Journal of Colloid And Interface Science (v.287, #1)

Clays were compared with organoclays for the sorption of paraquat from aqueous solution. Sepiolite (S), bentonite (B), and illite (I) were used as clay samples. Organoclays were prepared by the modification of the clays with nonyl- and dodecylammonium chlorides, denoted as NS, DS, NB, DB, NI, and DI, respectively. Specific surface area and pore size distribution of the samples were determined by N2 adsorption–desorption at 77 K using the BET method. X-ray powder diffraction analysis of the samples was used to determine the effects of modifying agents on the layer structure of the clays. In the adsorption experiments, C m values increased from 0.038 mmol/g for DS to 0.223 mmol/g for NI. K d 0.3 values ranged from 0.177 for DS to 0.843 for NI. The adsorption data indicated that illite and NI are the most effective adsorbents among these clays and organoclay samples, respectively.
Keywords: Paraquat; Adsorption; Clay; Organoclay; Bentonite; Sepiolite;

The effect of solution temperature and the determination of the thermodynamic parameters of adsorption of methylene blue (MB), Cibacron Reactive black C-NN (RB) and Cibacron Reactive golden yellow MI-2RN (RY) onto manganese-oxides-modified diatomite (MOMD), such as activation energy, E, enthalpy of activation, Δ H ∗ , entropy of activation, Δ S ∗ , and free energy of activation, Δ G ∗ , on the adsorption rates is important in understanding the adsorption mechanism. The rate and the transport/kinetic processes of dye adsorption onto the adsorbents were described by applying various kinetic adsorption models. This would lead to a better understanding of the mechanisms controlling the adsorption rate. The pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behaviour of RB onto MOMD, suggesting that the adsorption mechanism might be a chemisorption process. The activation energies, E, for RB, RY and MB were −6.74, 56.65 and 99.80 kJ/mol, respectively. The negative value of the activation energy suggested that the rise in the solution temperature did not favour RB adsorption onto MOMD. Moreover, the activation energy of the diffusion process, E ′ , for RB, RY and MB increased as MB > RY ≫ RB. It means that the RB molecules are much faster moving and a lower energy is needed to diffuse into MOMD than RY and MB molecules. E ′ , the activation energy for adsorption into pores, of RY is higher than E, indicating that the rate-limiting step of RY adsorption onto MOMD might be diffusion controlled, while the activation energy of the diffusion process, E ′ , of MB is slightly lower than E, suggesting that the rate-limiting step is a combination of chemical and diffusion adsorption.
Keywords: Adsorption; Diatomite; Modified adsorbents; Thermodynamic parameters; Kinetic studies; Reactive dyes; Methylene blue;

Adsorption of phenols from wastewater by M. Ahmaruzzaman; D.K. Sharma (14-24).
The present work involves an investigation of the possible use of coal, residual coal, and residual coal treated with H3PO4 as a means of removal of phenol from wastewater. The study was realized using batch experiments, with synthetic wastewater having phenol concentration of 1000 ppm. Other low-cost adsorbents such as petroleum coke, coke breeze, rice husk, and rice husk char have also been used. The effect of system variables such as pH, contact time, and temperature has been investigated. The suitability of the Freundlich, Langmuir, and Redlich–Peterson adsorption models to the equilibrium data was investigated for each phenol–adsorbent system. The results showed that the equilibrium data for all the phenol–sorbent systems fitted the Redlich–Peterson model best. Kinetic modeling of removal of phenols was done using the Lagergren first-order rate expression. A series of column experiments were performed to determine the breakthrough curves.
Keywords: Adsorption; Equilibrium isotherm; Freundlich isotherm; Redlich–Peterson isotherm; Langmuir isotherm; Kinetics; Column;

In this paper, adsorption equilibrium and kinetics of three reactive dyes from their single-component aqueous solutions onto activated carbon were studied in a batch reactor. Effects of the initial concentration and adsorbent particle size on adsorption rate were investigated Adsorption equilibrium data were then correlated with several well-known equilibrium isotherm models. The kinetic data were fitted using the pseudo-first-order equation, the pseudo-second-order equation, and the intraparticle diffusion model. The respective characteristic rate constants were presented. A new adsorption rate model based on the pseudo-first-order equation has been proposed to describe the experimental data over the whole adsorption process. The results show that the modified pseudo-first-order kinetic model generates the best agreement with the experimental data for the three single-component adsorption systems.
Keywords: Adsorption; Equilibrium; Kinetics; Pseudo-first-order equation; Pseudo-second-order equation; Intraparticle diffusion model;

In this study, we use the quartz crystal microbalance with dissipation monitoring (QCM-D) to study the immobilization of the enzyme horseradish peroxidase (HRP) on poly(ethylene-co-acrylic acid) (PEAA) films. The surface polarity of spin-coated PEAA films was varied by heat treatments in air or in a 30% NaOH aqueous solution leading to COOH-depleted or COOH-enriched surfaces, respectively. Two reaction schemes, direct adsorption and amine coupling, were employed for HRP immobilization on the two surfaces. The shifts in frequency and dissipation, Δf and ΔD, measured by QCM-D and the ratio Δ D / Δ f were used to evaluate the binding amount and the conformation of the adsorbed enzyme. It is found that HRP immobilized via covalent linkages forms rigid and little dissipative films. In contrast, directly adsorbed HRP films exhibit a highly dissipative structure. HRP-catalyzed oxidation of the 4-chloro-1-naphthol in the presence of H2O2 was used to characterize the catalytic activity of the HRP films. The results show that the enzymatic activity of the covalently immobilized HRP tends to be higher.
Keywords: QCM-D; Polyethylene-co-acrylic acid; HRP;

