Journal of Colloid And Interface Science (v.292, #1)


Defluoridation of drinking water using activated titanium rich bauxite by Nigamananda Das; Pragyan Pattanaik; Rita Das (1-10).
The potential of thermally activated titanium rich bauxite (TRB) for adsorptive removal of excess fluoride from drinking water was examined. Adsorption with respect to variation of pH, adsorbent dose, initial fluoride concentration, presence of interfering ions and heat treatment were investigated by batch equilibrium experiments. Thermal activation at moderate temperatures (300–450 °C) greatly increased the adsorption capacity of TRB. The rate of adsorption was rapid and maximum level was attained within 90 min. The uptake of fluoride increased with increasing pH, reached to a maximum at pH 5.5–6.5 and thereafter decreased. The adsorption kinetics was found to follow first order rate expression and the experimental equilibrium adsorption data fitted reasonably well to both Langmuir and Freundlich isotherm models. The presence of common interfering ions in drinking water did not greatly affect the uptake of fluoride from aqueous solution indicating F specific sorption behaviour of TRB. Nearly complete desorption of adsorbed fluoride from loaded bauxite was achieved by treating with aqueous solutions of pH ⩾ 11.1 ([NaOH] ⩾ 0.015 mol/dm3).
Keywords: Defluoridation; Adsoption; Bauxite; Thermal activation; Desorption;

Surface modification of sepiolite with quaternary amines by Jovan Lemić; Magdalena Tomašević-Čanović; Mirjana Djuričić; Tanja Stanić (11-19).
This study was aimed at elucidating the mechanism of adsorption of quaternary amines, stearyldimethylbenzylammoniumchloride (SDBAC), as monomers and as micelles, distearyldimethylammoniumchloride (DDAC) and hexadecyltrimethylammoniumchloride (HTAC), on the surface of sepiolite. The adsorption capacity for these surfactants onto sepiolite, calculated by fitting the experimental data to the Langmuir–Freundlich equation, were 324% (SDBAC), 278% (DDAC), and 258% (HTAC) of the cation exchange capacity of sepiolite. The Mg2+ ions released during the exchange process were higher than the CEC value of sepiolite because of the simultaneous dissolution of the present minerals. The water adsorption decreased with the increasing surfactant loading up to 250 mmol/kg of sepiolite, which can be ascribed to an intensification of the hydrophobic properties. With loadings above 250 mmol/kg, the water adsorption increases. Simple kinetic analysis of SDBAC adsorption was performed. The properties and the type of bonding between the surfactants and sepiolite were investigated by DT, TG, and DTG analysis. During the gradual heating in oxidizing atmosphere, the adsorbed organic material is oxidized giving rise to significant exothermic peaks. The exothermic peak temperatures in the range 200–500 °C depended on the surfactant loadings and provided evidence of the formation of multilayers on the sepiolite surface.
Keywords: Sepiolite; Quaternary amine; Surface modification; Adsorption isotherm; Thermal analysis;

Sequential heavy metals extraction from polluted solids: Influence of sulfate overconcentration by Sonia Pénilla; François Bordas; Jean-Claude Bollinger (20-28).
The effect of sulfate on the chemical partitioning of Cu, Cd, and Pb in solid phases was assessed in this study. Modified BCR sequential extraction, speeded up by focused ultrasound, was systematically applied to various mixtures of typical geochemical solid phases (an artificial goethite spiked with Cu, Cd, and Pb and natural clays), with or without the addition of calcium sulfate. Sulfate was added so that three different concentrations were found in sequential extracts: 0.5, 1, and 1.5 g/L of sulfate. First, the results suggested that the goethite-surface adsorption sites for sulfate are limited. Then, significant changes in Cd and Pb fractionation were observed in the presence of sulfate, whereas Cu remained strongly adsorbed on the solid phases. The main modifications observed in all the studied samples were a decrease in metal amounts in the first three fractions to the profit of an increase in the residual fraction. These results suggested that the adsorption of metals onto the studied solids was enhanced by the presence of sulfate. From these considerations, some hypotheses are advanced to describe the behavior of Cu, Cd, and Pb and their adsorption mechanisms on solid phases in sulfate-rich systems. The presence of sulfate in mixtures of typical geochemical solid phases (synthetic goethite spiked with Cu, Cd, and Pb mixed with clay) involves significant changes in the metals fractionation.Display Omitted
Keywords: Sequential extraction; Cu; Cd; Pb; Clays; Goethite; Sulfate;

The build-up of multilayers constructed from polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) under different pH conditions was continuously monitored using the quartz crystal microbalance with dissipation. The adsorbed amount of polymer as well as the amount of coupled water was determined. Furthermore, from dissipation measurements, it was possible to determine the visco-elastic properties of the adsorbed multilayer. These properties were highly dependent on the polyelectrolyte present in the outermost layer. The multilayer was far more rigid and elastic with PAA as the outermost layer. Furthermore, a link has been established between the conformability or rigidity of a multilayer covered surface and the adhesion between such surfaces. Adhesion measurements using the atomic force microscope showed a greater pull-off force when the more viscous PAH was present in the outermost layer. Visco-elastic properties of polyelectrolyte multilayer films were studied as a function of pH. A rigid film was formed when polyacrylic acid capped the film and a more open film was formed when polyallylamine hydrochloride capped the film.Display Omitted
Keywords: Polyelectrolyte; Multilayer; Quartz crystal microbalance with dissipation; Atomic force microscope; Adhesion; Visco-elastic properties;

Polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) have been used to build up polyelectrolyte multilayers (PEM) on wood fibres and on silicon oxide surfaces, under various pH conditions. Consecutive adsorption onto silicon oxide surfaces of PAH and PAA were studied using stagnation point adsorption reflectometry, and the results showed a steady build-up of multilayers. Furthermore, by altering pH, the build-up of the multilayer could be made either linear or exponential in terms of adsorbed amount. Nitrogen analysis of sheets prepared from modified fibres showed that the adsorbed amount of PAH increased throughout PEM build-up, the amount of increase depending on pH during adsorption. Strength measurements of the sheets, i.e., stress at break and strain at break, showed significant improvements ranging from 60 to 200%, depending on both pH during adsorption and type of polyelectrolyte in the outer layer. A good correlation between the adsorbed amount of PAH and the improved strength properties of the paper was also found. The polyelectrolyte multilayer technique has been used to modify wood fibres. It was shown that pH during the film formation and the type of polyelectrolyte used in the outer layer did affect the strength properties of sheets made from modified fibres.Display Omitted
Keywords: Polyelectrolyte; Multilayer; Adsorption; Optical reflectometry; Refractive index increment; Paper strength; Tensile index; Strain at break; Fibre bonding;

A rheological investigation of the self-assembly and adsorption behavior of a surfactant salt by Genhai G. Liang; Brian S. Hawkett; Roger I. Tanner (46-53).
The properties of a surfactant salt obtained by neutralizing oleic acid with an ethoxylated stearylamine were determined in blends of water and propylene glycol. The adsorption of this surfactant salt onto the surface of a commercial TiO2 dispersed in blends of water and propylene glycol was studied using a rheometer. At low propylene glycol content the dispersions exhibited Newtonian behavior, but became shear-thinning fluids with high viscosity at propylene glycol contents above a critical concentration. The observed behavior is consistent with a model involving a surfactant bilayer below the critical point, moving to a monolayer above the critical point. The high viscosity above the critical point is generated by reversible flocculation via hydrophobic forces. The viscosity of the dispersion flocculated by the hydrophobic forces was found to be much higher than that caused by flocculation via van der Waals forces in the absence of surfactant. Changing both the total concentration of the surfactant in the dispersion and the dispersion temperature resulted in a reversible transition between the bilayer and the monolayer. Although the surfactant was always above its critical micelle concentration (CMC) the amount on the particle surface varied appreciably with both propylene glycol and surfactant concentration. Without surfactant particles at their point of zero charge interacted via van der Waals forces to yield a median viscosity. The viscosity had a maximum, due to hydrophobic forces, at monolayer coverage while at bilayer coverage and beyond the viscosity was low and Newtonian.Display Omitted
Keywords: Reversible flocculation; Hydrophobic force; Bilayer; Monolayer; Titanium dioxide; Surfactant salt; Dispersion; Viscosity; Cloud point; CMC;

The process of 1,10-phenanthroline adsorption at pH 5 on Ca-montmorillonite, activated carbon, and silica gel mixtures was studied as a function of the equilibrium concentration and the composition of the mixture. A model is presented for determining adsorption of the main component (the variable in the system) of the mixture, based on the thermodynamic concept of apparent and partial quantities, in combination with an equation representing total adsorption of the other two adsorbents as a function of the weight fraction of one of them and introducing the concept of mean total adsorption. The partial specific adsorption of orthophenanthroline (OP) on Ca-montmorillonite is strongly influenced by the presence of activated carbon and silica gel. Owing to a phenomenon of cationic exchange, adsorption on the clay is higher at low proportions in the mixture, but the strong effect of carbon and silica gel becomes apparent at increasing amounts of clay in the mixture. The partial specific adsorption of orthophenanthroline on activated carbon and silica gel was determined using a total adsorption equation for the two adsorbents as a function of the weight fraction of one of them and shows behavior inverse to that of adsorption on clay. A model for adsorption of 1,10-phenanthroline (OP) on a ternary mixture is presented, to determine the adsorption in the main component of the mixture, based on the thermodynamic concept of the apparent and partial quantities in combination with an equation representing total adsorption of the other two adsorbents as a function of the weight fraction of one of them and introducing the concept of mean total adsorption.Display Omitted
Keywords: Activated carbon; Apparent specific adsorption; Mixtures; Montmorillonite; Partial specific adsorption; 1,10-Phenanthroline; Silica gel;

Water-based latex dispersions by Catherine Boissier; Jan-Erik Löfroth; Magnus Nydén; Staffan Schantz (63-70).
The equilibrium residence times of the nonionic surfactant nonylphenol ethoxylate (NP100) in a latex dispersion were determined using NMR diffusometry. At 16% w/w particle concentration and 0.12, 0.43 and 0.81% w/w NP100, the residence times of the surfactant were 0.16, 1.02 and 4.73 s in solution ( τ A ) and 0.3, 0.37 and 0.61 s on the surface of the particles ( τ B ), respectively. At even higher particle concentration (>45% w/w), τ A and τ B were 1.47 and 2.2 s. Calculating the number of collisions that ought to result in adsorbed species, at 16% w/w, only 2, 5 and 2 (corresponding to 0.12, 0.43 and 0.81% w/w NP100) resulted in adsorption, whereas at >45% w/w, only 12 resulted in adsorption, which suggested that the surfactant was irreversibly adsorbed on the particles. The small increase in collision frequency with increased particle concentration could be a result of a diffusion controlled adsorption, while an energy barrier for desorption controlled the overall exchange dynamics in the dispersion. The slow dynamics in the dispersion was controlled, mainly by the nonylphenol group, which gave NP100 a strong preference to surfaces. In addition, the chain length of the poly(ethylene glycol) (PEG) group changed the solution behavior from being that of a typical surfactant to that of a polymer.
Keywords: Dispersion; Exchange dynamics; Nonionic surfactant; Residence times; Self-diffusion; Relaxation; NMR;

