Journal of Colloid And Interface Science (v.284, #2)
Editorial Board (OFC).
Surface chemical studies on the competitive adsorption of poly(ethylene glycol) and ammonium poly(methacrylate) onto alumina by L. Saravanan; S. Subramanian (363-377).
The adsorption of poly(ethylene glycol) (PEG) and ammonium poly(methacrylate) (APMA) onto alumina has been examined both individually and in combination. The adsorption density of APMA was found to be higher than that of PEG onto alumina. The adsorption isotherms of PEG and APMA for alumina exhibited a Langmuirian behavior. The adsorption density of PEG was significantly reduced in the presence of APMA, but the reverse was not true. About 60% desorption of PEG from alumina was achieved, while in the case of APMA the amount desorbed was only 10% in the pH range of 3–6. The zeta potential values of alumina were decreased and the isoelectric point (i.e.p.) values were shifted toward acidic pH values, proportional to the concentration of APMA added. However, such changes in the electrokinetic behavior were not observed by the addition of PEG. The dispersion behavior of alumina in the combined presence of PEG and APMA essentially followed the trends obtained for the alumina–APMA system, corroborating the electrokinetic measurements. Coprecipitation tests confirmed complexation between aluminum species and APMA in the bulk solution, but not with PEG. The interaction between alumina and PEG is primarily governed by hydrogen-bonding forces, while both hydrogen bonding and chemical interaction are involved in the case of the alumina–APMA system. FTIR spectroscopic studies provided evidence in support of the interaction mechanisms proposed.
Keywords: Alumina; Poly(ethylene glycol); Ammonium poly(methacrylate); Adsorption; Desorption; Electrokinetics; Stability; Coprecipitation; FTIR; Interaction mechanisms;
The fabrication and characterization of a formaldehyde odor sensor using molecularly imprinted polymers by Liang Feng; Yongjun Liu; Xiaodong Zhou; Jiming Hu (378-382).
The fabrication and characterization of odor sensors based on molecularly imprinted polymers is reported as the first case of imprinting formaldehyde. A quartz crystal microbalance is employed as a sensitive apparatus of a sensor for the determination of odor formaldehyde. An equation is deduced to characterize the interaction between molecularly imprinted films and the template. A linear relationship between the frequency shifts and the concentration of analyte in the range of 1.25 to 14.25 μM is found. The detection limit is about 20.5 μM. The sensor can selectively distinguish gaseous formaldehyde. It is envisaged that this novel and handy method could be employed to determine formaldehyde gas in the atmosphere.
Keywords: Formaldehyde; Molecular imprinting; Quartz crystal microbalance;
Surface complexation modeling of the sorption of 2-, 3-, and 4-aminopyridine by montmorillonite by Jaslin Ikhsan; Michael J. Angove; John D. Wells; Bruce B. Johnson (383-392).
The sorption of 2-, 3-, and 4-aminopyridine on K-saturated Wyoming (SWy-K) and Texas (STx-K) and Ca-enriched Texas (STx-Ca) montmorillonite was measured at 25 °C with 10 mM KNO3 or 3.3 mM Ca(NO3)2 as the background electrolyte. The aminopyridines adsorbed to montmorillonite at low pH, but not at high pH. Extended constant capacitance surface complexation models (ECCMs) and attenuated total reflectance-FTIR data indicate that aminopyridines sorb to the silica-like faces by cation exchange, forming outer-sphere complexes between aminopyridinium ions and permanent negatively charged surface sites (X−). X-ray diffraction data and sorption kinetics suggest that sorption occurs not only at external X− sites but also at those in the interlayer spaces. Differences in the sorption behaviors of 2-, 3-, and 4-aminopyridine result from differences in their p K a s. The extent of sorption of aminopyridines by the montmorillonite samples (SWy-K > STx-K > STx-Ca) results from the higher cation-exchange capacity of SWy-K, and from the fact that Ca2+ is much more effective than K+ in competing with protonated aminopyridines for the X− sites.
Keywords: Montmorillonite; Kaolin; Kaolinite; 2-Aminopyridine; 3-Aminopyridine; 4-Aminopyridine; Extended constant capacitance model; ATR-FTIR; XRD;
Effect of low-molecular-weight organic anions on exchangeable aluminum capacity of variable charge soils by Jiuyu Li; Renkou Xu; Shuangcheng Xiao; Guoliang Ji (393-399).
Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes, such as nutrient availability, translocation of metals, fate of heavy metals, and mineral weathering. In this paper, the effect of the LMW organic anions on the exchangeable aluminum of two variable-charge soils was examined. The results showed that the organic anions induced an increase or a decrease in the exchangeable Al, and the extent and direction of the effect depended on the nature of organic anions, surface chemical properties of soils, and pH. For example, at pH 4.5, the quantity of exchangeable Al of Oxisol in the control system was 2.65 mmol kg−1, whereas the values in the citrate, oxalate, malonate, malate, tartarate, salicylate, and lactate systems increased by 3.25, 1.93, 1.95, 1.82, 1.28, 0.88, and 0.45 times, respectively. In contrast, the quantity of the exchangeable Al of Ultisol at pH 4.5 in the oxalate and the citrate systems decreased by 8.8 and 19.6%, respectively. The increase in the exchangeable Al was caused mainly by the increase in negative surface charge of the soils due to the specific adsorption of organic anions. The ability of organic anions at low concentrations to increase exchangeable Al for Oxisol followed the order citrate > oxalate and malonate > malate > tartarate > salicylate > maleate > lactate. This order is consistent with that of the effect of the adsorption of anions on the increase in the negative surface charge and/or the decrease in the positive surface charge of the soil. On the other hand, the organic anions could depress the exchangeable Al through the formation of soluble Al–organic anion complexes under certain conditions. The anions with small stability constants of Al–organic anion complexes, such as lactate, caused an increase in exchangeable Al with the change in surface charge of the soils, while those with large stability constants, such as citrate and oxalate, caused an increase in the exchangeable Al at low concentration and a decrease at high concentration.
Keywords: Organic anions; Exchangeable Al; Variable-charge soil;
Cosorption of Zn(II) and 2-, 3-, or 4-aminopyridine by montmorillonite by Jaslin Ikhsan; Michael J. Angove; Bruce B. Johnson; John D. Wells (400-407).
Data from acid–base titrations at 25 °C of Zn(NO3)2 and 2-, 3-, or 4-aminopyridine in 10 mM KNO3 as background electrolyte suggested that soluble complexes ZnL2+ and Zn(OH)L+ form, where L represents aminopyridine. Zinc–hydroxyaminopyridine complexes have not been reported previously. The cosorption of Zn(II) with each of the aminopyridines to K-saturated Wyoming (SWy-K) and Texas (STx-K), and Ca-enriched Texas (STx-Ca) montmorillonites was measured at 25 °C, with 10 mM KNO3 or 3.3 mM Ca(NO3)2 as background electrolyte. Comparison with previous data for sorption of Zn(II) and the aminopyridines separately and surface complexation modeling of the cosorption data showed that under acid conditions competition between Zn2+ and aminopyridinium ions for the permanent negatively charged sites of montmorillonite results in suppression of the uptake of each sorbate by the other, but only when a large excess of the competing sorbate is present. Under alkaline conditions the sorption of Zn(II) was not affected by the presence of even a large excess of aminopyridine, but the sorption of 4-aminopyridine in particular was slightly enhanced when a large excess of Zn(II) was present. The enhancement was attributed to the formation of metal-bridged ternary surface complexes at the variable-charge sites on the edges of the montmorillonite crystals.