A spectroscopic study of mechanochemically activated kaolinite with the aid of chemometrics by Onuma Carmody; János Kristóf; Ray L. Frost; Éva Makó; J. Theo Kloprogge; Serge Kokot (43-56).
The study of kaolinite surfaces is of industrial importance. In this work we report the application of chemometrics to the study of modified kaolinite surfaces. DRIFT spectra of mechanochemically activated kaolinites (Kiralyhegy, Zettlitz, Szeg, and Birdwood) were analyzed using principal component analysis (PCA) and multicriteria decision making (MCDM) methods, PROMETHEE and GAIA. The clear discrimination of the Kiralyhegy spectral objects on the two PC scores plots (400–800 and 800–2030 cm−1) indicated the dominance of quartz. Importantly, no ordering of any spectral objects appeared to be related to grinding time in the PC plots of these spectral regions. Thus, neither the kaolinite nor the quartz, are systematically responsive to grinding time according to the spectral criteria investigated. The third spectral region (2600–3800 cm−1 ―OH vibrations), showed apparent systematic ordering of the Kiralyhegy and, to a lesser extent, Zettlitz spectral objects with grinding time. This was attributed to the effect of the natural quartz on the delamination of kaolinite and the accompanying phenomena (i.e., formation of kaolinite spheres and water). With the MCDM methods, it was shown that useful information on the basis of chemical composition, physical properties and grinding time can be obtained. For example, the effects of the minor chemical components (e.g., MgO, K2O, etc.) indicated that the Birdwood kaolinite is arguably the most pure one analyzed. In another MCDM experiment, some support was obtained for the apparent trend with grinding time noted in the PC plot of the OH spectral region.
Keywords: Kaolinite surfaces; Chemometrics; PCA; PROMETHEE and GAIA; DRIFT;

Effects of oxygen and carbon dioxide plasmas on the surface of poly(ethylene terephthalate) by M. Carmen Almazán-Almazán; J.I. Paredes; M. Pérez-Mendoza; M. Domingo-García; F.J. López-Garzón; A. Martínez-Alonso; J.M.D. Tascón (57-66).
Poly(ethylene terephthalate) was exposed to oxygen and carbon dioxide plasmas for different periods of time. The surface-modified samples were characterized by infrared spectroscopy, atomic force microscopy, and inverse gas–solid chromatography. The main difference between both types of plasma was connected to the time scale of degradation, which was much faster when using oxygen plasma. Aggregate globular features were produced by different treatments due to chain scission and further recombination of evolved products. Oxygenated functionalities were introduced in significant amounts after long exposure times to the oxygen plasma. As a consequence, the specific component of the surface free energy was clearly observed to increase after these long treatments.
Keywords: O2 and CO2 plasmas; Surface modifications; DRIFTS; AFM; Inverse gas–solid chromatography;

Polyaniline film was electropolymerized in organic acidic media (CF3COOH) and then investigated by cyclic voltammetry, AC impedance, and galvanostatic charging and discharging tests in ionic liquid—1-methyl-3-butylimidazolium hexafluorophosphate (BMIPF6) and the mixture electrolytes of BMIPF6 and acetonitrile (ACN) with different ratios. The results showed that the polymer in mixture of BMIPF6 and ACN have lower solution resistance, higher cycle life, and higher electrochemical capacitance. The relationship of the peak current to the scan rates provides some insight into the nature of the polyaniline film switching reaction in different electrolytes.
Keywords: Ionic liquid (IL); Electropolymerization; Polyaniline; Diffusion coefficient;

Retention of tannic acid and condensed tannin by Fe-oxide-coated quartz sand by J. Kaal; K.G.J. Nierop; J.M. Verstraten (72-79).
This paper intends to shed light on the interactions between tannin and mineral soil particles. For that purpose, aqueous solution of condensed tannin (CT) (derived from Black pine (Pinus nigra var. maritima)) and commercially available tannic acid (TA) were added to purified quartz (Qtz) sand and quartz sand coated with either goethite (Gt) or ferrihydrite (Fh). After solvent removal by evaporation the samples were extracted by water. The extracts were analysed for organic carbon, total phenolics and CT. The extractability of the two tannins was small and increased in the order Qtz–Fh < Qtz–Gt < Qtz. For all mineral samples, TA was more extractable than CT. Bonding of tannins to the mineral samples and the partial peptisation of the Fe oxide coatings upon the binding resulted in complex tannin release curves. Our results suggest that the inextractability of tannins from natural soils and the absence of tannins in soil leachates might be caused by strong adsorption on soil minerals such as Qtz and Fe (oxy)(hydr)oxides. The results of competition experiments with mixtures of both tannins demonstrate that the CTs, and TA in particular, can release large amounts of Fe (oxides), suggesting that the tannins are excellent metal-mobilising agents. We therefore suggest that the fate of tannins in the mineral soil environment is highly dependent on the abundance of weakly bonded secondary oxides.
Keywords: Adsorption; Tannic acid; Condensed tannin; Synthetic minerals; Fe oxides;

c-Jun interacts with phospholipids and c-Fos at the interface by Maximiliano Del Boca; Beatriz L. Caputto; Bruno Maggio; Graciela A. Borioli (80-84).
We describe c-Jun, a widely studied transcription factor that participates in cell proliferation, differentiation, and tumorigenesis, as amphitropic. We show that c-Jun forms stable monolayers and interacts favorably, although in a nonselective manner, with phospholipids at the interface. The surface activity of c-Jun, together with that of c-Fos, its common partner in AP-1 transcription heterodimers, drives interfacial complex formation. We show that AP-1 is very stable at the air–water interface and suggest that AP-1 may not be substantially formed in solution as a stable equimolar association of both proteins.
Keywords: AP-1; c-Jun; Surface activity; Monolayers;

Aggregation and gelation of micellar casein particles by Maud Panouillé; Dominique Durand; Taco Nicolai; Eric Larquet; Nicolas Boisset (85-93).
Micellar casein particles (submicelles) are formed by removing calcium phosphate from native casein. The submicelles aggregate and eventually form a gel with a rate that increases strongly with increasing temperature and casein concentration. At low casein concentrations the gel is very weak and collapses under its own weight so that a precipitate is formed. The structure of the aggregates is studied using light scattering and cryo-electron microscopy. It is found that the aggregates have a self-similar structure with fractal dimension 2. The viscoelastic properties of the gel are studied by frequency scans of the loss and storage moduli during the gelation process. The bonds between the submicelles probably involve calcium phosphate complexes.
Keywords: Casein; Submicelles; Light scattering; Rheology; Aggregation; Gel;

