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A new microcolumn flotation cell for determining the wettability and floatability of minerals.

Flotation is one of the most important physicochemical processes for mineral separations and other recovery operations. Flotation machines have been developed since the beginning of the 19th century and are still under intensive research and development. The cell we devised is a combination of the Canadian column flotation cell and the Partridge-Smith cell. The materials used for the construction of the new cell are cheap and use available laboratory accessories and aquarium materials. The cell functions well in terms of its scale, control, and sample requirement. It can be used both in the laboratory for research and in classrooms for demonstrations of experiments. Some of the data obtained by the flotation method using this cell are in good agreement with data measured independently on the same minerals by the contact angles method. The critical values of surface tension of wetting (gamma(c)) for talc, sulfur, and chemically treated surfaces of calcite and barite obtained by the contact angle measurements were 31, 26, 30.5, and 31.2 mN/m, respectively. On the other hand, the gamma(c) values of those minerals, obtained using our new designed flotation cell, were 30, 28, 31.4, and 34.5 mN/m, respectively. The measurements obtained in our experiment are also comparable to those previously published for the same minerals.

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Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar fibers/epoxy composites using micromechanical techniques and surface wettability.

Comparison of interfacial properties and microfailure mechanisms of oxygen-plasma treated poly(p-phenylene-2,6-benzobisoxazole (PBO, Zylon) and poly(p-phenylene terephthalamide) (PPTA, Kevlar) fibers/epoxy composites were investigated using a micromechanical technique and nondestructive acoustic emission (AE). The interfacial shear strength (IFSS) and work of adhesion, Wa, of PBO or Kevlar fiber/epoxy composites increased with oxygen-plasma treatment, due to induced hydrogen and covalent bondings at their interface. Plasma-treated Kevlar fiber showed the maximum critical surface tension and polar term, whereas the untreated PBO fiber showed the minimum values. The work of adhesion and the polar term were proportional to the IFSS directly for both PBO and Kevlar fibers. The microfibril fracture pattern of two plasma-treated fibers appeared obviously. Unlike in slow cooling, in rapid cooling, case kink band and kicking in PBO fiber appeared, whereas buckling in the Kevlar fiber was observed mainly due to compressive and residual stresses. Based on the propagation of microfibril failure toward the core region, the number of AE events for plasma-treated PBO and Kevlar fibers increased significantly compared to the untreated case. The results of nondestructive AE were consistent with microfailure modes.

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Predicting the wettability of quartz surfaces exposed to dense nonaqueous phase liquids.

A model incorporating electrostatic and van der Waals forces was used to predict the wetting behavior of several systems containing a quartz surface, water, and a dense nonaqueous phase liquid (DNAPL). The disjoining pressure between the quartz/water and a water/DNAPL interface as a function of their separation was calculated from a full solution of the nonlinear Poisson-Boltzmann equation allowing for charge regulation. For the pH regime associated with a positive pressure barrier, the quartz surface was predicted to be water wetting. The model takes into account the charging behavior of an isolated DNAPL/water interface as inferred from electrophoretic mobility data. It provides excellent predictions for four coal tar and creosote samples tested, where the quartz was oil-wetting below pH 5 and water-wetting at higher pH values. In a fifth sample, the transition occurred above pH 7, which indicates that mechanisms other than electrostatic interaction control the wetting behavior in this sample.

Environmental Pollution↗

Imidazolium ion-terminated self-assembled monolayers on Au: effects of counteranions on surface wettability.

Self-assembled monolayers presenting imidazolium ions at the tail ends (SAMIMs) having different counteranions have been prepared on Au, and the measurement of water contact angles of the surfaces proved to be an extremely valuable simple technique for quantifying the effects of counteranions on hydrophilicity and hydrophobicity of SAMIMs, which will be extrapolated to the water miscibility of the related ionic liquids.

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Molecular origins of wettability of hydrophobic poly(vinylidene fluoride) microporous membranes on poly(vinyl alcohol) adsorption: Surface and interface analysis by XPS.

Irreversible adsorption of poly(vinyl alcohol) (PVA) on hydrophobic, porous poly(vinylidene fluoride) (PVDF) membranes was carried out using aqueous PVA solution. Water permeation was observed in PVDF microporous membranes after PVA adsorption, and maximum permeability was obtained after treatment with 4% PVA solution. Water permeability increased linearly with increasing PVA concentration up to 4%, and then a marginal decrease with a further increase in PVA concentration occurred. PVA adsorbed PVDF membranes were subjected to intense physicochemical analysis, especially with XPS. XPS results display the presence of an interface between PVA and PVDF, and the binding energy (BE) of the interface is low for the PVDF membranes treated with 4% PVA. Carbon from CF2-groups and F 1s core level clearly showed a decrease in its content on the surface after PVA adsorption and showed a minimum fluorine content at 4% PVA. F 1s BE shifts by 0.5 eV upon PVA adsorption and is independent of PVA concentration. EDAX analysis indicates that the bulk oxygen content remains within 4.5 +/- 0.6% and is independent of the PVA concentration. Nonetheless, a large amount of surface atom percentage of oxygen (20 +/- 4%) from O 1s core level shows an increase in PVA content on the surface of PVDF, and it is restricted mostly to the surface. The 4% PVA treated PVDF membrane clearly shows a broadening of O 1s core level to lower BE and indicates the interaction between PVDF and PVA which is significantly different compared to any other compositions. A new valence band feature at low BE, which is nonexistent on PVDF, develops after PVA adsorption. This indicates that the shift in the nature of the highest occupied molecular orbital (HOMO) derived mostly from oxygen; simultaneously, a suppression in the PVDF derived band indicates the change in nature of the PVA adsorbed surfaces from hydrophobic to hydrophilic. The above observations also suggest an irreversible electronic interaction between PVA and PVDF, possibly through charge transfer.