This paper compares the adsorption behavior of 1,4-phenylene diisocyanide (PDI) and terephthalic acid (TA) on Ni, Cu and Pt surfaces. Following competitive adsorption from two-component equimolar solutions of PDI and TA, chemical analysis by XPS confirmed the preferential adsorption of PDI over TA on Ni and Cu. The ability to form “chemically sticky” surfaces on Ni, Cu and Pt surfaces by self-assembly into organized organic thin films (OOTFs) was also investigated. PM-IRRAS analysis revealed a tendency for PDI to bond in a terminal fashion through one isocyanide group, on both Ni and Cu. In contrast, PDI adsorbed in a flat configuration on Pt. Chemically sticky OOTFs have potential for utilization as coupling agents to achieve a high cross-link density and enhance stress transfer between the nanoclusters and the organic matrix molecules in metal-nanocluster-filled polymer matrix nanocomposites. The results of this work indicate that 1,4-phenylene diisocyanide is a suitable choice as a coupling agent for metal nanoclusters of Ni and Cu.
Keywords: Self-assembly; XPS; PM-IRRAS; Terephthalic acid; Phenylene diisocyanide;

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration. Addition of monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones, followed by a structural reorganization of the PEO/SDS complex.Display Omitted
Keywords: Poly(ethylene oxide); SDS; Dynamic light scattering; Isothermal titration calorimetry; Aggregation;

A capillary wave technique was used to study the viscoelastic properties of floating polyelectrolyte multilayers of (PSS/PAH) n at the air–water interface. Oppositely charged polyelectrolyte layers were adsorbed onto two different Langmuir monolayers, either the lipid dimethyldioctadecylammonium bromide (DODAB) or the block copolymer poly(styrene-b-sodium acrylate) (PS-b-PAA). The results allow to propose a schematic representation of the multilayers in three zones: Zone I as a precursor, representing the adhesion between the Langmuir monolayer and the bulk polyelectrolyte multilayer. Zone II forms a bulk or core zone of the multilayer. Zone III as an outer zone in direct contact with the aqueous phase. The results show an increase of the elasticity after the formation of four polyelectrolyte layers accompanied by an apparent negative viscosity. This behaviour was interpreted as a translation of elasticity dominance from zone I to zone II. The Young modulus of seven layers was in the same order of magnitude as observed for planar polyelectrolyte multilayer films.
Keywords: Polyelectrolyte multilayers; Langmuir monolayer; Capillary wave;

In this study we modified the surface of silica nanoparticles with methyl methacrylate by UV-induced graft polymerization. It is a surface-initiated polymerization reaction induced by ultraviolet irradiation. The resulting organic–inorganic nanocomposites were near-monodisperse and fabricated without homopolymerization of the monomer. Substantial increase in mean particle size was observed by SEM image analysis after UV-induced grafting of methyl methacrylate onto pure silica particles. FT-Raman spectroscopy and X-ray photoelectron spectroscopy studies of these materials revealed the successful grafting of methyl methacrylate onto the silica surface. The formation of a covalent bond between the grafted PMMA chains and silica surface was indicated by FT-Raman spectra. Thermogravimetric analysis of the PMMA-grafted silica particles indicated the polymer contents in good agreement with SEM photographs.In this study we modified the surface of silica nanoparticles with methyl methacrylate by UV-induced graft polymerization. It is a surface-initiated polymerization reaction induced by ultraviolet irradiation. The resulting organic–inorganic nanocomposites were near-monodisperse and fabricated without homopolymerization of the monomer. Substantial increase in mean particle size was observed by SEM image analysis after UV-induced grafting of methyl methacrylate onto pure silica particles. FT-Raman spectroscopy and X-ray photoelectron spectroscopy studies of these materials revealed the successful grafting of methyl methacrylate onto the silica surface. The formation of a covalent bond between the grafted PMMA chains and silica surface was indicated by FT-Raman spectra. Thermogravimetric analysis of the PMMA-grafted silica particles indicated the polymer contents in good agreement with SEM photographs.
Keywords: Silica nanoparticles; Surface modification; UV-induced graft polymerization; PMMA;

Collagen adsorption and structure on polymer surfaces observed by atomic force microscopy by Sara E. Woodcock; William C. Johnson; Zhan Chen (99-107).
The structure and adsorption patterns of type I and type III collagen were imaged on various polymer substrates with atomic force microscopy. Type I collagen had higher adsorption on polystyrene than on a series of polymethacrylates and formed a network of tightly, interwoven strands. Upon adsorption to different polymethacrylates, with varying side chain lengths, the collagen molecules formed long, branching fibrils. Types I and III collagen had different adsorption patterns, in some cases, on the identical substrate material. For example, instead of forming a tightly packed network, type III forms long, branching fibers on the polystyrene surface. On other materials, such as poly(n-butyl methacrylate), the two types of collagen showed similar adsorption pattern and structure. Adsorbed collagen was also imaged on various blends of polystyrene and polymethacrylates to determine how the polymer surface chemical structure and surface topography mediates protein adsorption.

Covalently bonded WO3/polyvinylimidazole (C-WO3/PVI) core-shell microspheres in sizes of 250 nm were prepared. The microstructures of C-WO3/PVI core-shell microspheres were characterized by TEM, IR, and XRD. It is found that the chemical and thermal stabilities of C-WO3/PVI core-shell microspheres are higher than those of pure WO3 nanoparticles and noncovalently bonded WO3/polyvinylimidazole (NC-WO3/PVI) core-shell microspheres. This is attributed to the strengthened interaction of the WO3 nanoparticle core and the PVI shell resulting from the interaction of covalent bonds. The electrochromic device made by the C-WO3/PVI core-shell microspheres was studied. It is suggested that the C-WO3/PVI core-shell microspheres exhibit better electrochromic properties than pure WO3 nanoparticles or NC-WO3/PVI core-shell microspheres.
Keywords: Covalently bonded; Core-shell microspheres; WO3; Electrochromic;