Rheological properties of nanopowder alumina coated with adsorbed fatty acids by Nelson S. Bell; Megan E. Schendel; Marcin Piech (94-106).
The rheological properties of a nanosized alumina powder coated with fatty acid steric stabilizers of varying chain length were investigated. The storage and loss moduli of the complex modulus were measured to characterize the behavior of the flocculated systems. As chain length increased, there was a transition from an elastic response to fluid behavior. However, the fluid system developed elastic characteristics at relatively low volume fractions of 22%. The length of the steric barrier required to produce the fluid dispersion was estimated to be ∼2 nm and correlates with attractive interactions on the order of the system thermal energy. Moreover, in the flocculated systems, the storage modulus was found to be higher than reported previously in the literature. These higher values were related to the additional attractive forces due to van der Waals attractions between the hydrocarbon tails of the adsorbed fatty acid layers.
Keywords: Nanoparticle dispersion; Oscillatory rheology; Alumina; Steric separation;

The solid-state synthesis of metal nanoparticles from organometallic precursors by Dorota Wostek-Wojciechowska; Jeremiasz K. Jeszka; Catherine Amiens; Bruno Chaudret; Pierre Lecante (107-113).
Nanoparticles (NPs), average size of 2–5 nm, of ruthenium, cobalt, and rhodium have been prepared by an original method, namely the solid-state decomposition under dihydrogen of an organometallic precursor either dispersed in polymer films or directly as nanocrystals. The NPs dispersion, size, and morphology are investigated by transmission electron microscopy, and their structure by wide angle X-ray scattering. Infrared spectroscopy, after adsorption of carbon monoxide on the metal NPs surfaces, evidences a nonoxidized surface of high reactivity.
Keywords: Metal nanoparticles; Polymer; Organometallic precursor; Dihydrogen; TEM; WAXS;

Study on hydration layers near nanoscale silica dispersed in aqueous solutions through viscosity measurement by S. Song; C. Peng; M.A. Gonzalez-Olivares; A. Lopez-Valdivieso; T. Fort (114-120).
On the basis of the Einstein theory of viscosity of dispersion, a parameter, termed as solvation factor, is presented to evaluate the solvation degree of nanoscale particles dispersed in a liquid in this work. The value of the parameter is obtained through the measurements of relative viscosity of the dispersions as a function of the volume fraction of dry particles. The solvation factor has been used to study the hydration layers near nanoscale silica particles dispersed in water and aqueous electrolyte (NaCl and CaCl2) solutions in this work. The experimental results have shown that a strong hydration indeed applied to the silica surfaces in aqueous solutions, leaving a large volume of hydration layers on the surfaces. Also, it has been found that the hydration of the nanoscale silica particles could be greatly enhanced if they were dispersed in aqueous NaCl or CaCl2 solutions, which might be attributed to that the hydrated cations (Na+ or Ca2+) bind onto the silica/ water interface and thus increase the volume of the hydration layers.
Keywords: Hydration layers; Viscosity; Nanoscale silica; Electrolyte; Solvation factor;

Interactions between particles with an undulated contact line at a fluid interface: Capillary multipoles of arbitrary order by Krassimir D. Danov; Peter A. Kralchevsky; Boris N. Naydenov; Günter Brenn (121-134).
A colloidal particle adsorbed at a fluid interface could have an undulated, or irregular contact line in the presence of surface roughness and/or chemical inhomogeneity. The contact-line undulations produce distortions in the surrounding liquid interface, whose overlap engenders capillary interaction between the particles. The convex and concave local deviations of the meniscus shape from planarity can be formally treated as positive and negative “capillary charges,” which form “capillary multipoles.” Here, we derive theoretical expressions for the interaction between two capillary multipoles of arbitrary order. Depending on the angle of mutual orientation, the interaction energy could exhibit a minimum, or it could represent a monotonic attraction. For undulation amplitudes larger than 5 nm, the interaction energy is typically much greater than the thermal energy kT. As a consequence, a monolayer from capillary multipoles exhibits considerable shear elasticity, and such monolayer is expected to behave as a two-dimensional elastic solid. These theoretical results could be helpful for the understanding of phenomena related to aggregation and ordering of particles adsorbed at a fluid interface, and for the interpretation of rheological properties of particulate monolayers. Related research fields are the particle-stabilized (Pickering) emulsions and the two-dimensional self-assembly of microscopic particles.
Keywords: Capillary force between particles; Capillary multipoles; Colloid particles—self-assembly; Interfacial rheology; Particles at interfaces; Particulate monolayers; Pickering emulsions; Surface shear elasticity;

Highly charging of nanoparticles through electrospray of nanoparticle suspension by Jeongsoo Suh; Bangwoo Han; Kikuo Okuyama; Mansoo Choi (135-140).
Patterned deposition of nanoparticles is a prerequisite for the application of unique properties of nanoparticles in future nanodevices. Recent development of nanoxerography requires highly charged aerosol nanoparticles to avoid noise deposition due to random Brownian motion. However, it has been known that it is difficult to charge aerosol nanoparticles with more than two elementary charges. The goal of this work is to develop a simple technique for obtaining highly charged monodisperse aerosol nanoparticles by means of electrospray of colloidal suspension. Highly charged aerosol nanoparticles were produced by electrospraying (ES) and drying colloidal suspensions of monodisperse gold nanoparticles. Size and charge distributions of the resultant particles were measured. We demonstrate that this method successfully charges monodisperse nanoparticles very highly, e.g., 122 elementary charges for 25.0 nm, 23.5 for 10.5 nm, and 4.6 for 4.2 nm. The method described here constitutes a convenient, reliable, and continuous tool for preparing highly charged aerosol nanoparticles from suspensions of nanoparticles produced by either wet chemistry or gas-phase methods.
Keywords: Charging; Patterning; Monodisperse particles; Electrospray;

The critical phenomenon of nonaqueous microemulsion was studied for the first time. The coexistence curves of ( T , n ), ( T , ϕ ), and ( T , ψ ) (n and ϕ are refractive index and volume fraction, respectively; ψ is defined as ψ = ϕ / [ ϕ + ϕ c ( 1 − ϕ ) / ( 1 − ϕ c ) ] ) for a ternary microemulsion [ ϕ  (AOT–DMA) + ( 1 − ϕ ) decane] at constant pressure and a constant molar ratio ( ω = 2.86 ) of DMA to AOT have been determined within about 7 K from the critical temperature T c by measurements of refractive index. The critical exponent β has been deduced from ( T , n ), ( T , ϕ ), and ( T , ψ ) coexistence curves within 1 K below T c . They all were 0.329 ± 0.005 and were consistent with the 3D Ising value. The experimental results in a temperature range of ( T c − T ) < 7   K also have been analyzed to obtain critical amplitudes and the Wegner correction terms, to examine the diameters of the coexistence curves.
Keywords: Critical phenomenon; Nonaqueous microemulsion; Coexistence curve; Critical exponent;

