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Journal of Colloid And Interface Science (v.347, #1)

Cover 1 (pp. ofc).

Effect of sodium oleate as a buffer on the synthesis of superparamagnetic magnetite colloids by Wen Jiang; Yao Wu; Bin He; Xiaobo Zeng; Kuilin Lai; Zhongwei Gu (pp. 1-7).
Sodium oleate exerts an important buffering effect assisting the preparation of superparamagnetic monodisperse magnetite colloids with similar properties.In this study, superparamagnetic monodisperse magnetite colloids, around 5nm in size, were prepared by dissolving iron chlorides, sodium hydroxide (NaOH) and sodium oleate (NaOL), in toluene/ethanol/water mixtures and refluxing for 4h. The concentrations of NaOH and NaOL were varied to systematically investigate the effect on the surface properties, size, dispersion, and magnetic properties of magnetite nanoparticles (MNPs). The samples were characterized via XRD, FTIR, TGA, TEM, SAED, DLS, and VSM. The results indicated that the surface coatings of MNPs could be manipulated from oleate to hydroxyl groups via increasing the molar ratio of NaOH/Fe(II) more than 8. The amount of NaOH had no obvious influence on the size and the saturation magnetization of MNPs. Therefore NaOH was not a necessary reactant for forming magnetite crystals. On the contrary, NaOL was shown to be the most important component for synthesizing stable magnetite colloids. The NaOL acted as both a key reactant to buffer the pH environment and a surfactant to keep the MNPs stable in nonpolar solvent media.

Keywords: Magnetite colloid; Superparamagnetism; Monodisperse; Sodium oleate; Surfactant; Buffer effect

Facile synthesis of silica monolith doped with meso-tetra( p-carboxyphenyl)porphyrin as a novel metal ion sensor by Radchada Buntem; Amarawan Intasiri; Warunee Lueangchaichaweng (pp. 8-14).
The silica sol was mixed with meso-tetra( p-carboxyphenyl)porphyrin to produce the porphyrin-doped silica monolith for sensing variety of metal ions.A novel metal ion sensor was prepared using silica monolith doped with meso-tetra( p-carboxyphenyl)porphyrin. The doped material was prepared using TEOS:EtOH:H2O:HCl:porphyrin molar ratios of 1:5:7:3.1×10−2:2.3×10−5, respectively. The mixture was kept 16days for the gelation process and then the wet gel was dried at 55–60°C for 3days. The porphyrin-doped monolith obtained was kept in 1M metal salt solution for 2days. The visible spectrum of the metal-coordinated porphyrin-doped monolith was compared with the uncoordinated porphyrin-doped monolith. The spectra show the characteristic maxima for Cu2+ at 543nm, for Zn2+ at 522, 559, and 596nm, for Pb2+ at 531 and 559nm, and for Ni2+ at 522 and 551nm. The metal coordinated to the silica monolith could be removed by washing with 1M HNO3. However the Cu2+ could not be eluted by acidic solution due to its strong bonding to the porphyrin. The results show that this porphyrin-doped monolithic biomaterial is a promising sensor for metal ions in wastewater and other harsh environments.

Keywords: Metal ion sensor; Optical sensor; Porphyrin-doped silica monolith; Sol gel silica

Interaction of cement model systems with superplasticizers investigated by atomic force microscopy, zeta potential, and adsorption measurements by Lucia Ferrari; Josef Kaufmann; Frank Winnefeld; Johann Plank (pp. 15-24).
In multiphase suspensions with positively and negatively charged particles, the adsorption of superplasticizers is rather different depending on particles charge. Therefore dispersion forces due to PCE are steric and/or electrostatic.Polyelectrolyte-based dispersants are commonly used in a wide range of industrial applications to provide specific workability to colloidal suspensions. Their working mechanism is based on adsorption onto the surfaces of the suspended particles. The adsorbed polymer layer can exercise an electrostatic and/or a steric effect which is responsible for achieving dispersion. This study is focused on the dispersion forces induced by polycarboxylate ether-based superplasticizers (PCEs) commonly used in concrete. They are investigated by atomic force microscopy (AFM) applying standard silicon nitride tips exposed to solutions with different ionic compositions in a wet cell. Adsorption isotherms and zeta potential analysis were performed to characterize polymer displacement in the AFM system on nonreactive model substrates (quartz, mica, calcite, and magnesium oxide) in order to avoid the complexity of cement hydration products. The results show that PCE is strongly adsorbed by positively charged materials. This fact reveals that, being silicon nitride naturally positively charged, in most cases the superplasticizer adsorbs preferably on the silicon nitride tip than on the AFM substrate. However, the force–distance curves displayed repulsive interactions between tip and substrates even when polymer was poorly adsorbed on both. These observations allow us to conclude that the dispersion due to PCE strongly depends on the particle charge. It differs between colloids adsorbing and not adsorbing PCE, and leads to different forces acting between the particles.