Colloidal dispersions of rhodium (Rh) nanoparticles have been synthesized by the reduction of Rh ions (III) in high-temperature and high-pressure water, ethanol, or water–ethanol mixture under the existence of the protective polymer of poly(N-vinyl-2-pyrrolidone). The possibility of the regulation of the particle size and size distribution has been tested under several solvents at various temperatures and pressures. At 473 K and 25 MPa, particularly, concentrated colloidal dispersions of Rh particles of 2.5 ± 0.5 nm were synthesized from the ionic solution of ethanol ([Rh] = 15 mM) within a few seconds. Dilute colloidal dispersions of Rh particles were also synthesized from the dilute ionic solution ([Rh] = 1.5 mM) with a diameter of 2.0 ± 0.4 nm . From the water solution, Rh particles tended to form aggregates, especially for the lower concentration solution. In the case of solutions in water and ethanol mixture, the average diameter of Rh particles tended to be larger than in ethanol solution, and their distribution became broad. TEM image and size distribution of the Rh colloidal solutions produced from 15 mM Rh and 15 g dm−3 PVP with a flow rate of 3 cm3 min−1 at 25 MPa and 200 °C in the mixture of water and ethanol solvent.Display Omitted
Keywords: Colloidal dispersions; Rhodium nanoparticles; Supercritical fluids; High temperature and high pressure; Continuous synthesis; EXAFS;

Preparation and characterization of aqueous colloids of Pt–Ru nanoparticles by Yuzuru Shimazaki; Yoshio Kobayashi; Shinji Yamada; Takao Miwa; Mikio Konno (122-126).
A synthetic method for platinum–ruthenium (PtRu) nanoparticles in aqueous media is proposed. This method employs citric acid as a capping agent and NaBH4 as a reducing agent with the aid of pH control. The number-averaged size of the PtRu nanoparticles was ca. 2 nm. The crystal phase and chemical composition of the nanoparticles was investigated by X-ray diffraction measurement and scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis, which indicated that the nanoparticles mainly consisted of an alloy of Pt and Ru. Electrochemical measurement showed that the PtRu nanoparticles had catalytic activity for methanol oxidation. A synthetic method for platinum–ruthenium (PtRu) nanoparticles in aqueous media is proposed. This method employs citric acid as a capping agent and NaBH4 as a reducing agent with the aid of pH control.Display Omitted
Keywords: Nanoparticle; Colloid; Alloy; Direct methanol fuel cell; XRD; STEM-EDX;

Facile, alternative synthesis of lanthanum phosphate nanocrystals by ultrasonication by Suree S. Brown; Hee-Jung Im; Adam J. Rondinone; Sheng Dai (127-132).
Highly luminescent, rhabdophane (Ce0.33La0.66)PO4nH2O nanorods and nanoparticles were prepared in aqueous solutions by ultrasonication, at pH 1 and pH 12, respectively. Both nanorods (5 to 9 nm wide and several tens to several hundreds nm long) and nanoparticles (elongated, connected 5 nm particles) were as small and as uniform as products obtained from methods that utilize complexing agents or surfactants, only with no complexing agent. This method of synthesis by ultrasonication is a fast and simple method and it is expected to be applicable for the synthesis of other nanocrystalline lanthanide phosphates.
Keywords: Lanthanum phosphate; Nanocrystals; Nanorods; Ultrasonication;

Removal of aqueous ammonium with magnesium phosphates obtained from the ammonium-elimination of magnesium ammonium phosphate by Shigeru Sugiyama; Masahiko Yokoyama; Hisaaki Ishizuka; Ken-Ichiro Sotowa; Tahei Tomida; Naoya Shigemoto (133-138).
In order to recycle magnesium ammonium phosphate (MgNH4PO4⋅6H2O: MAP) obtained from MAP process, which is one of the attractive processes for removal of aqueous ammonium and phosphate from wastewater, ammonium elimination from MAP to magnesium phosphates and ammonium incorporation into the magnesium phosphates have been investigated in the present study. It is confirmed that magnesium hydrogen phosphate (MgHPO4) is favorably obtained from the ammonium elimination from MAP at temperatures greater than 353 K, although magnesium phosphate (Mg3(PO4)2) and magnesium pyrophosphate (Mg2P2O7) have been suggested as possible candidates. Based on the dissolution–precipitation mechanism for the removal of aqueous ammonium with magnesium phosphates, three magnesium phosphates were employed for the removal of aqueous ammonium. The order of the removal rate of the aqueous ammonium was MgHPO4  > Mg3(PO4)2  > Mg2P2O7, as expected from the solubility of those magnesium phosphates. The removability of the solid obtained from ammonium elimination of MAP is also confirmed. The present results show that MAP can be employed as an advanced material for the removal/recovery of ammonium, although it is generally accepted that an excess of MAP obtained from the wastewater treatment can be only used as a slow-acting fertilizer.
Keywords: Recycle; Aqueous ammonium; MAP; Magnesium phosphates; Removal/recovery process;

Effects of heat treatment on the aggregation and charging of Stöber-type silica by Motoyoshi Kobayashi; Michal Skarba; Paolo Galletto; Dusko Cakara; Michal Borkovec (139-147).
Colloidal silica is known to be stable at high salt concentrations and low pH, where silica is basically uncharged. This observation is in qualitative disagreement with the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO), which predicts rapid aggregation (or coagulation) under these conditions. This study reports a very different behaviour for Stöber-type silica heated at 800 °C, as these particles follow DLVO theory quantitatively. Unheated samples behave approximatively according to DLVO theory, but they show systematic deviations, in particular, featuring higher stability at low pH. The heat treatment also substantially modifies the charging properties, as heated particles show titratable surface charge densities in the range expected for the water–silica interface, while much higher charge densities are observed for the unheated samples. The electrophoretic mobilities, on the other hand, are hardly influenced by the heat treatment. We suspect that the suspension stability of the unheated particles is influenced by the presence of a hairy-layer of polysilicilic acid chains on the surface. Colloidal stability of Stöber-type silica, when heated at 800 °C, follows the predictions of DLVO theory quantitatively.Display Omitted
Keywords: Aggregation; Coagulation; Colloid stability; Silica; Heat treatment; DLVO theory;

