensional non-conserved phase field variable to parametrize the Gibbs free energy of the three-dimensional system. Contact line tension and contact angle hysteresis arise from the gradient term in the free energy and the kinetic coefficient respectively. A significant advantage of this approach is in the constitutive specification of hysteresis. The advancing and receding angles of a surface, the liquid–vapor interfacial energy and three-phase line tension are the only required constitutive inputs to the model. We first simulate hysteresis on a smooth chemically homogeneous surface using this theory. Next we show that it is possible to study heterogeneous surfaces whose component surfaces are themselves hysteretic. We use this theory to examine the wetting of a surface containing a circular heterogeneous island. The contact angle for this case is found to be determined solely by the material properties at the contact line in accord with recent experimental data.
Keywords: Wetting; Contact angle; Hysteresis; Heterogeneous; Phase field theory;

Hydrodynamic control of droplet division in bifurcating microchannel and its application to particle synthesis by Masumi Yamada; Saki Doi; Hirosuke Maenaka; Masahiro Yasuda; Minoru Seki (401-407).
We describe herein a microfluidic system for active and precise control of droplet division at a bifurcation point in a microchannel. Water-in-oil or oil-in-water droplets, which were initially formed at a T-junction, were introduced into the bifurcation point, and then divided into two daughter droplets. By continuously introducing ‘tuning flow’ into the downstream of one of the branch channels, and by controlling the flow rates distributed into the two branch channels, the sizes of the daughter droplets could be precisely tuned. The ratio of the volumetric flow rates into the branch channels was estimated by regarding the microchannel network as a resistive circuit. In addition, we performed synthesis of monodispersed polymer particles with controlled sizes utilizing the presented system. The ability to hydrodynamically control the droplet sizes will open new possibilities not only for producing useful emulsions, but also for conducting controlled chemical and biochemical reactions in a confined space.The division ratio of droplets at a microfluidic bifurcation could be rigidly controlled by introducing ‘tuning flow’ into the downstream of one of the two branch channels.
Keywords: Microfluidic device; Emulsification; Droplet division; Particle synthesis;

Microstructure and structural transition in microemulsions stabilized by aldonamide-type surfactants by Katarzyna Zielińska; Kazimiera A. Wilk; Adam Jezierski; Teofil Jesionowski (408-417).
Significant efforts were undertaken to characterize the microstructure and structural properties of water-in-oil (w/o), oil-in-water (o/w), and bicontinuous (bc) microemulsions composed of N-alkyl-N-methylgluconamides (n-alkyl = n-C12H25, n-C14H29, n-C16H33) and n-alcohols (ethanol, n-propanol, n-butanol) or iso-alcohols (iso-propanol, iso-butanol) as cosurfactants, as well as iso-octane and water. The internal structure of so created four-component system was elucidated by means of an analysis of isotropic area magnitudes in phase diagrams and conductivity measurements. Dynamic light scattering (DLS) measurements provided the microemulsion size and polydispersity. Polarity and viscosity of microemulsion microenvironment were acquired by means of electron paramagnetic resonance (EPR), UV–vis absorption spectroscopy (in the case of w/o droplets), and steady-state fluorescence (SSF) (in the case of o/w droplets). The results show that both the surfactant and the cosurfactant types affect the shape and extent of microemulsions. The size of droplets depends strongly on the type of examined microemulsion and the type of cosurfactant (linear or brunched) but is almost independent of the length of the surfactant alkyl chain. The size of microemulsion droplets ranges from 8.1 to 22.6 nm and from 3.7 to 14.3 nm respectively, for o/w and o/w microemulsions, making them good candidates for both template-based reactions and household components solubilizing media.
Keywords: o/w and w/o Microemulsions; Phase diagram; Glucose-derived surfactant; Light scattering technique; Microenvironment polarity; Spin probe technique;

The actual mechanism of cholesterol reduction by phytosterols is yet to be explored. One hypothesis states that cholesterol and phytosterols compete on the solubilization locus within gastric bile salt micelles. In this study competitive solubilization within microemulsions as vehicles for dietary intake of cholesterol and phytosterols was studied by pulse gradient spin-echo nuclear magnetic resonance. The loaded microemulsions undergo phase transitions as a function of dilution, the type of solubilized sterol, and the weight ratio of the cosolubilized sterols. Microemulsions containing 10–20 wt% of aqueous phase, show similar diffusivity of the oil and aqueous phases in all examined systems (excluding PS-loaded one) reflecting the minor influence of these solubilizates on the structure of the inner and the outer phases. The closeness of these structures enables the mobility of water molecules between them. Upon further dilution (>20 wt% aqueous phase), significant differences in decrease rate of the oil and increase of the water phases mobilities (occurring upon inversion), were detected within the studied systems. It was concluded that the solubilized sterols influence the structural transitions based on their location within the structures and their competitive solubilization. The phytosterols solubilized mostly in the continuous oil phase and between the surfactant tails. Cholesterol is solubilized in the vicinity of the surfactant headgroups and affects the surface curvature. In mixtures of cholesterol and phytosterols, structural changes are dictated mostly by the presence of the cholesterol.Competitive solubilization of cholesterol and phytosterols and structural transitions in dilutable microemulsions studied by pulse gradient spin-echo nuclear magnetic resonance.
Keywords: Cholesterol; Phytosterols; Nonionic surfactant; Solubilization capacity; PGSE-NMR; Self-diffusion coefficient; SAXS; Microemulsion; Dilution line; R-(+)-limonene;