Keywords: Montmorillonite; 2-Aminopyridine; 3-Aminopyridine; 4-Aminopyridine; Surface complexation model; Extended constant capacitance model;
Ammonium sorption from aqueous solutions by the natural zeolite Transcarpathian clinoptilolite studied under dynamic conditions by Myroslav Sprynskyy; Mariya Lebedynets; Artur P. Terzyk; Piotr Kowalczyk; Jacek Namieśnik; Bogusław Buszewski (408-415).
The scope of this study is ammonium-ion uptake from synthetic aqueous solutions onto raw and pretreated forms of the natural zeolite Transcarpathian clinoptilolite under dynamic conditions. Hydrogen ions displaced exchangeable cations on the clinoptilolite in distilled water (sodium ions) and hydrochloric acid (sodium, potassium, and calcium ions) and destroyed the zeolite framework structure in the last case. Ammonium uptake onto the zeolite occurs by exchange with Na+, Ca2+, and K+ ions. Although Na+ ions were observed to be more easily exchanged for both hydrogen and ammonium ions, the role of Ca2+ ions increased with zeolite saturation by NH+ 4 ions. The maximum sorption capacity of the clinoptilolite toward NH+ 4 ions, estimated under dynamic conditions, is significantly higher than that measured under static conditions; proximity of the values of a distribution coefficient and a retardation factor for different conditions (215–265 dm3/kg and 979–1107, respectively) allows us to use these parameters to model ammonium uptake onto the clinoptilolite. Slowing down or interruption in filtration resulted in the improvement of ammonium sorption properties of the zeolite. The ammonium removal improves with use of the finer fractions of the clinoptilolite up to 0.35 mm. A recycling study results confirmed the importance of external diffusion for ammonium sorption by the clinoptilolite. Preliminary treatment of the sorbent confirmed the predominant importance of the ion-exchange mechanism. The advantage of prior NaCl treatment of the clinoptilolite in improvement of ammonium removal over the other techniques was shown.
Keywords: Zeolite; Clinoptilolite; Ammonium sorption; Filtration effect; Ion exchange; Water purification;
Adsorption of lignite-derived humic acids on coal-based mesoporous activated carbons by Ewa Lorenc-Grabowska; Grażyna Gryglewicz (416-423).
The adsorption by a coal-based mesoporous activated carbon of humic acids (HAs) isolated from two Polish lignites was studied. For comparison, a commercial Aldrich humic acid was also included into this study. The differences in chemical structure and functional groups of HAs were determined by elemental analysis and infrared spectroscopy DRIFT. Two activated carbons used differed in terms of mesopore volume, mesopore size distribution, and chemical properties of the surface. The kinetics of adsorption of HAs have been discussed using three kinetic models, i.e., the first-order Lagergren model, the pseudo-second-order model, and the intraparticle diffusion model. It was found that the adsorption of HAs from alkaline solution on mesoporous activated carbon proceeds according to the pseudo-second-order model. The correlation coefficients were close to 1. The intraparticle diffusion of HA molecules within the carbon particle was identified to be the rate-limiting step. Comparing the two activated carbons, the carbon with a higher volume of pores with widths of 10–50 nm showed a greater removal efficiency of HA. An increase in the Freundlich adsorption capacity with decreasing carbon content of HA was observed. Among the HAs studied, S-HA shows characteristics indicating the highest contribution of small-size fraction. The S-HA was removed by both activated carbons to the highest extent. The effect of pH solution on the adsorption of HA was examined over the range pH 5.4–12.2. It was found that the extent of adsorption decreased with decreasing pH of the solution.
Keywords: Adsorption; Humic acids; Kinetics; Activated carbons;
A mathematical simulation of H+ ion chemisorption by anilinepropylsilica xerogels by Yuriy Khoroshevskiy; Sergiy Korneev; Sergiy Myerniy; Yuriy V. Kholin; Flávio A. Pavan; José Schifino; Tania M.H. Costa; Edilson V. Benvenutti (424-431).
A study of the basic characteristics of the hybrid anilinesilica xerogel, which shows promising metal sorption properties, was carried out. A simplified approach to describe the protonization affinity of the immobilized aniline was developed, and it was compared with the known values for free aniline. The proton chemisorption process may be described properly by a model based on the Langmuir equation, and the immobilized groups have less affinity to the H+ than to the free aniline.
Adsorption equilibrium and transport kinetics for a range of probe gases in Takeda 3A carbon molecular sieve by S.W. Rutherford; J.E. Coons (432-439).
Measurements of adsorption equilibria and transport kinetics for argon, oxygen and nitrogen at 20, 50, and 80 °C on commercially derived Takeda carbon molecular sieve (CMS) employed for air separation have been undertaken in an effort to elucidate fundamental mechanisms of transport. Results indicate that micropore diffusion which is modeled by a Fickian diffusion process, governs the transport of oxygen molecules and the pore mouth barrier controls argon and nitrogen transport which is characterized by a linear driving force (LDF) model. For the three temperatures studied, the pressure dependence of the diffusivity and the LDF rate constant appear to be well characterized by a formulation based on the chemical potential as the driving force for transport. Isosteric heat of adsorption at zero loading and activation energy measurements are compared with predictions made from a previously proposed molecular model for characterizing CMS.
The physical and surface chemical characteristics of activated carbons and the adsorption of methylene blue from wastewater by Shaobin Wang; Z.H. Zhu; Anthony Coomes; F. Haghseresht; G.Q. Lu (440-446).
Adsorption of a basic dye, methylene blue, from aqueous solutions onto as-received activated carbons and acid-treated carbons was investigated. The physical and surface chemical properties of the activated carbons were characterized using BET-N2 adsorption, X-ray photoelectron spectroscopy (XPS), and mass titration. It was found that acid treatment had little effect on carbon textural characteristics but significantly changed the surface chemical properties, resulting in an adverse effect on dye adsorption. The physical properties of activated carbon, such as surface area and pore volume, have little effect on dye adsorption, while the pore size distribution and the surface chemical characteristics play important roles in dye adsorption. The pH value of the solution also influences the adsorption capacity significantly. For methylene blue, a higher pH of solution favors the adsorption capacity. The kinetic adsorption of methylene blue on all carbons follows a pseudo-second-order equation.
Keywords: Activated carbon; Dye adsorption; Acid treatment; Methylene blue;
The effects of 1-hydroxyethane-(1,1-diphosphonic acid) on the adsorptive partitioning of metal ions onto γ-AlOOH by María C. Zenobi; Lidia Hein; Elsa Rueda (447-454).
1-Hydroxyethane-(1,1-diphosphonic acid) (HEDP) is extensively used in many technical applications. This work, as a first stage, examines the adsorption of aqueous HEDP, Cu(II), and Zn(II) onto boehmite (γ-AlOOH) as single solutes. These processes can be described by the formation of inner-sphere complexes by the surface complexation model with constant capacitance. Uncomplexed HEDP is adsorbed as a mononuclear complex over the entre pH range. Four mononuclear surface complexes with different protonation levels are required to obtain a good fit of the experimental data for the pH range studied. Cu(II)– and Zn(II)–HEDP complexes at equimolar concentrations are studied at high and low surface density. The results indicate that the presence of HEDP significantly promotes metal adsorption at low pH in defects of surface sites. However, metal adsorption exhibits the same trend in the absence and presence of HEDP when the surface sites are in excess. The constant capacitance model successfully describes the experimental data through a ligand-like adsorption complex at low pH. At high pH, the model predicts separate adsorption of divalent metal and HEDP onto different surface sites as the preferred adsorption form.
Keywords: Boehmite; 1-Hydroxyethane-(1,1-diphosphonic acid) (HEDP); Cu(II)–HEDP adsorption; Zn(II)–HEDP adsorption; Constant capacitance model;
Salt softening of polyelectrolyte multilayer microcapsules by Olga V. Lebedeva; Byoung-Suhk Kim; Krasimir Vasilev; Olga I. Vinogradova (455-462).