A new method for gaining insight into the chemistry of drying mineral surfaces using ATR-FTIR by Catherine E. Dowding; Michael J. Borda; Martin V. Fey; Donald L. Sparks (148-151).
Although it is understood that the chemical environment at a drying surface is likely to be quite different from that at a fully hydrated surface, the difficulty of quantitative measurement has meant that this potentially crucial aspect of surface chemistry has gone largely overlooked. As a result, most of our understanding comes from measurement before and after drying, with a gray region of speculation in between. An interesting natural example is the paradoxical reduction of Mn oxides in moist soils as they dry, because drying is usually considered an oxidative process. This phenomenon indicates that important chemical changes are occurring during drying and an approach is needed to probe the chemistry of drying surfaces. Here we show the suitability of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy for real-time, in situ investigation of the drying solid–water interface, using the change in surface pH as an example. This was achieved by adsorbing thymol blue pH indicator (p K a = 1.65 ) onto a natural Mn-rich clay and observing the real-time pH change, which dropped from pH 5 to below pH 1.65 with the removal of free water from the surface.
Keywords: Thymol blue; Surface acidity; Drying; ATR-FTIR;

Study on the NaNp/Al2O3–SiO2 system by Paweł Maksimowski; Wincenty Skupiński (152-159).
Sodium naphthenide (NaNp) deposited on aluminosilicate gels yields systems that exhibit signals in EPR spectra, the multiplicity, g values, and intensities of which depend on the gel composition. The symmetrical singlets (I) of a g 1 value of 2.00 were assigned to the complexed naphthalene anion radicals on the centers occurring on the surfaces of the alumina and silica phases of the gels studied. Those radicals complexed on surface Lewis acid sites are of hard acid/hard base type according to Pearson theory, and are bonded to the sites by ionic forces. The asymmetrical singlets (II) described by the value g 2 = 1.95 were assigned to naphthalene anion radicals, which are complexed by the centers of the aluminosilicate phase of the gels studied. Those radicals, complexed on surface Lewis acid sites of this phase, are of soft acid/soft base type and are bonded to the sites by covalent bonds. The highest intensity of the singlet (I) was recorded for the system containing alumina gel. The highest intensity of the singlet (II) was recorded for the system containing Al2O3 in 30 wt%. The structures of Lewis acid sites on aluminosilicate phases are postulated to be different from those on alumina or silica phases. M–O–M[ ]–O–M [ ] is hard Pearson acid site; M = Si for silica and Al for alumina phases Al[ ]–O–Si–  ⇄  –Al–O–Si[ ]– [ ] acidic sites spread on aluminum and silicon atoms create soft Pearson acid sites on aluminosilicate phase EPR study of NaNp/Al2O3–SiO2 system established presence of Pearson hard and soft acid sites on alumina–silica gels.
Keywords: Alumina–silica; Sodium naphthenide; EPR; Soft and hard acid centers;

Studies of the surface charge of amorphous aluminosilicates using surface complexation models by Alejandra A. Jara; Sabine Goldberg; M.L. Mora (160-170).
Synthetic noncrystalline aluminosilicates with variable charge, similar to allophanes present naturally in volcanic soils, were studied. The surface charge behavior was determined by zero point charge (ZPC) measured by electrophoretic mobility (isoelectric points, IEP) and determined by potentiometric titration (point of zero salt effect, PZSE). The ZPC calculated by Parks model (ZPCc), compared with IEP values, showed that the aluminosilicate (AlSi) surface was slightly enriched by AlOH (34% Al2O3 and 66% SiO2) compared with the bulk composition (29% Al2O3 and 71% SiO2). For aluminosilicate coated with iron oxide (AlSiFe) the ZPCc (4.4) was lower than the IEP (8.46), showing that the surface composition is formed mainly from iron oxide. The PZSE values for AlSi and AlSiFe were 6.2 and 4.8, respectively. The differences between the IEP and PZSE are attributed to the formation of Si―O―Fe or Si―O―Al bonds; therefore, the reactivity of Fe and Al atoms was modified on the surface. Two mechanistic models, the constant capacitance model (CCM) and the triple layer model (TLM), using the program FITEQL 3.2 were able to describe the surface behavior of both synthetic aluminosilicates. The acidity constants determined using both models for the aluminosilicates showed differences with respect to pure oxide, mainly attributed to the presence of SiOH sites on the internal surfaces. The ionic strength showed a good relation with the parameters obtained using the CCM ( p K a 1 int , p K a 2 int and capacitance values) and the TLM ( p K a 1 int , p K a 2 int , p K Cl − int , p K K + int , and inner capacitance) for both aluminosilicates. However, the TLM was able to describe the acidity and complexation constants better since it considered the formation of the outer sphere complex between the background electrolyte and the surface. Then, the TLM makes it possible to describe real systems.
Keywords: Amorphous aluminosilicates; Surface charge; Zero point charge; Point of zero salt effect; Isoelectric point; Mechanistic models;

Describing chlorophenol sorption on variable-charge soil using the triple-layer model by Mara Cea; J.C. Seaman; Alejandra A. Jara; M.L. Mora; M.C. Diez (171-178).
The sorption of 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol by a variable-charge soil from southern Chile was studied in a series of batch experiments. The chlorophenol sorption behavior was evaluated as a function of pH (pH range 4–8) at two different ionic strengths, 0.01 and 0.1 M KCl (25 °C). Chlorophenol sorption depended on pH and a downward shift in the soil PZC was observed with increasing chlorophenol concentration. Chlorophenol sorption decreased with increasing pH, suggesting that the undissociated species is sorbed more readily and that electrostatic repulsion may inhibit partitioning as pH increases. Data from the sorption experiments were fitted by the triple-layer model, in which monodentate outer- and inner-sphere complexes were formed between deprotonated organic molecules and active sites on the variable-charge soil.
Keywords: Variable charge soil; Chlorophenol; Surface complexation modeling;

