George V. Franks

Surface chemistry-rheology relationships in concentrated mineral suspensions

The rheological behaviour of concentrated mineral suspensions can be controlled by understanding and regulating the net particle-particle interaction force. This paper summarizes the experimental results from studies showing how the yield stress of concentrated aqueous suspensions of alumina, zirconia and kaolin can be controlled through regulation of pH and the addition of inorganic electrolytes, ionic surfactants and polyelectrolytes (polymers). The various kinds of inter-particle forces acting in aqueous suspensions of fine particles are reviewed and used to interpret the experimental results.

The isoelectric points of sapphire crystals and alpha-alumina powder

Abstract Streaming potential measurements and atomic force microscropy (AFM) were used to determine the zeta potentials, diffuse layer potentials and isoelectric points (ieps) of alpha alumina sapphire single crystals for four different crystallographic orientations: c-plane, 0001 ; a-plane, 11 2 0 ; m-plane, 10 1 0 ; and r-plane, 1 1 02 . Both types of measurements indicated that the iep of the sapphire crystals was between about pH 5.0 and 6.0 for all crystals investigated. Unfortunately the techniques utilized could not conclusively differentiate any subtle differences in iep between the four orientations studied. The iep of alpha alumina powder was found to be pH 9.4. The difference in the ieps of the two different types of alpha alumina (single crystal and powder) is attributed to the presence of different types of surface hydroxyl groups on the two different types of surfaces. Powder is likely to have a greater fraction of singly coordinated surface hydroxyl groups (that have a high p K a ), while sapphire single crystals are more likely to have most surface hydroxyls multiply coordinated (which have a low p K a ).

Effect of interparticle forces on shear thickening of oxide suspensions

The rheological behavior of concentrated nearly monodisperse, nearly spherical silica, and narrow particle size distribution alumina suspensions was measured in Couette geometry. Attractive forces between the particles (suspensions flocculated at or near the isoelectric point) produce high viscosities. Repulsive forces between the particles produce dispersed suspensions and decrease the viscosity at low and intermediate shear rates. The viscosity of the dispersed suspensions shear thickens (abruptly increases and approaches that of the flocculated suspensions) at a critical shear rate (and therefore stress) provided the volume fraction of solids of the suspension is sufficiently high. As the pH is adjusted farther from the isoelectric point the critical shear rate (and stress) increase. At pH away from the isoelectric point the addition of salt decreases the critical shear rate (and stress). Thus shear thickening is not only dependent upon hydrodynamic interactions but also depends on surface forces, in p...

Solid–liquid separations with a temperature-responsive polymeric flocculant: Effect of temperature and molecular weight on polymer adsorption and deposition

The effects of temperature and molecular weight of the temperature-responsive polymer, poly(N-isopropylacrylamide) (PNIPAM) were investigated in the solid-liquid separation of silica and alumina mineral particles. Suspensions dosed with PNIPAM at 25 degrees C were stable and did not settle. When the temperature was raised above the polymer lower critical solution temperature (LCST) (>32 degrees C), the suspensions were found to have high settling rates, large particle aggregate sizes and high suspension shear yield stresses (tau(y)). The sediment bed solids volume fraction (phi(f)), of these suspensions was found to increase after a temperature decrease below the polymer LCST and was attributed to a decrease in the attractive particle-particle interactions as shown by a corresponding decrease in shear yield stress, with decreasing temperature. Settling rates were found to increase with molecular weight when suspensions were dosed at 25 degrees C and settled at 50 degrees C. Increasing polymer molecular weight resulted in increased molecular polymer adsorption at 25 degrees C. Greater initial adsorbed amounts of polymer on the surface produced more nucleation sites for deposition of additional polymer as the temperature was increased from 25 degrees C to above the LCST where polymer phase separation occurred. When the polymer was dosed at 50 degrees C, the rate of sedimentation was very low. Under these conditions, the polymer molecules associate with each other to form polymer aggregates of typically 1250 nm diameter. These colloidal polymer aggregates do not readily deposit on the particles surfaces such that mineral particle aggregation does not readily occur.

Temperature controlled surface hydrophobicity and interaction forces induced by poly (N-isopropylacrylamide)

Poly (N-isopropylacrylamide) (PNIPAM) is a temperature responsive polymer. At temperature below its lower critical solution temperature (LCST 32 degrees C) PNIPAM is soluble in water and hydrophilic. At temperature above the LCST, the polymer becomes hydrophobic and insoluble in water. At temperatures above the LCST, PNIPAM has been shown to induce flotation of previously hydrophilic minerals. The mechanism is believed to be an increase in particle hydrophobicity when PNIPAM adsorbs to the particle surfaces at high temperature. This paper investigates the interaction forces between bare silica surfaces in PNIPAM solutions. The influence of three phase contact angles on these interactions, in the presence of polymers of different molecular weights, is also examined. It was found that the presence of PNIPAM on silica surfaces significantly increases their hydrophobicity at a temperature above the LCST. The AFM measurements of surface forces at high temperature also showed that strong adhesion is present between the PNIPAM coated surfaces, which is absent in the absence of polymer. These findings lead to the conclusion that the detected attractive force and subsequent adhesion result from hydrophobic attraction induced by PNIPAM at temperature above the LCST.

