John Ralston

A Three-Color, Solid-State, Three-Dimensional Display

A three-color, solid-state, volumetric display based on two-step, two-frequency upconversion in rare earth-doped heavy metal fluoride glass is described. The device uses infrared laser beams that intersect inside a transparent volume of active optical material to address red, green, and blue voxels by sequential two-step resonant absorption. Three-dimensional wire-frame images, surface areas, and solids are drawn by scanning the point of intersection of the lasers around inside of the material. The prototype device is driven with laser diodes, uses conventional focusing optics and mechanical scanners, and is bright enough to be seen in ambient room lighting conditions. QuickTime movie of the three-dimensional display.

Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of “green” applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Functionalized gold nanoparticles: Synthesis, structure and colloid stability

Gold nanoparticles and their arrays are some of the most studied nanomaterials, with promising applications in many fields such as electronics, optoelectronics, catalysis and biology. In order to protect bare gold nanoparticles from aggregation, to manipulate the optical, electronic and catalytic properties of the gold core, as well as to control interfacial properties, the gold nanoparticles are generally capped by an organic layer. Previous studies [C.D. Bain, G.M. Whitesides, J. Am. Chem. Soc. 110 (1988) 3665-3666] have revealed that many phenomena (e.g., wetting, friction and adhesion), are sensitive to the top few angstroms of a surface. The interfacial properties of a gold surface derivatized with a self-assembled monolayer will thus be dictated by the functionalities present on the outer side of the monolayer. The synthesis, functionalization and surface structure of monolayer-protected gold nanoparticles have been intensively studied in recent times [M.-C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293-346]. In addition, the aggregation and dispersion of colloidal nanoparticles is one of the key issues related to their potential applications. The forces that govern the colloid stability of nanoparticle dispersions, and how to control them, have yet to be fully investigated. Here special attention has been paid to control of colloid stability using external stimuli. In this feature article, the following five areas are reviewed: synthesis and applications of nanostructured particles; formation and structure of self-assembled monolayer protected gold nanoparticles; colloid stability-DLVO and non-DLVO forces; photochemistry, photochromism and pyrimidine; and manipulation of colloid stability with external stimuli.

Particle-bubble collision models--a review

A critical review of the various models existing in the literature for the calculation of the collision efficiency between particles and single, rising gas bubbles is presented. Although all of these collision models predict that the collision efficiency increases with particle size, their dependence on the latter is different because of the various assumptions and hydrodynamic conditions used in each model. Collision efficiencies of quartz particles with single bubbles have been obtained from experimental flotation experiments under conditions where the attachment and stability efficiencies were at, or near, unity. These collision efficiencies were then used to test various collision models. Good agreement between the experimental and calculated collision efficiencies was only obtained with the Generalised Sutherland Equation. The differences in collision efficiencies obtained between the various models were mainly explained in terms of, firstly, the degree of mobility of the bubble surface and, secondly, a consideration of the inertial forces acting on the particles.

Differential capacitance of the double layer at the electrode/ionic liquids interface

The differential capacitance of the electrical double layer at glassy carbon, platinum and gold electrodes immersed in various ionic liquids was measured using impedance spectroscopy. We discuss the influence of temperature, the composition of the ionic liquids and the electrode material on the differential capacitance/potential curves. For different systems these curves have various overall shapes, but all include several extremes and a common minimum near the open circuit potential. We attribute this minimum to the potential of zero charge (PZC). Significantly, the differential capacitance generally decreases if the applied potential is large and moving away from the PZC. This is attributed to lattice saturation [A. A. Kornyshev, J. Phys. Chem. B, 2007, 111, 5545] effects which result in a thicker double layer. The differential capacitance of the double layer grows and specific adsorption diminishes with increasing temperature. Specific adsorption of both cations and anions influences the shapes of curves close to the PZC. The general shape of differential capacitance/potential does not depend strongly on the identity of the electrode material.

High-resolution in situ x-ray study of the hydrophobic gap at the water–octadecyl-trichlorosilane interface

The knowledge of the microscopic structure of water at interfaces is essential for the understanding of interfacial phenomena in numerous natural and technological environments. To study deeply buried liquid water–solid interfaces, high-energy x-ray reflectivity measurements have been performed. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane, provide strongly hydrophobic substrates. We show interfacial density profiles with angstrom resolution near the solid–liquid interface of water in contact with an octadecyl-trichlorosilane layer. The experimental data provide clear evidence for the existence of a hydrophobic gap on the molecular scale with an integrated density deficit ρd = 1.1 Å g cm−3 at the solid–water interface. In addition, measurements on the influence of gases (Ar, Xe, Kr, N2, O2, CO, and CO2) and HCl, dissolved in the water, have been performed. No effect on the hydrophobic water gap was found.

