Journal of Colloid And Interface Science (v.306, #2)

The adsorption mechanism of mixed cationic alkyl diamine and anionic sulfonate/oleate collectors at acidic pH values was investigated on microcline and quartz minerals through Hallimond flotation, electrokinetic and diffuse reflectance FTIR studies. In the presence of anionic collectors, neither of the minerals responded to flotation but the diamine flotation of the minerals was observed to be pH and concentration dependent. The presence of sulfonate enhanced the diamine flotation of the minerals by its co-adsorption. The difference in surface charge between the minerals at pH 2 was found to be the basis for preferential feldspar flotation from quartz in mixed diamine/sulfonate collectors. The infrared spectra revealed no adsorption of sulfonate collector when used alone but displayed its co-adsorption as diamine–sulfonate complex when used with diamine. The presence of sulfonate increased the diamine adsorption due to a decrease in the electrostatic head–head repulsion between the adjacent surface ammonium ions and thereby increasing the lateral tail–tail hydrophobic bonds. The mole ratio of diamine/sulfonate was found to be an important factor in the orientation of alkyl chains and thus the flotation response of minerals. The increase in sulfonate concentration beyond diamine concentration leads to the formation of soluble 1:2 diamine–sulfonate complex or precipitate and the adsorption of these species decreased the flotation since the alkyl chains are in chaotical orientation with a conceivable number of head groups directing towards the solution phase.The adsorption mechanism of mixed cationic alkyl diamine and anionic sulfonate/oleate collectors at acidic pH values was investigated on microcline and quartz minerals through Hallimond flotation, electrokinetic and diffuse reflectance FTIR studies. It is the first time that the zeta-potential results show the increased adsorption of cationic collector in the presence of anionic collector apart from its own co-adsorption.
Keywords: Microcline; Quartz; Adsorption; Flotation; Zeta-potential; FTIR;

Kinetics of adsorption of p-hydroxy benzoate and phthalate on hematite–electrolyte interface were investigated at a constant ionic strength, I = 5 × 10 −4   mol dm −3 , pH 5 and at three different temperatures. The state of equilibrium for the adsorption of p-hydroxy benzoate onto hematite surfaces was attained at 70 h, whereas it was 30 h for phthalate–hematite system. None of the three kinetics models (Bajpai, pseudo first order and pseudo second order) is applicable in the entire experimental time period; however, the pseudo second order kinetics model is considered to be better than the pseudo first order kinetics model in estimating the equilibrium concentration both the p-hydroxy benzoate–hematite and phthalate–hematite systems. The variation of adsorption density of p-hydroxy benzoate and phthalate onto hematite surfaces as a function of concentration of adsorbate was studied over pH range 5–9 at a constant ionic strength, I = 5 × 10 −4   mol dm −3 and at constant temperature. The adsorption isotherms for both the systems were Langmuir in nature and the maximum adsorption density ( Γ max ) of p-hydroxy benzoate is ∼1.5 times more than that of phthalate on hematite at pH 5 and 30 °C in spite of an additional carboxylic group at ortho position in phthalate. This is due to the more surface area coverage by phthalate than that of p-hydroxy benzoate on hematite surface. The activation energy was calculated using Arrhenius equation and the activation energy for adsorption of p-hydroxy benzoate at hematite–electrolyte interface is ∼1.8 times more than that of phthalate–hematite system. The negative Gibbs free energy indicates that the adsorption of p-hydroxy benzoate and phthalate on hematite surfaces is favourable. The FTIR spectra of p-hydroxy benzoate and phthalate after adsorption on hematite surfaces were recorded for obtaining the bonding properties of adsorbates. The phenolic ν C―O appears at ∼ 1271   cm −1 after adsorption of p-hydroxy benzoate on hematite surfaces, which shifted by 10 cm−1 to higher frequency region. The phenolic group is not deprotonated and is not participating in the surface complexation. The shifting of the ν as (―COO) and ν s (―COO) bands and non-dissolution of hematite suggest that the p-hydroxy benzoate and phthalate form outer-sphere surface complex with hematite surfaces in the pH range of 5–7.
Keywords: Adsorption; p-Hydroxy benzoate; Phthalate; Hematite; FTIR; Kinetics;

Modeling synergistic adsorption of phenol/aniline mixtures in the aqueous phase onto porous polymer adsorbents by W.M. Zhang; Q.J. Zhang; B.C. Pan; L. Lv; B.J. Pan; Z.W. Xu; Q.X. Zhang; X.S. Zhao; W. Du; Q.R. Zhang (216-221).
The adsorption equilibria of phenol and aniline on nonpolar polymer adsorbents (NDA-100, XAD-4, NDA-16 and NDA-1800) were investigated in single- and binary-solute adsorption systems at 313 K. The results showed that all the adsorption isotherms of phenol and aniline on these adsorbents can be well fitted by Freundlich and Langmuir equations, and the experimental uptake of phenol and aniline in all binary-component systems is obviously higher than predicted by the extended Langmuir model, arising presumably from the synergistic effect caused by the laterally acid–base interaction between the adsorbed phenol and aniline molecules. A new model (MELM) was developed to quantitatively describe the synergistic adsorption behavior of phenol/aniline equimolar mixtures in the binary-solute systems and showed a marked improvement in correlating the binary-solute adsorption of phenol and aniline by comparison with the widely used extended Langmuir model. The newly developed model confirms that the synergistic coefficient of one adsorbate is linearly correlated with the adsorbed amount of the other, and the larger average pore size of adsorbent results in the greater synergistic effect of phenol/aniline equimolar mixtures adsorption.A new model (MELM) was developed to quantitatively describe the synergistic adsorption property of phenol/aniline equimolar mixtures in the binary-solute solution on porous polymer adsorbent surfaces. MELM equation : Q e a cal = K l a Q m a C e a 1 + K l a C e a + K l b C e b ( 1 + a a ( C 0 b − C e b ) V 1 / W + b a ) , Q e b cal = K l b Q m b C e b 1 + K l a C e a + K l b C e b ( 1 + a b ( C 0 a − C e a ) V 1 / W + b b ) .
Keywords: New model; Synergistic adsorption; Phenol; Aniline; Isotherm; Porous polymer adsorbent;