The preparation of different samples of vanadia supported on ultrastable zeolite ( VO x /USY ) is discussed. The samples were prepared in order to obtain highly dispersed V-species, avoiding the formation of crystalline vanadia and the destruction of the zeolite framework. Two methods were employed for preparing VO x /USY samples: an organic route using V(AcAc)3 and an inorganic route using NH4VO3. The characterization of the samples was performed with XRD, TPR, NH3-TPD, and N2 isotherms. From these results it is concluded that when VO x is supported on the surface of USY from acidic aqueous solution of ammonium metavanadate, the destruction of the zeolite framework is accomplished. For higher pH values in the impregnating solution, undesired V2O5 is formed on the USY surface. On the other hand, VO x /USY prepared from the organic precursor shows no destruction of the USY structure. In addition, highly dispersed VO x are formed, though for relatively high V loadings (6%) an obstruction of the zeolite windows takes place. The samples are tested as catalysts for gas phase dehydrogenation of n-butane to olefins. The catalysts prepared from NH4VO3 are almost inactive for the reaction. On the other hand, both samples prepared from V(AcAc)3 present initial conversion levels in the 8–12% range. However, the selectivity depends on the V loading, the catalysts with 6% loading being the most selective (75%). The catalytic patterns of the samples (activity and selectivity) are in agreement with the physicochemical features of the VO x /USY surface. NH3 thermal desorption for (a) zeolite USY, (b) and (c) VO x supported on USY surface. The addition of VO x to USY surface strongly decreases the acidity of the zeolite. This diminution renders the system more selective as a catalyst for the dehydrogenation of n-butane.Display Omitted
Keywords: Surface acidity; VO x ; USY zeolite; Dehydrogenation of n-butane; Supported vanadia;

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate by Md. Mufazzal Hossain; Toshiyuki Suzuki; Teiji Kato (186-194).
Surface phase behavior of n-hexadecyl phosphate (n-HDP) and its mixture with l-arginine (L-arg), which behaves as l-argininium cation (L-arg+) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at ⩽20 °C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at ⩾10 °C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the π – t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg+.
Keywords: Gibbs monolayers; Phase transition; Surface pressure; Surface tension; Brewster angle microscopy; n-Hexadecyl phosphate; l-Arginine;

Contact angle hysteresis (CAH) is critical to superhydrophobicity of a surface. This study proposes a free energy thermodynamic analysis (of a 2-D model surface) that significantly simplifies calculations of free energy barrier associated with CAH phenomena. A microtextured surface with pillar structure, typical of one used in experimental studies, is used as an example. We demonstrate that the predicted CAH and equilibrium contact angles are consistent with experimental observations and predictions of Wenzel's and Cassie's equations, respectively. We also establish a criterion for transition between noncomposite and composite wetting states. The results and methodology presented can potentially be used for designing superhydrophobic surfaces. This work proposes a simple thermodynamic analysis for calculations of free energy and free energy barrier associated with contact angle hysteresis of microtextured surfaces.Display Omitted
Keywords: Contact angle; Superhydrophobic; Wetting; Hysteresis; Ultrahydrophobic; Free energy analysis; Surface tension; Interfacial tension;

The role of adsorption compression in nanocapillarity by G.L. Aranovich; M.D. Donohue (202-209).
Influence of adsorption compression on nanocapillarity is discussed. Kelvin's equation for a compressible liquid is written in a form that takes into account not only Laplace's pressure, but also adsorption compression. This leads to a simple analytical equation for pressure in nanocapillaries. It is shown that the ratio of Laplace's pressure to the adsorption compression pressure determines different types of nanocapillary behavior. When the Laplace pressure dominates, it results in classical capillarity that is well studied and understood. There is an intermediate range where Laplace's pressure is partially or fully compensated by adsorption compression, and the resulting pressure in a capillary is an interplay between attraction to walls and repulsions from neighboring molecules in compressed adsorbed fluid. If the adsorption compression pressure dominates, it results in inversion of capillary pressure and the fluid adsorbed in the nanocapillary presses on walls from inside. This phenomenon has been observed experimentally for fluids in nanoporous solids; in particular, high-precision measurements have shown significant expansion of nanoporous adsorbents loaded with various fluids. It is also shown that oscillatory adhesion forces and internal forces in nanoporous adsorbents have a common thermodynamic origin and can be discussed in the framework of adsorption compression mechanisms.
Keywords: Nanocapillarity; Kelvin's equation; Adsorption compression; Capillary pressure inversion; Adhesion forces; Nanopores;

Studies on the mechanism of Indigo Carmine removal by solvent sublation by Yujuan Lu; Jizhen Li; Xueli Zhang; Jie Tang; Bo Wei; Jianhong Liu (210-218).
Indigo Carmine (C16H8N2Na2O8S2), an anionic dye, was removed from aqueous solution by solvent sublation of Indigo Carmine–cetyltrimethylammonium bromide (CTAB) complex (sublate) into 2-octanol. A stoichiometric amount of surfactant (surfactant:dye = 2:1) was demonstrated to be able to remove over 93% IC from the aqueous solution in 5 min. The apparent activation energy of attachment of the sublate to bubbles was calculated as 1.3 kJ/mol. Parameters were considered. At the same time, on the base of the complete transport mechanism, a mathematical model for the dye–surfactant complexation was obtained. Furthermore, the simulation of the mathematical and experimental data was made with good results.
Keywords: Indigo Carmine; Solvent sublation; Mechanism; Kinetics; Mathematical model;