By using a combination of atomic force and confocal microscopy, we explore the effect of 1:1 electrolyte (NaCl) on the stiffness of polyelectrolyte microcapsules. We study the “hollow” and “filled” (with polystyrene sulfonate) capsules. In both cases the shells are composed of layers of alternating polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH). The stiffness of both “hollow” and “filled” capsules was found to be largest in water. It decreases with salt concentration up to ∼3 mol/L and gets quasi-constant in more concentrated solutions. The “filled” capsules are always stiffer than “hollow.” The observed softening correlates with the salt-induced changes in morphology of the multilayer shells detected with the scanning electron microscopy. It is likely that at concentrations below ∼3 mol/L the multilayer shell is in a “tethered” state, so that the increase in salt concentration leads to a decrease in number of ionic cross-links and, as a result, in the stiffness. In contrast, above the critical concentration of ∼3 mol/L multilayer shells might be in a new, “melted,” state. Here the multilayer structure is still retained, but sufficient amount of ionic cross-links is broken, so that further increase in salt concentration does not change the capsule mechanics. These ideas are consistent with a moderate swelling of multilayers at concentrations below ∼3 mol/L and significant decrease in their thickness in more concentrated solutions measured with surface plasmon spectroscopy.
Keywords: Polyelectrolytes; Multilayer microcapsules; Salt softening;
Size and shape of soil humic acids estimated by viscosity and molecular weight by Masayuki Kawahigashi; Hiroaki Sumida; Kazuhiko Yamamoto (463-469).
Ultrafiltration fractions of three soil humic acids were characterized by viscometry and high performance size-exclusion chromatography (HPSEC) in order to estimate shapes and hydrodynamic sizes. Intrinsic viscosities under given solute/solvent/temperature conditions were obtained by extrapolating the concentration dependence of reduced viscosities to zero concentration. Molecular mass (weight average molecular weight ( M ¯ w ) and number average molecular weight ( M ¯ n )) and hydrodynamic radius ( R H ) were determined by HPSEC using pullulan as calibrant. Values of M ¯ w and M ¯ n ranged from 15 to 118 × 10 3 and from 9 to 50 × 10 3 (g mol−1), respectively. Polydispersity, as indicated by M ¯ w / M ¯ n , increased with increasing filter size from 1.5 to 2.4. The hydrodynamic radii ( R H ) ranged between 2.2 and 6.4 nm. For each humic acid, M ¯ w and [η] were related. Mark–Houwink coefficients calculated on the basis of the M ¯ w − [ η ] relationships suggested restricted flexible chains for two of the humic acids and a branched structure for the third humic acid. Those structures probably behave as hydrated sphere colloids in a good solvent. Hydrodynamic radii of fractions calculated from [η] using Einstein's equation, which is applicable to hydrated sphere colloids, ranged from 2.2 to 7.1 nm. These dimensions are fit to the size of nanospaces on and between clay minerals and micropores in soil particle aggregates. On the other hand, the good agreement of R H values obtained by applying Einstein's equation with those directly determined by HPSEC suggests that pullulan is a suitable calibrant for estimation of molecular mass and size of humic acids by HPSEC.
Keywords: Viscosity; HPSEC; Weight average molecular weight; Number average molecular weight; Polydispersity; Hydrodynamic radius; Flexible chain; Branched structure; Humic acid; Ultra filtration;
Rheological properties of protein–surfactant based gels by Mauro Roversi; Camillo La Mesa (470-476).
Water-based protein–surfactant gels, formed by mixing bovine serum albumin (BSA) and sodium dodecyl sulfate in water, were investigated by rheological methods. The measurements were performed for many different protein-to-surfactant ratios as a function of the applied frequency, stress, or strain, as well as by changing the temperature, in the range between 15 and 65 °C. The rheological behavior of the gels as a function of applied frequency is interpreted in terms of the overlapping of at least two viscoelastic relaxation processes. The rheological results indicate the presence of thermal transitions from essentially viscous to mainly elastic regimes, in analogy with the thermal gelation processes observed in polymer solutions. The thermal gelation threshold in the present system is modulated by the protein/surfactant ratio. Differential scanning calorimetry measurements were also performed to determine whether thermal gelation is somehow concomitant to protein denaturation. The results indicate that the thermal denaturation of BSA in protein–surfactant based gels occurs at slightly higher temperatures than in the bulk. Scanning electron microscopy indicates the occurrence in the gel structure of globules formed by the arrangement of fibrils.
Keywords: Gels; Protein–surfactant mixtures; Rheology; Viscous and elastic behavior; Relaxation times; Differential scanning calorimetry; Scanning electron microscopy;
An evaluation of the colloidal stability of metal working fluid by Adrienne Menniti; Kishore Rajagopalan; Timothy A. Kramer; Mark M. Clark (477-488).
The effect of calcium on the stability of a commercial MWF is characterized through the experimental determination of the stability ratio, W. Three experimental methods of stability ratio evaluation are investigated. (1) The initial slope of the absorbance versus time curve is used to estimate the rate of coagulation. (2) Absorbance measurements are used to estimate N 0 / N with time. The stability ratio is determined from the slope of N 0 / N versus time. (3) Photon correlation spectroscopy (PCS) measurements of the volume distribution with time are used to estimate N 0 / N with time. Electrophoretic mobility was also measured and used to determine the fast coagulation concentration of the MWF. The accuracy of the experimentally determined stability ratios is evaluated using a population balance coagulation model. The model predicts the population distribution of a coagulating dispersion with time based on an initial particle size distribution and stability ratio. The model results were compared with the PCS-measured distributions to determine which stability ratio evaluation method best describes the stability of the MWF emulsion studied. Using the initial slope of the absorbance versus time curve to determine the fast coagulation concentration correlates well with electrophoretic mobility measurements. However, using absorbance measurements to determine the rate of coagulation underestimates the stability ratio of the MWF studied by orders of magnitude. N 0 / N values calculated from absorbance measurements provide a reasonable estimate of the stability ratio but inconsistencies in the method decrease its reliability. The stability ratio derived from PCS measurements appears to provide the most accurate, reliable description of MWF stability.
Keywords: Colloidal stability; Stability ratio; Population balance model; Metal working fluid;
Sonochemical synthesis of stable hydrosol of Fe3O4 nanoparticles by R. Abu Mukh-Qasem; A. Gedanken (489-494).
The sonolysis of an aqueous solution of Fe(CO)5 in the presence of sodium dodecyl sulfate leads to the formation of a stable hydrosol of amorphous Fe3O4 nanoparticles. The amorphicity of iron oxide nanoparticles was determined by X-ray diffraction and differential scanning calorimetry. The nanoparticles were characterized by elemental analysis, EDX, transmission electron microscopy, dynamic light scattering, Raman spectroscopy, XPS, and spot test.
Keywords: Sonochemical synthesis; Hydrosol; Iron oxide; Nanoparticles;
Synthesis and characterization of CdS nanoparticles embedded in a polymethylmethacrylate matrix by Lucia Pedone; Eugenio Caponetti; Maurizio Leone; Valeria Militello; Valentina Pantò; Stefano Polizzi; Maria Luisa Saladino (495-500).
CdS nanopowder capped with sodium bis(2-ethylhexyl)sulfosuccinate was synthesized by using water-in-oil microemulsions. The CdS nanoparticles of about 5 nm obtained were embedded in polymethylmethacrylate matrix by a photocuring process. The transparent yellow solid compound was characterized by optical absorption and emission spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The properties of this compound were compared with those of the nanopowder dispersed in heptane and in methylmethacrylate. The results obtained indicate that the nanoparticles are homogeneously dispersed in the matrix and do not change in size during the embedding process. Even if the surface slightly changes its luminescence properties, as a consequence of the different new chemical environment, the final product seems to be suitable for practical applications.