The mechanism for hydrothermal growth of zinc oxide

The mechanism for hydrothermal growth of ZnO was studied in ammonium hydroxide solution at pH near 11 (0.3 M NH4OH). The products formed at 20–90 °C and ambient pressure were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photon Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectroscopy (SIMS). Under these conditions, the growth of ZnO occurs via the initial precipitation of e-Zn(OH)2 (Wulfingite), which subsequently dehydrates, to form Wurtzite ZnO. Isotope tracking experiments show that most of the oxygen atoms do not mix with water during the conversion from Wulfingite to ZnO, and thus that the reaction proceeds primarily in the solid phase rather than through dissolution then reprecipitation. XPS results show that the surface of hydrothermal ZnO consists primarily of zinc hydroxide at lower temperatures, and there is a significant fraction of zinc hydroxide persisting at higher temperatures. Together these results show that much of the solid-solution interface is mainly comprised of zinc hydroxide not ZnO. These findings may have implications for understanding how small organic molecules can be used to control the morphology of zinc oxide crystals grown under hydrothermal conditions.

Temperature responsive flocculation and solid-liquid separations with charged random copolymers of poly(N-isopropyl acrylamide).

Temperature-responsive random copolymers based on poly(N-isopropyl acrylamide) (PNIPAM) with 15 mol% of either acrylic acid or dimethylaminoethyl acrylate quaternary chloride were prepared. The effect of the charge and its sign were investigated in the solid-liquid separation of silica and alumina mineral suspensions. The results were compared to PNIPAM homopolymer of similar molecular weight. PNIPAM copolymers of the same charge as the particles (co-ionic PNIPAM) act as dispersants at both 25°C and 50°C. Flocculation occurs when counter-ionic PNIPAM facilitates selective aggregation and rapid sedimentation of minerals at both 25°C and 50°C. Adsorption and desorption studies showed that, unlike non-ionic PNIPAM, little desorption of the counter-ionic copolymers from the oxides occurred after cooling a suspension from 50°C to below the lower critical solution temperature. Thus, incorporation of counter-ionic charge into the temperature sensitive polymer PNIPAM was found to reduce the sediment bed consolidation upon cooling when compared to PNIPAM homopolymers. The lack of secondary consolidation upon cooling is attributed to attractive inter-particle forces, such as conventional polyelectrolyte flocculation mechanisms (bridging, charge neutralization or charge patch) which persist at both 25°C and 50°C when counter-ionic PNIPAM is used. On the other hand, it was possible to obtain rapid sedimentation with the counter-ionic PNIAPMs even when they were added to the suspension already at 50°C, a process which has not been possible with neutral PNIPAM homopolymers.

First principles pKa calculations on carboxylic acids using the SMD solvation model: effect of thermodynamic cycle, model chemistry, and explicit solvent molecules.

Aqueous pK(a) values are calculated from first principles for a set of carboxylic acids using the SMD solvation model with various model chemistries, thermodynamic cycles, and treatments of explicit solvation. In all, 108 unique theoretical protocols are examined. The direct (D) and water proton exchange (PX) cycles are trialled along with a new approach, termed the semidirect (SD) cycle. The SD thermodynamic cycle offers some improvements over the D and PX schemes, as it bypasses the gas-phase heterolytic bond dissociation calculation required in the conventional D approach while also avoiding an aqueous OH(-) calculation required by the PX method when using water as the reference acid. With all three cycles, the recommended model chemistry employs M05-2X/cc-pVTZ Gibbs energies of solvation with a single discrete water molecule and a high-level composite method for the gas-phase reaction energies. With the SD cycle, these calculations result in a mean unsigned error of less than 1 pK(a) units, with respective mean signed error and maximum unsigned error of less than 0.5 and 2 pK(a) units. Similar results are obtained with the D and PX cycles, and further improvement is required in both the gas and aqueous phase ab initio energy calculations before we can truly discriminate between the thermodynamic cycles investigated here.

Photochromic, Metal-Absorbing Honeycomb Structures

We demonstrate the synthesis and use of a spiropyran functional polymer to form highly ordered honeycomb materials by the breath figure technique, which is based on the self-assembly of water droplets. These materials undergo rapid and intense color changes both in solution and as porous films by irradiation with light (UV or visible). We also demonstrate the metal binding ability of these polymers ultimately to create hybrid organic-inorganic porous structures. Furthermore, by reduction of the metal and calcination of the organic materials, unique palladium microrings can be prepared. The methods described are general techniques that may be applied to a range of heavy metals.

Particle sizes and stability of UHT bovine, cereal and grain milks

Abstract Commercial ultra high temperature (UHT) processed bovine, oat, rice and soy milks were analysed for stability and particle size. Product stability was determined using a turbidity analyser to follow particle migration over time. Changes in the level of backscattering produced by the sample as a function of time were related to the stability of the sample. The results indicated that the level of stability of the milks varied and the type of destabilisation phenomenon (sedimentation or creaming) also varied between the milks. The relative stability of the samples studied was determined to be: rice≪oat