Synthesis and self-organization of soluble monodisperse palladium nanoclusters by Meng Chen; Joshua Falkner; Wen-Hua Guo; Jun-Yan Zhang; Christie Sayes; Vicki L. Colvin (146-151).
Control over size and size distribution of nanoparticles has been the subject of much interest during the past decade. In the present study, a simple strategy is described for obtaining monodisperse Pd nanoparticles with size less than 5 nm and size distribution around 10%. Without size-selective precipitation but with a simple reducing agent, hypophosphite, the synthesized Pd nanoparticles can form 2D well-ordered arrays on the TEM grids. When reducing agents were changed, no obvious size change of Pd nanoparticles was observed within experimental errors but the size distribution varied dramatically. In addition, stability, self-organized pattern, and solubility can be controlled by changing the capping agent. The present route is very simple and reproducible, and further study on the properties of the Pd nanoparticles is underway.
Keywords: Monodisperse nanoparticles; Palladium; Reverse micelle; Self-organization;

Through cross-flow filtration (CFF) with a 1-kDa regenerated cellulose Pellicon 2 module, the ultrafiltration characteristics of river organic matter from Longford Stream, UK, were investigated. The concentration of organic carbon (OC) in the retentate in the Longford Stream samples increased substantially with the concentration factor (cf), reaching approximately 40 mg/L at cf 15. The results of dissolved organic carbon (DOC) and colloidal organic carbon (COC) analysis, tracking the isolation of colloids from river waters, show that 2 mg/L of COC was present in those samples and good OC mass balance (77–101%) was achieved. Fluorescence measurements were carried out for the investigation of retentate and permeate behaviour of coloured dissolved organic materials (CDOM). The concentrations of CDOM in both the retentate and permeate increased with increasing cf, although CDOM were significantly more concentrated in the retentate. The permeation model expressing the correlation between log [CDOM] in the permeate and log cf was able to describe the permeation behaviour of CDOM in the river water with regression coefficients ( r 2 ) of 0.94 and 0.98. Dry weight analysis indicated that the levels of organic colloidal particles were from 49 to 71%, and between 29 and 51% of colloidal particles present were inorganic. COC as a percentage of DOC was found to be 10–16% for Longford Stream samples.
Keywords: Cross-flow ultrafiltration; Colloids; Colloidal organic carbon; Dissolved organic carbon; River water;

Using polyvinylpyrrolidone (PVP) as a stabilizing agent, stable colloidal solutions of platinum nanoparticles of different size distributions have been prepared by reducing H2PtCl6 with hydrogen. The UV–vis adsorption peaks at 258 nm due to the adsorption of Pt(IV) species disappear completely, indicating that the Pt(IV) species has been used up and colloidal Pt has been formed. The electrodes have been prepared from aqueous Pt colloids and glassy carbon (GC). The effect of platinum particle size of Pt/GC catalyst electrode on the electrocatalytic oxidation of carbon monoxide has been investigated. The voltammetry shows that a higher potential is needed for the oxidation of absorbed carbon monoxide with a decrease of the platinum particle size for particle sizes larger than 1 nm. But for particle sizes smaller than 1 nm, the potential remains constant while the activity decreases with decreasing the size. The snowlike, well-dispersed, and highly ordered platinum nanoparticles demonstrate high activity in the oxidation reaction of carbon monoxide. The reason may be due to the geometric structure of platinum nanoparticles.
Keywords: Platinum nanoparticles; Carbon monoxide; Electrocatalytic oxidation; Fuel cell; Size effect;

Dispersion and phase separation of carbon nanotubes in ultrathin polymer films by Jeffrey Foster; Srikanth Singamaneni; Ramesh Kattumenu; Valery Bliznyuk (167-172).
The inner structure and nanoscale distribution of the stiffness was studied for polymer–single-wall carbon nanotube composites. Dispersion of nanotubes in a polystyrene and polyurethane polymer matrix was achieved by a proper choice of the organic solvent (NMP) and sonification of polymer/SWNT solutions. Ultrathin nanocomposite films were prepared through a dip-coating procedure and possessed a noticeable degree of nanotube orientation in the direction of the applied shear force. Peculiarities of the phase separation in the films were studied by atomic force microscopy (with application of force modulation mode to map the nanotube distribution within the polymer matrix) and Raman spectroscopy.
Keywords: Nanocomposite; Atomic force microscopy; Single-wall carbon nanotubes;

We fabricated colloidal crystals on a fiber by a dip-coating method. The self-assembly of monodisperse colloidal particles was affected by the curvature of the fiber (the reciprocal of the fiber radius). As the fiber became smaller in diameter, fewer layers of the colloidal spheres were coated for a given lift-up speed. The hollow colloidal crystal cylinders were used as a template for creating macroporous structure having three-dimensionally interconnected air cavities. Specifically, the polymer precursor was infiltrated into the colloidal crystal template and the macroporous polymer structures were obtained after the selective etching of colloidal particles.
Keywords: Colloidal crystals; Dip coating; Ordered macropores; Self-assembly; Colloidal templates; FCC lattice;

In the present study, the competitive adsorption characteristics of binary and ternary heavy metal ions Pb2+, Cu2+, and Cd2+ on microporous titanosilicate ETS-10 were investigated in batch systems. Pure microporous titanosilicate ETS-10 was synthesized with P25 as the Ti source and characterized by the techniques of X-ray diffraction (XRD), field emission-scanning electron microscope (FESEM), nitrogen adsorption, and ζ-potential. Equilibrium and kinetic adsorption data showed that ETS-10 displays a high selectivity toward one metal in a two-component or a three-component system with an affinity order of Pb2+  > Cd2+  > Cu2+. The equilibrium behaviors of heavy metals species with stronger affinity toward ETS-10 can be described by the Langmuir equation while the adsorption kinetics of the metals can be well fitted to a pseudo-second-order (PSO) model.
Keywords: Microporous titanosilicate ETS-10; Heavy metal ions; Competitive adsorption; Selectivity;