Quaternary ammonium compounds (QACs) are recognized as membrane active agents widely used as biocides. The main purpose of this work was to investigate the influence of the QAC head group and acyl chain length on their permeability-perturbing power and on their affinity for lipidic membranes. Permeability perturbations were assessed by measuring the release of calcein entrapped inside vesicles. The affinity of QACs for bilayers was investigated by isothermal titration calorimetry (ITC). QACs bearing C16 chain were found to be more efficient to decrease the membrane permeability than their C12 analogues. On the other hand, the chemical nature of the ammonium head group has practically no influence on the permeability perturbations caused by QACs bearing C16 chains. It was difficult to assess the partitioning of the QACs between the aqueous and lipid phases since the ITC signals could also be associated to morphological changes such as vesicle aggregation. For the systems for which reliable thermodynamic parameters could be obtained, the Gibbs energy of transfer was similar to that for the micellization. The entropy variation represented the main contribution to the Gibbs energy, indicating that the insertion of QACs inside lipidic bilayers is driven by hydrophobic interactions. The paper reports the influence of molecular details of quaternary ammonium molecules on their affinity for lipid membranes and on their ability to disrupt membrane permeability.Display Omitted
Keywords: QAC; POPC; Permeability; ITC; Affinity;

Dielectric relaxation in binary mixtures containing particles or lamellae with complex geometry has been simulated within the quasielectrostatic approximation by a three-dimensional finite-difference method. The method was tested using simple models corresponding to water-in-oil (W/O) and oil-in-water (O/W) emulsions with volume fraction P up to 0.5. The dielectric spectra calculated by the finite difference method agreed with those expected from Wagner's equation at P ⩽ 0.3 and approached those from Hanai's equation at P > 0.4 . This method was applied to more complicated binary mixtures of oil and water: a bicontinuous cubic structure, a suspension of particles with projections, and a planar bilayer with a rippled or interdigitated interface. The bicontinuous phase that is supposed to appear near the transition between W/O and O/W emulsions showed dielectric properties similar to those of the O/W emulsion. The undulation of the particle surface and the interface of the planar bilayer affected all parameters of dielectric relaxation, especially the relaxation intensity. Simulations of dielectric dispersion in W/O and O/W emulsions and bicontinuous phase (BC).Display Omitted
Keywords: Dielectric relaxation; Interfacial polarization; Heterogeneous systems; Finite difference method; Complex permittivity; Emulsions; Planar layers; Bicontinuous structure; Surface roughness;

The effect of binary aqueous mixtures of ethylene glycol (EG), formamide (FA), N-methylformamide (NMF), dimethylformamide (DMF), and their pure phase on the apparent molar volume ϕ V of the bolaamphiphile decamethonium bromide (C10Me6) has been investigated at 298.15 K. The behavior of standard molar volumes V 2 0 and transfer volumes Δ t ϕ V of C10Me6 from water to solvent/water (S/W) binary mixtures, shows different minima and maxima depending on the composition of the solvent. This behavior is influenced by the nature of the cosolvent and on the type of the solute and more or less corresponds to volumetric changes in the S/W mixture. The investigation of the transfer volumes in different fixed concentrations reveals an inversion of Δ t ϕ V values between the compositions, which suggests a differentiation of the effects of different volume contributions on the partial molar volume of ions. The correlation of Δ t ϕ V with the dielectric constant of the aqueous amide mixtures shows that the behavior of Δ t ϕ V vs x amide reflects the changes of ɛ E vs x amide .
Keywords: Apparent molar volume; Transfer volume; Bolaamphiphile; Ethylene glycol/water; Formamide/water; N-Methylformamide/water; Dimethylformamide/water mixtures;

The influence of structure and composition of a reverse SDS microemulsion on enzymatic activities and electrical conductivities by Pierre Bauduin; Didier Touraud; Werner Kunz; Marie-Pierre Savelli; Sylviane Pulvin; Barry W. Ninham (244-254).
The activity of the enzyme horse radish peroxidase (HRP) is studied in a series of reverse microemulsions composed of dodecane, aqueous buffer, sodium dodecylsufate (SDS) and alcohols of the homologous series 1-butanol to 1-octanol. The HRP catalyzed reaction is the oxidation of a classical water soluble substrate, the 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) by hydrogen peroxide. In parallel electrical conductivity measurements are performed on the same solutions. The structural changes in the microemulsions, as inferred by the conductivity measurements, correlate remarkably well with the changes in the enzymatic activities. In particular it is found that (a) the maximum activity of the enzyme is always related to its optimum hydration and that this hydration can be related to the microemulsion structures, (b) the enzyme inhibition caused by the alcohols in microemulsions is a consequence of both the solubility of the alcohols in the buffer and the rigidity of the interfacial film. Consequently, it can be concluded that enzymatic activity measurements are a valuable tool to study confined systems such as microemulsions and, in particular, the amount of available hydration water. Enzymatic activities can be finely tuned by small changes in microemulsion structures, probably in a predictive way. Reverse SDS microemulsion containing HRP enzyme molecules, ions, alcohols and surfactant molecules.Display Omitted
Keywords: Anionic surfactant; Reverse microemulsion; Enzymatic reaction; Alcohols; Horse radish peroxidase; Conductivity; Hydration;

Self-assembly properties of a single-tailed chiral cationic surfactant, (1R,2S)-(–)-N-dodecyl-N-methylephedrinium bromide (DMEB), have been studied in water. The molecular self-assemblies of the amphiphile have been characterized by surface tension, fluorescence probes, light scattering, and microscopic techniques. The results have been compared with those of dodecyltrimethylammonium bromide (DTAB) surfactant. The critical aggregation concentration of DMEB was found to be much less than that of DTAB. Surface tension and fluorescence probe studies have suggested formation of micellar structures at low temperature (<28 °C) and spontaneous formation of giant vesicles in water above 28 °C. The mean size of the aggregates has been measured by a dynamic light scattering method. The micropolarity and microviscosity of the self-assemblies were determined by fluorescence probe technique. The 1H NMR and FTIR spectra were recorded to elucidate the role of the hydrophobic head group towards the formation of bilayer structures. The phase transition temperatures of the vesicular aggregates were determined by measurement of fluorescence anisotropy at various temperatures.
Keywords: Vesicle; Surface tension; Fluorescence; Microviscosity; Light scattering; Microscopy;