Keywords: Atomic force microscopy; Zeta potential; Adsorption; Superplasticizer; Cement model system

Use of small-angle neutron scattering to investigate modifications of internal structure in self-assembled grains of nanoparticles synthesized by spray drying by D. Sen; Arshad Khan; J. Bahadur; S. Mazumder; B.K. Sapra (pp. 25-30).
SANS reveal the modifications of particle correlation and specific surface area, in the assembled grains. Overall external spherical shape of the grains remains un-altered.Micrometric spherical grains consisting of self-assembled silica nanoparticles have been synthesized by spray drying of colloidal suspension. Inter-particle correlation and available specific surface area of silica and void interfaces, in the assembled grains, were modified by addition of electrolyte in initial colloidal dispersion prior to self-assembly process but keeping the overall spherical shape of the assembled grains un-altered. While the external morphology of the assembled grains was probed by scanning electron microscopy, small-angle neutron scattering technique has been employed to investigate the modifications in the internal structure and the inter-particle correlation inside the assembled grains. It is revealed that a sticky hard sphere type of inter-particle correlation between the constituent particles gets altered to a fractal type of correlation with addition of electrolyte. Further, the specific surface area of the silica-void interface gets somewhat enhanced by addition of electrolyte and particularly at higher electrolyte concentration due to formation of some hollow and buckled assembled grains.

Keywords: SANS; Self-assembly; Spray drying; Mesoscopic structure

Drug transport in HEMA conjunctival inserts containing precipitated drug particles by Chhavi Gupta; Anuj Chauhan (pp. 31-42).
Conjunctival inserts has been developed to release cyclosporine A at approximately zero order for 20–25days. Effect of drug loading, crosslinking and convection in the fluid on rate of release from inserts has been studied. An exhaustive model has been developed to predict drug release from a system/device which contain drug as particles.This paper focuses on exploring the mechanism of cyclosporine A transport in hydroxyethyl methacrylate (HEMA) rods to develop conjunctival inserts for extended ocular delivery. Cylindrical conjunctival HEMA inserts were prepared by thermal polymerization in presence of drug at high loadings to create rods containing particles of drug dispersed in the matrix. The drug release rates were measured to explore the effect of length, drug loading, crosslinking, and mixing in the release medium. Also microstructure of the inserts was characterized by SEM imaging. The inserts release the drug for a period of about a month at therapeutic rates. The rates of drug release are zero order and independent of drug loading and crosslinking for certain period of time. These effects were shown to arise due to a mass-transfer boundary layer in the fluid and a mathematical model was developed by coupling mass transfer in the insert with that in the boundary layer in the surrounding fluid. The model with diffusivity in the insert and boundary layer thickness as parameters fits the experimental data and explains all trends in release kinetics. The fitted diffusivity is about twice that obtained by direct measurements, which agreed well with the value obtained by using the Brinkman’s equation but only after accounting for drug binding to the polymer.

Keywords: Key words; Particle dispersion in gels; Ophthalmic drug delivery; Dry eyes; Conjunctival inserts; Cyclosporine; HEMA; Model; Diffusion; Mass transfer resistance

Influence of the zeta potential on the sorption and toxicity of iron oxide nanoparticles on S. cerevisiae and E. coli by Heiko Schwegmann; Andrew J. Feitz; Fritz H. Frimmel (pp. 43-48).
The influence of the zeta potential on the sorption between microorganisms ( Saccharomyces cerevisiae and Escherichia coli) and iron oxide nanoparticles is demonstrated in a model salt solution at two different pH-values. There was only a 1% survival rate of E. coli (4.5×107cells/mL) in the presence of 24mg/L nanoparticulate iron oxide at pH 4. S. cerevisiae were less affected by the presence of the nanoparticulate iron oxide. The extent of iron oxide nanoparticle coverage on the surface of the microorganisms appears to be related to electrostatic interaction forces. Furthermore, the toxic effect of the nanoparticle concentration follows the sorption isotherm for E. coli. Based on the resulting hydrodynamic size distributions in the supernatant after sorption experiments, it could be shown that predominantly smaller particle aggregates oxide were sorbed onto E. coli. This was evident by a shift in the particle size distribution towards a larger mean particle size. The effect was observed to a lower extent for S. cerevisiae. The extent of iron oxide nanoparticle sorption on E. coli quickly reached a maximum and remained constant during a 24h period compared to S. cerevisiae where sorption increased over time.

