Lidia Alexandrova

Flotation de-inking studies using model hydrophobic particles and non-ionic dispersants

Abstract A series of non-ionic alcohol ethoxylated surfactants (with HLB within the range of 11.1–12.5) were used as dispersants during flotation of mondisperse hydrophobised silica particles (representing ink particles) in de-inking formulations. Laboratory scale flotation experiments, contact angle, dynamic surface tension and thin film drainage experiments were carried out. The reduction in dynamic surface tension at the air/solution interface (which is dependent on the adsorption kinetics) followed the order C10E6>C12E8≈C12E6>C14E6 and these values were lower than sodium oleate, which is commonly used in de-inking systems. In addition the non-ionics adsorbed on the hydrophobised silica particles reducing the contact angle. These results indicated that the non-ionic surfactant with the highest CMC (C10E6) gave (a) the highest rate of adsorption at the air/solution interface (b) the froth with the greatest water content and higher froth volume (c) the lowest reduction in contact angle and (d) the highest flotation efficiency at concentrations above the CMC. It was also observed that flotation occurred, in spite of the fact that thin-film measurements indicated that the adsorption of non-ionic at the air/solution and silica/solution interfaces reduced the hydrophobicity of the particles, as indicated by an increase in stability of the aqueous thin film between the particle and air-bubble. This result suggests that the bubble-ink particle captures mechanism (occurring through rupture of the thin aqueous film separating the interfaces) is not the only mechanism controlling the flotation efficiency and that other parameters (such as the kinetics of surfactant adsorption, foaming characteristics, and bubble size) need to be taken into account. The kinetics is important with respect to the rate of adsorption of surfactant to both interfaces. Under equilibrium conditions, this may give rise to repulsive steric forces between the air-bubble and the particles (stable aqueous thin-films). However, a lower amount of surfactant adsorbed at a freshly formed air bubble or inkparticle (caused by slow adsorption rates) will produce a lower steric repulsive force allowing effective collection of particles by the bubble. Also, it was suggested that the influence of alcohol ethoxylates on bubble-size could effect the particle capture rate and mechanical entrainment of particles in an excessively buoyant froth, which will also play an important role in the flotation recovery.

Investigation of thin films formed from liposome suspensions on quartz substrate

The stability of thin wetting films formed from 0.15 M NaCl solutions containing small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles of different concentrations on quartz surface has been investigated by the microinterferometric method. The intensity of monochromatic light reflected from both film surfaces has been recorded as a function of the time of film thinning. Two temperatures were used in the experiments (20 and 35 degrees C). Films containing 10(-3), 5x10(-3) and 10(-2) mg/ml DMPC were unstable and ruptured, while films with 10(-1) and 1 mg/ml DMPC were stable. Film stability was explained on the basis of hydrophobic interactions. Film thickness dependence on time was calculated. The kinetics of film thinning did not obey Reynolds equation and a linearization was observed in co-ordinates ln(h) as a function of time. This phenomenon was explained by a non-homogeneous thinning process, which might be due to the existence of some areas of different structure of the DMPC adsorption layers.

Kinetics of air/vesicle-suspension/quartz three-phase contact

Abstract The stability of thin liquid films of vesicle suspensions in water formed on a quartz surface is studied by measuring the film lifetime τ and the kinetics of the three-phase contact line formed after the film rupture. The experiments were performed at two temperatures, 20 and 30°C, spanning the melting phase-transition temperature Tc of the DMPC lipid used. At the upper temperature it is observed that τ exhibits a pronounced minimum as a function of the vesicle concentration, whereas the dynamic contact angle shows a maximum. To explain the experimental results, the existence of a specific hydrophobic attractive force between the film surfaces is required. Thus, a scenario for the film rupture is proposed that provides an explanation of the results above Tc and of the unlimited stability of the films below Tc.

The three-phase contact parameters of thin water films on mineral surfaces

Abstract A new method for the characterization of the wettability of opaque solid surfaces by measuring 9a) the lifetime of thin liquid films, (b) the kinetics of expansion of the three-phase contact (TPC), (c) the equilibrium contact angle is developed. The method is tested with galexa-xanthate and copper mineral-pyrite-Aerodri 104 systems. The experimental results from TPC parameter measurements of galena and flotation tests indicate the important role of the TPC expansion rate in flotation. The effect of Aerodri 104 on three-phase froth stability is investigated. It is found that the breakdown of the three-phase froth can be correlated directly with the equilibrium contact angle.

Polyoxyalkylated diethylenetriamine polymeric surfactants: molecular structure effect on thin wetting films

Thin wetting films from aqueous solution of four polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants (named A, B, C, and D) are studied. Surfactants A, B, and C have a star-like structure differing only by the number of polymeric branches: four, six, and nine in the mentioned order while the forth one, D, is of a dendritic type with four to six primary and two to three secondary branches. The receding contact angles θr of the solution on hydrophilic SiO2 glass surface and the contact angle θaq of a drop of doubly distilled water on SiO2 glass surface pretreated with DETA polymeric surfactant solution are measured. The θr values on hydrophobicity of SiO2 glass surface, respectively, increase in the following order: surfactant A, surfactant C, surfactant B, and surfactant D. The equilibrium thickness heq of wetting films from DETA aqueous solution on hydrophilic SiO2 glass surface is measured using the micro-interferometric method. Results show an unexpected course of the heq vs. Cs curves with a maximum. Results from the studies indicate that differences in polymeric surfactant molecular structure affect the properties exhibited at air/liquid and solid/liquid interfaces, such as the value of surface tension, degree of hydrophobicity of solid surface, equilibrium film thickness, etc.

Wetting properties of phospholipid dispersion on tunable hydrophobic SiO2–glass plates

We study the wetting properties of very small droplets of salty aqueous suspensions of unilamellar liposomes of DMPC (dimyristoylphosphatidylcholine), situated on SiO2-glass surfaces with different levels of hydrophobicity. We evaluated two different measures of hydrophobicity of solid surfaces - receding contact angles and the thickness of wetting films trapped between an air bubble and the solid surface at different levels of hydrophobicity. We established a good correlation between methods which differ significantly in measurement difficulty and experimental setup. We also reveal details of the mechanism of wetting of different surfaces by the DMPC liposome suspension. Hydrophilic surfaces with water contact angles in the range of 0° to 35° are readily hydrophobized by the liposomes and only showed corresponding contact angles in the range 27°-43°. For same range of surface hydrophobicities, there was a clear reduction of the thickness of the wetting films between the surface and a bubble, reaching a minimum in the 35°-40° range. At higher levels of hydrophobicity both pure water and the liposome suspension show similar contact angles, and the thickness of wetting films between a bubble and those surfaces increases in parallel. Our analysis showed that the only force able to stabilize the film under these experimental conditions is steric repulsion. The latter suggests that nanobubbles adsorbed on hydrophobic parts of the surface, and coated with a DMPC layer, may be the cause of the 40-70 nm thickness of wetting films we observe.