Fluorescence emission from PAMAM and PPI dendrimers by Dongjun Wang; Toyoko Imae; Masao Miki (222-227).
A strong fluorescence emission from poly(amido amine) (PAMAM) dendrimers with different terminal groups or a poly(propylene imine) (PPI) dendrimer was studied under different conditions by varying experimental parameters such as pH value, aging time, temperature, and concentration. The increase of fluorescence intensity was fast at low pH or high temperature but linear with respect to dendrimer concentration. It was reasonable that the formation of a fluorescence-emitting moiety had a close relation to protonated tertiary amine groups in PAMAM or PPI dendrimers. Furthermore, oxidation of the tertiary amines was confirmed to play an important role, which was evidently caused by oxygen in air. The results of fluorescence decay indicated that the deactivation of luminescence was raised with increasing temperature. Dendrimers emitted blue photoluminescence along fiber chain templates on a fluorescent microscope.Strong fluorescent emission from poly(amido amine) dendrimers with different terminals or a poly(propylene imine) dendrimer was studied under different conditions. The formation of a fluorescence-emitting moiety has a close relation to the protonated tertiary amine groups in dendrimers. Oxidation of the tertiary amines was believed to play a critical role. The deactivation of luminescence was raised with increasing temperature. The blue photoluminescence on the dendrimer-treated fibers was observed on a fluorescence microscope.
Keywords: Fluorescence; Emission; Photoluminescence; Poly(amido amine) dendrimer; Poly(propylene imine) dendrimer; Dendrimer; Oxidation; Fluorescence lifetime; Fluorescence microscope;

Water sorption in silicone foam containing diatomaceous earth by S.W. Rutherford; J.E. Coons (228-240).
Room temperature vulcanizing (RTV) silicone foams are commonly used for compression sealing, structural support, packaging, and damping applications. The presence of sorbed water in foams can affect the mechanical and chemical properties of these materials. In order to investigate water sorption behavior, a silicone foam containing diatomaceous earth filler was synthesized and studied for water uptake characteristics at 20, 50, and 80 °C. Type II equilibrium and bimodal kinetic behavior that was governed by a rapid initial uptake followed by a prolonged sorption over a larger time scale was observed. In order to explain this bimodal behavior, the major components of this foam—the silicone polymer and the diatomaceous earth—were independently studied for their water equilibrium behavior and uptake kinetic characteristics. Type II equilibrium was observed for both components. The kinetic behavior of the silicone polymer was governed by a very rapid uptake of water. The kinetic behavior of the diatomaceous earth was governed by a rapid initial uptake followed by a prolonged sorption over a larger time scale. A physically based and thermodynamically consistent mathematical model describing the water equilibrium and kinetics in diatomaceous earth and silicone polymer components, was employed to characterize the data. This model formed the basis of a predictive model for estimation of water sorption in filled silicone foam. The predictive model was tested against sorption and desorption data yielding favorable results for a range of temperatures.Water sorption equilibrium isotherm for silicone foam containing diatomaceous earth.

Two kinds of polymer–metal compounds, heterogeneous complexes (metal-chelating copolymer microspheres, MCP) and homogeneous complexes (water-soluble metal-chelating polymers, WSMCP), were synthesized to act as nucleation agents for YIG precursor preparation in this text. Both of the metal-chelating polymers have the same chelating group and high metal ion adsorption ability from the FTIR and ICP measurement. Furthermore, good YIG crystals can be obtained by treating the MCP precursor with a low calcination temperature at 600 °C from the XRD spectra and TEM micrograph. However, the YIG crystal obtained using a WSMCP precursor should be synthesized at a higher calcination temperature (>900 °C) due to the different components of the YIG precursor. In addition, the YIG crystal obtained by using the MCP precursor had nearly superparamagnetic behavior after VSM examination.Good YIG crystals can be prepared simply by using a heterogeneous additive, the metal-chelating polymer, at the low calcination temperature 600 °C. In addition, this YIG crystal has nearly superparamagnetic behavior.
Keywords: YIG; Polymer–metal chelated; Precursor; Complex;

Lignocellulosic fiber charge enhancement by catalytic oxidation during oxygen delignification by Dongcheng Zhang; Xin-Sheng Chai; Yunqiao Pu; Arthur J. Ragauskas (248-254).
A series of one-stage oxygen delignification treatments with a softwood (SW) kraft pulp were studied employing 0.0–0.5% of a bismuth ruthenium pyrochlore oxide catalyst. The results demonstrated that a 0.09–0.18% charge of catalyst in an oxygen stage provided a 52.2–116.0% increase of carboxylic acid groups in the cellulosic component of kraft pulps without a significant decrease in fiber viscosity. A 3-factor at 3-level (L933) orthogonal experimental design was used to identify the main factors influencing acid group formation in pulp carbohydrates. The relative significance of experimental parameters for polysaccharide acid group formation was the molar equivalent NaOH, oxygen pressure, and finally, reaction temperature under the experimental conditions studied. The optimized reaction parameters for fiber charge development were shown to be 85–100 °C, 2.5% NaOH, and 800–960 kPa oxygen pressure. Pulps with higher fiber carboxylic acid content introduced by catalytic oxidation during oxygen delignification yielded a 10.9–33.7% increase in fiber charge after elemental chlorine free (ECF) pulp bleaching. The enhanced fiber charge resulted in 6.7–17.1% increase in paper sheet tensile index at comparable pulp viscosity.A 0.09–0.18% of bismuth ruthenium pyrochlore oxide employed during an oxygen stage resulted in 52.2–116.0% increase of carboxylic groups in the polysaccharides of oxygen delignified kraft pulps without significantly decreasing pulp viscosity.
Keywords: Carboxylic acid; Catalytic oxidation; Fiber charge; Holocellulose; Lignin; Oxygen delignification; Pulp polysaccharide;

Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles by Craig J. Taylor; Laura E. Dieker; Kelly T. Miller; Carolyn A. Koh; E. Dendy Sloan (255-261).
Adhesion forces between tetrahydrofuran (THF) hydrate particles in n-decane were measured using an improved micromechanical technique. The experiments were performed at atmospheric pressure over the temperature range 261–275 K. The observed forces and trends were explained by a capillary bridge between the particles. The adhesion force of hydrates was directly proportional to the contact force and contact time. A scoping study examined the effects of temperature, anti-agglomerants, and interfacial energy on the particle adhesion forces. The adhesion force of hydrates was found to be directly proportional to interfacial energy of the surrounding liquid, and to increase with temperature. Both sorbitan monolaurate (Span20) and poly-N-vinyl caprolactam (PVCap) decreased the adhesion force between the hydrate particles.Particle–particle adhesive forces were measured for tetrahydrofuran clathrate hydrates with preload, temperature, interfacial tension, and with additions of anti-agglomerants and kinetic inhibitors.
Keywords: Tetrahydrofuran clathrate hydrate; Particle adhesion; Micromechanical testing; Capillary bridging; Sorbitan monolaurate (Span20); Poly-N-vinyl caprolactam (PVCap);

High pressure dispersion nozzles of 2.5–10 mm length and 125 μm diameter have been characterized in terms of fluid dynamics and dispersion experiments at 100–1400 bar. Elongational stresses at the nozzle entry ( 5 × 10 5   Pa ) and turbulent stresses up to 105 Pa at a Reynolds number of 25,000 in turbulent channel flow are identified crucial for desagglomeration and aggregate fragmentation. Maximum stresses are calculated on representative particle tracks and related to agglomerate breakage. Agglomerates in the experimental study are in the range of the Kolmogorov micro scale (100–400 nm) and therefore break due to turbulent energy dissipation in viscous flow. Bond strength distributions could be determined experimentally from particle size distributions and fluid dynamics simulations, with primary particle erosion determined as dispersion mechanism for diffusion flame silica particles. Nanoscale agglomerates show a power law scaling for breakage with scaling exponents diverging from theory of floc dispersion. This is attributed to their strong bonding by sinter necks.Turbulent hydrodynamic stresses exert shear on nanoscale agglomerates resulting in fragmentation. High pressure dispersion builds up these stresses.
Keywords: Dispersion; Turbulent flow; Hydrodynamic stress; Nanoparticles;

Dispersion stability of TiO2 nanoparticles covered with SiO x monolayers in water by Hiroaki Tada; Osamu Nishio; Naoko Kubo; Hideo Matsui; Masakuni Yoshihara; Tetsuro Kawahara; Hiroshi Fukui; Seishiro Ito (274-280).
The repetition of a two-step route consisting of chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and its subsequent photooxidation has formed SiO x monolayers on the surfaces of TiO2 particles layer by layer (SiO x ( n ⩽ 4 )/TiO2, n =  repeated number ). The trace of these processes using diffuse reflectance FT-IR and solid-state 29Si NMR spectroscopy reveals that TMCTS is chemisorbed on TiO2 through Ti–O–Si bonds followed by growth of Si–O–Si networks via dehydration–condensation of the Si–OH groups generated by TiO2 photocatalytic oxidation of the Si–H and Si–CH3 groups. The point of zero charge of the TiO2 particles decreases from 6.5 to 5.4 at n = 1 , reaching 4.9 at n ⩾ 2 . As a result of coverage with the SiO x monolayers, the dispersion stability of TiO2 particles in neutral water is significantly improved without changing their optical properties. The dispersion stability further increases with accumulation of SiO x monolayers.The coverage of TiO2 particles on SiO x monolayer drastically improved their dispersion stability in neutral water.
Keywords: TiO2 particles; SiO x monolayer; 29Si NMR; Photocatalysis; Dispersion stability;

Metastable tetragonal phase CdWO4 nanoparticles synthesized with a solvothermal method by Adam J. Rondinone; Michelle Pawel; Dustin Travaglini; Shannon Mahurin; Sheng Dai (281-284).
CdWO4 has only previously been reported in the monoclinic, or wolframite, phase. Here we report the first metastable, tetragonal or scheelite, CdWO4 nanopowder. The tetragonal CdWO4 was synthesized by a propylene glycol solvothermal method. The scheelite phase is stabilized by a combination of high surface area and surface complexation by the propylene glycol. The CdWO4 is stable at 1 bar to 300 °C, and converts back to the monoclinic wolframite phase between 300 and 500 °C. The nanopowder exhibits cubic morphology and the average particle size of the nanopowder is around 50 nm. Scheelite CdWO4 nanopowders are synthesized with a solvothermal approach.
Keywords: Metastable; CdWO4; Nanopowders;