In this paper, we used two simplified methods to understand the influence of surfactant molecular structure on the properties of surfactant–DNA complexes. First, we selected Langmuir balance technique, a two-dimensional (2D) method, which allows complex formation under equilibrium-like conditions, avoiding some of the inherent problems involved in solution. Secondly, two series of simple quaternary ammonium surfactants were used. The cationic surfactant–DNA complex monolayers were formed at the air–water interface through the electrostatic interaction between the ammonium groups of the surfactants and the phosphate groups of DNA at the air–water interface. Combining the results of πA isotherms, πt isotherms, and atomic force microscopy (AFM) measurements, it was found that the surfactant molecular structures affect the surface properties and morphologies of 2D surfactant–DNA complexes. We expect that the study of the properties of 2D surfactant–DNA complexes will help us to understand the physicochemical properties of surfactant–DNA complexes, which are important for gene delivery.

Surface pressure–area isotherms and Brewster angle microscopy images of mixed binary films of dimyristoylphosphatidylcholine (DMPC) and the diacetylene 10,12-tricosadiynoic acid (TRCDA) were recorded at different temperatures and mole ratios to investigate the molecular interactions and cooperative properties of the films. The experiments revealed that segregation, on the one hand, and significant intermolecular interactions, on the other hand, both contribute to the thermodynamic properties of the phospholipids and the diacetylene assemblies. In particular, the data demonstrate that higher temperatures and greater percentage of DMPC promote repulsion between the liquid-condensed phospholipid monolayer and the TRCDA domains. In contrast, at high TRCDA mole ratios, film contraction occurred (lower molecular areas) due to TRCDA multilayer formation (at high temperature) or intermolecular affinities (at low temperature).
Keywords: Langmuir films; Polydiacetylene; Phospholipid films; Brewster angle microscopy;

Organic–inorganic composite membranes based on poly(vinyl alcohol)/SiO2 were prepared via an aqueous dispersion polymerization route and anion-exchange groups were introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired content. These membranes were extensively characterized for their surface morphology, thermal stability, water content, and surface-charge properties using SEM, TEM, FTIR, TGA, water uptake, and ion-exchange capacity measurements. Counterion transport numbers across these membranes were estimated from membrane potential data. Membrane conductance measurements were also performed and these data were used for the estimation of values of counterion diffusion coefficients in the membrane phase. Physicochemical and electrochemical properties of these membranes and equivalent pore radius (estimated from electroosmotic flux measurements) were found to be highly dependent on the 4-vinylpyridine (4-VP) content in the membrane phase. It was also observed that for better selectivity and membrane conductivity of anion-exchange membranes complete optimization of the loading of 4-VP in the membrane phase is necessary. Furthermore, among these, membrane with 25% loading with 4-VP exhibited very good selectivity, water content, and ion-exchange capacity along with moderate membrane conductivity, which may be used for their application in electro-driven separation at elevated temperatures or for other electrochemical processes.
Keywords: Composite membranes; Organic–inorganic composite; Anion-exchange membrane; Membrane conductance; Electroosmosis;

A series of multilayer films were fabricated alternately from poly(4-vinylpyridine) solution and mixed solutions of two carboxyl-terminated dendrimers. UV-Vis absorption spectroscopy was applied to study the composition of the multilayer films. In contrast to mixed solutions of linear polymers, the total amount of dendrimers in multilayer film is independent of the ratio of dendrimers in mixed solution, which is possibly due to the globular conformation of the dendrimer. Furthermore, the ratio of two dendrimers in films is linearly dependent on the ratio of the two dendrimers in solutions, which facilitates control of the composition in multilayer films.
Keywords: Composition; Multilayer film; Dendrimer; Mixed solution; Surface density;

Elliptic solution to the Young–Laplace differential equation by E. Hernández-Baltazar; J. Gracia-Fadrique (213-216).
The Young–Laplace equation differential form can be solved under the elliptic representation for a fluid–fluid interface in the range 0 ⩽ ϕ ⩽ 90 ° . For a characteristic point ( X max , Y = 90 ° ) we find a simple analytical relation between the curvature radius and the elliptic parameters that yields the surface tension in the range 0.125 < β < 100 . The solution to the differential equation gives an error lower than 2.5% for published normalized data. The origin of the coordinates of the profile drop is the maximum diameter and the distance from the equator to the drop apex. Through this mechanism, the need for numerical methods and published algorithms prior to this work is eliminated. To challenge the method, the procedure is used with published data of numerical solutions. The dimensions of the parameters used are invariant with respect to the coordinate system. For practical applications, this useful equation can be used in pendant drop, sessile drop, rising bubble, spinning drop, and capillary methods. Finally, to increase the sensitivity of the procedure, the elliptic segment data under the maximum diameter can be treated with a linear regression ( y 2 vs x 2 ) to obtain the elliptic parameters (a and b) in order to apply the final equation.
Keywords: Elliptic solution; Young–Laplace; Drop profile; Surface tension; Pendant drop; Sessile drop; Spinning drop; Capillary method;

Wettability of Freon hydrates in crude oil/brine emulsions by S. Høiland; K.M. Askvik; P. Fotland; E. Alagic; T. Barth; F. Fadnes (217-225).
The surface energy of petroleum hydrates is believed to be a key parameter with regard to hydrate morphology and plugging tendency in petroleum production. As of today, the surface energy of natural gas hydrates is unknown, but will depend on the fluids in which they grow. In this work, the wettability of Freon hydrates is evaluated from their behavior in crude oil emulsions. For emulsions stabilized by colloidal particles, the particle wettability is a governing parameter for the emulsion behavior. The transition between continuous and dispersed phases as a function of brine volume in crude oil–brine emulsions containing Freon hydrates has been determined for 12 crude oils. Silica particles are used for comparison. The results show that phase inversion is highly dependent on crude oil properties. Based on the measured points of phase inversion, the wettability of the Freon hydrates generated in each system is evaluated as being oil-wet, intermediate-wet, or water-wet. Generation of oil-wet hydrates correlates with low hydrate plugging tendency. The formation of oil-wet hydrates will prevent agglomeration into large hydrate aggregates and plugs. Hence, it is believed that the method is applicable for differentiating oils with regard to hydrate morphology.
Keywords: Hydrates; Crude oil emulsions; Phase inversion; Wettability;