The aggregation behavior of cetylpyridinium chloride (CPyCl) in N , N -dimethylformamide (DMF)–water mixed solvents was investigated using electrical conductivity and spectroscopic techniques. Micellar and thermodynamic parameters ( Δ G m 0 , Δ H m 0 , Δ S m 0 and Δ m C p 0 ) were obtained from the temperature dependence of critical micelle concentrations in various aqueous mixtures of DMF. The differences in the Gibbs energies of micellization of CPyCl between water and binary solvents were determined to evaluate the influence of the cosolvent. The effect of cosolvent on the Krafft temperature ( K T ) and on the aggregation number was also analyzed. Micellar micropolarity was examined spectrophotometrically using two different probes, methyl orange (MO) and methylene blue (MB), and was found to increase with DMF addition, accompanied by an enhanced solvation. The mechanism of docking of surfactant and the probe molecules in the system were obtained by using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy.The micellization behavior of the cationic surfactant CPyCl in water–DMF binary mixtures has been investigated using electrical conductivity and spectroscopic techniques.
Keywords: Electrical conductivity; Spectroscopic techniques; Temperature dependence; Critical micelle concentration; Krafft temperature; Micellar micropolarity; Enhanced solvation;

Development of a microemulsion-based process for synthesis of cobalt (Co) and cobalt oxide (Co3O4) nanoparticles from submicrometer rods of cobalt oxalate by Jahangeer Ahmed; Tokeer Ahmad; Kandalam V. Ramanujachary; Samuel E. Lofland; Ashok K. Ganguli (434-441).
Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from ∼6.5 μm at 50 °C to ∼2.5 μm at 150 °C) while keeping the diameter nearly constant (200–250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H2) leads to nanoparticles of Co3O4 and Co, respectively, while in a helium atmosphere a mixture of Co and CoO nanoparticles is obtained. Co3O4 nanoparticles (∼35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (∼25 nm). The oxalate rods and Co3O4 nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively.Cobalt oxalate rods of diameter ∼300 nm were obtained at room temperature. This yields pure and uniform Co3O4 (35 nm) and Co (25 nm) nanoparticles in a controlled atmosphere (air and H2).
Keywords: Nanomaterials; Reverse micellar synthesis; Cobalt oxalate; Cobalt oxide; Magnetism;

Conductivity, static fluorescence and 1H NMR measurements have been carried out to study the micellar behavior of aqueous solutions of dodecyldimethylethylammonium bromide (DDAB), dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TDAC) in absence and presence of α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), hydroxypropyl-β-cyclodextrin (HPβ-CD) and γ-cyclodextrin (γ-CD). The conductivity measurements were carried out at 298.15 K. The influence of cyclodextrins on the micellar parameters, such as cmc (apparent critical micellar concentration), β (degree of ionization) have been analyzed. Thermodynamics of the systems was discussed in terms of the change in standard free energy of micellization, Δ G m 0 . Micellization was found to be less spontaneous in presence of cyclodextrins. The fluorescence intensity of the surfactant solutions is enhanced by the addition of cyclodextrins. The association constants obtained from conductivity and fluorescence data suggest the binding of γ-CD with the surfactants to be strongest among all the cyclodextrins used. 1H NMR chemical shift changes provide powerful means for probing the cyclodextrin–micellar interactions and inclusion of surfactant is shown by the change in the chemical shift of some of the guest and host protons in comparison with the chemical shifts of the same protons in the free compounds.Physicochemical characterization of the association between cyclodextrins and cationic surfactants is carried out using conductivity and spectroscopic studies. Micellization process gets delayed and becomes less spontaneous in presence of cyclodextrin.
Keywords: DDAB; DTAC; TDAC; Cyclodextrin; Conductivity; Fluorescence; 1H NMR;