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle–particle (at the interface) and particle–interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.Appearance of the paraffin oil-in-water emulsions stabilized by LDHs particle 48 h after preparation: (a) stabilized by 0.5 wt% LDHs; (b) stabilized by 2 wt% LDHs; (c) real picture of the emulsions stabilized by 2 wt% LDHs.
Keywords: Layered double hydroxides (LDHs) particles; Dispersion; pH; Adsorption; Structural strength; Gel-like structure; Pickering emulsions; Formation and stability;

Janus Cu2(OH)2CO3/CuS microspheres were prepared via a Pickering emulsion route for the first time. By treating the Janus Cu2(OH)2CO3/CuS microspheres with dilute hydrochloric acid, ringent Cu2(OH)2CO3/CuS core/shell microspheres and ringent CuS shells were obtained. The hatch size of the ringent CuS shells increased with the increase of the hydrophobicity of the precursor Cu2(OH)2CO3 microspheres. Scanning electron microscopy, X-ray diffraction, energy dispersion spectra, and particle size analysis were used to characterize the products thus formed.Janus Cu2(OH)2CO3/CuS microspheres, ringent Cu2(OH)2CO3/CuS core/shell microspheres, and ringent CuS shells could be prepared via a Pickering emulsion strategy.
Keywords: Janus particles; Core/shell structures; Cupric salts; Pickering emulsion;

The dispersion-stability diagram of boehmite nanoparticles in aqueous AOT solutions by Oliver Weichold; Thomas Dederichs; Martin Möller (300-306).
The dispersion stability of hydrophilic boehmite nanoparticles in aqueous sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solutions was studied in a wide range of particle and surfactant concentrations. The two experimental parameters significantly influence the dispersion stability and span a stability diagram. With increasing surfactant concentration and decreasing particle concentration, the system changes from stable via moderately stable to unstable and back. In concentrated AOT solutions, fully redispersed particles are present, stabilized by a surfactant bilayer or admicelles on the surface. The redispersion can be reversibly induced by dilution or concentration of the samples. The positions of two transitions, namely for complete precipitation and for beginning redispersion, can be fitted accurately using a simple model based on H-type adsorption and including the specific surface area of the particle and the molar area of the surfactant. The transitions are controlled by the concentration of free surfactant molecules in solution as well as the saturation surface coverage and were corroborated by turbidity measurements.The dispersion stability of boehmite nanoparticles in aqueous AOT solutions is displayed in a stability diagram with the surfactant and the particle concentration as axes and exhibits 5 five distinct scenarios.
Keywords: Dispersion stability; Stability diagram; Self-assembled bilayer; Nanosized boehmite; AOT;

Novel silver–gold bimetallic nanostructures were prepared by seeding with silver nanoplates in the absence of any surfactants. During the synthesis process, it was found that the frameworks of silver nanoplates were normally kept though the basal plane of silver nanoplates became rugged. The real morphology of these nanostructures depended on the molar ratio of gold ions to the seed particles. When the molar ratio of gold ions to silver atoms increased from 0.5 to 4, porous or branched silver–gold bimetallic nanostructures could be made. The growth mechanism was qualitatively discussed based on template-engaged replacement reactions and seed-mediated deposition reactions. Due to the unusual structures, they exhibited interesting optical properties. Moreover, they were shown to be an active substrate for surface-enhanced Raman scattering measurements.
Keywords: Silver nanoplates; Seed-mediated growth; Silver–gold bimetal; Surface-enhanced Raman scattering; Branched particles;

The objective of this study was the investigation of energy transfer between the laser dyes rhodamine 3B (R3B) and oxazine 4 (Ox4) adsorbed on the surface of synthetic Sumecton saponite (Sum). The process of energy transfer was studied for both saponite dispersions and oriented solid films. The electronic properties, luminescence, and the energy transfer process were described by UV–vis absorption and fluorescence spectroscopy. For the efficiency of the energy transfer process, the concentrations of energy donor and acceptor components on a clay mineral surface were found to be essential. A side reaction of the molecular assembly formation reduced both the luminescence and energy-transfer yields, mainly due to fluorescence quenching. The quenching was more problematic for the solid film specimens, where an appropriate modification of the inorganic host with hydrophobic alkylammonium cations was used to achieve a higher luminescence. Due to the higher tendency of Ox4 to form nonluminescent aggregates at higher concentrations, the lowering of the Ox4 concentration further improved the luminescent properties of the films. In this case, the energy transfer occurring in the solid film from R3B to Ox4 was clearly proven.Energy transfer from R3B to Ox4 adsorbed on the clay mineral surface was easily achieved in smectite dispersions. Due to the quenching of luminescence the efficiency of energy transfer in the films did not achieve a significant amount.
Keywords: Fluorescence resonance energy transfer; Smectite dispersion; Spin-coated films; Laser dyes; Fluorescence spectroscopy;

Iron dissolution in aqueous AOT solution by I.L. Lehr; S.B. Saidman; P.C. Schulz (323-327).
The effects of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) on the electrochemical behavior of iron were studied by potentiodynamic and potentiostatic techniques and open-circuit potential measurements. Experiments were made in both neutral and alkaline AOT solutions (pH 7 and pH 12). It was found that AOT-assisted dissolution is initiated on a passivated iron surface and that the oxidation current leads to the formation of a gel-like film on the electrode surface. This dissolution process was investigated as a function of pH, potential, and electrode rotation rate and the corrosion products were characterized by polarizing microscopy, SEM/EDX, and IR spectroscopy. The gel-like material is a mixed NaAOT–Fe(AOT)3 lamellar mesophase and a structure for this mesophase is proposed.The interaction between iron and AOT has been studied by using electrochemical and surface analysis techniques. The oxidation of iron leads to the formation of a mixed lamellar mesophase.
Keywords: Iron; AOT; Dissolution; Electrochemistry; Lamellar mesophase;

