Journal of Colloid And Interface Science (v.326, #2)

The goethite surface structure has been extensively studied, but no convincing quantitative description of its highly variable surface reactivity as inversely related to its specific surface area (SSA) has been found. The present study adds experimental evidence and provides a unified macroscopic explanation to this anomalous behavior from differences in average adsorption capacities, and not in average adsorption affinities. We investigated the chromate anion and lead(II) cation adsorption behavior onto three different goethites with SSA varying from 50 to 94 m2/g, and analyzed an extensive set of published anion adsorption and proton charging data for variable SSA goethites. Maximum chromate adsorption was found to occupy on average from 3.1 to 9.7 sites/nm2, inversely related to SSA. Congruency of oxyanion and Pb(II) adsorption behavior based on fractional site occupancy using these values, and a site density analysis suggest that: (i) ion binding occurs to singly and doubly coordinated sites, (ii) proton binding occurs to singly and triply coordinated sites (ranging from 6.2 to 8 total sites/nm2, in most cases), and (iii) a predominance of (210) and/or (010) faces explains the high reactivity of low SSA goethites. The results imply that the macroscopic goethite adsorption behavior may be predicted without a need to investigate extensive structural details of each specific goethite of interest.Adsorption congruency of site occupancy helped determine reactive site densities of differing specific surface area goethites.
Keywords: Goethite; Chromate; Surface saturation; Site density; Specific surface area; Site occupancy; Anion; Pb(II); Adsorption; Surface charging;

It has been known that pre-adsorbed polymers on a solid surface can block some adsorption sites for a post-added polymer on the same substrate. If the charge of pre-adsorbed polymer is the same as that of post-adsorbed polymer, the repulsion force between these two polymers will change not only the polymer adsorption amount but also the conformation and the properties of the polymer on the substrate. The site blocking effect is a possible mechanism in many commonly used flocculation programs. However, no research has been able to confirm the proposed theory of its effects on adsorbed polymer conformation. This work reports, for the first time, detailed information regarding the effects of site blocking on an adsorbed polymer's conformation using scanning probe microscopy. Using both polymers and nanoparticles as site blocking additives, experiments were performed on single cationic polyacrylamide polymers. This work illustrates that the increased thickness in adsorbed polymer layers, reported by previous researchers, is due to a dramatic increase in the tail portion of the adsorbed polymer. The previously postulated increase in adsorbed polymer loop lengths was not present in these experiments.Site blocking agents will change the conformation of cationic polyacrylamide molecules absorbed on a mica surface, resulting in a significant increase in the tail length of the adsorbed polymer.
Keywords: Site blocking; Polymer adsorption; Conformation; Loop length;

Contact angles of diiodomethane on silicon-doped diamond-like carbon coatings in electrolyte solutions by Konstantin B. Borisenko; Evangelos A. Evangelou; Qi Zhao; Eric W. Abel (329-332).
The influence of surrounding electrolyte type and concentration on the contact angle of hydrophobic diiodomethane on silicon-doped diamond-like carbon (DLC) coatings was examined to provide insight into how the presence of electrolytes in the solution influences adhesion of hydrophobic material to doped DLC surfaces. There was a small but statistically significant increase of contact angle with increasing electrolyte concentration over the range from 0 to approximately 0.01 M, after which the contact angle was virtually unaffected by further increase in the concentration of electrolyte. It was shown that CaCl2 has a stronger influence on the change of the contact angle than NaCl, and that an increase in Si content in the DLC coatings increased the change in the contact angle of diiodomethane for all types of electrolyte. These observations suggest that the adhesion to the Si-doped DLC surfaces is reduced by addition of the electrolytes to the surrounding solvent. This could be explained by increased ion adsorption on the DLC surface with increase in silicon doping, causing the surfaces to be more hydrophilic.Contact angle on silicon doped DLC increases slightly as electrolyte concentration increases for low molar concentration of electrolytes. This might imply that the cleaning process of removing hydrophobic materials such as proteins from polar surfaces using aqueous solutions could be improved by increasing electrolyte concentration.
Keywords: Adhesion; Ion–solid interactions; Amorphous surfaces; Coatings; Wetting; Surface energy;

Alkanethiol-oxidized copper interface: The critical influence of concentration by G. Fonder; F. Laffineur; J. Delhalle; Z. Mekhalif (333-338).
In this contribution, self-assembled monolayers of n-dodecanethiol (C12H25SH) at different concentrations on polycrystalline copper have been elaborated. Using XPS, PM-IRRAS, and electrochemical methods (cyclic voltammetry curves and cathodic desorption), the effect of the C12H25SH concentration on the reduction of the oxide layer has been studied. In all cases, a monolayer of good quality has been obtained. Results provide proof that while the concentration is increased, the thickness of the oxide layer is decreased, to a point that leads to metallic copper for the higher concentration. The results presented in this publication indicate the importance of controlling the interface when forming SAMs of organothiols on oxidizable metals.
Keywords: Copper; SAM; Oxide reduction; Thiol;