Mobility of permeable fractal agglomerates in slip regime by P. Vainshtein; M. Shapiro (501-509).
Hydrodynamic drag and mobility of fractal aggregates in the slip creeping flow regime are calculated. A theoretical continuum model of the gas slip flow past and within agglomerates is developed. It accounts for effects of flow rarefaction and porous fractal structure upon the molecular mean free path, apparent viscosity, and effective permeability of agglomerates. It is shown that flow rarefaction significantly diminishes the aggregates' drag to an extent that cannot be predicted by the Cunningham's drag correction factor. The developed model allows calculation the agglomerates' transport properties in a wide range of fractal dimensions. For low D f agglomerates the drag force agrees with the Friedlander's expression based on the Epstein's single sphere drag in the free molecular regime.
Keywords: Agglomerate; Porosity; Slip flow; Fractal dimension; Drag;
Size-controlled preparation of Cu2O octahedron nanocrystals and studies on their optical absorption by Ping He; Xinghai Shen; Hongcheng Gao (510-515).
We report herein the size-controlled preparation of monodispersed cuprous oxide octahedron nanocrystals smaller than 100 nm. The method is based on the reduction of copper nitrate in Triton X-100 water-in-oil (w/o) microemulsions by γ-irradiation. The average edge length of the octahedron-shaped nanocrystals varies from 45 to 95 nm as a function of the dose rate. The quantum confinement effect was illustrated by the blueshift in the optical absorption. In addition, the growth process was also traced by absorption spectra.
Keywords: Cuprous oxide; Nanooctahedron; γ-Irradiation; Microemulsion;
Effect of layer thickness on the luminescence properties of ZnS/CdS/ZnS quantum dot quantum well by Lixin Cao; Shihua Huang; Shaozhe Lü; Jiuling Lin (516-520).
ZnS/CdS/ZnS quantum dot quantum well was prepared. The optical properties of ZnS/CdS/ZnS QDQW with different thickness of CdS well and ZnS shell were studied. Absorption spectra, emission spectra, and luminescence lifetimes were measured. The observed luminescence was assigned to the bulk donor–acceptor pair recombination of CdS and can be enhanced by increasing the thickness of the CdS well or coating an appropriate thickness of ZnS shell on the surface of the CdS well. The luminescence enhancement was caused by the relative reduce in the surface effect. The luminescence lifetimes were influenced strongly by the surface state.
Keywords: Quantum dot quantum well; ZnS; CdS;
Synthesis of spherical silver nanoparticles by digestive ripening, stabilization with various agents, and their 3-D and 2-D superlattice formation by Alexander B. Smetana; Kenneth J. Klabunde; Christopher M. Sorensen (521-526).
Capped nanoparticles of silver were synthesized via the solvated metal atom dispersion (SMAD) technique followed by a digestive ripening procedure producing gram quantities of monodisperse spherical nanoparticles. This shows for the first time that a digestive ripening protocol is possible for an element other than gold. The particle size and optical spectra were found to be dependent on the capping agent used. Particles capped with dodecane thiol had a mean diameter of 6.6 ± 1 nm, while trioctyl phosphine capped particles were 6.0 ± 2 nm determined via TEM microscopy. These particles were found to organize into two- and three-dimensional superlattices with a well defined geometry through self-assembly in a liquid solution, that was dictated by the ligand used resulting in a triangular or circular lattice.
Keywords: Digestive ripening; Self-assembly; 3-D superlattice; Gram scale;
Stabilization of magnetorheological suspensions by polyacrylic acid polymers by J.L. Viota; J. de Vicente; J.D.G. Durán; A.V. Delgado (527-541).
This work is devoted to the synthesis and stabilization of magnetorheological suspensions constituted by monodisperse micrometer-sized magnetite spheres in aqueous media. The electrical double-layer characteristics of the solid/liquid interface were studied in the absence and presence of adsorbed layers of high molecular weight polyacrylic acids (PAA; Carbopol). Since the Carbopol-covered particles can be thought of as “soft” colloids, Ohshima's theory was used to gain information of the surface potential and the charge density of the polymer layer. The effect of the pH of the solution on the double-layer characteristics is related to the different conformations of the adsorbed molecules provoked by the dissociation of the acrylic groups present in polymer molecules. The stability of the suspensions was experimentally studied for different pH and polymer concentrations, and in the absence or presence of a weak magnetic field applied. The stability of the suspensions was explained using the classical DLVO theory of colloidal stability extended to account for hydration, steric, and magnetic interactions between particles. Diagrams of potential energy vs interparticle distance show the predominant effect of steric, hydrophilic/hydrophobic, and magnetic interactions on the whole stability of the system. The best conditions to obtain stable suspensions were found when strong steric and hydrophilic repulsions hinder the coagulation between polymer-covered particles, simultaneously avoiding sedimentation by the thickening effect of the polymer solution. When a not too high molecular weight PAA was employed in a low concentration, the task of a long-time antisettling effect compatible with the desired magnetic response of the fluid was achieved.
Keywords: Magnetorheological fluids; Carbopol polymers; Polyacrylic acid; Magnetite; Electrophoretic mobility; Surface potential; Extended DLVO theory;
Effect of base addition rate on the preparation of partially neutralized ferric chloride solutions by Ta-Kang Liu; Edward S.K. Chian (542-547).
Cationic polymers prepared from partially neutralized ferric chloride solutions were investigated using photon correlation spectroscopy (PCS) techniques. To study the speciation and yield of the polymers, FeCl3 solutions were hydrolyzed by adding NaHCO3 solution for a range of hydrolysis ratios ( B = [ OH ] / Fe T ) of 0 to 2.0 under different base addition rates. It was observed that the prepared solution has a higher degree of polymerization at a higher base addition rate. The “B rate” ( R B , 1/time), i.e., the rate of change of B during preparation, was found to have a dramatic effect on the speciation of the partially neutralized ferric chloride solution. High-yield inorganic iron(III) polymer ( ∼ 80 % ) can be produced in a specific range of B rates. These iron(III) polymers, having a size ranging from 3 to 7 nm, were stable without precipitation of ferric hydroxide microcolloids after aging for 2 weeks.
Keywords: Iron(III) polymers; Polymeric iron chloride (PICl); Polymer yield; Polymerization of iron(III); Base addition rate;
Influence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium by Frédéric Gruy; Michel Cournil; Patrick Cugniet (548-559).
The aim of this work was to determine and to interpret the influence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium. Two silica granular samples were studied: one was naturally hydrophilic; the other was made hydrophobic. Aggregation in an aqueous ethanol solution was followed by in situ turbidimetry. The influence of stirring rate and deaeration was determined. Aggregates of hydrophilic particles were small and fragile, whereas aggregates of hydrophobic particles were large and solid. Moreover, they differred greatly in optical properties. Within the proposed approach, different features of the aggregate morphology were identified: fractal dimension, maximum size, and gas content of the hydrophobic clusters. These elements are taken into account in the models of aggregation dynamics proposed here.
Keywords: Aggregation; Fragmentation; Silica; Nonwetting; Hydrophobic; Turbidimetry; Light scattering; Fractal;
Interactions of hairy latex particles with cationic copolymers by Pascal Borget; Françoise Lafuma; Cécile Bonnet-Gonnet (560-570).