Grand canonical Monte Carlo simulations are carried out for the basic Stern model of the electrical double layer formed at the energetically heterogeneous metal oxide/electrolyte interface. The effect of the global (macroscopic) and local (microscopic) adsorption energies correlations as well as the influence of the model parameter on the surface charge density curves were investigated. The linear dependence of point of zero charge (PZC) as a function of H+ ion adsorption energy proves that the acidic/basic properties of the system are mainly governed by proton uptake/release. Two kinds of systems were taken into account: one neglecting lateral interactions and the other one including them. The effect of electrolyte concentrations as well as the surface heterogeneity on the surface charge density curves were shown too. The presented simulation algorithm allows to model two experimentally observed instances of the metal oxide/electrolyte interface: one possessing a common intersection point (CIP) at pH = PZC and the other one with CIP ≠ PZC.
Keywords: Monte Carlo simulation; Double electrical layer; Metal oxide/electrolyte interface; Surface heterogeneity; Adsorption energy correlations;

To clarify the cause of time-dependent separation behavior, the pervaporation performance with operating time through pure poly(vinyl alcohol) (PVA) membrane and glutaraldehyde (GA) cross-linked PVA membranes was investigated. The results showed that the water concentration in the permeate for the air-side surface of the PVA membrane increased dramatically from 92.2 to 95.7% in about 110 min and then remained almost unchanged. However, the water selectivity for the glass-side surface did not change with operating time. Similar results were observed for the GA cross-linked PVA membranes. Furthermore, the contact angle of water on the air-side surfaces of those membranes decreased with the time of contact with the feed. These results revealed that this dynamic pervaporation process was mainly attributable to the reconstruction of hydroxyl groups at the air-side surfaces of PVA membranes in response to the change of their surrounding medium during pervaporation. The reconstruction at the glass-side surface of the membrane did not occur because of the preferential localization of hydroxyl groups at the interface between the membrane and the glass plate during film formation of PVA solution. The above conclusion was further confirmed by the following results. The water concentration in the permeate through PVA membranes with the air-side surface facing the feed reached equilibrium more quickly with increasing operation temperature or decreasing degree of cross-linking, which was consistent with the fact that the rate of surface reconstruction accelerated with the increase of temperature or the decrease of the degree of cross-linking.The time dependence of pervaporation performance for PVA membrane was mainly caused by the reconstruction at the membrane surfaces in response to the change of their surrounding media during pervaporation.
Keywords: Pervaporation; Poly(vinyl alcohol) membrane; Dynamic separation process; Surface reconstruction; Water concentration in the permeate; Contact angle;

Study of lithium ion exchange by two synthetic zeolites: Kinetics and equilibrium by R. Navarrete-Casas; A. Navarrete-Guijosa; C. Valenzuela-Calahorro; J.D. López-González; A. García-Rodríguez (345-353).
We examined the exchange of univalent cations (Na+ and H+) retained on two commercially available synthetic zeolites with Li+ ions present in aqueous solutions in contact with the solids with a view to preparing effective controlled–release pharmaceutical forms. The studied zeolites were manufactured by Merck and featured channel diameters of 0.5 (Zeolite 5A, Ref. 1.05705.250, designated Z-05 in this work) and 1.0 nm (Zeolite 13X, Ref. 1.05703.250, designated Z-10 here). The XRD technique revealed that Z-05 possesses an LTA structure derived from that of sodalite and Z-10 a faujasite-type structure. Their exchange capacities were found to be 2.72 and 3.54 meq/g. The Z-Na + Li+/Z-Li + Na+ and Z-H + Li+/Z-Li + H+ ion-exchange processes were found to be reversible and their kinetic laws to obey the equation ( − d C / d t ) = k a ⋅ C n ⋅ ( 1 − θ ) − ( k d ⋅ θ ) , with n = 1 for Z-10 and n = 2 for Z-05. Based on the equilibrium results, the overall processes involve one (with Z-05) or two single ion-exchange processes (with Z-10). In both cases, the equations that govern equilibrium are direct results of the kinetic laws. Thus, the first process—the one with only Z-05—involves the retention of Li+ cations at anionic sites on the outer surface of the solid and their access to the larger pores; the second process—which occurs with Z-10 only—involves the retention of lithium(I) cations within the zeolite channels. In both systems, the exchange with Li+ (from the aqueous solution) is easier than that with H+; this is consistent with our kinetic, equilibrium, and thermodynamic results.We studied the exchange of univalent cations (Na+ and H+) retained on two synthetic zeolites with Li+ ions present in aqueous solutions in contact with the solids, with a view to preparing effective controlled release pharmaceutical forms.
Keywords: Zeolites; Ion-exchange; Lithium; Kinetics; Adsorption; Equilibrium;

Phase separation in PS/PVME thin and thick films by Khalil El-Mabrouk; Mohamed Belaiche; Mosto Bousmina (354-367).
Phase separation in both thin and thick films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) was studied by small-angle laser light scattering (SALLS), atomic force microscopy (AFM), optical microscopy, and X-ray photoelectron spectroscopy (XPS). Blend films with controlled thickness were obtained by spin-coating polymer–toluene solutions with various concentrations. Films with thicknesses smaller and larger than the maximum wavelength of concentration fluctuations were considered. Morphology of the blend films was characterized during and after phase separation. The obtained peculiar morphology was related to surface enrichment with the lower-surface-energy component, as was verified by XPS analyses.Cloud point determination by SALLS for PS/PVME (25/75) with a films thickness of 30 and 180 nm. The dramatic increase of the scattering intensity marks the phase separation temperature T c .
Keywords: Phase separation; PS/PVME blends; Spinodal decomposition mechanism; Nucleation and growth;