Keywords: Microorganisms; Nanoparticles; Zeta potential; Sorption; Toxicity

Intercalation of anionic organic ultraviolet ray absorbers into layered zinc hydroxide nitrate by Ana Cristina Trindade Cursino; José Eduardo Ferreira da Costa Gardolinski; Fernando Wypych (pp. 49-55).
Layered zinc hydroxide nitrate was topotactically exchanged with anionic species of organic UV absorbers. After intercalation/grafting, the absorption ability was improved in relation to the use pristine material.Layered zinc hydroxide nitrate (ZHN) was synthesized and nitrate ions were topotactically exchanged with three different anionic species of commercial organic ultraviolet (UV) ray absorbers: 2-mercaptobenzoic acid, 2-aminobenzoic acid, and 4-aminobenzoic acid. The exchange reactions were confirmed by X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR), ultraviolet visible (UV–Vis) spectroscopy, and thermal analysis (thermogravimetry, TGA, and differential thermal analysis, DTA). In all the anionic exchanged products, evidence of grafting of the organic species onto the inorganic matrix was obtained. In general, after intercalation/grafting, the UV absorption ability was improved in relation to the use of the parent organic material, showing that layered hydroxide salts (LHS) can be good alternative matrixes for the immobilization of organic species with UV-blocking properties in cosmetic products.

Keywords: Layered materials; Sunscreen; Intercalation; Ultraviolet absorbers; Exchange reactions

Interaction of oligonucleotide-based amphiphilic block copolymers with cell membrane models by L. Caseli; C.P. Pascholati; F. Teixeira Jr.; S. Nosov; C. Vebert; A.H.E. Müeller; O.N. Oliveira Jr. (pp. 56-61).
Oligonucleotide-based block copolymer mixed with DPPC at the air–water interface to form thin films.Oligonucleotides have unique molecular recognition properties, being involved in biological mechanisms such as cell-surface receptor recognition or gene silencing. For their use in human therapy for drug or gene delivery, the cell membrane remains a barrier, but this can be obviated by grafting a hydrophobic tail to the oligonucleotide. Here we demonstrate that two oligonucleotides, one consisting of 12 guanosine units (G12), and the other one consisting of five adenosine and seven guanosine (A5G7) units, when functionalized with poly(butadiene), namely PB–G12 and PB–A5G7, can be inserted into Langmuir monolayers of dipalmitoyl phosphatidyl choline (DPPC), which served as a cell membrane model. PB–G12 and PB–A5G7 were found to affect the DPPC monolayer even at high surface pressures. The effects from PB–G12 were consistently stronger, particularly in reducing the elasticity of the DPPC monolayers, which may have important biological implications. Multilayers of DPPC and nucleotide-based copolymers could be adsorbed onto solid supports, in the form of Y-type LB films, in which the molecular-level interaction led to lower energies in the vibrational spectra of the nucleotide-based copolymers. This successful deposition of solid films opens the way for devices to be produced which exploit the molecular recognition properties of the nucleotides.

Keywords: Oligonucleotide-based amphiphilic block copolymer; DPPC; Langmuir monolayers; Langmuir–Blodgett films; Biosensor

Preparation of organic/inorganic composites by deposition of silica onto shell layers of polystyrene (core)/poly[2-( N, N-dimethylamino)ethyl methacrylate] (shell) particles by Tatsuo Taniguchi; Takuya Kashiwakura; Takumi Inada; Yusuke Kunisada; Masakatsu Kasuya; Michinari Kohri; Takayuki Nakahira (pp. 62-68).
Organic/inorganic composites were prepared by catalytic hydrolysis and subsequent condensation of tetraethoxysilane in shell layers of polystyrene (core)/poly[2-( N, N-dimethylamino)ethyl methacrylate] (shell) particles.Organic/inorganic composites were prepared by catalytic hydrolysis and subsequent condensation of tetraethoxysilane (TEOS) in a shell layer of core–shell polymer particles. First, core–shell particles were prepared by emulsifier-free emulsion polymerization of styrene (St) with 2-chloropropionyloxyethyl methacrylate (CPEM) using potassium persulfate as an initiator, followed by surface-initiated activator generated electron transfer–atom transfer radical polymerization (AGET–ATRP) of 2-( N, N-dimethylamino)ethyl methacrylate (DMAEMA). Dynamic light scattering revealed that hydrodynamic diameter of the particle increased from 482 to 931nm after AGET–ATRP of DMAEMA. The amount of grafted DMAEMA was determined to be ca. 10mol% with respect to (wrt) St by1H NMR. Second, the composite particles were prepared by adding TEOS into a water/methanol dispersion of the P(St-CPEM)- g-P(DMAEMA). The P(St-CPEM)- g-P(DMAEMA)–SiO2 composite particles containing ca. 50wt.% of silica wrt the total weight were obtained. Hollow silica shell particles were also obtained by extraction of polymer components from the composites with tetrahydrofuran.