A simplified method is proposed to estimate the thermodynamic parameters of clusterization at the air/water interface for various classes of amphiphilic compounds with a single alkyl chain. The method is based on the calculation of thermodynamic characteristics only for one of the homologous series of dimers ( n = 6 – 16 ) governing the formation of infinite clusters. The method is used to calculate the thermodynamic parameters of clusterization for alcohols, thioalcohols, carboxylic acids and amines, and the dependencies of the Gibbs energy of clusterization on the alkyl chain length are evaluated. It is shown that the alkyl chain length, at which the spontaneous clusterization begins, as calculated using the proposed simplified method, is in fact the same as that calculated using the additive scheme developed earlier. The simplified method proposed was verified using alkylnitriles as example. In contrast to alcohols, thioalcohols and amines, infinite ‘rhombic’ clusters are formed rather than ‘rectangular’ clusters for this class of compounds. Spontaneous clusterization of nitriles is shown to start for alkyl chains containing 18–19 carbon atoms. This value agrees with that obtained from experimental data with 17–18 carbon atoms. The proposed simplified method introduces an exact and suitable tool for the estimation of thermodynamic parameters of the clusterization of amphiphilic compounds.
Keywords: Quantum chemistry; Air/water interface; Clusterization; Alkylnitriles; Thermodynamic parameters;

Arsenic, selenium, and vanadium are major anionic elements of concern in drinking water. This research investigated the adsorption characteristics of As(V), Se(IV), and V(V) onto a commercial activated alumina (AA) under different pH, surface loading, and ionic strength conditions using batch systems. The results indicated that the adsorption of these elements was significantly affected by pH and the surface loading. However, ionic strength generally did not impact their adsorption, indicating that the electrostatic effect on the adsorption of these elements was relatively not important compared to surface chemical reactions. A speciation-based adsorption model was used to simulate the adsorption of As(V), Se(IV), and V(V) by activated alumina and to determine the adsorption constants of different element species. This model can satisfactorily predict the adsorption of these elements in a broad pH range from 1.5 to 12 and a wide surface loading range from 1.0 to 50 mg/g activated alumina for different sorbent concentrations, using the same set of adsorption constants.As(V), Se(IV), and V(V) adsorption onto activated alumina was investigated using batch experiments, and quantified using a speciation-based surface complexation model with only three adsorption constants for each element.
Keywords: Arsenic; Selenium; Vanadium; Adsorption; Activated alumina; Speciation-based model;

Homogeneous nucleation is fundamentally important in emulsion polymerization. A molecular theory is proposed to quantify primary particle formation in the process. The proposed model divides a polymerization system into three portions: the domains formed by growing radicals, their surrounding aqueous solution, and dispersed monomer droplets. In general, the total free energy of the domains is contributed from the mixing among the molecules including monomer and water, the elasticity of oligomeric radical chains, and the transferable free energy of electrolytes; while that of the outer solution is from the mixing of monomer and water molecules and the transferable energy of electrolytes. Application of this theory to vinyl acetate emulsion polymerization has shown that the critical degree of polymerization ( j cri ) predicted is in a good agreement with the value derived from experimental data reported in literature. Furthermore, this model can also estimate the concentrations of VAc and water in the domain at the degrees of polymerization around j cri .Primary particle formation in emulsion polymerization is modeled by a molecular theory, which considers the domains formed by oligomeric radicals and their surrounding aqueous solution in equilibrium.
Keywords: Emulsion polymerization; Nucleation; Primary particle formation; Molecular theory; Model;

A superhydrophobic polymeric surface was prepared through a very simple bulk photografting method. A thin layer of acrylic acid (AA), a highly hydrophilic monomer, was sandwiched between two high-density polyethylene (HDPE) sheets, followed by UV irradiation for a short time, and then the two sheets were pulled apart and dried. The contact angles on the two grafted surfaces decreased very quickly with irradiation time in the first several seconds, and then increased with irradiation time to a level higher than that on pristine HDPE surface. When using a scraped PE surface as the bottom one, it showed superhydrophobicity after 35 s irradiation. XPS investigations show that strong rearrangement of the poly(acrylic acid) molecules has taken place on both surfaces, especially on the bottom surface, which provides the low surface free energy. The scraping and the grafting process led to the formation a unique micro- and nanostructure on the surface. These two factors lead to the superhydrophobicity. The as-prepared surface possesses superhydrophobic properties in a wide range of pH values, stimuli-responsive properties and low or very high adhesion under different situations.A superhydrophobic polymeric surface was fabricated by the bulk photografting of a highly hydrophilic monomer acrylic acid (AA) onto scraped high-density polyethylene (HDPE) surface.
Keywords: Superhydrophobic; Photografting; High-density polyethylene; Acrylic acid;

Organic inorganic dye filler for polymer: Blue-coloured layered double hydroxides into polystyrene by Rafael Marangoni; Christine Taviot-Guého; Abdallah Illaik; Fernando Wypych; Fabrice Leroux (366-373).
A series of blue dye molecules, Evans blue (EB), Chicago sky blue (CB), Niagara blue (NB) were incorporated by direct co-precipitation within the galleries of negatively charge layered double hydroxide (LDH). The materials of cation composition Zn/Al = 2 lead to well-defined organic inorganic assemblies. The molecular arrangement of the interleaved dye molecule is proposed by 1D electronic density projection along the stacking direction for the hydrothermally treated samples with alternatively a highly inclined orientation of EB and CB and a parallel-bilayer arrangement for NB. Blue coloured LDH assemblies were subsequently dispersed into polystyrene (PS). It was found that the hybrid fillers do not interfere in the radical polymerization of styrene, giving rise to similar molecular weight and polydispersity than filler free PS, while higher glass transition temperatures were obtained for the nanocomposites. This was consistent with the rheological behaviour with the observation for LDH/NB filler based nanocomposite of shear thinning exponent different from zero, underlining frictional interaction between filler and PS chain. The absorption maximum slightly blue-shifted for the hybrid filler in comparison to the corresponding organic dye was found unmodified for the PS nanocomposite, thus giving rise to blue coloured plastic films, reminiscent somehow of the blue Maya effect.Interleaved large organic dyes into inorganic LDH gap lead to well-defined organic–inorganic assemblies, which can be used as multifunctional filler in polymer nanocomposites.
Keywords: Kayered double hydroxide; Blue-coloured dye; Polystyrene; Nanocomposite;