Drop manipulation and surgery using electric fields by L.Y. Yeo; R.V. Craster; O.K. Matar (368-378).
We study the dynamics of a slender drop sandwiched between two electrodes using lubrication theory. A coupled system of evolution equations for the film thickness and interfacial charge density is derived and simplified for the case of a highly conducting fluid. The contact line singularity is relieved by postulating the existence of a wetting precursor film, which is stabilised by intermolecular forces. We examine the motion of the drop as a function of system parameters: the electrode separation, β, an electric capillary number, C , and a spatio-temporally varying bottom electrode potential. The possibility of drop manipulation and surgery, which include drop spreading, translation, splitting and recombination, is demonstrated using appropriate tuning of the properties of the bottom potential; these results could have potential implications for drop manipulation schemes in various microfluidic applications. For relatively small β and/or large C values, the drop assumes cone-like structures as it approaches the top electrode; the latter stages of this approach are found to be self-similar and a power-law exponent has been extracted for this case.Droplet splitting and recombination using applied electric fields.
Keywords: Thin films; Electric fields; Microfluidics; Spreading; Wetting; Lubrication;

How the capillary burst microvalve works by Hansang Cho; Ho-Young Kim; Ji Yoon Kang; Tae Song Kim (379-385).
The capillary burst microvalve offers an attractive means to regulate microliquid flow owing to its simple structure and operation process. However, there existed no rigorous theoretical work to elucidate how the valve works and consequently to predict the valve-bursting condition. Therefore, here we report the theoretical investigation of how the capillary burst valve can stop the advancing liquid meniscus and when it bursts. We confirm our theory with experiments using a centrifugal microfluidic valve system fabricated by soft lithography.We provide a theoretical framework to explain how the capillary burst valve, as shown in this figure, works.
Keywords: Capillary burst valve; Microfluidics; Contact angle; Interface; Wetting; Surface tension;

4-(N-Stearoylamino)-2-amino-azobenzene (AzoNH2C18) and 4-(N-stearoylamino)-azobenzene (AzoC18) have been synthesized. The inclusion complex formation of AzoNH2C18 and β-cyclodextrin (β-CyD) at the air/water interface was investigated and compared to that of AzoC18. It has been found that both the amphiphiles can form stable monolayer films on water surface. When the amphiphiles were spread on the aqueous solution of β-CyD, AzoNH2C18 can form inclusion complexes with the β-CyD molecules at the interface while AzoC18 cannot. The inclusion complex formation was confirmed by the changes in the isotherms and the circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra of the transferred LS films. Atomic force microscopy (AFM) observation found morphological changes in the course of complex formation. It was suggested that the additional amino group in the azobenzene ring plays an important role in forming the inclusion complex in situ at the air/water interface.The interfacial assemblies of two amphiphiles, 4-(N-stearoylamino)-2-amino-azobenzene (AzoNH2C18) and 4-(N-stearoylamino)-azobenzene (AzoC18), with β-cyclodextrin were investigated at the air/water interface. It has been found that while AzoNH2C18 formed inclusion complexes with β-CD in situ, AzoC18 could not.
Keywords: Induced chirality; β-Cyclodextrin; Langmuir–Blodgett film; Inclusion;

Four different phases and four different first-order phase transitions have been shown to exist in Gibbs adsorption layers of mixtures containing n-hexadecyl dihydrogen phosphate (n-HDP) and l-arginine (l-arg) at a molar ratio of 1:2. These conclusions have been made from surface pressure–time ( π – t ) adsorption isotherms measured with a film balance and from monolayer morphology observed with a Brewster angle microscopy (BAM). The observed four phases are gas (G), liquid expanded (LE), liquid condensed (LC) and LC ′ phases. Three first-order phase transitions are G–LE, LE–LC and LC–LC ′ . However, the thermodynamically allowed G–LC phase transition in a 1.2 × 10 −4 M mixture at 2 °C, which is below the so-called triple point, is kinetically separated into the G–LE and LE–LC phase transitions. The most interesting observation is that the homogeneous LC phase shows a new first-order phase transition named as LC–LC ′ at 2 or 5 °C. The LE and LC phases represent circular and fractal shaped domains, respectively, whereas the LC ′ phase shows very bright, anisotropic and characteristic shaped domains.Different phase transitions observed in Gibbs adsorption layers at 2 °C.
Keywords: Gibbs adsorption layers; Phase transition; Brewster angle microscopy; n-Hexadecyl dihydrogen phosphate; l-arginine; Triple point;

Poly(vinyl pyrrolidone) (PVP) was grafted onto Pluronic F127 (PEO–PPO–PEO) to produce novel amphiphilic PVP–g–F127 graft copolymers. A controlled synthesis method was used to graft PVP onto different parts of F127. Two types of graft polymers were obtained: one has PVP grafted onto the PEO part of F127 and the other has PVP grafted onto the PPO part of F127. The association behavior of the two modified polymers was examined using differential scanning calorimetry, surface tension measurements, and dynamic light scattering.Selective modification of the copolymer chains.
Keywords: Pluronic; Graft copolymer; Poly(vinyl pyrrolidone); 1H NMR; Surface tension (ST); Dynamic light scattering (DLS);

