Colloids and Surfaces最新文献

The stability of high surface charge latex dispersions containing non-adsorbing polyelectrolyte was investigated. Poly-(styrene/sodium p-vinylphenylsulphonate) copolymer latices and sodium polystyrenesulphonate systems were used. Flocculation was confirmed using three observational methods — transmission measurement, microscopy and dynamic laser light scattering particle analysis. It was found that in flocculation the cluster size of the flocculated particles split into two peaks — a small peak consisting almost entirely of single particles and a larger peak which increased with time. Moreover, the floc which formed was in an ordered state and changed into a crystalline phase after sedimentation.

Tin tailing slimes from three ex-mining ponds located in different parts of Malaysia and with different clay compositions were studied. Pengkalan slime contained about 8% of montmorillonite clay; Serendah (SH) slime had a high content of amorphous materials (24%) while the main constituents of all samples were kaolinite and illite with a small amount of organic materials and microorganisms as exemplified by Malim Nawar slime. The surface nature of the clay minerals in these slimes was characterised using the particle microelectrophoresis technique. The electrokinetic behaviour of the slime basically followed that of kaolinite but differences due to compositional variations were evident. This is especially so for the SH slime where the amorphous materials present completely modified the surface property of the kaolinite. The magnitude of the electrophoretic mobility depended strongly upon the exchangeable cations associated with the clay particles and pore fluid chemistry of the slime samples. The evaluation of the isoelectric point of the edges of clay particles by the electrokinetic method is discussed. The importance of understanding the charged surfaces of the clay minerals in the slimes in relation to the design of an efficient dewatering programme is briefly discussed.

The solubilization of the copper(II) complexes of N-alkyl-β-alanines, NAA (butyl, NBuA; hexyl, NHeA; octyl, NOA; decyl, NDeA), in aqueous micellar solutions of NAA has been investigated. It was found that a crystalline coordination compound of composition Cu(I);NAA(2), the complex trans-[Cu(NAA)₂(H₂O)₂], was formed by direct mixing of NAA with copper(II) chloride in an aqueous solution. The amount of the insoluble stoichiometric 1:2 complex increased with increasing concentration of NAA in the concentration region below the critical micelle concentration (CMC) of NAA, but the 1:2 complex began to redissolve above the CMC and disappeared at a certain concentration of NAA, i.e. the insoluble 1:2 complex was solubilized in the micelles of NAA.

Visible spectroscopic data suggested that the dielectric environment around the coordination sphere of the copper(II) ion in the solubilized 1:2 complex molecule was similar to that in the crystalline 1:2 complex molecule. Lamellar structures of NAA micelles solubilizing the 1:2 complexes were characterized by small-angle X-ray diffractometry. A model for the solubilization was proposed: two hydrocarbon chains in a 1:2 complex molecule of trans form were interlinked with the NAA bilayers. The free energy of solubilization was estimated from the solubilized amounts of the 1:2 complexes for the NDeA and NOA systems. It was found that the change in the free energy of solubilization per methylene group was −1.04 kJ mol⁻¹, which is roughly half of the corresponding value for soap micellization.

The influence of addition of a high concentration of non-ionic surfactants on the rheology of a concentrated (volume fraction φ_p = 0.4) polystyrene latex suspension was investigated. Four different surfactants containing the same hydrophobic group (C₉H₁₉C₆H₅(CH₂CH(CH₃)O)₁₃) and increasingly large hydrophilic chains (poly(ethylene oxide) with 27, 48, 79 and 174 ethylene oxide (EO) units) were used. The rheology was studied using steady-state shear stress (τ)—shear rate (γ) measurements as well as oscillatory measurements. From the τ—γ curves, the extrapolated yield stress τ_β and plastic viscosity η_pl were obtained as a function of surfactant concentration. From oscillatory measurements, the complex modulus G*, the storage modulus G′ and loss modulus G″ were obtained as a function of frequency at different surfactant concentrations. All results showed a rapid increase in the rheological parameters with increase of surfactant concentration, once the latter reached a critical concentration. This rapid increase (non-Newtonian flow) was considered to be due to depletion flocculation. The latter was thought to occur at a surfactant concentration in excess of that required for maximum random packing of surfactant micelles, which were assumed to be spherical. A simple calculation (using a value of 0.63 for random packing) predicted the surfactant concentration at the onset of depletion flocculation, using the aggregation number of surfactant molecules in the micelle (obtained from surface-tension or light-scattering measurements).

The extrapolated yield value was used to calculate the energy of separation E_sep between contact points in a floc. E_sep was compared with the theoretical free energy of depletion attraction G_dep that was calculated using the Asakura and Oosawa theory. Agreement between the calculated G_dep and experimental E_sep values was fair.