Dielectric and electrical properties of electrorheological carbon suspensions by K. Negita; Y. Misono; T. Yamaguchi; J. Shinagawa (452-458).
Measurements of electrorheological (ER) effects, dielectric properties, and electrical conductivity are made on an ER suspension composed of carbonaceous particles and silicone oil to understand the mechanism which governs the ER effect. In the frequency dependence of the dielectric permittivities, a dielectric dispersion is observed in the first-order dielectric permittivity, while in the third-order dielectric permittivity a resonance peak, which is due to shear-induced particle rotation, is recognized, showing that such dielectric properties are closely related to ER properties of the suspension. An electrical conduction mechanism, which is responsible for the ER effect, is also investigated from the dependence of dc electrical conductivity on electric-field strength and temperature.In the spectra of third-order dielectric permittivity, a negative peak due to shear-induced particle rotation is observed.
Keywords: Electrorheological fluid; Carbon suspension; Dielectric properties; Nonlinear dielectric properties; First-order dielectric permittivity; Third-order dielectric permittivity; Shear-induced particle rotation; Electrical conduction;

Swollen and collapsed lyotropic lamellar rheology by Mohamed Youssry; Luigi Coppola; Isabella Nicotera; Carmen Morán (459-467).
We have investigated linear rheological properties and the structure–flow relationship of the swollen (Lam1) and collapsed (Lam2) lamellar phases, formed on didodecyldimethylammonium bromide (DDAB)/lecithin/water ternary system at 25 °C. Both lamellar phases behaved like Bingham fluids and showed remarkable yield stresses. At rest the Lam1 phase, which is characterized by densely packed vesicles whose sizes increase as the water content decreases in accordance to evolution of 2H NMR spectral profiles of D2O, resulted in a strong elastic gel-like response. On the other hand, the Lam2 phase, formed at high surfactant concentrations, showed a weak-gel viscoelasticity and 2H NMR spectral patterns which are typical of planar bilayered structures. The increase of the quadrupole splitting as the water content decreases was assumed as a strong evidence of size increasing of the lamellar domains. We have demonstrated that by using dynamic rheology and the derived relaxation time spectra, along with 2H NMR spectra of D2O, it is possible to differentiate between equilibrium lamellar structures occurring in a broad interval of total surfactant concentration. In addition, a shear-thickening regime, observed at intermediate shear-rate values, highlighted the onset of out-equilibrium lamellar structures which were present both on Lam1 and Lam2 phases.The variation of the rheological parameters (shear modulus, G 0 ; yield stress, σ y ; longest relaxation time, τ R ) and the quadruple splitting Δν as a function of water content could distinguish between different bilayered structures.
Keywords: Bilayered structures; Dynamic rheology; Shear-induced structures; 2H NMR;

Synthesis of Cd-free water-soluble ZnSe1−x Te x nanocrystals with high luminescence in the blue region by Chunliang Li; Kazuhiro Nishikawa; Masanori Ando; Hiroyuki Enomoto; Norio Murase (468-476).
Cd-free core–shell nanocrystals (ZnSe1−x Te x /ZnS, 0 ⩽ x ⩽ 0.5 ) emitting in the pure blue region were prepared using an aqueous colloidal method followed by post-preparative ultraviolet irradiation. The photoluminescence (PL) efficiency and peak wavelength were maximized (40%, 448 nm) through a heat treatment of the nanocrystals during irradiation in a Zn2+-containing thiol solution. Because of the small lattice mismatch between ZnSe1−x Te x and ZnS, post-preparative irradiation at high temperature resulted in the formation of a thick ZnS shell (ca. 1.6 nm) around the core (ca. 2.2 nm in diameter) without deterioration of PL efficiency. Surface substitution of Te by S and size-selective precipitation of nanocrystals before ultraviolet irradiation resulted in intense PL. Quantum mechanical calculations show that the wave function of the electron of the exciton in the Te-containing core extends well into the ZnS shell. The calculations also reveal that a thick shell can confine the electrons inside the particles and thereby improve the PL efficiency and stability against the pH of the solution. Nanocrystals that had been post-preparatively irradiated showed good stability in solution and in a glass matrix even after months of storage in air.Forming a thick shell through a heat treatment during irradiation, the photoluminescence efficiency of Cd-free core–shell nanocrystals (ZnSe1−x Te x /ZnS) was increased to 40% and the emission peak was tuned into the pure blue range (448 nm). Quantum mechanical calculations revealed that the thick ZnS shell confines the electron inside the particle and then improves the PL efficiency and stability of nanocrystals.
Keywords: ZnSe1−x Te x ; Nanocrystals; Cd free; Irradiation; Photoluminescent efficiency; Quantum mechanical calculation; Glass;