Addition of an electrolyte such as NaCl to ionic micelles such as sodium dodecyl sulfate (SDS) alters the ionic atmosphere of the bulk solvent, thereby changing both the micellar properties and the interaction pattern of micelles with a molecule in the solvent. In this study, we show how added NaCl in the presence of SDS micelles modulates the surface charge of the micelles, which in turn fine-tunes the switchover equilibrium between anionic and global neutral forms of piroxicam. The presence of salt alters the CMC and aggregation number of SDS micelles. The binding of the global neutral form of piroxicam with SDS is found to be strongly modulated by the presence of counterion in the bulk solvent. The switchover equilibrium between anionic and neutral forms of piroxicam in presence of SDS micelles can be fine-tuned by the modulation of surface charge of micelles by the addition of NaCl.Display Omitted
Keywords: Surfactants; Salt effect; Switchover; Apparent p K a ; Change in free energy (ΔG);

A method to create assembly of polydiacetylene vesicles on solid substrates in a novel fashion is described. The formation of the assembly is based on electrostatic layer-by-layer deposition using negatively charged 10,12-pentacosadiynoic acid (PCDA) vesicles and polyelectrolyte polyethylenimine, or positively charged PCDA- 2 ′ -aminoethylamide (PCDANH2) vesicles. This is an efficient method for preparing the chromatic sensor films of polydiacetylene vesicles after ultraviolet light irradiation. The strategy would be useful in the development of polydiacetylene-based chemosensors and biosensors. PCDA vesicle/PCDANH2 vesicle assembly upon PEI-modified quartz slide.Display Omitted
Keywords: Polydiacetylene vesicle; Assembly; Sensor;

Calculation of the dynamic impedance of the double layer on a planar electrode by the theory of electrokinetics by Hao Zhou; Matthew A. Preston; Robert D. Tilton; Lee R. White (277-289).
Applications of microelectromechanical systems in the biotechnological arena (bioMEMS) are a subject of great current interest. Accurate calculation of electric field distribution in these devices is essential to the understanding and design of processes such as dielectrophoresis and AC electroosmosis that drive MEMS-based devices. In this paper, we present the calculation of the electrical double-layer impedance ( Z el ) of an ideally polarizable plane electrode using the standard model of colloidal electrokinetics. The frequency variation of the electrical potential drop across the double layer above a planar electrode in a general electrolyte solution is discussed as a function of the electrode zeta potential ζ, the Debye length κ −1 , the electrolyte composition and the bulk region thickness L.
Keywords: Electrical double-layer impedance; Planar electrodes; Colloidal electrokinetics; Microelectromechanical systems;

Drag on two coaxial, nonuniformly structured flocs in a uniform flow field by Jyh-Ping Hsu; Shu-Jen Yeh; Duu-Jong Lee (290-298).
The drag on two coaxial flocs of nonuniform structure in a Newtonian fluid is evaluated for Reynolds number ranges from 0.1 to 40. A two-layer model is adopted to simulate various possible structures of a floc. The influences of the key parameters of the problem under consideration, including the separation distance between two flocs, the Reynolds number, and the ratios (inner radius/outer radius) and (permeability of outer layer/permeability of inner layer), on the drag acting on two flocs are investigated. We show that if Reynolds number is small, the drag on the leading floc is about the same as that on the rear floc. However, if Reynolds number is sufficiently large, because wakes are formed in the rear region of the leading floc, the drag on it is greater than that on the rear floc. For a fixed mean permeability, the more nonuniform the floc structure is, the more important is its influence on the drag, and the more appreciable the deviation of the drag coefficient–Reynolds number curve from a Stokes'-law-like relation. Drag on two coaxial flocs of nonuniform structure, two-layer model.Display Omitted
Keywords: Drag; Two coaxial flocs; Nonuniform structure; Two-layer model;

AFM studies on LB films of poly(n-hexyl isocyanate), poly(vinyl acetate), and their binary mixtures by Masaki Ohkita; Masahiro Higuchi; Masami Kawaguchi (300-303).
LB films of rigid-rod-like poly(n-hexyl isocyanate) (PHIC), flexible poly(vinyl acetate) (PVAc), and binary mixtures of PHIC as well as of PHIC and PVAc transferred on a mica surface from the air–water interface were observed by AFM. The grain structure of three individual PHIC samples in the AFM images changed shape from a rigid rod to a coiled rod with an increase in the molecular weight due to changes in the chain rigidity of PHIC. On the other hand, the AFM image of PVAc was similar to that of a mica surface, indicating that PVAc forms a uniform and homogeneous film. For the binary mixtures of PHIC, the grain structure in the AFM image of the highest molecular weight PHIC was expanded with a similar shape after the addition of the smallest one, whereas it lost its shape after the addition of the middle one. Their peak-to-valley values in the AMF images were similar to those of the individual PHIC samples. For the binary mixtures of PHIC with the highest mass and PVAc, the grain in the AFM image of the PHIC lost its shape after the addition of PVAc and it changed shape from a connected partial lost coil to an extended bundle rod with an increase in the PVAc component. AMF images (top) and distributions of grain size (bottom) of PHIC-138 and PHIC-138/PHIC-18.Display Omitted
Keywords: LB films; AFM; Binary mixtures; Poly(n-hexyl isocyanate); Poly(vinyl acetate);