Adsorption↗

Reversible wettability of a chemical vapor deposition prepared ZnO film between superhydrophobicity and superhydrophilicity.

A superhydrophobic ZnO thin film was fabricated by the Au-catalyzed chemical vapor deposition method. The surface of the film exhibits hierarchical structure with nanostructures on sub-microstructures. The water contact angle (CA) was 164.3 degrees, turning into a superhydrophilic one (CA < 5 degrees) after UV illumination, which can be recovered through being placed in the dark or being heated. The film was attached tightly to the substrate, showing good stability and durability. The surface structures were characterized by scanning electron microscopy and atomic force microscopy.

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Thermally- and photoinduced changes in the water wettability of low-surface-area silica and titania.

The surface properties of silica and titania are mainly determined by the presence, density, and type of terminal hydroxyl groups (Si-OH "silanol" and Ti-OH "titanol"). Thermal treatment at elevated temperatures causes dehydroxylation on both surfaces, confirmed by streaming potential and ToF-SIMS measurements. The magnitude of the zeta potential markedly decreases after heat treatment, but the IEP is not affected. The intensity ratio MOH(+)/M(+) (M = Si or Ti), which reflects the surface density of OH groups, also decreases noticeably after high-temperature treatment. The mechanism is condensation of adjacent silanol/titanol groups into siloxane/titanoxane bonds. Ultraviolet light (lambda = 254 nm) has little effect on silica but rapidly induces hydrophilicity on titania surfaces. There is a strong correlation between the amount of hydrocarbons adsorbed on the surface and the density of titanol groups (thence the water contact angle). The effect of UV radiation can be entirely attributed to photolytic decomposition of organic contaminants. Dehydroxylated titania and silica (at 1050 degrees C) show very different wetting behavior: silica is moderately hydrophobic (water contact angle of about 40 degrees), while titania is hydrophilic (0 degrees). This dissimilarity can be explained with a simple model estimating the van der Waals and acid-base interfacial interactions.

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Nanostructuring of a polymeric substrate with well-defined nanometer-scale topography and tailored surface wettability.

This study demonstrates a simple and highly reproducible method for fabricating well-defined nanostructured polymeric surfaces with aligned nanoembosses or nanofibers of controllable aspect ratios, showing remarkable structural similarity with interesting natural biostructures such as the wing surface of Cicada orni and the leaf surface of Lotus. Our studies on the present biomimetic surfaces revealed that the wetting property of the nanostructured surface of a given chemical composition could be systematically controlled by rendering nanometer-scale roughness. The nanofabricating method we developed can be readily extended to other thermoplastic polymeric materials (e.g., light-emitting polymers, conducting polymers, block copolymers, liquid crystalline polymers), and it could be applied to developing a new generation of optical and electronic devices.

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Aggregation states and surface wettability in films of poly(styrene-block-2-perfluorooctyl ethyl acrylate) diblock copolymers synthesized by atom transfer radical polymerization.

Well-defined poly(styrene-block-2-perfluorooctyl ethyl acrylate) [P(St-b-PFA)] copolymers with various chemical compositions were synthesized by atom transfer radical polymerization. Films of P(St-b-PFA) were structurally characterized, from bulk to surface, on the basis of transmittance electron microscopic observation and small-angle X-ray scattering, X-ray photoelectron spectroscopic, and contact angle measurements. For a comparison, poly(styrene-random-2-perfluorooctyl ethyl acrylate) [P(St-ran-PFA)] copolymers were also synthesized by conventional free radical polymerization. While P(St-b-PFA) with the 2-perfluorooctyl ethyl acrylate (PFA) content higher than 18.7 mol % formed a typical phase-separated cylinder structure, P(St-b-PFA) with a lower PFA content and P(St-ran-PFA) were in a miscible state. Since the perfluoroalkyl groups possess extremely low surface energy, they were preferentially segregated at the film surface, resulting in the formation of the PFA surface layer. This was the case for all P(St-b-PFA) films examined, although the aggregation state at the surface was strongly dependent on the PFA content. In the case of the P(St-b-PFA) with the PFA content higher than 18.7 mol %, both advancing and receding contact angles for water were 120 degrees and even larger with almost no hysteresis. In addition, extremely excellent oil-repellent surface properties such as advancing and receding contact angles for dodecane of 76 degrees and 75 degrees were also observed. However, these intriguing liquid-repellent properties were not observed for the films of miscible P(St-b-PFA) and P(St-ran-PFA). Therefore, it can be concluded that the internal structure beneath the surface as well as the surface itself should be deeply considered to design excellent and stable liquid-repellent materials.