Keywords: ATRP; Core–shell particles; Silica; Composites; Hollow particles

Enzymatic proteolysis of alpha gliadin monolayer spread at the air–water interface by K. Mircheva; Tz. Ivanova; I. Panaiotov; V. Ducel; F. Boury (pp. 69-73).
In this article an appropriate kinetic model describing the proteolytic action of a peptidase at a model monolayer of a plant protein was applied.The mechanism of the enzymatic hydrolysis under the proteolytic enzyme action of a plant protein alpha gliadin organized as a model monolayer system at the air/water interface was studied. The advantage of the monolayer technique is the ability to control and modify easily the interfacial organization of the molecules and the possibility to optimize the conditions for the hydrolysis. Enzymatic hydrolysis was studied by using a traditional barostat surface balance. The hydrolysis kinetic was followed by measuring simultaneously the decrease of the surface area and change of the surface potential with time. The decrease with time in film area is result of the random scission of the peptide bonds of polypeptide chain and their solubilization in the aqueous subphase. The interpretation of the surface potential data is based on the contribution of the dipole moments of the intact and broken peptide groups. An appropriate kinetic model describing the proteolytic action of a peptidase was applied and the global kinetic constant was obtained. The random scission of the protein chains gave kinetic constants comparable with those measured during the hydrolytic scission of polyester macromolecules but quite different to the values obtained with short-chain lipids.

Keywords: Monolayers; Hydrolysis; Alpha gliadin

Organoclay particles as reinforcing agents in polysaccharide films by J.L. Viota; M. Lopez-Viota; B. Saake; K. Stana-Kleinschek; A.V. Delgado (pp. 74-78).
Electrophoretic mobility of organically modified montmorillonite clay particles in the presence of different concentrations of inulin, as a function of KBr concentration.In this work an investigation is described on the use of organically modified montmorillonite clay particles as stabilizers of bioplastic films based on xylan. With the aim of facilitating the incorporation of the nanoparticles to the films, the former were treated with a non-ionic surfactant, inulin. In order to evaluate the possible role of electrostatic interactions in the formation of montmorillonite/xylan complexes, an exhaustive electrokinetic characterization of the modified montmorillonite was carried out. Because montmorillonite has been modified by adsorption of the cationic surfactant DSDMAC, the electrophoretic mobility of montmorillonite in the absence of inulin is positive in a wide range of concentration of NaCl and CaCl2. On the contrary, addition of KBr provokes a charge inversion when the salt concentration is around 0.05M, suggesting adsorption of Br ions. In the presence of inulin, the positive electrophoretic mobility decreases with the concentration of this surfactant, and this can be explained by assuming that inulin adsorption is accompanied by simultaneous DSDMAC desorption, eventually producing charge inversion, particularly in the presence of bromide ions. A thorough characterization of the wettability of the xylan films demonstrated that it is dominated by acid–base interactions and that incorporation of inulin-coated montmorillonite leads to a considerable reduction of the hydrophilic character of the films.

Keywords: Inulin; Xylan; DSDMAC; Nanofil 8; Organoclay; Polysaccharides; Electrophoresis

Effect of layer-by-layer confinement of polypeptides and polysaccharides onto thermoresponsive microgels: A comparative study by Ana M. Díez-Pascual; John E. Wong (pp. 79-89).
A comparative study of the layer-by-layer assembly of polypeptides (PLL/PGA) and polysaccharides (CHIT/DEX) onto soft and porous thermoresponsive P(NiPAM-co-MAA) microgel was undertaken by dynamic light scattering and electrophoretic measurements.The confinement of polyelectrolyte multilayers of poly-l-lysine (PLL)/poly-l-glutamic acid (PGA) and chitosan (CHIT)/dextran sulfate (DEX) onto soft and porous thermoresponsive poly(N -isopropylacrylamide-co-methacrylic acid) (P(NiPAM-co-MAA)) microgel was studied by dynamic light scattering (DLS) and electrophoretic measurements. DLS demonstrates an “odd–even” effect in the hydrodynamic radius depending on the type of polyelectrolyte in the outermost layer and that coated microgels retain their native thermoresponsive property. Strong hysteresis is found between the swelling and deswelling processes of microgels coated with polypeptides, whilst for those coated with polysaccharides are nearly reversible. Electrophoretic mobility results indicate charge reversal after each layer deposition, which magnitude decreases with increasing number of layers. Microgels coated with polysaccharides exhibit higher mobility values. Stability studies reveal spatial and temporal reorganization of the polymer chains over several weeks. Upon increasing time, the hydrodynamic radius of polypeptide-terminated microgels decreases, whereas that of polysaccharide-terminated increases to almost the original size of the uncoated microgel. Both systems exhibit an exponential growth of the bilayer thickness with the number of deposition steps, more pronounced for microgels coated with polypeptides. Our results demonstrate the feasibility of the layer-by-layer assembly of these biopolymers onto microgels, which could have potential application for storage and release of biomolecules.