The effect of the addition of cyclodextrins (CD) viz., α-, β-, HPβ- and γ-CD to the aqueous solutions of the most widely prescribed anti-inflammatory drug, diclofenac sodium (DS), has been fully investigated by means of spectroscopic (UV–vis, steady-sate fluorescence, 1H NMR and ROESY) and thermodynamic (conductivity) techniques. The global picture of the results indicates that diclofenac sodium penetrates the CD cavity. The apparent association constants for all the inclusion complexes were estimated from fluorescence data. Conductivity measurements of aqueous solutions of diclofenac sodium were performed both as a function of DS concentration and CD concentration, at different temperatures ranging from 15 to 40 °C. Results suggested the existence of 1:1 complex between DS and CD. The thermodynamics of the system was discussed in terms of change in Gibbs free energy. Free energy of the DS/W system was found to decrease on addition of cyclodextrin, which points towards the energetically favorable interactions between drug and cyclodextrin molecules in solution phase. 1H NMR chemical shift changes and ROESY spectra provide powerful means for probing CD:DS interactions.The inclusion complexes between diclofenac sodium, a nonsteroidal anti-inflammatory drug, and different cyclodextrins were assessed using spectroscopic and thermodynamic studies. Studies confirmed the formation of 1:1 complex in the solution.
Keywords: Diclofenac sodium; Cyclodextrin; Conductivity; UV–visible; Fluorescence; 1H NMR; ROESY;

Errors in two particle tracking at close distances by M. Gyger; F. Rückerl; J.A. Käs; J. Ruiz-García (382-386).
Tracking single and multiple particles is of great importance for many physical investigations in a variety of different areas. It is essential to find and eliminate sources of systematic errors in the particle position determination (PPD) and to determine the limits of its applicability to a given problem. Particularly when measuring the interactions between colloids at close distances, artifacts in the image taking process pose a great problem. By means of a simulation technique, we investigated the accuracy of the PPD using two-dimensional Gaussian and Gaussian-like fitting functions. For the distance between the two colloidal particles this revealed a systematic overestimation of the inter-particle distance of up to 1.9% of the particle diameter for the Gaussian fitting function. This deviation can be explained by the differences between the intensity distribution of the overlap of the simulated particles and the linear superposition of the Gaussian functions. Modifications of the fitting functions can reduce the systematic error significantly.
Keywords: Single/multiple particle tracking; Colloids; Particle position determination; Video microscopy; Simulation;

A gold nanosphere in water is considered to attain special stability in derivatization like an artificial atom when the octet rule is satisfied by forming four covalent bonds with two 5′-phosphorothioate-modified oligonucleotide molecules. Owing to this, the hybridization of two mutually complementary gold-bound oligonucleotides makes gold nanospheres preferentially connected linearly by duplexes to produce strands like linear artificial molecules. We have then fixated the linear strands of DNA-linked gold nanospheres by reducing Ag+ ion clusters immobilized around duplexes to show the absorption spectrum of silver-coated artificial-molecular nanorods.A gold nanosphere is considered to attain special stability in derivatization when the octet rule is satisfied by forming four covalent bonds with two 5′-phosphorothioate-modified oligonucleotide molecules.
Keywords: Artificial atoms; Artificial molecules; DNA; Nanoconjunction; Octet rule; Self-assembly; Silver;

The synthesis and optical properties of the heterostructured ZnO/Au nanocomposites by Guiye Shan; Mingya Zhong; Shuang Wang; Yajun Li; Yichun Liu (392-395).
ZnO/Au heterostructured nanoparticles were formed through epitaxial growth of Au on the ZnO seeds. The morphology and structure of ZnO/Au nanocomposites were investigated by TEM and XRD analysis. The nanocontact between Au and ZnO results in red-shift of surface plasmon of the Au and increase the intensity of Raman signals of ZnO. Heterostructured ZnO/Au nanocomposites also enhance chemical stability of ZnO in aqueous solution.HRTEM image shows that single ZnO nanoparticle has been connected with Au nanoparticle. The resonant Raman scattering spectra of ZnO/Au nanocomposites with increasing the amount of Au were changed. The enhancement is predominant for the first-order LO phonon mode of ZnO. The enhancement of surface optical phonons in RRS experiments can be rationalized by the fact that surface optical phonons execute vibrational motion only within a few atomic layers near the surface, and the Raman scattering volume of this mode can entirely be subjected to the electromagnetic interaction with the Au surface plasmon.
Keywords: ZnO; Au; Heterostructure; Resonant Raman scattering;