Salt effect on the interactions between gemini surfactant and oppositely charged polyelectrolyte in aqueous solution by Yingying Pi; Yazhuo Shang; Honglai Liu; Ying Hu; Jianwen Jiang (405-410).
The effect of alkali halides (NaBr, NaCl, KCl) on the interactions between the cationic gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and the anionic polyelectrolyte sodium polyacrylate (NaPAA) in aqueous solution has been investigated by fluorescence emission spectroscopy, UV transmittance, zeta potential, and transmission electron microscopy (TEM). With increased addition of NaBr, a counterbalancing salt effect on the critical aggregation concentration (CAC) is observed. At low concentrations, NaBr facilitates the formation of micelle-like structures between surfactant and polyelectrolyte and results in a smaller CAC. At high concentrations, NaBr screens the electrostatic attraction between surfactant and polyelectrolyte and leads to a larger CAC. Upon the formation of micelle-like structures at high surfactant concentrations, the addition of NaBr is favorable for larger aggregates. The microstructure detected by TEM show that a global structure is generally formed in the presence of NaBr. The interactions also depend on ion species. Compared to NaBr, the addition of NaCl or KCl yields a smaller CAC.Microstructure of 12-6-12/NaPAA solution in the presence of NaBr.
Keywords: Gemini surfactant; Polyelectrolyte; Interactions; Salt; Alkali halide;

This series examines the effects of added silica nanoparticles on the properties and behavior of an aqueous suspension of kaolinite particles. Part I focused on the structural changes induced by the nanoparticles, primarily through scanning electron microscopy images. In this manuscript, we describe the changes in the rheological behavior of the kaolinite suspensions upon addition of the nanoparticles. In the absence of any additives, kaolinite platelets quickly aggregate and settle. When nanoparticles and salt (NaCl in these experiments) are added together, however, the suspensions begin to stabilize. When the salt and nanoparticle concentrations each exceed specific lower limits, the suspensions undergo a transition to a gel and develop a finite yield stress. Increasing the nanoparticle concentration or added salt concentration substantially increases the measured yield values, such that for the strongest samples, the yield stress exceeds the maximum for the rheometer to shear (3500 Pa). Plots of the complex viscosity, | η ∗ | , versus time suggest two different time scales for the gelation process—a short, initial time (e.g., less than 2 h) in which | η ∗ | increases rapidly, followed by a gradual rise over a much longer period. Measurement of the phase lag, δ, between the applied stress and response strain indicates that the long-term state of the suspension is either completely viscous ( δ = π / 2 ) or completely elastic ( δ = 0 ). Values of δ between 0 and π / 2 were only seen with suspensions that were transitioning from a liquid to a gel state.The addition of silica nanoparticles and sodium chloride induced a sol–gel transition in aqueous kaolinite suspensions. The gels displayed relatively strong yield stresses that increased with both salt and nanoparticle concentration.
Keywords: Clay suspensions; Liquid–solid phase transition; Nanoparticle/colloid mixtures; Rheology of clay suspensions;

Dynamic electrophoresis of droplet dispersions at low surface potentials by Jyh-Ping Hsu; Wei-Lun Min; Eric Lee (421-427).
The dynamic electrophoresis of a dispersion of spherical droplets under conditions of low surface potential and arbitrary double-layer thickness and droplet volume fraction is analyzed. A cell model with the Shilov–Zharkikh boundary condition for the electric potential is adopted to simulate a dispersion, and the governing equations and the associated boundary conditions are solved by a pseudo-spectral method based on Chebyshev polynomials. The influence of the frequency of the applied electric field, the volume fraction of the droplets, the thickness of the double layer, and the relative magnitude of the viscosity of the droplet fluid on the electrophoretic behavior of a dispersion is discussed.
Keywords: Dynamic electrophoresis; Droplets; Cell model; Low surface potential;

A simple process is demonstrated to transfer the self-assembly of submicrospheres from solid substrate to liquid surface in fabricating synthetic opal. Irradiating light and controlled airflow are used to control the evaporation of solvent. This process induces solid layer with colors. SEM images of the solid layers show that those layers are synthetic opals for well ordered polystyrene spheres in a hexagonal close-packed structure over a large scale. An absorption peak is also observed at 541 nm in normal transmission spectrum of the fabricated sample. Furthermore, investigation of the phase transitions indicates that nuclei of synthetic opals grow up through accumulating at their bottom and extending at their edges during self-assembly.A method is reported to self-assemble submicrospheres rapidly at liquid surface instead of on substrate in traditional method. Polystyrene spheres are packed in a large scale with low crack.
Keywords: Photonic crystal; Synthetic opal; Colloid crystal; Liquid surface;

The binary oxide composite, consisting of rock salt-type SrO and spinel Co3O4 nano-domains, exhibits soft ferromagnetic properties at ambient temperature. This ferromagnetism is originated from interface-induction, and the magnitude of the magnetic properties can be enhanced when the spinel phase of the composite is doped by a small amount of Ln2O3 (Ln = La, Nd, for instance). In this work, we study the composites of tri-oxide, 1/2( 1 − x )Ln2O3-xSrO/1/3Co3O4, where 0.01 ⩽ 1 − x ⩽ 0.6 , by focusing on three areas: (i) generation of nano-composite dominant by interfacial phase via the pyrolysis of preceramic metallo-organic gel; (ii) influence of post-pyrolysis calcination and Ln2O3 content on the phase composition of the composite; and (iii) elucidation of different magnetic responses caused by the nature of Ln2O3 dissolved in the Co3O4 phase. The Ln3+-doped Co3O4 oxide displays only paramagnetic behavior at room temperature, but the ferromagnetic response is attained upon its mixing with SrO in nano-scale. The SrO phase plays the role in assisting Co3O4 phase with aligning unpaired electrons through interfacial induction.HR-TEM image depicts the heterogeneous tri-oxide comprising respective oxide domains in nano-scale, 1/2( 1 − x )Ln2O3-xSrO/1/3Co3O4. The delicate mixing triggers the interfacial induction effect leading to magnetic properties at room temperature, as shown by its hysteresis loop.
Keywords: Interfacial; Ferromagnetism; Lanthanide-doped Co3O4; Pyrolysis; Preceramics;