Keywords: Layer-by-layer; Polyelectrolytes; Thermoresponsive microgels; Biopolymers; Temporal stability; Bilayer thickness

Surface-enhanced Raman spectroscopy of arsenate and arsenite using Ag nanofilm prepared by modified mirror reaction by Zhonghou Xu; Jumin Hao; Fasheng Li; Xiaoguang Meng (pp. 90-95).
The mirror reaction was modified to prepare a sensitive silver nanofilm which can detect ppb level As(V) and As(III).A modified mirror reaction was developed to prepare a sensitive and reproducible Ag nanofilm substrate for the surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) and arsenite (As(III)). A good linear relationship between the SERS intensity of As(V) and As(III) and their concentrations in the range from 10 to 500μg-As/L was achieved using the SERS substrate. As(V) and As(III) appear to be adsorbed on the Ag nanofilm through formation of surface complexes with Ag, based on comparisons of the Raman spectra of the arsenic species in solutions, on the SERS substrate, and in silver arsenate and arsenite solids. As(V) and As(III) species on the SERS substrate and in the solids had the same Raman band positions at 780 and 721cm−1, respectively. The effect of eight ions in natural waters on the SERS analysis of As(V) was studied. K+, Na+,SO42-,CO32-, andNO3- in the range of 0.1–100mg/L did not interfere with the SERS detection of As(V) for a As(V) concentration greater than 100μg-As/L. While Cl (50mg/L), Mg2+ (10mg/L), and Ca2+ (1mg/L) were found to quench the SERS intensity of 100μg/L As(V). Cl (at concentrations >10mg/L) formed silver chloride with the adsorbed Ag+ and decreased the SERS detection limits for arsenic species. The mechanism of the Ca2+ effect on the SERS analysis of As(V) was through the formation of surface complexes with As(V) in competition with Ag. When the Ca2+ concentration increased from 0 to 100mg/L, the amount of As(V) adsorbed in Ag nanoparticles was reduced from 38.9 to 11.0μg/mg-Ag. When the Ca2+ concentration increased to values higher than 1mg/L in the As(V) solution, the As(V) peak height was decreased in the corresponding SERS spectra and the peak position shifted from 780 to 800cm−1. The fundamental findings obtained in this research are especially valuable for the development of sensitive and reliable SERS methods for rapid analysis of arsenic in contaminated water.

Keywords: SERS; As(V); As(III); Mirror reaction

Interaction of bovine serum albumin with gemini surfactants by Silvia Tardioli; Adalberto Bonincontro; Camillo La Mesa; Rita Muzzalupo (pp. 96-101).
Scheme of a gemini surfactant interacting with BSA.The interactions between bovine serum albumin and cationic gemini surfactants were investigated as a function of concentration, under different pH conditions. The investigation deals with dielectric relaxation, dynamic light scattering, ζ-potential, circular dichroism, and UV spectroscopy. The interactive behavior of the anionic form is quite different from the cationic species. It indicates that protein–surfactant interactions are mostly electrostatic in nature and depend on the state of charge of bovine serum albumin. The results indicate the presence of both hydrophobic and electrostatic contributions in the interactions of gemini with bovine serum albumin. Comparison of dynamic light scattering, dielectric relaxation, electrophoretic mobility, and optical circular dichroism allows drawing some preliminary hypotheses on the different contributions to surfactant binding and supports former studies on the formation of complexes between the bovine serum albumin and the above species.

Keywords: Gemini; Protein interactions; Dielectric spectroscopy; Zeta potential

An analysis of current transients during electrophoretic deposition (EPD) from colloidal TiO2 suspensions by C. Baldisserri; D. Gardini; C. Galassi (pp. 102-111).
A resistive control of background electrochemistry during electrophoretic deposition of TiO2 is proposed to explain the observed trend of faradic currents. Derived numerical parameters are tentatively used for the structural characterization of the EPD deposit.This is a simple quantitative analysis of the electrical current transients recorded during the electrophoretic deposition (EPD) of TiO2 particles from ethanol-based suspensions in which the linear correlation between the mass deposited and the charge passed was verified experimentally. Using this experimental knowledge as our starting point, we were able to test a simple electrical model of a deposition cell for its consistency with electrical current density data measured during EPD. Assuming that the background electrochemistry was controlled resistively rather than diffusively, we then tentatively exploited the electrochemical data to gain information on the structure of the deposit during its growth.