Interactions between polycations and core–corona particles are governed by ion-exchange reactions, entropically favored by the release of counterions. This complexation process allows the chains to penetrate into the shell, leading to adsorbed amounts greater than 1 mg m−2. The destabilization occurs quickly, the domain of flocculation becomes larger when the concentration of monovalent salts is increased, and aggregates are composed of small and very compact clusters in a more or less self-similar structure at large scale. The adsorption of copolymers of low cationicity is characterized by still larger adsorbed amounts and layers thicker than the radius of gyration of the macromolecules. Depending on the charge content, the enhancement of the ionic strength can either promote the destabilization of the suspension or conversely induce the desorption of the chain. In pure water the structure of the flocs is long-range ordered and it becomes more heterogeneous in ionic media.
Keywords: Core–corona particle; Soft surface; Cationic copolymer; Flocculation; Adsorption; Small angle neutron scattering;
On the self-similar solution of fragmentation equation: Numerical evaluation with implications for the inverse problem by M. Kostoglou; A.J. Karabelas (571-581).
It is well known that the fragmentation equation admits self-similar solutions for evolving particle-size distributions (PSD); i.e., if the shape of PSD is independent of time after an initial transient period. Although an analytical derivations of the self-similar PSD cases have been studied extensively, results for cases requiring numerical solutions are rare. The aim of the present work is to fill this gap for the case of homogeneous breakage functions. The known analytical and approximate solutions for the self-similar PSD are reviewed and a general algorithm for the numerical solution is proposed. Results for a broad range of breakage functions (kernel and rate) are presented. Further, the work is focused on the sensitivity of the relation between self-similar PSD and breakage kernel and its influence on the inverse breakage problem, i.e., that of estimating the breakage kernel from experimental self-similar PSDs. Useful suggestions are made for tackling the inverse problem.
Keywords: Population balance; Fragmentation; Breakage; Self-similar size distribution; Inverse breakage problem;
Effects of a central metal on the organization of 5,10,15,20-tetra-(p-chlorophenyl)–rare earth porphyrin hydroxyl compound at the air/water interface and in Langmuir–Blodgett films by Xiaoling Li; Weiqing Xu; Tamitake Itoh; Akifumi Ikehata; Bing Zhao; Bofu Li; Yukihiro Ozaki (582-592).
Langmuir monolayers and Langmuir–Blodgett (LB) films of 5,10,15,20-tetra-(p-chlorophenyl) terbium/gadolinium porphyrin hydroxyl compound (TbOH and GdOH) and their mixtures with stearic acid (SA) in a molar ratio of 1:1 were investigated by Brewster angle microscopy (BAM), ultraviolet–visible (UV–vis), and infrared (IR) spectroscopy and atomic force microscopy (AFM). π–A isotherms showed that well-defined Langmuir monolayers were formed at an air/water interface for the porphyrins and their mixture with SA. The BAM observations suggest that the π – π interaction between the GdOH molecules is stronger than that between the TbOH molecules. This result can be further confirmed by the AFM measurements. After the introduction of SA, the π – π interaction between the TbOH molecules is broken and thus two phases formed in the mixed LB film. However, it cannot break the stronger π – π interaction between the GdOH molecules. Therefore, no phase separation is observed in the GdOH/SA LB film. IR reflection–absorption (RA) spectra showed that the COOH groups of SA are partly converted to COO− groups, suggesting that there is an interaction between MOH and SA in the films. This interaction leads the benzene rings of TbOH to rotate toward parallel to the substrate and those of GdOH to rotate toward perpendicular to the substrate. All these results have demonstrated that the central metal ions have great effects on the organization and formation of the films.
Keywords: Interface; Central metal; LB; AFM; IR;
Carbon nanotubes as a secondary support of a catalyst layer in a gas diffusion electrode for metal air batteries by Hui Huang; Wenkui Zhang; Meichao Li; Yongping Gan; Jinhua Chen; Yafei Kuang (593-599).
In this paper, we report the use of binary carbon supports (carbon nanotubes (CNTs) and active carbon) as a catalyst layer for fabricating gas diffusion electrodes. The electrocatalytic properties for the oxygen reduction reaction (ORR) were evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) in an alkaline electrolyte. The binary-support electrode exhibits better performance than the single-support electrode, and the best performance is obtained when the mass ratio of carbon nanotubes and active carbon is 50:50. The results from the electrode kinetic parameters indicate that the introduction of carbon nanotubes as a secondary support provides high accessible surface area, good electronic conductivity, and fast ORR kinetics. Furthermore, the effect of CNT support on the electrocatalytic properties of Pt nanoparticles for binary-support electrodes was also investigated by different loading-reduction methods. The electrocatalytic activity of the binary-support electrodes is improved dramatically by Pt loading on CNT carbon support, even at very low Pt loading. Additionally, the EIS analysis results indicate that the process of ORR may be controlled by diffusion of oxygen in the electrode thin film for binary-support electrodes with or without Pt catalyst.
Keywords: Carbon nanotubes; Electrocatalysis; Gas diffusion electrodes; Metal–air batteries; Oxygen reduction; Carbon support;
Adsorption and redox reactions of heavy metals on Fe–Mn nodules from Chinese soils by Wenfeng Tan; Fan Liu; Xionghan Feng; Qiaoyun Huang; Xueyuan Li (600-605).
Adsorption of heavy metals and redox reactions of Cr(W) ions on Fe–Mn nodules from five soils of China were investigated by chemical analysis, equilibrium adsorption/redox, and X-ray photoelectron spectroscopy (XPS). Results show that Mn is mainly present as Mn3+ and Mn4+ forms in Fe–Mn nodules. The maximum adsorption amounts for different heavy metal ions follow the order Pb2+ ≈ Cu2+ > Zn2+ > Co2+ > Ni2+ > Cd2+. The adsorption capacity for heavy metals by Fe–Mn nodules from calciaquert in Shandong province (N5-1) is the highest, while that from hapludalf in Shandong province (N6-1) is the lowest. About 44–100% of the heavy metals adsorbed on Fe–Mn nodules were dissolved in 0.1 mol/L hydroxylamine hydrochloride (HAHC). The maximum amounts of Cr(VI) production by Fe–Mn nodules follow the order of N1-1 (69 mmol/kg) > N4-1 (57 mmol/kg) > N2-1 (52 mmol/kg) > N5-1 (44 mmol/kg). Based on the content of MnO2 in Fe–Mn nodules dissolved in HAHC, the amount of Cr(VI) production by Mn oxides in N1-1, N2-1, N4-1, and N5-1 is 326, 624, 726, and 482 mmol/kg (MnO2), respectively. We propose that the amounts of Cr(VI) production through oxidation Cr(III) by Mn oxides are related to the types of Mn oxides in Fe–Mn nodules.
Keywords: Manganese oxide; Nodule; Adsorption; Redox; Heavy metal; Soil;
Catalytic dehydration of ethanol using transition metal oxide catalysts by T. Zaki (606-613).
The aim of this work is to study catalytic ethanol dehydration using different prepared catalysts, which include Fe2O3, Mn2O3, and calcined physical mixtures of both ferric and manganese oxides with alumina and/or silica gel. The physicochemical properties of these catalysts were investigated via X-ray powder diffraction (XRD), acidity measurement, and nitrogen adsorption–desorption at −196 °C. The catalytic activities of such catalysts were tested through conversion of ethanol at 200–500 °C using a catalytic flow system operated under atmospheric pressure. The results obtained indicated that the dehydration reaction on the catalyst relies on surface acidity, whereas the ethylene production selectivity depends on the catalyst chemical constituents.
Keywords: Catalysis; Dehydration; Ethanol; Iron; Manganese;
Evaluation of thermoporometry for characterization of mesoporous materials by Takuji Yamamoto; Akira Endo; Yuki Inagi; Takao Ohmori; Masaru Nakaiwa (614-620).