Use of dielectric relaxation for measurements of surface energy variations during adsorption of water on mordenite by J.M. Douillard; G. Maurin; F. Henn; S. Devautour-Vinot; J.C. Giuntini (440-448).
This paper tries to assess simply and quantitatively the link between classical adsorption theories and dielectric spectroscopy, in order to demonstrate that dielectric spectroscopy can be used as a tool of determination of surface energy variations due to movements of charge carriers at the surface of solids. A simple theory is developed to analyze hops of cations at the surface of mordenite, which are detected by complex impedance spectroscopy during adsorption of water. An energy of extraction of the cation can be determined from measurements and its dependence on the quantity of water molecules adsorbed is shown and qualitatively and quantitatively explained, using relationships developed in order to interpret adsorption phenomena generally. The agreement with other determinations of the adsorption energies and solid surface energy is correct.The energy of extraction of a Na cation from the internal surface of a Na-mordenite, in the presence of adsorbed water, is deduced from complex impedance spectroscopy using a simple physical model. The influence of the adsorbed water quantity is interpreted using well-known relationships describing adsorption. Data are in good agreement with enthalpies of adsorption and estimates of solid surface energy, showing that the CIS could be a valuable tool for studying adsorption of polar molecules at the surfaces of complex solids.
Keywords: Complex impedance spectroscopy; Cations; Mordenite; Adsorption enthalpy; Solid surface tension; Water;

Determination of the thickness of emulsion films by using the film interferometric images is usually less accurate than that of foam films, due to the close values of the refractive indices of the liquid film and adjacent liquid phases (hence, low contrast and high level of noise at high magnification). A new technique was developed to improve the thickness determination by obtaining the interferometric images without directly filtering the illuminating light, as is usually done in the classical Scheludko interferometric technique. The new method then uses digital filtration during the off-line image post-processing to obtain monochromatic interferometric images required for the thickness determination. The technique was tested with foam films stabilised by sodium dodecyl sulfate and successfully applied to determine thickness of toluene–water–toluene emulsion films using the green and red digital filters. Results for emulsion film thickness determined by either the green or red digital filtration are comparable, thus validating the new technique developed here for emulsion films.Thickness of toluene–water–toluene emulsion films can be better determined by using digital (green or red) filters than by directly filtering the illuminating light, as in the classical Scheludko interferometric technique.
Keywords: Emulsion film; Foam film; Interferometry;

A study on pore-opening behaviors of graphite nanofibers by a chemical activation process by Byung-Joo Kim; Young-Seak Lee; Soo-Jin Park (454-458).
In this work, porous graphite nanofibers (GNFs) were prepared by a KOH activation method in order to manufacture porous carbon nanofibers. The process was conducted in the activation temperature range of 900–1100 °C, and the KOH:GNFs ratio was fixed at 3.5:1. The textural properties of the porous carbons were analyzed using N2 adsorption isotherms at 77 K. The BET, D-R, and BJH equations were used to observe the specific surface areas and the micro- and mesopore structures, respectively. From the results, it was found that the textural properties, including the specific surface area and the pore volumes, were proportionally enhanced with increasing activation temperatures. However, the activation mechanisms showed quite significant differences between the samples activated at low and high temperatures.The figure illustrates possible pore development mechanisms by a chemical activation of graphite nanofibers. According to the types of pore development, there can be three mechanisms, such as breaking, expansion, and exfoliation. Breaking means just breaking fibers and forms shorter fibers. Expansion indicates the opening of the pores, so this mechanism does not cause the demolition of fibrous shapes of GNFs. Exfoliation means the co-reaction of both the separation of layers and the breaking of the fibers, resulting in the collapse of fiber shapes.
Keywords: Graphite nanofibers; Chemical activation; Mesopores; Porous structures;

Applying grazing incidence X-ray reflectometry (XRR) to characterising nanofilms on mica by Wuge H. Briscoe; Meng Chen; Iain E. Dunlop; Jacob Klein; Jeffrey Penfold; Robert M.J. Jacobs (459-463).
Molecularly smooth mica has hitherto not been widely used as a substrate for the X-ray reflectometry (XRR) technique. That is largely due to the difficulty of achieving flatness over a sufficiently large area of mica. Here we show that this difficulty can be overcome by slightly bending the mica substrate over an underlying cylinder; the enhanced rigidity of the bent mica sheet along the axis of the cylinder provides sufficient flatness along this axis for XRR measurements. To test this method, we have employed it to characterise three types of nanofilms on mica in air: (A) Cr–Au thin films; (B) a surface-grown zwitterionic polymer brush; and (C) a Langmuir–Blodgett (LB) phospholipid monolayer, using a table-top X-ray reflectometer. Fitting the obtained reflectivity curves with the standard Parratt algorithm allows us to extract the structural information of the nanofilms (both thickness and apparent roughness). Our simple method points to how XRR may be exploited as a useful characterisation tool for nanofilms on mica.
Keywords: XRR; Thin films; Nanofilms; X-ray reflectometry; Mica; Surface-grown polymer brushes; Surface characterisation;