An analytical expression for the shape of the capillary meniscus formed around a vertical cylinder or spherical particle near a vertical wall is derived by using the method of matched asymptotic expansions. The forces of capillary interaction between the particle (cylinder) and the wall are calculated. The resulting expressions are valid when the distance between the particle (cylinder) and the wall, as well as the particle radius, are much smaller than the capillary length. This range corresponds to colloidal and micron-size particles. The theory predicts attraction between such a particle and the wall. The results can be useful for a better understanding of processes such as surface coagulation and two-dimensional ordering of colloidal particles or protein molecules attached to a fluid interface.

Experimental results with different colloidal systems (ionic and non-ionic micelles, swollen micelles, latex suspensions) are presented to demonstrate the universality of the phenomenon termed film stratification (stepwise thinning of liquid films). A general explanation of the phenomenon is proposed which assumes that the stratification is caused by the layer-by-layer expulsion of a colloid crystal-like structure formed by spherical particles (micelles, latex particles, etc.) inside the film. The effect of different factors on the stratification such as surfactant and electrolyte concentrations, temperature, etc. is explained from this viewpoint.

Uniform copper particles were prepared by reduction of copper(I) oxides with hydrogen at 69–111°C. Three types of precursor particles having average modal sizes of O.46, 0.62 and 1.26 μm, respectively were produced by reducing Fehling's solutions of different concentrations with glucose. The reduction of fine copper(I) oxide particles took place through an autocatalytic process, while the reaction rates of rather coarse oxide particles were controlled by the diffusion of gaseous species through the copper layers formed at a later stage of the process. The resulting copper particles retained the original morphology of the oxides. A moderately oxygen-rich layer about 30 Å thick was detected on exposure to air by means of photoelectron surface microanalysis (XPS).

The mechanism of the formation and growth of spherical amorphous chromium hydroxide particles of narrow size distribution by aging chrome alum solutions at elevated temperatures is described. Since the precipitation is controlled by the presence of the sulfate ion, the reaction was followed by monitoring the change in the concentrations of this anion using the sulfur-35 radioisotope and of the chromium ion by atomic absorption.

The charges and zeta potentials of the particles, determined by potentiometric titration and electrophoretic measurements, showed that the particles behaved as rigid gels.

The ternary phase diagram of the dodecyldimethylamine oxide (DDMAO)—hexanol—water system is reported for low volume fractions of the surfactant. Besides the well known L₁, L(α) and L₃ phases, surprisingly three more single-phase regions are observed in the phase diagram and called L*₁, L*₃ and L(α)₁. The L*₁ and L*₃ phases are optically isotropic, slightly turbid and viscous. The L(α)₁ phase is birefringent and highly viscous. It is conceivable that these seemingly single-phase regions are in reality subphases of the normal L(α) phase. With increasing cosurfactant concentration, the sequence of the phases is L₁, L*₁, L(α)₁, L*₃, L(α)_h and L₃. The last five phases all have a bilayer-type structure. The L*₃ and L*₁ phases consist of single and multilamellar vesicles. The L*₃ phase disappears at higher surfactant concentrations and two L(α) phases then border on each other but seem to be separated by a two-phase region. At low volume fractions of about 1%, both L(α) phases show iridescent colours which are due to interference of white light from the ordered bilayers. The colours of the L(α)_h phase are much brighter than those of the L(α)₁-phase. The L*₁, L(α)₁ and the L*₃ phases are viscoelastic phases with very long structural relaxation times. For all three phases both the storage and the loss moduli are about the same and independent of the oscillating frequency over several decades. The L(α)_h and the L₃ phases have a low viscosity.

Some results are also given for the ternary system tetradecyldimethylamine oxide—hexanol—water. Results of electric birefringence and viscosity measurements are reported for the L₁ phase.

In some of these studies, increasing amounts of tetradecyldimethylamine oxide were substituted by the cationic surfactant tetradecyltrimethylammonium bromide. The L₁/L*₁ phase boundary shifts linearly with increasing mole fraction of the cationic surfactant to a higher cosurfactant/surfactant ratio. The solutions with the mixed surfactants contain globular micelles in the absence of hexanol. With increasing hexanol concentration, a sphere—rod transition is observed and the rods grow in length with the hexanol concentration up to the phase boundary. The lengths of the rods at the L₁/L*₁ phase boundary decrease with increasing molar ratio of the cationic surfactant. The growth of the rods with increasing hexanol concentration in the L₁ phase is reflected in the viscosities of the solutions. These increase continuously tip to the phase boundary for all surfactant mixtures which contain some cationic surfactants. In contrast, the viscosities and the rotational diffusion time constants of the ro