The accuracy of thermoporometry (TPM) in terms of the characterization of SBA-15 is examined based on a model that classifies the water in the mesopores into two different types: freezable pore water, which can form cylindrical ice crystals, and nonfreezable pore water, which cannot undergo a phase transition during a differential scanning calorimetry (DSC) measurement. Applying the empirical relationship between the sizes of the ice crystals formed in the mesopores and the solidification temperature of the freezable pore water to a thermogram (a recording of the heat flux during the solidification of the freezable pore water) yielded a size distribution of the ice crystals. The size of the ice crystals increased slightly with repetitive freezing, indicating that the mesopores were enlarged by formation of the ice crystals. Adding the thickness, t nf , of the nonfreezable pore water layer to the ice crystal-size distribution calculated from the thermogram allowed for the determination of the porous properties of SBA-15. The porous properties attained from TPM experiments were compared with the results attained through the combination of Ar gas adsorption experiments and nonlocal density functional theory (NLDFT) analysis. The porous properties determined by TPM were confirmed to be quite sensitive to the t nf value.
Keywords: SBA-15; Pore-size distribution; Surface area; Pore volume; Pore water; Thermoporometry; DSC; Ar gas adsorption method; NLDFT;
Theory of mercury intrusion in a distribution of unconnected wedge-shaped slits by Pierre Bracconi; Michael Sipple; Jean-Claude Mutin (621-629).
Effective mercury intrusion in a wedge-shaped slit is gradual, the intruded depth increasing with applied pressure. The Washburn equation must be modified accordingly. It relates the distance, e, separating the three-phase contact lines on the wedge faces to the hydrostatic pressure, P, wedge half-opening angle α, mercury surface tension γ, and contact angle θ: e = ( − 2 γ / P ) cos ( θ − α ) if θ − α > π 2 . The equations relating the volume of mercury in a single slit to hydrostatic pressure are established. The total volume of mercury V Hg tot ( E 0 , e ) intruded in a set of unconnected isomorphous slits (same α value) with opening width, E, distributed over interval [ E 0 , 0 ], and volume-based distribution of opening width, f V ( E ) , is written as V Hg tot ( E 0 , e ) = − ∫ E 0 e f V ( E ) d E + ( 1 − b ) e 2 ∫ E 0 e f V ( E ) d E E 2 − tan α ∫ E = e 0 G ( X ( E , e ) ) f V ( E ) d E , where G ( X ) = ( sin −1 X − X 1 − X 2 ) / X 2 and X ( E , e ) = − cos ( θ − α ) E e . The exact relation between total internal surface area and integral pressure work is S tot = − 1 γ Hg ( cos θ + sin α ) ∫ 0 V Hg tot P d V Hg tot .
Keywords: Mercury; Porosimetry; Intrusion; Theory; Wedge; Slit;
Quantifying the effect of ionic strength on colloidal fouling potential in membrane filtration by Gurdev Singh; Lianfa Song (630-638).
Ultrafiltration experiments were conducted to study the fouling potential of colloidal suspensions under different ionic strengths and colloid concentrations. A linear relationship was found relating the colloidal fouling potential to the logarithm of the Debye–Hückel parameter, a characteristic for electrical double layers of colloids. This finding provided a useful quantitative linkage between the colloidal fouling potential and the water chemistry. Considering the linear dependence of colloidal fouling potential on the colloid concentration, a bilinear model was proposed to explain the coupling effects of colloid concentration and ionic strength of the suspension on the fouling potential. The model predictions of fouling potential were found to fit accurately with experimentally determined fouling potential values. Further analysis of the model showed that ionic strength can significantly affect colloidal fouling, for example, a 10-fold increase in ionic strength from 0.001 to 0.01 M for a given feed concentration has the same membrane fouling effect as doubling the feed concentration. The model allows for a quick and reliable assessment of fouling potential without even performing any experiments. This could then be used to design the membrane process or pretreatment stages required to mitigate membrane fouling.
Keywords: Colloidal feed waters; Water chemistry; Ionic strength; Fouling potential; Quantitative;
Characterization of latex particle arrays by gas adsorption by M.C. Carbajo; E. Climent; E. Enciso; M.J. Torralvo (639-645).
Assemblies of colloidal particles are frequently used in novel applications, and this requires nondestructive methods allowing overall characterization of the sample and collection of information about the quality of the arrays. From suspensions of polystyrene, poly[styrene–co-(2-hydroxyethylmethacrylate)], poly[styrene–co-acrylic acid], and poly[styrene–co-methacrylic acid], assemblies of spherical particles were obtained by elimination of the solvent in different ways—evaporation, gravity deposition, and filtration. These latex particle packings were characterized by scanning and transmission electron microscopy and by gas adsorption to determine the efficiency of packing. The surface area, total pore volume, and pore size distributions obtained from the adsorption and desorption data were related to characteristic parameters calculated for cubic close-packed spherical particles.
Keywords: Colloidal particle assemblies; Gas adsorption characterization; Latex particles; Self-assembly behavior; Ordered particle arrays; Electron microscopy;
Synthesis of functional microcapsules containing suspensions responsive to electric fields by Huilin Guo; Xiaopeng Zhao; Jianping Wang (646-651).
A sort of functional microcapsules, which contain a suspension responsive to electric fields, is prepared by in situ polymerization of urea and formaldehyde. The suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. In order to solve the particles' separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influences of different emulsifiers on the microencapsulation are investigated. It is found that the dispersion extent and lipophilicity of the PPG particles are improved due to their surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. The interfacial tension between C2Cl4 and H2O with urea–formaldehyde prepolymer descends from 43 to 35 mN/m, which indicates that the polymer has certain surface activity. However, water-soluble emulsifiers have an important influence during the microencapsulation because they can absorb on the surfaces of internal phase and prevent the resin of urea–formaldehyde from depositing there. From the SEM images of shell surface and cross section, the microcapsules have relatively smooth surfaces and the average thickness is about 4.5 μm. When the microcapsules are prepared with agitation rates of 1000 and 600 rpm, the mean diameters of the obtained microcapsules are 11 and 155 μm, respectively. The particles in the capsules move toward positive electrode with a responsive time of several hundred milliseconds while providing an electric field.
Keywords: Microcapsules; Suspension; Contact angle; Interfacial tension; Electric response;
Wetting films of polar and nonpolar liquids by Rosa Vazquez; Rui Nogueira; Sandra Busquets; José Luís Mata; Benilde Saramago (652-657).
Disjoining pressure isotherms for water and n-octane films on glass obtained using a recently developed apparatus, based on the interferometric technique, are compared with other values reported in the literature. Our method, including a simple cleaning procedure, yields experimental results in excellent agreement both with other experimental results and with theoretical predictions. An error analysis was made which quantifies the uncertainty of our results and simultaneously allows the minimization of the errors.
Keywords: Wetting films; Interferometric technique; Disjoining pressure isotherms; Water; n-Octane;
Self-aggregation of binary mixtures of alkyltriphenylphosphonium bromides: a critical assessment in favor of more than one kind of micelle formation by M. Prasad; S.P. Moulik; R. Palepu (658-666).