Keywords: Electrophoretic deposition; Current transient modeling; Resistivity; Film processing; Colloidal suspension; TiO; 2

Analysis of phosphate adsorption onto ferrihydrite using the CD-MUSIC model by Juan Antelo; Sarah Fiol; Claudio Pérez; Silvia Mariño; Florencio Arce; Dora Gondar; Rocío López (pp. 112-119).
The CD-MUSIC model is able to successfully predict phosphate speciation on ferrihydrite nanoparticles under a wide range of conditions, pointing out that the dominant species are the bidentate surface complexes.Ferrihydrite nanoparticles may dominate the ion binding properties of the natural oxide fraction present in soil and aquatic systems. A correct description of the adsorption properties of ferrihydrite nanoparticles may be useful for gaining a better insight into the adsorption processes in natural systems and at the same time will be essential for developing surface complexation models able to describe these processes. In the present study, phosphate speciation in ferrihydrite has been analyzed combining the available spectroscopic data and molecular information with modeling calculations. For this purpose, a new data set that analyzes the effect of pH and ionic strength on the phosphate adsorption onto ferrihydrite has been used. Description of the phosphate adsorption process onto ferrihydrite nanoparticles, for the entire pH and ionic strength range, has been made taking into account the presence of protonated and nonprotonated bidentate surface complexes. The presence of monodentate complexes, protonated and nonprotonated, was also analyzed, but no significant improvement in the description of the results was observed. The surface complexation constants obtained with the CD-MUSIC modeling calculations are comparable to the values found in the literature for phosphate surface complexes in goethite particles.

Keywords: Iron oxides; Ferrihydrite; Goethite; Nanoparticles; Phosphate; Adsorption; Surface complexation modeling; CD-MUSIC

Methodology to obtain exchange properties of the calcite surface—Application to major and trace elements: Ca(II),HCO3-, and Zn(II) by E. Tertre; C. Beaucaire; A. Juery; J. Ly (pp. 120-126).
Concentrations of sorbed/desorbed Ca(II) andHCO3- on a calcite surface calculated for the sole reversible component of sorption, plotted as a function of time.Sorption of inorganic elements onto carbonate minerals has been intensively described in the literature by two reaction steps: (1) a first one rapid and completed within a few hours and (2) a second one slower, eventually irreversible, and occurring at a constant rate. The first step is often attributed to an ion-exchange process, but its reversibility is rarely investigated. Consequently, discrimination of the global sorption phenomenon into two different mechanisms is not always justified. In this study, we investigated, by batch experiments, both sorption and desorption of Ca(II),HCO3-, and Zn(II), radiolabeled with isotopes45Ca(II),H14CO3-, and65Zn(II), respectively, onto synthetic pure calcite. Solutions were preequilibrated with atmospheric pCO2 and saturated with respect to calcite. Therefore, our purpose was to: (1) obtain experimental distribution coefficients of major elements (Ca(II) andHCO3-) and a trace element (Zn(II)) onto calcite from sorption and desorption experiments, (2) test the validity of a first-occurring ion-exchange process generally noted in the literature, by calculating distribution coefficients for the “sole” exchange process, and (3) quantify the amounts of Ca(II),HCO3-, and Zn(II) sorbed on the calcite surface by the sole “exchange process” and compare them with surface crystallochemical data. Ca(II) orHCO3- sorption experimental data suggest that a significant fraction of these two elements was sorbed irreversibly onto or in the calcite. By using a method based on isotopic ratios, the Ca(II) orHCO3- concentrations, which are reversibly adsorbed on the calcite, have been quantified. These concentrations are respectively estimated at 4.0±2.0×10−4 and 7.0±1.5×10−4mol/kg. The obtained Ca(II) surface concentration value is one order of magnitude lower than the one obtained from isotopic measurement by former authors [Geochim. Cosmochim. Acta 55 (1991) 1549; Geochim. Cosmochim. Acta 51 (1987) 1477; Geochim. Cosmochim. Acta 52 (1988) 2281] at the same pH. On the other hand, the kinetics of Zn(II) sorption onto calcite was followed over more than 1000h. Sorption/desorption experimental results suggest that the sorption is totally reversible at least when total aqueous Zn concentration is less than 10−6mol/L and when experiments are performed in equilibrium with both calcite and pCO2=10−3.5 atm. Under these conditions and at pH 8.3, the occupancy rate of Zn(II) onto the calcite surface is estimated to represent approximately 1% of the total surface-site density.