The origin of the surface potential of calcium carbonate in aqueous dispersions and the dissolution of calcite in systems containing excess Ca2+ and CO2− 3 have been the subjects of this study. In addition, stabilization of calcite particles with an anionic polyelectrolyte (sodium polyacrylate (NaPA)) and the effect on surface potential and dissolution of calcite have been studied. Preferential dissolution of either Ca2+ or CO2− 3 from the surface, which is governed by the partial pressure of CO2 in solution and the pH of the solution, mainly determines the surface potential. Both lattice ions (Ca2+ and CO2− 3) adsorb onto the surface and thus alter the surface potential. NaPA adsorbs strongly onto the calcite surface regardless of background electrolyte concentration, and reverses the surface potential to negative values. Chelation of the surface due to NaPA can be partly prevented by adding Ca2+ to the dispersion.
Keywords: Calcite; Sodium polyacrylate; Zeta potential; Surface charge; Dissolution;

Influence of long-term aqueous exposure on surface properties of plasma-sprayed oxides Cr2O3 and Cr2O3–25 wt% TiO2 by Mika Harju; Mikael Järn; Per Dahlsten; Juha-Pekka Nikkanen; Jarl B. Rosenholm; Tapio Mäntylä (403-410).
The influence of water exposure on the surface properties of plasma-sprayed Cr2O3 and Cr2O3–25 wt% TiO2 was studied. It was shown that both plasma-sprayed materials contained Cr(VI) hydrous oxide phases, which dissolved rapidly at the beginning of water exposure. The dissolution continued slowly during the whole water exposure time. The Cr(VI) dissolution was accompanied by a rapid increase in surface IEP value. Both Cr2O3 and Cr2O3–25 wt% TiO2 showed similar dissolution, zeta potential, and surface oxidation states. Thus the addition of TiO2 did not influence the surface properties of the plasma-sprayed Cr2O3.The influence of aqueous aging treatment on surface properties of plasma sprayed Cr2O3 and Cr2O3–25 wt% TiO2 coating materials was evaluated.
Keywords: Plasma spraying; Chromia; Zeta potential; IEP; Dissolution; Surface state;

A new synthesis of underivatized gold nanoparticles (Au-NPs) in water stabilized by the highly water soluble 28-membered macrocyclic polyammonium chloride, [28]ane-( NH 2 + )6O2⋅6Cl (28-MCPAC) is reported. In addition to providing stability, 28-MCPAC with its cationic form functionalizes the Au-NPs for sensing anions in water. The 28-MCPAC-Au-NPs show a surface plasmon band in the visible region (>520 nm). By tuning the 28-MCPAC:HAuCl4 ratio, Au-NPs with different core diameters ranging from 4 nm to 6 nm, as determined by TEM analysis, can be obtained. Particles are spherical, discrete, and appeared to have narrow size distributions. Raman spectroscopy confirms that the physisorption is responsible for the interaction between Au-NP surface and 28-MCPAC. The potential of the as-synthesized particles for sensing monophosphorylated nucleosides (nucleotides): 5-adenosine monophosphate (5-AMP), 5-cytosine monophosphate (5-CMP), 5-guanine monophosphate (5-GMP), and 5-uridine monophosphate (5-UMP) is investigated spectroscopically. Nucleotides-assisted agglomerations of 28-MCPAC-Au-NPs follow the order: 5 -UMP > 5 -GMP > 5 -CMP > 5 -AMP . An attempt is taken to prepare Au-NPs in water at pH 4.55 without an added stabilizer. Particles without an added stabilizer are short lived, and the TEM image shows that the particles aggregate following a quasi-two-dimensional self-assembly array.Physical adsorption of 28-MCPAC on Au-NP surface renders electrostatic repulsion as a dispersion force between neighboring particles.
Keywords: Gold nanoparticles; Macrocyclic polyammonium chloride; Water dispersion; Nucleotides;

The synthesis of cationically modified poly(vinyl alcohol), CPVA, by copolymerization of vinyl acetate and diallyldimethyl ammonium chloride (DADMAC), followed by alkaline hydrolysis was systematically studied. The application of the resulting polymer to the fine clay flocculation was also reported. The charge density and the structure of the resulting CPVA were characterized by polyelectrolyte titration and NMR. A photometric dispersion analyzer was used to conduct the dynamic flocculation experiments. Under fine clay experimental conditions, the CPVA alone contributed little to inducing clay flocculation. However, in conjunction with anionic polyacrylamide-based polymer with high molecular weight and low charge density, significant improvement in the flocculation of fine clay particles was achieved. The influence of factors such as pH and shear force on clay flocculation was also investigated to identify optimum application conditions for clay flocculation. The electrostatic interactions between the clay and CPVA, as well as those between the CPVA pre-treated clay and anionic polymer, were studied to explore the flocculation mechanism.In conjunction with an anionic polymer, cationic-modified PVA induced effective flocculation of fine clay particles.
Keywords: Cationic poly(vinyl alcohol) (CPVA); NMR; Photometric dispersion analyzer; Clay flocculation; Dual-component flocculants;

Hybrid films composed of pseudoisocyanine (PIC) and layered silicates were prepared by direct adsorption from dye solution. Properties of the films were characterized by absorption and fluorescence spectroscopy, which indicated the formation of two types of J-molecular aggregates. Molecular arrangement and structure of the J-aggregates were investigated by means of linearly polarized absorption spectroscopy and X-ray diffraction (XRD). Structural models of the two main types of J-aggregates with oblique and those of parallel molecular alignments were designed. The spectral properties of these species were studied in detail. All the films contained a mixture of both types of the J-molecular assemblies. Predominance of either species likely depends on the layer charge of the silicate template.Two types of pseudoisocyanine J-aggregates are formed in the films of layered silicates dependent on the surface properties of the inorganic carrier.
Keywords: Pseudoisocyanine; Layered silicates; UV/vis absorption spectroscopy; Fluorescence; J-aggregates;