The micellization behavior of binary combinations of alkyltriphenylphosphonium bromides (ATPBs) with alkyl chain carbons 10, 12, 14, and 16 has been studied by conductometry and calorimetry. The combinations C10–C12, C10–C14, C10–C16, C12–C14, C12–C16, and C14–C16 were found to form two cmc's by both the methods, with good agreement, except C14–C16TPB, which has evidenced only a single cmc by calorimetry for all combinations. The combinations C10–C12 (for both cmc1 and cmc2) and C10–C14TPB (for cmc2) formed ideal mixtures, whereas the rest were nonideal. In the nonideal binary mixtures, the ATPB components showed antagonistic interaction with each other. The cmc, interaction parameter (β), mixed micellar composition, extent of counterion binding, and thermodynamic parameters for the micellization process have been reported and discussed. The enthalpy of mixed micelle formation has been found to have a fair correlation with a Clint-type relation applicable to ideal binary mixtures of surfactants.
Keywords: ATPBs; Binary mixtures; Calorimetry; Conductometry; Interaction parameter; Thermodynamics;
Interaction between cationic surfactants and montmorillonites under nonequilibrium condition by Seung Yeop Lee; Won Jin Cho; Kang Joo Kim; Jung Ho Ahn; Minhee Lee (667-673).
Surfactant adsorption by two different montmorillonites was characterized by examining the time dependence of surfactant behavior on clay surfaces. Surfactants with different micelle concentrations were conducted in our experiment to observe a nonequilibrium activity of cationic surfactant on the clay over reaction periods ranging from 0.1 min to 11 days. Compared with Ca-montmorillonite (SAz), a more active intrusion of surfactant molecules into the interlayers was found in Na-montmorillonite (SWy). During a short “initiation” stage, the basal spacing of SWy montmorillonite increased rapidly with logarithmic time. For SAz montmorillonite, however, the abrupt basal spacing increase occurred at a later stage of the reaction. From the results, it is assumed that the difference in the adsorption behavior exhibited by the two montmorillonite types partly arises from their intrinsic nature; that is, inorganic cations originally existed on the clay surfaces. Additionally, the micelle concentration of the surfactants affects the development of organomontmorillonite, especially in the intercalant formation and stabilization under nonequilibrium.
Keywords: Surfactant adsorption; Montmorillonite; Micelle; Nonequilibrium; Interlayers;
Effect of AOT on enzymatic activity of the organic solvent resistant tyrosinase from Streptomyces sp. REN-21 in aqueous solutions and water-in-oil microemulsions by Janina Rodakiewicz-Nowak; Masaaki Ito (674-679).
The effect of AOT (sodium-bis(2-ethylhexyl sulfosuccinate)) on enzymatic activity of the organic solvent resistant tyrosinase (OSRT) in aqueous phosphate buffer solutions and in water-in-oil microemulsions of the water/AOT/isooctane system has been investigated. In contrast to mushroom tyrosinase, AOT does not activate OSRT in aqueous solutions, altering its activity very little at concentrations lower than 2 mM. Increasing contents of AOT in isooctane reduce the observed initial reaction rates of oxidation of t-butylcatechol (tBC) and 4-methylcatechol (4-MC). Similarly to mushroom tyrosinase, the effect has been described using an equation based on preferential binding of the substrates by surfactant interface layers. The apparent Michaelis–Menten substrate binding constants increase linearly with AOT concentration (with slopes of 0.12 ± 0.02 and 0.051 ± 0.006 for tBC and 4-MC, respectively), and the effective enzyme turnover number in the microemulsions remains practically constant.
Keywords: AOT; Tyrosinase; Water-in-oil microemulsions; Reverse micelles; OSRT;
Fluorescence studies of interactions of ionic surfactants with poly(amidoamine) dendrimers by Mandeep Singh Bakshi; Aman Kaura (680-686).
The pyrene fluorescence measurements have been carried out for the micelle formation of sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dimethylene bis(dodecyldimethylammonium bromide) (12-2-12) in the presence of fixed different amounts of various generations of poly(amidoamine) (PAMAM). The critical micelle concentration (cmc) of SDS decreases with an increase in the fixed amount of PAMAM, suggesting the facilitation of micellization due to the participation of SDS–PAMAM complex in the micelle formation. This behavior has not been observed for DTAB/12-2-12 in the presence of various generations of PAMAM. The results indicate that SDS always has stronger interactions with all the generations of PAMAM in comparison to those of DTAB and 12-2-12.
Keywords: Poly(amidoamine) dendrimers; Fluorescence measurements; Ionic surfactants; Quenching;
Physical chemistry of nanostructured molecular sieves by the study of phase diagrams: the case of the cetyltrimethylammonium bromide–tetramethylammonium silicate–water system by A. Albuquerque; C. Vautier-Giongo; H.O. Pastore (687-693).
A phase diagram for the system cetyltrimethylammonium bromide (CTAB)/tetramethylammonium silicate (TMASi)/water has been constructed in order to better understand the interactions between these precursors of the MCM-41 mesoporous molecular sieves. Three different CTAB concentration regions were analyzed: the dilute and semidilute regions, where simple surfactant species, such as monomers and spherical and nonspherical CTAB micelles, are found, and the concentrated region, involving liquid-crystalline phases. In the dilute and semidilute regions, the formation of a white nanostructured solid, having a hexagonal array similar to that found in MCM-41 materials, was observed. Precipitation of this solid requires some degree of surfactant monomer aggregation, which is favored by the presence of silicate anions. If micelles have already been formed, the material can be obtained at any CTAB concentration above a threshold concentration of silicate anions. These facts suggest that silicate anions have an important role in changing the aggregation and/or the shape of the surfactant aggregates. In the concentrated region, precipitation of the solid was not observed, but the presence of the silicate anions alter the characteristics of the liquid-crystalline phase formed by the surfactant. The system shows very complex and rich behavior and its investigation may be very useful in understanding the processes of nanostructured solid formation.
Keywords: Nanostructured molecular sieves; Phase diagram; Cetyltrimethylammonium bromide; MCM-41; Liquid crystals;
Molecular modeling study on the relative stabilities of the flotation products for arsenic-containing minerals: dixanthogens and arsenic(III) xanthates by Meftuni Yekeler; Hülya Yekeler (694-697).
The interactions of As(III) ion with C2H5OCS− 2 and C2H5SCS− 2, known as the most popular collector ions, were studied by density functional theory (DFT) at the B3LYP/6-31G** level in connection with the arsenic-containing minerals realgar, orpiment, and arsenopyrite. The dixanthogen formations of these ions were also investigated at the same level of theory. The central purpose of this paper is to compare the stabilities of the major flotation products, namely As(III) xanthates and dixanthogens. The results show that the magnitudes of the interaction energies for the formations of As(III) xanthates and dixanthogens increase when the oxygen atom is replaced by the sulfur atom in C2H5OCS− 2. Therefore, the C2H5OCS− 2 ion is preferred in these formations. The results obtained are in agreement with the experimental data reported.
Keywords: Flotation; Molecular modeling; Sulfide minerals; Thiol collectors;
Transition from micelle to vesicle in aqueous mixtures of anionic/zwitterionic surfactants studied by fluorescence, conductivity, and turbidity methods by Limin Zhai; Jiyu Zhang; Qingxiu Shi; Wenjun Chen; Mei Zhao (698-703).
Vesicles form spontaneously in a aqueous mixture of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT) and lauryl sulfonate betaine (LSB). Different from catanionic vesicles, the formation or disaggregation of such zwitterionic/anionic vesicles may be easily controlled by adjusting the relative amount of LSB and salinity. The participation of LSB reduces the polydispersity of the vesicles and even results in the formation of monodispersed vesicles at a certain salinity. But as LSB exceeds a certain proportion, vesicles cannot form at any concentration and salinity, making convenient the study of the structural transitions. We applied pyrene as a fluorescence probe and monitored the transition among the monomer, micelle, and vesicle through the variation of I 1 / I 3 , accompanied by conductivity and turbidity measurements. In LSB solution and LSB-rich mixture, an abrupt change of the ratio of I 1 / I 3 was found in the transition from monomer to micelle with increasing concentration, as well as in the transition from micelle to vesicle with increasing salinity, which shows that a difference of the polarity of the microenvironment between the micelle and the vesicle bilayer resulted from the composition change. But in AOT solution and AOT-rich mixture, only a gradual change in the transition is observed due to the existence of intermediate structures, which have different microenvironments from micelles and vesicles. So the formation of vesicle experiences a process of monomer to premicelle to micelle to bilayer segment with increasing concentration by combining the conductivity method. The ratio of I 1 / I 3 is independent of the vesicle size once formed.
