Consolidation behavior in sedimentation of TiO(2) suspensions in the presence of electrolytes

The consolidation of TiO(2) suspensions (anatase and rutile) due to gravity sedimentation in the presence of electrolytes has been investigated as a function of pH. Sodium and barium nitrate were used as flocculating electrolytes. The particle interaction was related to the zeta potential and the thickness of the electrical double layer, kappa(-1), by utilizing the repulsive barrier in the classical DLVO theory. The stability of the suspensions was represented as the average final solids content of the sediment cake, phi(fin). The batch sedimentation process was followed by scanning the sample cell with X-rays, from which the solids content and the particle size were calculated. Generally, dense sediments, with phi(fin) up to volume fractions of 0.5, were found for stable suspensions. Flocculated suspensions produced sediments with low phi(fin). The phi(fin) was observed to increase linearly with increasing repulsive barrier. However, at pH values only slightly higher than the isoelectric point (pH(iep)) the phi(fin) remained low until it returned to linearity at a pH much higher than pH(iep). This was attributed to the stronger affinity of sodium than of nitrate for the particle surface, which may be explained by the higher negative hydration energy of sodium. The stronger affinity of sodium was also shown as unsymmetrical distribution of phi(fin) around pH(iep), with stronger flocculation at pH>pH(iep). The interpretation of phi(fin) as a function of the repulsive barrier (or kappa(-1)) also made it possible to distinguish between the adsorption mechanisms of ions from solution. Addition of electrolyte at a fixed low and high pH (surface positively and negatively charged, respectively) clearly showed the specificity in adsorption and consequent flocculation of the barium ion from the indifferent nitrate. Sodium was, however, again observed to flocculate the TiO(2) suspensions slightly more strongly than nitrate.     

Enthalpies of Lyotropic Liquid Crystalline Phases. : 2. The Partial Molar Enthalpies of the Lamellar and Hexagonal Phases in the System N-Pentanol-Sodium N-Octanoate-Water

The relative partial molar enthalpies of the components in the system N-pentanol-sodium N-octanoate-water (C₅OH-NaC₈-H₂O) have been determined in the lamellar (D-) and hexagonal (E-) liquid crystalline phases formed in this system at 25°C. The enthalpies are correlated with Bragg spacings and earlier nmr studies. They indicate that (a) the minimum amount of water needed to hydrate the polar groups limits the region of existence of both D- and E-phase towards low water contents, (b) the interaction between -OH and -COO is especially important in stabilizing the D-phase containing large amounts of water, (c) the Bragg spacings and the enthalpies both change their concentration dependence markedly when the maximum amount of water that can be bound by primary hydration of the polar groups and the counter ions in the D-phase is exceeded, (d) enthalpies of transition from one phase to another are small compared to enthalpy changes with composition within the phases; the same holds for Bragg spacings. Hence, the composition of the sample (above all, the mole fraction of NaC₈) is more important than the phase structure in determining these properties. It is obvious that further studies of enthalpies in similar systems can give important information on the factors governing phase stabilities and structural changes within the phases.     

Synthesis of polyester from renewable feedstock: a comparison between microwave and conventional heating

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Wettability of model fountain solutions: The influence on topo-chemical and -physical properties of offset paper

The surface chemical and physical character of offset paper was studied before and after application of model fountain solutions based on isopropyl alcohol and an alcohol-free surfactant solution. The paper surface features were characterised with atomic force microscopy and the surface energies were determined by contact angle measurements. Changes in the surface chemical properties induced by the fountain solutions were investigated with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. Coated papers wetted with the surfactant solution revealed a slight increase in the root mean square roughness, but the isopropyl alcohol solution led to no observable changes. The change in sub-micro roughness is ascribed not only to substrate swelling or migration of coating constituents but also to the presence of surfactant on the surface. A change in the surface energy and particularly the polar contribution was observed after application of the surfactant solution. The X-ray photoelectron spectroscopy showed an increase in the oxygen-to-carbon ratio, which confirms the presence of surfactant on the surface. Time-of-flight secondary ion mass spectroscopy showed that the isopropyl alcohol solution did not change the elemental composition of the surface whereas the surfactant solution clearly did so. The distribution of surfactant on the surface was confirmed by mapping the characteristic fragments of the molecule.     

Nickelmodifiedlarge poremesoporoussilicas ascatalysts for methanol decomposition

Summary Ni-modifiedlarge poremesoporous silicasarecharacterized by XRD, N₂physisorption and TPR with H₂. Theeffectof the supportpore structure on their catalytic behaviorin methanol decomposition to H₂, CO and CH₄is studied.     

Electrokinetic studies of metal oxides in the presence of alkali trichloroacetates, trifluoroacetates, and trifluoromethanesulfonates

The electrokinetic potentials of anatase (titanium dioxide) and alpha-alumina were studied by means of various instruments in the presence of sodium and potassium trichloroacetates, trifluoroacetates, and trifluoromethanesulfonates at various pH, ionic strengths, and solid-to-liquid ratios. At concentrations up to 0.01 M, these salts do not induce substantial shifts in the isoelectric point of the oxides. Such behavior is typical of inert electrolytes.     

Thermogravimetric Analysis of γ-Aminopropyl-trimethoxysilane Adsorbed on Silica Support

When ethylene-vinyl acetate copolymer, EVA, is heated, a two-stage thermal degradation occurs following its melting. The vinyl acetate content of the copolymer was determined to be 43.8% by using TA 2950 and TA 2050 thermogravimetric instruments. TG/FTIR was used to detect the evolved gas. Acetic acid and trans-1-R-4-R'-cyclohexane were the main products evolved from EVA in the first and second stage, respectively. The apparent activation energies were determined for both stages by differential methods. This revised version was published online in August 2006 with corrections to the Cover Date.