Effect of PSS on morphology and optical properties of ZnO by Jiaguo Yu; Chuan Li; Shengwei Liu (433-438).
ZnO micrometer-sized rods with tunable aspect ratios and 3D hollow spherical superstructures are selectively fabricated by a simple poly(sodium 4-styrene-sulfonate) (PSS)-mediated hydrothermal crystallization and assembly strategy. When PSS concentration is relatively low (0–0.5 g L), the aspect ratios of as-obtained microrods steadily decrease with increasing PSS concentration due to the selective adsorption of PSS on the polar ZnO (001) crystal plane. When PSS concentration is relatively high (1 g L), 3D nanosheets-built hollow microspheres form probably due to the organic–inorganic interfacial cooperative assembly. Raman, photoluminescence and UV–vis diffuse reflectance spectra show that the optical properties of as-obtained ZnO microstructures are highly related to their specific morphologies.ZnO micrometer-sized rods with tunable aspect ratios and 3D hollow spherical superstructures are selectively fabricated by a simple PSS-mediated hydrothermal crystallization and assembly strategy.
Keywords: ZnO; Micrometer-sized rods; Hollow microspheres; Polymer; Optical properties;

Synthesis of ordered mesoporous silica membrane on inorganic hollow fiber by Jiansheng Li; Yan Zhang; Yanxia Hao; Jiangyan Zhao; Xiuyun Sun; Lianjun Wang (439-444).
This paper reports on a new method for the preparation of mesoporous silica membranes on alumina hollow fibers. A surfactant–silica sol is filled in the lumen of an α-alumina hollow fiber. The filtration technique combined with an evaporation-induced self-assembly (EISA) process results in the formation of a continuous ordered mesoporous silica layer on the outer side of α-alumina hollow fibers. X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen isothermal adsorption measurements reveal that these membranes possess hexagonal (P6mm) mesostructures with pore diameters of 4.48 nm and BET surfaces of 492.3 m2 g−1. Scanning electron microscopy (SEM) studies show that the layers are defect free and energy-dispersive spectroscopy (EDS) mapping images further confirm the formation of continuous mesoporous silica layer on the outer side of α-alumina hollow fibers. Nitrogen and hydrogen permeance tests show that the membranes are defect free.A new method for the preparation of ordered mesoporous silica membranes on alumina hollow fibers was reported. The method combines a filtration technique combined with an evaporation-induced self-assembly (EISA) process.
Keywords: Synthesis; Mesoporous membranes; Alumina hollow fiber; Filtration technique; Evaporation-induced self-assembly (EISA);

Collective behavior during the exit of a wetting liquid through a network of channels by Charles N. Baroud; Xin C. Wang; Jean-Baptiste Masson (445-450).
The exit of a wetting fluid from a thin microchannel into a sudden expansion is studied experimentally. In the case of the exit from a single channel, the advancing interface converges to a parabolic shape after an initial transient, in accordance with the lubrication limit analysis of a spreading drop. The experiments are then repeated for the exit from two parallel channels. At early times, the two exiting drops behave independently and display the same evolution as a single exiting droplet, while at late times we recover a single parabolic profile. The transition between the early and late states is due to the merging of the two drops, which is associated with a sudden increase in the flow rate. This is the signature of a collective effect which acts to redistribute the fluid spatially. Finally, the experiment is generalized to the case of seven parallel channels where a cascade of two-by-two mergings is observed, indicating that local interactions dominate the dynamics which lead to the global state of the system.We study the collective behavior arising as a wetting fluid exits through n parallel channels and compare the situation with a single channel or a doublet. The early and late times can be simply related to the single channel case, while the transition displays a sudden increase of the flowrate due to the droplet interactions.
Keywords: Imbibition; Porous network; Collective behavior; Microfluidic models;

Mechanisms of equilibrium shape transitions of liquid droplets in electrowetting by A.I. Drygiannakis; A.G. Papathanasiou; A.G. Boudouvis (451-459).
Liquid droplets bridging the gap between two dielectric-coated horizontal electrode plates suffer breakup instabilities when a voltage applied between the electrodes exceeds a threshold. Interestingly enough, broken liquid bridges (i.e. a pair of a sessile and a pendant drop) can spontaneously rejoin if the voltage is still applied to the electrodes. Here we study the electro-hydrostatics of the liquid bridges in the joined or broken state and we illuminate the mechanisms of the shape transitions that lead to bridge rupture or droplet joining. The governing equations of the capillary electro-hydrostatics form nonlinear and free boundary problems which are solved numerically by the Galerkin/finite element method. On one hand, we found that capillary bridges become unstable at a turning point bifurcation in their solution space. The solutions past the turning point are unstable and the instability signals the bridge rupture. On the other hand, the separate droplets approach each other as the applied voltage increases. However, solutions become unstable past a critical voltage at a turning point bifurcation and the droplets join. By studying the relative position of the turning points corresponding to bridge rupture and droplet joining, respectively, we define parameter regions where stable bridges or separate droplets or oscillations between them can be realized.Electrowetting by liquid drops exhibits transitions between liquid bridges and separated droplets. These are caused by instabilities which, depending on key parameter values, favor bridges, droplets, or oscillations between them.
Keywords: Drop breakup; Contact angle; Turning point; Finite element method; Electrowetting;