Keywords: Zwitterionic/anionic vesicle; Transition; I 1 / I 3 ; Micropolarity; Salinity;
Structures of micelles formed by synthetic alkyl glycosides with unsaturated alkyl chains by Götz Milkereit; Vasil M. Garamus; Koen Veermans; Regine Willumeit; Volkmar Vill (704-713).
Three new alkyl glycosides with similar molecular structures (oleyl and oleoyl alkyl chains and various head groups: disaccharide, trisaccharide and disaccharide with an additional amidoethoxy spacer) were synthesized and their supramolecular structure in aqueous solution was investigated. Small angle neutron scattering, surface tension measurement and the contact preparation method were applied to get molecular structure–property relationships. Although the chemical structures differ only in small details, their CMC values, lyotropic phase behaviour, surface area per surfactant molecule in the micelle and at the liquid–air interface, and the size and shape of the micelles are very different. We have found three different types of aggregates: spherical, cylindrical and polymer-like micelles in dilute solutions.
Keywords: Alkyl glycosides; Small angle neutron scattering; Polymer-like micelles; Surface tension;
Microstructural evolution of viscoelastic emulsions stabilised by sodium caseinate and xanthan gum by Thomas Moschakis; Brent S. Murray; Eric Dickinson (714-728).
The time-dependent evolution of the phase-separated microstructure of a caseinate-stabilised emulsion containing xanthan gum added before emulsification has been investigated by confocal laser scanning microscopy, image analysis and rheology. Moderately low levels of xanthan addition lead to depletion flocculation and gravity-induced phase separation. Increasing the polysaccharide concentration causes immobilisation of the microstructure due to an increase in the local viscoelasticity: that is, the emulsion structure cannot easily rearrange to expel xanthan-enriched aqueous serum phase because a weak gel-like network is generated. The effect of xanthan on the evolving microstructure of phase-separated regions, which reflects indirectly the local emulsion micro-rheology, has been estimated from image analysis of time sequences of confocal micrographs. A comparison has been made between object shape analysis using four different shape descriptors. The roundness parameter has been found to be a convenient descriptor for reliably quantifying the structural change in terms of the relaxation rate of xanthan-rich aqueous drops. The Taylor parameter has been used to link the kinetics of drop relaxation to the time-dependent small-deformation rheological behaviour. The analysis of the combined experimental data reveals the difficulty of relating the evolving microstructure to bulk rheological measurements.
Keywords: Confocal scanning laser microscopy; Image analysis; Phase separation; Depletion flocculation; Sodium caseinate; Xanthan gum; Emulsion rheology;
Drag force on a rigid spheroidal particle in a cylinder filled with Carreau fluid by Jyh-Ping Hsu; Yu-Heng Hsieh; Shiojenn Tseng (729-741).
The boundary effect on the drag acting on a rigid particle is investigated by considering a spheroid on the axis of a cylinder filled with a Carreau fluid. The result of numerical simulation reveals that the ratio (drag coefficient in Carreau fluid/drag coefficient in Newtonian fluid) has a maximum as the ratio (semiaxis in radial direction/radius of cylinder) varies. The presence of a wall has the effect of enhancing the convective motion in the rear part of a particle, and therefore, the formation of wakes. The influence of the shape of a particle on the drag force acting on it can be decreased either by increasing the shear-thinning effect of the fluid or by increasing the Reynolds number. The Reynolds number at which flow separation occurs is found to increase roughly linearly with the increase in the power-law exponent of the Carreau fluid.
Keywords: Wall effect; Spheroid in cylinder; Drag correction factor; Drag coefficient; Carreau fluid; Flow separation;
Shear-modulated electroosmotic flow on a patterned charged surface by Hsien-Hung Wei (742-752).
The effect of imposing shear flow on a charge-modulated electroosmotic flow is theoretically investigated. The flow structures exhibit either saddle points or closed streamlines, depending on the relative strength of an imposed shear to the applied electric field. The formation of closed streamlines could be advantageous for trapping nondiffusive particles at desired locations. Different time periodic alternating flows and their corresponding particle trajectories are also examined to assess strategies for creating efficient mixing.
Keywords: Patterned electroosmotic flow; Shear flow;
Timescales for relaxation to Boltzmann equilibrium in nanopores by Ho Sang Kwak; Ernest F. Hasselbrink (753-758).
In most models for electrokinetic phenomena at charged interfaces, Boltzmann equilibrium is assumed to be established. Here we show that a long nanopore with significant double layer overlap establishes equilibrium quite slowly and that centimeter-long nanopores can take O ( 10 5 ) s to establish Boltzmann equilibrium. The timescale is determined not by diffusion across the double layer, but by diffusion or convective transport along the length of the pore to reservoirs at its ends. An “intermediate equilibrium” state described by Qu and Li (J. Colloid Interface Sci. 224 (2000) 397) may exist for times between the (fast) EDL establishment timescale and (slow) axial transport timescale.
Keywords: Electroosmotic flow; Electric double layer (EDL); Transient evolution; Boltzmann equilibrium;
Microcalorimetric study on the interaction of dissymmetric gemini surfactants with DNA by Nan Jiang; Jinben Wang; Yilin Wang; Haike Yan; Robert K. Thomas (759-764).
The interaction of a series of dissymmetric gemini surfactants, [C m H2m+1(CH3)2N(CH2)6N(CH3)2C n H2n+1]Br2 (designated as C m C6C n Br2, with constant m + n = 24 , and m = 12 , 14, 16, and 18) with DNA in 10 mM NaCl solution has been investigated by isothermal titration microcalorimetry (ITC). The curves for titration of the surfactants into DNA solution show noticeable differences from those into 10 mM NaCl solution without DNA. It is attributed to the interaction between DNA and surfactants. The critical aggregation concentration (CAC), the saturation concentration ( C 2 ), and the thermodynamic parameters for the aggregation and interaction processes were obtained from the calorimetric titration curves. The results show that the dissymmetry degree ( m / n ) has a marked effect on the interaction of the C m C6C n Br2 surfactants with DNA. The CAC and C 2 tend to become smaller with increased m / n . The enthalpy change ( Δ H agg ) and the Gibbs free energy change ( Δ G agg ) for aggregation become more negative down the series, indicating that the hydrophobic interaction between the hydrophobic chains of the surfactant molecules increases and the aggregation process is more spontaneous with increased m / n . The entropy changes of aggregation ( Δ S agg ) are all positive and T Δ S agg is much larger than | Δ H agg | , revealing that the aggregation process is mainly entropy-driven. However, the calculated Gibbs free energy ( Δ G DS ) for the interaction between the gemini surfactants and DNA becomes less negative with increased m / n , which reveals that the interaction between the gemini surfactants and DNA tends to be weaker with increased m / n . This is induced by the disruption of the chain–chain hydrophobic interaction between the surfactant molecules at higher m / n , where the entropy change Δ S DS for the interaction process tends to be an unfavorable factor. In addition, the DNA concentration also has a remarkable influence on the interaction.
Keywords: Dissymmetric gemini surfactants; DNA; Interaction; Microcalorimetry;
Author Index for Volume 284 (765-766).