The influence of particle size and contact angle in mineral flotation

Abstract The flotation behaviour of quartz particles, whose surfaces have been hydrophobized to varying extents with an organosilane compound, has been studied over the particle-size range from 15 to 125 μm in diameter under conditions of known bubble size and relative turbulent velocity. For a given particle size, there is a unique contact angle below which the particle will not float. This leads to the concept of a flotation domain, a region determined by particle size and contact angle, within which flotation is possible. Coarse-particle behaviour is predicted by the kinetic theory of flotation proposed by Schulze, whereas there is only a qualitative agreement with Scheludkos theory of fine-particle flotation. Calculated induction times, in conjunction with observed flotation behaviour, indicate that the attachment process is most efficient for particles of about 38 μm in diameter under the experimental conditions. The dependence of rate constant on particle size was found to be essentially linear. Implications for flotation practice are discussed.

Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions

Abstract The influence of an anionic polyacrylamide–acrylate copolymer (PAM) and a nonionic polyethylene oxide (PEO) polymer on the surface chemistry, shear yield stress, settling rates and consolidation behaviour of kaolinite dispersions has been investigated at pH 7.5. The magnitude of the particle zeta potential decreased with increasing flocculant concentration for both flocculants but was much greater for PEO, due to an increased adsorbed layer thickness compared with PAM. Furthermore, the adsorption studies showed a higher affinity for PEO than PAM. The optimum flocculation for improved clarification rate occurred at less than half the plateau adsorbed amount for both polymers. At similar polymer concentrations, the kaolinite floc sizes were larger and the settling rates greater in the presence of PEO than PAM. Upon shear, the consolidation of pre-sedimented kaolinite slurries significantly improved under flocculation with PEO, while PAM produced no such effect. The magnitude of yield stress was strongly dependent upon the polymer structure, with greater yield stress (stronger interparticle bridging forces) observed for PEO than for PAM flocs. The difference in shear sensitivity of the flocculated slurries may be attributed to polymer structure-related adsorption and interfacial conformation behaviour. PEO polymer chains adsorb via hydrogen bonding interactions between the ether oxygen (Lewis base) and OH groups (Bronsted acid) associated with aluminol and silanol groups, which led to flexible and compressible floc structures. For PAM, however, although the adsorption is still via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and the polymers primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the polymers COO− pendant groups and the negatively charged kaolinite surface. This, in conjunction with possible steric hindrance due to PAM polymer chain branching, led to the formation of loose, open and fragile flocs. A strong correlation between the polymer structure type and flocculation, shear sensitivity and the dewatering behaviour of kaolinite dispersion was established.

Bubble particle heterocoagulation under turbulent conditions

An analytical model that enables the calculation of the flotation rate constant of particles as a function of particle size with, as input parameters, measurable particle, bubble, and hydrodynamic quantities has been derived. This model includes the frequency of collisions between particles and bubbles as well as their efficiencies of collision, attachment, and stability. The generalized Sutherland equation collision model and the modified Dobby-Finch attachment model developed previously for potential flow conditions were used to calculate the efficiencies of particle-bubble collision and attachment, respectively. The bubble-particle stability efficiency model includes the various forces acting between the bubble and the attached particle, and we demonstrate that it depends mainly on the relative magnitude of particle contact angle and turbulent dissipation energy. The flotation rate constants calculated with these models produced the characteristic shape of the flotation rate constant versus particle size curve, with a maximum appearing at intermediate particle size. The low flotation rate constants of fine and coarse particles result from their low efficiency of collision and low efficiencies of attachment and stability with gas bubbles, respectively. The flotation rate constants calculated with these models were compared with the experimental flotation rate constants of methylated quartz particles with diameters between 8 and 80 micro m interacting with gas bubbles under turbulent conditions in a Rushton flotation cell. Agreement between theory and experiment is satisfactory.

An electrokinetic study of pyrite oxidation

Abstract The electrokinetic properties of pyrite have been studied as a function of pH in various pretreatment conditions: conditioning in argon, nitrogen, air and oxygen atmospheres, at several conditioning pH values and for several conditioning times. The zeta potential of “virgin” pyrite is negative over the pH range from 3–10. Upon exposure to oxygen, a reversal of zeta potential from negative to positive occurs at low pH values. This reversal takes place more rapidly at lower conditioning pH values. The isoelectric point of “virgin” pyrite has been estimated to lie at a pH of 1.2 ± 0.4. A mechanism for dissolution of iron from the surface followed by electrostatic adsorption of positively charged iron hydroxide species onto negatively charged surface sites on pyrite has been proposed to account for the observed results. Good agreement has been found between the experimental and the calculated zeta potential data using electrical double layer theories.