Aggregation and supramolecular chirality of achiral amphiphilic metalloporphyrins by Wei Yu; Zhanshuang Li; Tianyu Wang; Minghua Liu (460-464).
Metalloporphyrin derivatives with three hydrophobic dodecyl chains and a hydrophilic ester or carboxylic acid substituent were designed in order to clarify the effect of the central metal ions on the aggregation as well as the supramolecular chirality in the Langmuir–Schaefer films. All the metalloporphyrins showed good spreading behavior on water surface and can be transferred onto solid substrates. The transferred films were characterized by a variety of methods including UV–visible spectroscopy, circular dichroism (CD) spectroscopy, FTIR spectroscopy, atomic force microscopy (AFM) and scanning electron microscope (SEM) measurements. It has been found that the copper derivative forms J-aggregates as well as H-aggregates in the film. Moreover, the film showed strong CD signals. Change from the ester substitution to carboxylic acid caused the decrease of the supramolecular chirality. On the contrary, the zinc derivative showed only a negligible CD signal although the corresponding free base could assemble into a chiral assembly. A possible mechanism for the subtle relationship between supramolecular chirality and molecular structures has been proposed.Achiral copper porphyrin derivative was found to assemble into chiral LB films, while the corresponding achiral zinc derivative could not.
Keywords: LB film; Supramolecular chirality; Aggregation; Metalloporphyrin;

UV and thermally stable superhydrophobic coatings from sol–gel processing by Yonghao Xiu; Dennis W. Hess; C.P. Wong (465-470).
A method for the preparation of inorganic superhydrophobic silica coatings using sol–gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol–gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 °C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 °C and UV testing times of 5500 h. Contact angles remained >160° with contact angle hysteresis ∼2°.The UV stability was significantly improved after the hydrophobic surface groups (isobutyl) were replaced by fluoroalkyl groups.
Keywords: Silica; Thin film; Superhydrophobicity; Fluoroalkyl silane; UV stability; Thermal stability; Accelerated UV weathering test;

3-D thermodynamic analysis of superhydrophobic surfaces by Ken Yamamoto; Satoshi Ogata (471-477).
Some microtextured surfaces strongly repel water. In particular, surfaces with contact angle (CA) higher than 150° are called superhydrophobic surfaces and many studies to obtain such surfaces have been reported. However, none of them could be a guide to achieve superhydrophobicity and the thermodynamic mechanisms are not well understood. In this study, two types of 3-D models—pillar-textured surfaces and cavity surfaces—were selected and thermodynamically analyzed focusing on the surface free energy. By calculations, equilibrium CA, free energy wall (FEW), and CA hysteresis are obtained. Based on these calculations, the factors to determine the sliding angle are indicated. Additionally, based on these results, one example of the optimal geometry for superhydrophobic surfaces is proposed.Using a 3-D model, equilibrium contact angle and so on are thermodynamically calculated. Actual contact line will not be circular, but as in the second figure.
Keywords: Superhydrophobic; Contact angle; Sliding angle; Free energy; Energy wall; Hysteresis; Cavity;

Superhydrophobic surface from Cu–Zn alloy by one step O2 concentration dependent etching by Weici Wu; Miao Chen; Shan Liang; Xiaolong Wang; Jianmin Chen; Feng Zhou (478-482).
The article reports on a very simple method to fabricate superhydrophobic surfaces with Cu–Zn alloy via changing the local oxygen concentration and formation of oxygen difference cell, which can be readily realized by covering or contacting the Cu–Zn alloy surface with a glass slide. This superhydrophobic film comes from the formation of a flower-like hierarchical structure due to the accelerated alloy etching. In contrast, the white film grown in the un-covered area showed a much lower hydrophobicity due to its different morphology. These superhydrophobic surfaces or superhydrophobic–hydrophobic surfaces are expected to find applications in making self-cleaning alloy surface, in metal anticorrosion, and in biomineralization, etc.One step chemical etching of Cu–Zn alloy results in different regions with contrast wettability from hydrophobicity to superhydrophobicity, which arises from different morphologies.
Keywords: Cu–Zn alloy; Superhydrophobic; Dezincification corrosion; O2-difference macrocell;

The cloud point phenomenon has been investigated for the solutions of polyoxyethylene (POE)-type nonionic surfactants (C12E5, C12E6, C12E7, C10E6, and C14E6) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), a typical room temperature ionic liquid (RTIL). The cloud point, T c , increases with the elongation of the POE chain, while decreases with the increase in the hydrocarbon chain length. This demonstrates that the solvophilicity/solvophobicity of the surfactants in RTIL comes from POE chain/hydrocarbon chain. When compared with an aqueous system, the chain length dependence of T c is larger for the RTIL system regarding both POE and hydrocarbon chains; in particular, hydrocarbon chain length affects T c much more strongly in the RTIL system than in equivalent aqueous systems. In a similar fashion to the much-studied aqueous systems, the micellar growth is also observed in this RTIL solvent as the temperature approaches T c . The cloud point curves have been analyzed using a Flory–Huggins-type model based on phase separation in polymer solutions.Cloud point phenomena have been studied for solutions of POE-type nonionic surfactants (C12E5, C12E6, C12E7, C10E6, and C14E6) in bmimBF4, a typical room temperature ionic liquid (RTIL).
Keywords: Polyethylene glycol alkyl ether; Nonionic surfactant; Cloud point; Room temperature ionic liquid; Solvophilicity; Solvophobicity;