Keywords: Exchange properties; Calcite surface; Sorption; Desorption; Reversible process; 45; Ca; H; 14; CO; 3; 65; Zn; Saturated solutions

Dissolution of fine and intermediate sized galena particles and their interactions with iron hydroxide colloids by Yongjun Peng; Stephen Grano (pp. 127-131).
Fine galena particles produce a much higher amount of lead oxidation products during dissolution than intermediate galena particles at pH 9 and have a greater affinity to iron hydroxide colloids.Dissolution of fine (−10μm) and intermediate (+10–53μm) galena particles was studied in the presence and absence of iron hydroxide colloids at pH 9 with nitrogen and oxygen purging. X-ray photoelectron spectroscopy (XPS) measurements and ethylene diamine-tetra acid (EDTA) extraction of the galena particles after dissolution indicate that galena dissolution is strongly dependent on particle size. Fine galena particles produced a much higher amount of lead hydroxide species per surface area than intermediate galena particles. Gas purging only affected galena dissolution slightly. More iron hydroxide colloids adsorbed on fine particles. Zeta potential measurements indicate that galena dissolution enhances the adsorption of iron hydroxide colloids due to the electrostatic attraction between lead hydroxide products and iron hydroxide colloids at pH 9. This explains the stronger affinity of iron hydroxide colloids to fine galena particles than intermediate galena particles. This study has an important implication in sulfide flotation where iron hydroxide colloids play a dominant role in mineral depression.

Keywords: Fine particles; Intermediate particles; Galena; Iron hydroxide colloids; Lead oxidation products

A method to determine zeta potential and Navier slip coefficient of microchannels by H.M. Park (pp. 132-141).
A simple method is devised to estimate the Navier slip coefficient and the zeta potential using measurements of volumetric flow rate and streaming potential.The no-slip boundary condition for liquid flows in microchannel has been applied successfully although it has no theoretical foundation. Liquid molecules, however, can slip at the liquid–solid interfaces if the liquid has a lower wettability. The velocity slip at the solid wall, called the Navier slip, is proportional to the velocity gradient at the wall for given wall physicochemical properties. Since the electroosmotic flow has a sharp velocity variation at the wall, the Navier slip of electroosmotic flow can be appreciable in a microchannel where there may be negligible Navier slip for a purely pressure-driven flow which has a smoother velocity variation at the wall. The Navier slip affects the volumetric flow rate and streaming potential significantly in electrokinetic flows and, therefore, one must be cautious about the possible occurrence of Navier slip in the design and operation of various microfluidic devices. In the present work, we have devised a simple method of estimating both Navier slip coefficient and zeta potential by measuring both the volumetric flow rate under a given pressure gradient after eliminating streaming potential and streaming potential under a given pressure gradient. Instead of streaming potential, one may adopt volumetric flow rate under a given external electric field. The method relies on a semi-analytic formula derived in the present work, which allows evaluation of streaming potential and volumetric flow rate without the necessity of numerical solution of nonlinear partial differential equations. The present method is found to estimate both slip coefficient and zeta potential reasonably accurately even using contaminated experimental data.

Keywords: Estimation; Navier slip coefficient; Zeta potential; Microchannel

Wall-induced lateral migration in particle electrophoresis through a rectangular microchannel by Litao Liang; Ye Ai; Junjie Zhu; Shizhi Qian; Xiangchun Xuan (pp. 142-146).
Particles migrate toward the channel center in electrophoretic motion through a rectangular microchannel due to a wall-induced electrical force.The fundamental study of particle electrophoresis in microchannels is relevant to many applications. It has long been accepted that particles move parallel to the applied electric field in a straight uniform microchannel. This paper presents the first experimental demonstration of lateral particle migration in electrophoresis through a rectangular microchannel. This wall-induced dielectrophoresis-resembled motion is driven by the electrical force arising from the non-uniform electric field around the particle. It decays rapidly when the particle is away from the channel wall, and vanishes in the center of the channel. The observed result is a focused particle stream flowing along the channel centerline. The measured widths of the particle stream at various electric fields agree reasonably with the predictions of an approximate analytical model.