Measurement of interfacial tension (IFT) using the micropipet technique involves the solid–liquid interface. At equilibrium, oil–water interfacial tension is determined from the interface curvature and the critical pressure, according to the Young–Laplace equation. This paper aims to examine the possible contribution of the solid–liquid interface on IFT measurement. Three different experimental configurations are used to examine the sought effect. The three configurations are straight, concentric, and tapered pipets with diameters ranging from 2.5 to 30 μm. For all three configurations, the critical pressure is found to depend only on the pipet diameter. However, when the Young–Laplace equation is applied to determine the IFT, a significant error was noticed at small pipet diameters. The IFT error was described by an exponential function whose asymptote approached the independently determined IFT value with a sufficiently large pipet diameter. The IFT error is anticipated to arise from the layerlike effect of an “ultrastructured” liquid near the solid surface. The solid-induced error in oil–water IFT is noted to fade away at lowered IFT by the addition of surfactant.
Keywords: Interfacial tension; Micropipet; Critical pressure; Oil–water interface;

Axisymmetric spreading of a liquid drop covered with an insoluble surfactant monolayer on a smooth solid substrate is numerically investigated. As the drop spreads, the adsorbed surfactant molecules are constantly redistributed along the air–liquid interface by convection and diffusion, leading to nonuniformities in surface tension along the interface. The resulting Marangoni stresses affect the spreading rate by altering the surface flow and the drop shape. In addition, surfactant accumulation in the vicinity of the moving contact line affects the spreading rate by altering the balance of line forces. Two different models for the constitutive relation at the moving contact line are used, in conjunction with a surface equation of state based on the Frumkin adsorption framework, to probe the surfactant influence. The coupled evolution equations for the drop shape and monolayer concentration profile are integrated using a pseudospectral method to determine the rate of surfactant-assisted spreading over a wide range of the dimensionless parameters governing the spreading process. The insoluble monolayer enhances spreading through two mechanisms; a reduction in the equilibrium contact angle, and an increase in the magnitude of the radial pressure gradient within the drop due to the formation of positive surface curvature near the moving contact line. Both mechanisms are driven by the accumulation of surfactant at the contact line due to surface convection. Although the Marangoni stresses induced at the air–liquid interface reduce the rate of spreading during the initial stages of spreading, their retarding effect is overwhelmed by the favorable effects of the aforementioned mechanisms to lead to an overall enhancement in the rate of spreading in most cases. The spreading rate is found to be higher for bulkier surfactants with stronger repulsive interactions. With the exception of monolayers with strong cohesive interactions which tend to retard the spreading process, the overall effect of an insoluble monolayer is to increase the rate of drop spreading. Simulation results for small Bond numbers indicate the existence of a power-law region for the time-dependence of the basal radius of the drop, consistent with experimental measurements.
Keywords: Surfactants; Drops; Spreading; Marangoni Stress;

This study develops and tests an experimental method to monitor in situ the dynamic spreading of individual toner particles on model substrates during heating, to simulate on laboratory scale the fusing sub-processes occurring in electrophotographic printing of paper. Real toner particles of cyan, magenta, yellow and black are transformed to perfect spheres by a temperature pre-treatment, then applied to the substrate, either high-energy clean glass or low-energy hydrophobised glass, and heated at rates up to 50 °C/min. The subsequent spreading as a function of time (and temperature) is recorded by an optical microscope and CCD camera mounted above the substrate, with the measured drop covering area used to calculate the corresponding toner–substrate–air contact angle. On the hydrophobic substrate the spreading is limited and equal for all four colours, while the substantially greater spreading on the hydrophilic substrate is accompanied by significant differences between the toner colours. In particular, the cyan and black toners are found to spread to almost twice the extent of the yellow particles. The dynamic spreading behaviour is interpreted in terms of complementary measurements of substrate and toner surface energy components and bulk toner rheology, and a simple empirical relation is proposed that fits very well the measurements for all toner and substrate types tested. In particular, the spreading relation is found to be determined only by the toner surface energy and its equilibrium contact angle, with no explicit dependence on toner viscosity.
Keywords: Electrophotography; Toner; Wetting; Spreading;

Coating of an inclined plane in the presence of insoluble surfactant by B.D. Edmonstone; O.K. Matar; R.V. Craster (261-272).
We consider the flow of a thin liquid film coating an inclined plane in the presence of an insoluble surfactant. A fully non-linear two-dimensional system of governing equations is formulated using lubrication theory to describe the dynamics. Numerical simulations of this system highlight a fingering instability present at the main fluid front and elucidate the role of surfactant in the destabilizing mechanism. A full parametric study is undertaken which reveals the dependence of the fingering characteristics on system parameters. Numerical solutions at low angles of inclination are also obtained in order to illustrate the connection between gravitationally driven fingering and the instability induced by surfactant on a flat substrate. The similarities and differences between the destabilizing mechanisms in each case are discussed.
Keywords: Thin liquid film; Surfactant; Instability; Fingering;

Solubilization of dodecanol–limonene binary oil mixtures has been studied in saturated Winsor type I and III sodium dihexyl sulfosuccinate microemulsions. The systems showed different oil solubilization behavior below and above dodecanol volume fraction 0.2. Below 0.2 dodecanol volume fraction regular Winsor type microemulsions formed. The oil solubilization was characterized in this concentration range by the optimum salinity and the maximum characteristic length. Dodecanol showed Langmuirian-type surface excess adsorption at the vicinity of the surfactant layer. Variation of the optimum salinity and middle phase characteristic length with increasing dodecanol concentration could be linked to changes in the dodecanol surface excess. These relationships were used to develop new mathematical models for the optimum salinity and characteristic length as a function of oil phase composition. Both models yield excellent agreement with the data. Above dodecanol volume fraction 0.2 regular Winsor type III microemulsions are not formed. Therefore our new models are not applicable in this concentration range.
Keywords: Microemulsion; Phase behavior; Anionic surfactant; Polar and nonpolar oil mixture; Solubilization; Curvature; Bending rigidity; Optimum salinity; Characteristic length; Synergism;