Synthesis and properties of ionic liquid-type Gemini imidazolium surfactants by Mingqi Ao; Guiying Xu; Yanyan Zhu; Yan Bai (490-495).
A series of ionic liquid-type Gemini imidazolium surfactants with four-methylene spacer groups were synthesized ([C n -4-C n im]Br2, n = 10 , 12, 14). The surface activity and thermodynamic properties of micellization between the Gemini imidazolium surfactants and their corresponding monomers ([C n mim]Br, n = 10 , 12, 14) were compared by means of surface tension and electrical conductivity measurements. The values of cmc, γ cmc , p c 20 , Γ max , and A min derived from surface tension measurement at 25 °C suggest that the surface activity of [C n -4-C n im]Br2 is higher than that of [C n mim]Br. While the thermodynamic parameters of micellization ( Δ G m ○ , Δ H m ○ , Δ S m ○ ) derived from electrical conductivity indicate that the micellization of [C n -4-C n im]Br2 is entropy-driven, aggregation of [C n mim]Br is entropy-driven at low temperature but enthalpy-driven at high temperature. Finally, the activation energy of conductance ( E a ) that is associated with the effective charge is also obtained for [C n -4-C n im]Br2 and it is constant below the cmc, but it increases above the cmc.The thermodynamic parameters of micellization obtained from conductivity measurements indicate that negative values of Δ G m ○ arise due to large positive Δ S m ○ values, and so the micellization process is entropy-driven.
Keywords: Ionic liquid-type Gemini imidazolium surfactant; Surface tension; Adsorption efficiency; Electrical conductivity; Thermodynamics of micellization; Activation energy;

Multiresponsive self-assembled liquid crystals with azobenzene groups by Miao Xu; Liqin Chen; Yifeng Zhou; Tao Yi; Fuyou Li; Chunhui Huang (496-502).
An optical and electric field-responsive self-assembled complex containing nitril azobenzene groups and 1,3,5-triazine-2,4-diamine was obtained and characterized. Both the azobenzene precursor and the complex form a liquid-crystalline phase in a certain temperature range. The transition temperature from crystalline phase to liquid-crystalline mesophase was obviously decreased in the complex by the self-assembling. The self-assembled liquid crystals revealed good response to both stimuli of light irradiation and electric field, and the induced molecular orientation could be held even after the removal of the stimuli. The structural and mechanical investigation proved that the formation of hydrogen bonds and assembly-induced molecular dipolar change contributed to the multiresponding action. This kind of self-assembled complex thus has potential applications in imaging and data storage.Optical and electric field-responsive self-assembled liquid crystals containing intermolecular hydrogen bonding azobenzenes have been developed.
Keywords: Azobenzene; Hydrogen bond; Liquid crystal; Multiresponse; Self-assembly;

Analysis of electroosmotic flow of power-law fluids in a slit microchannel by Cunlu Zhao; Emilijk Zholkovskij; Jacob H. Masliyah; Chun Yang (503-510).
Electroosmotic flow of power-law fluids in a slit channel is analyzed. The governing equations including the linearized Poisson–Boltzmann equation, the Cauchy momentum equation, and the continuity equation are solved to seek analytical expressions for the shear stress, dynamic viscosity, and velocity distribution. Specifically, exact solutions of the velocity distributions are explicitly found for several special values of the flow behavior index. Furthermore, with the implementation of an approximate scheme for the hyperbolic cosine function, approximate solutions of the velocity distributions are obtained. In addition, a generalized Smoluchowski velocity is introduced by taking into account contributions due to the finite thickness of the electric double layer and the flow behavior index of power-law fluids. Calculations are performed to examine the effects of κH, flow behavior index, double layer thickness, and applied electric field on the shear stress, dynamic viscosity, velocity distribution, and average velocity/flow rate of the electroosmotic flow of power-law fluids.Figure shows the dimensionless velocity distribution v x / V s (normalized by the generalized Smoluchowski velocity that is introduced in this study) for various values of the fluid behavior index, n.
Keywords: Microfluidics; Electroosmotic flow; Non-Newtonian power-law fluids; Generalized Smoluchowski equation; Fluid behavior index;

We present a finite-element/discrete-element numerical model for calculating full trajectories of cylindrical metallic colloids in liquid flows and subjected to non-uniform electric fields. The effect of the particle orientation relative to the liquid flow is investigated by considering barcode magnetic nanowires pinned in different directions by applying uniform magnetic fields. We compare the motion of free as well as vertically and horizontally pinned nanowires and demonstrate that their nanoassembly may accurately be tuned by magnetically controlling the orientation during the dielectrophoretic capture.
Keywords: Dielectrophoresis; Metallic colloids; Magnetic nanowires; Nanoassembly; Numerical modeling;

In this study, multifunctional nanoparticles containing thermosensitive polymers grafted onto the surfaces of 6-nm monodisperse Fe3O4 magnetic nanoparticles coated by silica were synthesized using reverse microemulsions and free radical polymerization. The magnetic properties of SiO2/Fe3O4 nanoparticles show superparamagnetic behavior. Thermosensitive PNIPAM (poly(N-isopropylacrylamide)) was then grafted onto the surfaces of SiO2/Fe3O4 nanoparticles, generating thermosensitive and magnetic properties of nanocomposites. The sizes of fabricated nanoparticles with core–shell structure are controlled at about 30 nm and each nanoparticle contains only one monodisperse Fe3O4 core. For thermosensitivity analysis, the phase transition temperatures of multifunctional nanoparticles measured using DSC was at around 34–36 °C. The magnetic characteristics of these multifunctional nanoparticles were also superparamagnetic.Multifunctional nanoparticles were synthesized by grafting PNIPAM onto the surfaces of SiO2/Fe3O4 nanoparticles. The physical properties of these nanoparticles show superparamagnetic behavior with thermosensitivity.
Keywords: Core–shell structure; Magnetic nanoparticles; Superparamagnetism; Multifunctional; Thermosensitive;