Preparation of chitosan nanoparticles using methacrylic acid by Márcia R. de Moura; Fauze A. Aouada; Luiz H.C. Mattoso (477-483).
In this work the preparation of chitosan nanoparticle was investigated using methacrylic acid in different conditions and studied by particle size analyzer, zeta-potential, Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM). The particle size was dependent on the chitosan concentration used during the preparation method. Nanoparticles with sizes as small as 60 nm were achieved, that can be extremely important for several applications. The nanoparticles solution was also pH-sensitive, due to swelling and aggregation of the nanoparticles. The nanoparticles obtained presented a very homogeneous morphology showing a quite uniform particles size distribution and a rather spherical shape.
Keywords: Natural polymer; Biopolymer; Nanoparticles formation; Chitosan;

Synthesis of cobalt ferrite core/metallic shell nanoparticles for the development of a specific PNA/DNA biosensor by Marcos Pita; José María Abad; Cristina Vaz-Dominguez; Carlos Briones; Eva Mateo-Martí; José Angel Martín-Gago; Maria del Puerto Morales; Víctor M. Fernández (484-492).
Controlled synthesis of cobalt ferrite superparamagnetic nanoparticles covered with a gold shell has been achieved by an affinity and trap strategy. Magnetic nanoparticles are functionalized with a mixture of amino and thiol groups that facilitate the electrostatic attraction and further chemisorption of gold nanoparticles, respectively. Using these nanoparticles as seeds, a complete coating shell is achieved by gold salt-iterative reduction leading to monodisperse water-soluble gold-covered magnetic nanoparticles, with an average diameter ranging from 21 to 29 nm. These constitute a versatile platform for immobilization of biomolecules via thiol chemistry, which is exemplified by the immobilization of peptide nucleic acid (PNA) oligomers that specifically hybridize with complementary DNA molecules in solution. Hybridation with DNA probes has been measured using Rhodamine 6G fluorescence marker and the detection of a single nucleotide mutation has been achieved. These results suggest the PNA-nanoparticles application as a biosensor for DNA genotyping avoiding commonly time-consuming procedures employed.Magnetic core–gold shell nanoparticles are synthesized through affinity and trap strategy. A PNA probe is immobilized to develop a fluorescence DNA biosensor.
Keywords: Magnetic nanoparticle; Gold shell; Peptide nucleic acid; DNA–PNA hybridization; Mutation detection; Rhodamine 6G;

Arsenic sorption onto laterite iron concretions: Temperature effect by Frederick Partey; David Norman; Samuel Ndur; Robert Nartey (493-500).
We investigated arsenate and arsenite sorption onto laterite iron concretions (LIC) to test its suitability for use in the low-tech treatment of arsenic-bearing drinking water. Batch experiments on crushed LIC from Prestea, Ghana were conducted at a series of temperatures, ionic strengths, and pHs. The point of zero net charge on laterite iron concretion was determined by potentiometric titrations yielding an average pHpZNC around 8.64. Experiments show that sorption capacity for both arsenite and arsenate increase with temperature. The equilibrium sorption capacity for arsenite was larger than that for arsenate over the 25 to 60 °C temperature range. A Langmuir model satisfactorily fits the arsenite and arsenate sorption isotherm data. Both arsenite and arsenate sorbed over the pH range of natural waters. Arsenite sorption increases with increasing solution pH to a maximum at pH 7, then decreases with further increase in solution pH. Arsenate sorption, on the other hand, shows little change with increasing solution pH. Increasing solution ionic strength 10-fold results in a slight increase in sorption. Ionic strength experiments show that an inner-sphere sorption mechanism is responsible for As (V) sorption on LIC, while As (III) sorption is by an outer-sphere mechanism. Gibbs free energy ( Δ G ° ) for arsenite and arsenate sorption onto LIC was calculated from Langmuir isotherms; the negative values agree with reaction spontaneity. The positive values of the standard enthalpy ( Δ H ° ) show the endothermic nature of arsenite and arsenate sorption onto LIC. Positive entropy ( Δ S ° ) values suggest the affinity of LIC for the arsenic species in solution. Analysis of the arsenic sorption data suggests that LIC can be used for low-tech natural-materials arsenic water treatment. Laterite iron concretions have a number of advantages for this use over commercial materials, including the ability to remove arsenic from waters with a wide range in pH, the ability to sorb both common arsenic aqueous species equally well, and cost less. Laterite iron concretion's positive sorption temperature dependence will enhance sorption in tropical climates, and more especially in areas where groundwater sources are related to geothermal springs.Experiments show that adsorption capacity for both arsenite and arsenate increases with temperature. The equilibrium adsorption capacity for arsenite is greater than that for arsenate over the 25 to 60 °C.
Keywords: Arsenate; Arsenite; Temperature; Sorption; Ghana; Laterite iron concretion;