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Temperature-actuated changes in wettability at elastomer/water interfaces.

Surface modification of 1,4-polybutadiene and cis-1,4-polyisoprene to introduce polar functional groups provided surfaces that reconstructed reversibly against water as a function of temperature. These surfaces became hydrophobic in contact with hot water, but their original hydrophilicity returned upon equilibration against cold water. Repeated cycling between hot and cold water, however, led to a damping of this reversibility. A series of parallel experiments on both the interfacial and bulk behavior of these elastomers strongly indicated that this damping was due to the alignment of extended interfacial chains during temperature cycling and to a decay of the restoring force on the interfacial chains under extension. These studies thus demonstrate that the interfacial behavior of elastomers can display close analogies to the bulk viscoelastic properties of the solid.

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Anisotropic surface energetics and wettability of macroscopic form I paracetamol crystals.

Advancing (theta(A)) and receding (theta(R)) contact angles were measured with several probe liquids on the external facets (201), (001), (011), and (110) of macroscopic form I paracetamol crystals as well as the cleaved (internal) facet (010). For the external crystal facets, dispersive surface energies gamma(d) calculated from the contact angles were found to be similar (34 +/- 1 mJ/m(2)), while the polar components varied significantly. Cleaving the crystals exposed a more apolar (010) surface with very different surface properties, including gamma(d) = 45 +/- 1 mJ/m(2). The relative surface polarity (gamma(p)/gamma) of the facets in decreasing order was (001) > (011) > (201) > (110) > (010), which agreed with the fraction of exposed polar hydroxyl groups as determined from C and O 1s X-ray photoelectron spectroscopy (XPS) spectra, and could be correlated with the number of non-hydrogen-bonded hydroxyl groups per unit area present for each crystal facet, based on the known crystal structures. In conclusion, all facets of form I paracetamol crystals examined exhibited anisotropic wetting behavior and surface energetics that correlated to the presence of surface hydroxyl groups.

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Wettability of zinc oxide surfaces with controllable structures.

Zinc oxide (ZnO) surfaces with controllable structures (i.e, microstructure, nanostructure, and micronanobinary structure) have been created by controlling pH at < 4 or > 10.5 in the Zn(gray) + H2O2 reaction. The resulting surface shows superhydrophobicity. It is found that the water contact angle (CA) of the surface with micronanobinary structure is greater than that of nanostructure and that of nanostructure is greater than that of the microstructure. Theoretical analysis is completely in agreement with the experimental results.

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Preparation and photocatalytic wettability conversion of TiO2-based superhydrophobic surfaces.

We present here a facile method for the preparation of TiO2-based superhydrophobic surfaces. It consists of two steps: (1) roughening of the TiO2 surface with a rf (radio frequency) plasma with CF4 as an etchant and (2) modification of the roughened TiO2 surface with an octadodecylphosphonic acid (ODP) monolayer. Plasma etching caused the thinning of the TiO2 film but at the same time enhanced its surface roughness. A discontinuous wedgelike surface microtexture was formed after etching for 30 s, which, after modification with a monolayer of ODP, showed Cassie-type water super-repellency with a contact angle (CA) hysteresis smaller than 2 degrees . The state of water super-repellency (water CA >165 degrees) could be converted to the state of superhydrophilicity (water CA approximately 0 degrees) by means of ultraviolet (UV) illumination as a result of the photocatalytic decomposition of the ODP monolayer by TiO2. Readsorption of ODP molecules leads directly to the recovery of water super-repellency.

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Wettable phosphorylcholine-containing polymers useful in blood filtration.

The use of phosphorylcholine (PC)-containing polymers has long been acknowledged as a method to improve the haemocompatibility of blood contacting devices. Such polymers were investigated for coating leukocyte filters, as a means of preserving precious platelet numbers and function. It was demonstrated that by use of such coated filters, the platelet recovery could be significantly increased by some 30%. This was however, balanced by a decrease in the leukocyte removal efficiency of the filter. More problematic however, was the poor critical wetting surface tension (CWST) of the filter (45 mNm(-1)). This was due to the surface expression of the hydrophobic groups of the polymer in air. In order to obtain a filter that could be immediately wetted by the blood and hence remove the need for any detrimental priming of the filter, the PC polymer was modified in order to reduce its hydrophobic content. A series of new cross-linkable polymers was developed that contained other hydrophilic monomers, yet could film-form to produce a coating stable to fluid contact. These materials demonstrated the required wetting characteristics (a CWST of >78 mNm(-1)), whilst retaining the excellent blood-contacting properties.

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