Surface charging of layered double hydroxides during dynamic interactions of anions at the interfaces by Zhi Ping Xu; Yonggang Jin; Shaomin Liu; Zheng Ping Hao; Gao Qing (Max) Lu (522-529).
In this research, we investigated the effect of dynamic anion adsorption/exchange on the surface charging property of Mg2Al―Cl―LDH and Mg2Al―CO3 ―LDH particles that show the average zeta potential of 41 and 34 mV in the as-prepared suspension, respectively. The addition of NaCl up to 3 × 10 −3   M in the suspension does not obviously affect the zeta potential of both LDHs, which can be attributed to the less affinity of Cl to LDH. The introduction of Na2CO3 severely reduces the zeta potential at the CO2− 3 concentration higher than 1 × 10 −4   M , and to the negative value in both LDH systems at ca. 2 × 10 −3   M , which is presumably resulted from the exchange and the re-orientation of CO2− 3 in a tilt/vertical style on the surface. All four organic anions (dodecyl sulfate, folate, citrate and polyacrylate) also significantly affect the zeta potential of the LDH particles. At the lower concentrations of organic anionic groups ( < 1 × 10 −4   M ) , the zeta potential was slightly affected, i.e. limited exchange/adsorption. However, the concentration increasing to some point suddenly decreases and reverses the zeta potential of the LDH particles, which is presumably caused by the hydrophobic interactions that bind the hydrophobic hydrocarbon chains (especially in dodecyl sulfate) into the micelle-like bilayer bunches on the LDH surface. In addition, the effect of pH in 5.5–11.0 on the LDH particle surface charging is mainly reflected through the conversion of CO2− 3 to HCO 3/H2CO3 when pH decreases from ca. 11 to 6, with limited contribution from protonation/deprotonation and exchange/adsorption.The adsorption/exchange of dodecyl sulfate and the self-assembly on the surface change the zeta potential of LDH particles from the positive to the negative.
Keywords: Layered double hydroxide; Zeta potential; Surface charging; Anion adsorption and exchange; Point of zero charge; Agglomeration;

Effects of unsaturation on film structure and friction of fatty acids in a model base oil by S.M. Lundgren; M. Ruths; K. Danerlöv; K. Persson (530-536).
The normal and friction forces between layers of three fatty acids (stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid) have been measured in n-hexadecane with a surface forces apparatus. Stearic, oleic, and dehydroabietic acid form loose-packed monolayers on mica surfaces when adsorbed from dry n-hexadecane. Linoleic acid forms an additional dimer layer between monolayer-covered surfaces, where it is stabilized by interactions between the double-bond-rich regions of the molecules. The monolayers formed by linoleic and dehydroabietic acid are thinner than the ones formed by stearic and oleic acid, but are not as easily removed from between the mica surfaces when the load or pressure is increased. The friction force increased linearly with load in all systems, and the friction coefficient increased with increasing unsaturation. Linoleic acid showed two regimes of linear friction with increasing load, corresponding to two different film thicknesses. Its friction was sensitive to sliding speed and adsorption time, and the thinner film observed at higher load had a lower friction coefficient. Such features were not observed for stearic and oleic acid, where the monolayers were removed and the friction coefficient changed to that of pure n-hexadecane at a pressure of 3.5 MPa.The friction coefficient measured between linoleic acid monolayers adsorbed on mica from n-hexadecane is lowered by a change in molecular orientation at a pressure of 2.5–3 MPa.
Keywords: Friction; Surface forces; Adsorption; Fatty acid; Rosin acid; Surface forces apparatus (SFA); Mica;

Fabrication of platy apatite nanocrystals loaded with TiO2 nanoparticles by two-step emulsion method and their photocatalytic activity by Mitsunobu Iwasaki; Yuki Miyamoto; Seishiro Ito; Tsutomu Furuzono; Won-Kyu Park (537-540).
Nanometer-sized TiO2 island structure on the platy hydroxyapatite nanocrystals (HAp) has been accomplished by two-step emulsion process. At the first step, platy HAp nanocrystals, of which size was in the range of 70–200 nm after heat-treatment at 1078 K for 1 h, were prepared using the W/O emulsion system. Before the second step, HAp nanocrystals were immersed in NaH2PO4 solution for the formation of hydroxyl group on their surface. In the following, titanium tetraisopropoxide reacted with the hydroxyl group of HAp surface to form TiO2 nanoparticles on the surface of HAp nanocrystals, which were dispersed in the micrometer-sized methanol droplets of polyethylene cetylether–cyclohexane mixture (methanol/oil emulsion). The resulting hydroxyapatite nanocrystals loaded with TiO2 nanoparticles showed the high photocatalytic activity comparing to the commercial TiO2 catalyst.Nanometer-sized TiO2 island structure with 10–20 nm in diameter was formed on the platy hydroxyapatite nanocrystal (HAp) in the range of 70–200 nm.
Keywords: TiO2 island structure; Platy hydroxyapatite nanocrystals; Methanol/oil emulsion; Photocatalytic activity;