Pyrene fluorescence and Krafft temperature measurements have been carried out for various combinations of cationic gemini ( m - 2 - m ) with zwitterionic surfactants by changing the length of the hydrophobic tail over the whole mixing range. The results have been evaluated by using the regular solution theory. All the mixtures of cationic gemini + zwitterionic surfactants indicate the presence of synergistic interactions which largely decrease at higher hydrophobicity of both components. A greater amount of gemini component in the mixed micelles induces stronger synergism which reduces with the increase in the length of hydrophobic tail of the gemini component. The Krafft temperature measurements also indicate the presence of strong synergistic interactions, which decrease with increase in the length of hydrophobic tail of both components.
Keywords: Fluorescence measurements; Mixed micelles; Synergistic interactions; Gemini surfactant; Zwitterionic surfactant;

Shear-induced permeation and fusion of lipid vesicles by Anne-Laure Bernard; Marie-Alice Guedeau-Boudeville; Valérie Marchi-Artzner; Thadeus Gulik-Krzywicki; Jean-Marc di Meglio; Ludovic Jullien (298-306).
This paper introduces a novel approach to controlling membrane permeability in free unilamellar vesicles using shearing in the presence of a detergent with a large head-group to tune pore formation. Such shear-induced permeation could offer a simple means of postencapsulating bioactive molecules to prepare vesicle vectors for drug delivery. Using UV absorption, fluorescence emission, dynamic light scattering, and electron microscopy, we investigated the membrane permeability and the morphology of unilamellar lipid vesicles (diameter in the range 50–400 nm) subjected to a shear stress in the presence of a small amount of nonionic surfactant (Brij 76). Shear-induced leakage and fusion events were observed. We analyzed the significance of the vesicle size, the shear rate, and the surfactant-to-lipid ratio for the observed phenomena. The present approach is evaluated for postloading of preformed vesicles.
Keywords: Vesicle; Shearing; Permeation; Surfactant; Fusion; Encapsulation;

Condensation properties of vesicles formed from an amphiphilic N-phosphorylamino acid by Hai-Yan Wang; Yan-Mei Li; Yi Xiao; Yu-Fen Zhao (307-311).
Stable unilamellar vesicles were formed in water under appropriate pH from dispersions of N-( O , O -di-n-hexadecyl)phosphorylalanine, an amphiphilic N-phosphorylamino acid. We found that condensation occurred in the vesicle solution after incubated at 40 °C, which may contribute to the stability of the vesicular system. Dipeptide derivative in the vesicle solution was identified by electrospray ionization mass spectrometry (ESI-MS), which suggests the peptide formation without any coupling reagents. Hydrogen bond and electrostatic interactions play important roles in the process of vesicle formation, while the suitable orientation and packing of the amphiphilic molecules at the vesicle/water interface together with certain conformational freedom in the vesicular bilayer are considered to be most favorable for the condensation in ordered systems as vesicles.
Keywords: Condensation; Vesicle; Amphiphilic N-phosphorylamino acid; Peptide; ESI-MS;

Stabilizing effect of low concentrations of urea on reverse micelles by Asima Chakraborty; Munna Sarkar; Soumen Basak (312-317).
Urea is a well-known destabilizing agent for biopolymers like proteins and molecular aggregates like micelles and reverse micelles. Several theories have been proposed to explain the destabilizing/denaturing effect of urea. In this work, we present evidence for a stabilizing effect of a low concentration (<1 M) of urea incorporated in the central pool of AOT/n-heptane/water reverse micelles. Static light-scattering experiments were performed to measure ( w 0 ) cr —the molar ratio of water to AOT beyond which the micelles become unstable—as a function of the concentration of urea in the central water pool. The stabilizing effect of urea is reflected in an increase in the value of ( w 0 ) cr at low urea concentrations over that in the absence of urea. Dynamic light-scattering experiments show that the hydrodynamic radius of the micelles is smaller at low urea concentrations (<1 M) than in the absence of urea. Size-distribution analysis shows that for w 0 = 20 the microemulsion containing 0.5 M urea in its pool is significantly more monodisperse than that containing no urea. Temperature-dependent studies in the range 15–65 °C indicate that the magnitude of this stabilizing effect decreases with increasing temperature, vanishing at temperatures higher than 65 °C. A model is proposed to explain the above results.
Keywords: AOT; Urea; Static light scattering; Dynamic light scattering; Stabilization; Renaturing effect;

Liquid phase deposition (LPD) of silica from soluble silicates has been performed in the presence of dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS) and sodium dodecylbenzyl sulfate (SDBS). The morphology of the silica varies between semi-ordered uniform spheres to low porosity agglomerates, with the choice and concentration of the surfactants. The agglomerate structures depend on the charge of the surfactant (and hence the retention of micelles under acidic LPD conditions and/or the ionic character of the surfactant solution), the critical micelle concentration (as compared to the concentration of the silica precursor), and the ionic strength of the solution. The application of surfactant micelles as templates for LPD silica is counter to a previous proposal that suggested the ionic strength of the silicate solution would cause the collapse of the ionic vesicles. The size of spherical silica particles is controlled by the relative concentration of the surfactant and the LPD precursor.
Keywords: Silica; Micelle; Liquid phase deposition; Surfactant; Spheres; Agglomerate;

The water/n-octane/octyl-β-d-glucoside/1-octanol system: Phase diagrams and phase properties by Johan Reimer; Markus Nilsson; Marta Álvarez Chamorro; Olle Söderman (326-332).
The partial phase diagram of D2O/n-octyl-β-d-alkyl-glucoside(C8G1)/n-octane has been determined at T = 25 ° C . The diagram contains a funnel-shaped micellar phase originating from the water corner of the phase diagram D2O/C8G1 with the stem forming a narrow three-phase region, in which the three phases in equilibrium are two microemulsions of similar composition and an excess oil phase. The microemulsions have been characterized with NMR self-diffusion measurements. At high surfactant concentration and no or low n-octane content, branched micelles exist. As the n-octane content is increased, discrete micelles are formed. Upon further addition of n-octane, the phase separation into two microemulsion phases is induced. Possible mechanisms causing the phase separation are discussed. The phase diagram of D2O/(C8G1)/1-octanol has been determined at 25 °C. Ten different phase regions were identified. The phases have been characterized with SAXS and deuterium heavy water NMR, and the swelling of the lamellar phase was investigated with SAXS.
Keywords: Octyl monoglucoside; Sugar surfactant; Microemulsion; Phase diagram; Microstructure; Phase separation; NMR diffusometry; SAXS;