Keywords: Particle electrophoresis; Wall effects; Lateral migration; Dielectrophoresis

Mechanical and wettable behavior of polyacrylonitrile reinforced fibrous polystyrene mats by Min Sun; Xiaohong Li; Bin Ding; Jianyong Yu; Gang Sun (pp. 147-152).
FE-SEM image of fibrous mat formed with the syringes ratio of 3/1 of PS/PAN. The insets show the larger magnifications and the profile of water droplet on fibrous mat.In this study, we have fabricated the polyacrylonitrile (PAN) reinforced super-hydrophobic fibrous polystyrene (PS) mats via a multi-syringe electrospinning technique. The composition ratio of PS/PAN in the blend mats could be controlled by tuning the number ratios of syringes of PS/PAN. The water contact angles (WCAs) of resultant fibrous mats was decreased from 155° to 143° with the decreasing the number ratios of syringes of PS/PAN from 4/0 to 1/3. The addition of the component of PAN nanofibers in fibrous PS mats significantly improved the mechanical properties of PS mats. At a critical syringe ratio of 3/1 (PS/PAN), the mat surface showed a WCA of 150° with a three times increased tensile strength compared with the pure PS mats. Additionally, the results of field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR), and mechanical properties indicated the multi-syringe electrospinning technique is an effective approach to fabricate the large-scale well-dispersed blend fibrous mats.

Keywords: Super-hydrophobic; Electrospun fibers; Mechanical properties; Polystyrene; Polyacrylonitrile

Is free surface free in micro-scale electrokinetic flows? by WooSeok Choi; Ashtosh Sharma; Shizhi Qian; Geunbae Lim; Sang Woo Joo (pp. 153-155).
In electrokinetic flows, the replacement from rigid to free boundary can lead to an increase (decrease) in shear stress (flow rate). This counterintuitive effect of Maxwell stress is presented.When a rigid boundary is replaced by a free surface, or by a liquid–liquid interface, the shear stress is reduced near the boundary because the no-slip on the rigid boundary is relaxed. A nonzero fluid velocity exists on the free surface, and the flow rate would increase by the replacement. For electrokinetic flows, however, the replacement from rigid to free boundary can lead to an increase (decrease) in shear stress (flow rate). This counterintuitive effect of free surface is presented in this note for a generic electroosmotic flow in a microchannel.

Keywords: Electroosmosis; Free surface; Electrokinetics; Microfluidics; Interfacial flows

On the possibility of superhydrophobic behavior for hydrophilic materials by X.S. Cui; W. Li (pp. 156-162).
Variation of normalized free energy with contact angle for noncomposite and composite states.It has been recognized well that it is necessary to achieve superhydrophobic surfaces on intrinsically hydrophobic materials. However, recently experiments have demonstrated that it is possible to fabricate superhydrophobic surfaces on intrinsically hydrophilic materials by creating adequate roughness. In this study, such a possibility for superhydrophobicity on a hydrophilic surface with an intrinsic contact angle (CA) of 80°, with a comparison to a hydrophobic surface with an intrinsic CA of 120°, is thermodynamically analyzed using a pillared microtexture. Based on the calculations of free energy (FE) and free energy barrier (FEB), it is found that for such hydrophilic materials, generally, the FE for noncomposite or Wenzel’s state is lower than that composite or Cassie’s state for various geometrical wetting systems. Furthermore, even if pillar height or roughness is adequately large, it is hard to realize superhydrophobic behavior because of the surface wicking resulted from its special FE state. In addition, due to the negative FEB of the noncomposite state, there is no transition between noncomposite and composite states no matter how surface geometry varies. The above results also indicates that once noncomposite state is formed, it can hardly be become composite state, or in other words, even if superhydrophobic behavior is possible, it could be temporary and unstable. The present theoretical investigation therefore keeps a reservation on the practicability of superhydrophobic surfaces built on hydrophilic materials.

Keywords: Superhydrophobic; Hydrophilic; Contact angle; Contact angle hysteresis; Free energy; Free energy barrier

Protrusion and rising of a gel-based precipitation layer by Akihito Suzuki; Shouhei Yamaguchi; Takahiko Ban; Akihisa Shioi (pp. 163-166).
A bird’s eye view of the rising structure of inorganic salt from the gel surface by a reaction-diffusion system.Two types of unstable growth of a precipitation layer in gel are discussed. A cation and an anion that are reactive diffuse from opposite ends of the gel to its center. A white turbid zone forms due to their reactions. When the concentration ratios for both the ions are far from stoichiometry, the turbid zone expands toward the lower-concentration side. However, when the ratio is nearly stoichiometric, unstable growth occurs. In a glass tube, a protrusion of the precipitation region from the turbid zone grows, which forms a long needle-like shape. When a free surface is present on the gel, the precipitation region protrudes from the gel surface to form a rising structure. Mapping the growing structure on a concentration diagram and using scanning electron microscopy to examine contained particles suggest that the reaction is restricted to a narrow region and the reaction product migrates through a path formed in the protrusive structure to form a bulk solid at the edge.

Keywords: Particle growth in gel; Unstable growth; Protrusion; Reaction diffusion

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