Thermodynamics of micellization for partially fluorinated cationic gemini surfactants and related single–chain surfactants in aqueous solution by Yajuan Li; Peixun Li; Jinben Wang; Yilin Wang; Haike Yan; Chuchuan Dong; Robert K. Thomas (333-337).
A series of partially fluorinated cationic gemini surfactants and their corresponding monomeric surfactants have been studied by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and enthalpy of micellization ( Δ H mic ) were obtained from calorimetric curves. The CMCs of the gemini surfactants are much lower than those of the corresponding monomeric surfactants and decrease with an increase in the number of fluorine atoms on the hydrophobic chain. The micellization of partially fluorinated cationic gemini surfactants is much more exothermic than that of the corresponding monomeric surfactants. Because of the incompatibility of hydrocarbon spacer and partially fluorinated chain, Δ H mic values of the surfactants with a C6 spacer are more negative than those of the surfactants with a C12 spacer. The variations in the architecture of the fluorocarbon chain segments may be the reason of the irregularities in the change of Δ H mic for the gemini surfactants. Moreover, the contribution of the enthalpy generally increases with an increase in the number of fluorine atoms.
Keywords: Critical micelle concentration; Enthalpy; Microcalorimetry; Gemini surfactants; Fluorinated surfactants;

A novel separation technique based on simultaneous application of AC dielectrophoresis and preferential transport through a semipermeable hydrophilic membrane is proposed for separation of small amounts of emulsified water droplets from a water-in-oil emulsion. Embedding an array of parallel microelectrodes on a membrane matrix, followed by application of an AC potential to these electrodes, can result in capturing the water droplets onto the membranes from the emulsion during a crossflow filtration process. The present paper describes the theoretical principles underlying such a process, and describes a simple mathematical framework based on trajectory analysis for assessing the separation efficiency of such a technique. The results indicate that superimposition of an AC dielectrophoretic field can significantly enhance the preferential transport of the emulsified water through the membrane in a crossflow filtration device. This can lead to a highly efficient continuous separation process for dilute emulsions.
Keywords: AC electrokinetics; Dielectrophoresis; Water-in-oil emulsion; Membrane separation; Trajectory analysis; Parallel electrodes;

Electrorheology (ER) is the name given to a set of phenomena related to the significant changes experienced by the rheological properties of certain fluids and suspensions upon application of external electric fields. It is mostly explained in terms of the formation of particle aggregates as a consequence of field-induced particle–particle interactions. In this work, we explore such structures by investigating the changes in optical absorbance of hematite/silicone oil suspensions associated to the application of an electric field. We have studied the effect of particle concentration, ϕ, electric field strength, E 0 , and viscosity, η m , of the liquid medium on the absorbance–time behavior of the suspensions. Photographs of the electrified suspensions helped in elucidating the structures formed. At low ϕ values, the absorbance A of electrified suspensions dramatically decreases with time until a constant plateau is reached. The absorbance fall is faster the higher the field, although at long times curves corresponding to different fields tend to merge. In these dilute suspensions particles are observed to migrate toward the electrodes thus clarifying the medium and reducing A. When the concentration of particles is increased, fibrils stretching between the electrodes can be observed in addition to particle deposition on them, as long as the field is kept low. At high fields, migration of the particles to the electrodes occurs whatever the volume fraction. Two mechanisms producing particle–particle interactions are suggested by these data: the conductivity mismatch between the particles and the medium brings about an interfacial or Maxwell–Wagner polarization of the particles; in addition, solids can acquire a net charge provoked by injection from the electrodes. The first mechanism will produce attractive dipole–dipole interactions and hence columns or fibrils. The second one should lead to electrophoretic migration. Structural observations suggest that the latter predominates at high fields. If the viscosity of the fluid phase is increased, the critical electric field values separating both regimes also increase: the electrophoretic motion is hindered and the particle–particle aggregation is enhanced.
Keywords: Electrorheological fluids; Particle aggregation; Polarization; Electrode deposition; Electrophoresis; Optical absorbance of suspensions;

Facile one-pot synthesis of gold nanoparticles stabilized with bifunctional amino/siloxy ligands by Haoguo Zhu; Zhengwei Pan; Edward W. Hagaman; Chengdu Liang; Steven H. Overbury; Sheng Dai (360-365).
A method for the direct one-pot synthesis of amine-stabilized gold nanoparticles using 3-(trimethoxysilylpropyl)diethylenetriamine (TMSP dien) is described. The amine groups of this bifunctional molecule act as a stabilizer for gold nanoparticles as they form by reduction of HAuCl4. Highly stable gold nanoparticles with sizes tunable between 8 and 20 nm can be readily obtained. This method is quite simple to implement and environmentally benign as there is no need to add an external reducing reagent. The incorporated siloxy functionality was subsequently used to form a silica shell around the gold particle.
Keywords: Synthesis; Gold; Nanoparticles; Silica; Core–shell;

Platinum states in citrate sols by EXAFS by Chia-Shiang Lin; Maksudur R. Khan; Shawn D. Lin (366-369).
Platinum sols have been prepared by citrate reduction in the temperature range of 343–363 K. The Pt state in the solution was examined by EXAFS (extended X-ray absorption fine-structure spectroscopy). It did not show any Pt―Pt bonding, a characteristic for reduced Pt sols. EXAFS model fitting further proved the presence of Pt―O with 4 oxygen neighbors, which suggests a tetraplanar coordination configuration. The possibility of neighboring Pt sharing oxygen ligand or the formation of PtO x is rejected by EXAFS model fitting. Citrate was found to be the most likely ligand to orient its oxygen end toward a charged Pt center. Thus we have revealed that the citrate treatment at this temperature range was clearly insufficient to reduce H2PtCl6(aq). Neither an extended period of reaction time nor an excess citrate reduced the Pt precursor. It is therefore highly recommended that the citrate sols should be carefully prepared and used.
Keywords: Pt sols; Citrate; EXAFS;

by Arthur Hubbard (370).