Colloids and Surfaces最新文献

The theory of reverse-osmosis separation was used to describe the rejection of poly(ethylene glycol) (PEG) molecules (molecular weight 1500–20 000) by ultrafiltration membranes.

Comparison of experimental data with theory allows us to obtain parameters such as the equilibrium partition coefficient γ, pore radius, thickness and porosity of the active layer of the membrane, coefficient of diffusion, and the thickness of the layer of concentration polarization. The optimum conditions for ultrafiltration are considered.

Rejection of PEG molecules may be explained in terms of the theory of surface forces by the action of forces of structural repulsion. Using the DLVO theory, the optimal pore sizes for charged membranes were estimated.

We have examined a sterically stabilised polypyrrole colloid in aqueous acidic media using dynamic light scattering techniques. Our results confirm that this dispersion consists of non-aggregated primary particles with an average hydrodynamic particle diameter of 161 nm. A comparison of these results with our transmission electron microscopy data allows us to calculate an upper limit layer thickness of approximately 25 nm for the adsorbed poly(4-vinylpyridine-co-n-butyl methacrylate) steric stabiliser.

ZrO₂ powders have been subjected to contact with molecular hydrogen at low temperatures T (25–200°C) and for variable duration (15 min–22 h).

X-ray photoelectron spectroscopy determinations showed that the hydrogen treatment, whatever the adopted condition, did not induce the formation of Zr³⁺ surface defects. Zr⁴⁺ spectra were, instead, found to be modified for treatments performed at T ⩾ 80°C. The same trend with the temperature of the treatment was observed for the reactivity of the electrical double layer. The values of the point of zero charge of samples treated at T ⩾ 80°C were observed to shift in the alkaline direction, the more so the longer the length of treatment. Static contact-angle measurements performed on ZrO₂ layers deposited on glass platelets indicated that the wettability for water decreased upon H₂ treatment. Possible relations between the different observed effects are discussed.

Hydroxyapatite has been widely used as a model substrate for studying biological systems involving mineralization and protein adsorption. In the present study, hydroxyapatite from three commercial sources was used to study the adsorption behavior of commercially available dextrans. Depending on the molecular weight of dextran and the technique used for dispersing the solid, maxima were observed in the adsorption isotherms for all three adsorbents. Scanning electron microscopy observations showed significant differences in the morphological properties of the hydroxyapatite samples. The adsorption maximum is attributed to the inability of the polymer molecules to diffuse into the interior of hydroxyapatite aggregates. The adsorption results have been analyzed to estimate the pore volume and the average pore size in the aggregates.

Aqueous solutions of 1-octanol polyoxyethylene glycol monoether with an average of 4.5 ethylene oxide units per molecule were subjected to ultrafiltration using hydrophobic membranes. Permeate concentrations always exceeded feed solution concentrations at temperatures below the cloud point and this fact together with the observed occurrence of permeation maxima and flux minima close to the critical micelle concentration was construed to indicate that surfactant adsorption onto the membrane pores induced the excess permeation. Observed concentration- and temperature-related variations in permeation for individual homologues are also discussed in terms of prevailing concepts regarding micelle-monomer partitioning and temperature effects on adsorption from aqueous solution.

Adsorption of Aerosol OT on alumina particles, preheated at 25°C or 300°C, from hexane and cyclohexanone was studied using two types of nitroxide probes. From the electron spin resonance spectra, the interaction between alumina surfaces having different amounts of water and Aerosol OT was discussed.

This paper presents a method for achieving low interfacial tension in surfactant formulation. Large surfactant micelles of sizes around 100 nm were found in aqueous surfactant solutions yielding ultralow tension between an oil—water interface. The dynamic interfacial tensions of the non-ionic surfactant—hydrocarbon systems were measured as a function of time with a spinning drop apparatus. A series of polvoxyethylene mono-n-dodecyl ethers (C₁₂H₂₆O(CH₂CH₂O)_xH.x = 3—6,8) and a series of hydrocarbons (C₈—C₁₅) were used in the experiments. The surfactant micelle size was measured by the dynamic light scattering technique using an He—Ne laser. At a spinning rate of 2000 rev min⁻¹ and a surfactant concentration of 0.024%, the process of lowering the interfacial tension seems to undergo a diffusion-controlled solubilization mechanism. It is suggested that one way of achieving low interfacial tension is to build up large surfactant micelles which are effective in solubilization, in the surfactant formulation.

The surface chemistry of bastnaesite, barite and calcite in aqueous carbonate solutions was investigated to delineate the performance of a flotation scheme used for separating barite and calcite from bastnaesite. Sodium carbonate additions to suspensions of these minerals affect both the pH of the system and the zeta potential of the mineral. Significant pH and zeta potential changes, however, are mineral dependent and occur at different levels of added carbonate. They appear to be controlled by the solubility of the mineral in the case of bastnaesite and calcite, and by the formation of a new surface compound, namely barium carbonate, in the case of barite. The conditions for the onset of barium carbonate precipitation in barite suspensions containing fixed concentrations of added carbonate were determined from computations of barite—aqueous carbonate equilibria, and they closely correlate with the experimental results. Using soda ash as a pH modifier in the flotation beneficiation of a rare-earth ore containing barite can lead to carbonation of the barium sulfate surface, thereby causing this mineral to exhibit the flotation behavior of barium carbonate.

A new interfacial activity model has been developed to predict, qualitatively as well as quantitatively, the equilibrium interfacial tension of alkali/acidic oil systems. The model accounts for a mixed interfacial layer and mixed micelle formation by ionized and unionized acid. Comparison of the model with experiments confirms the mechanisms proposed in Part I of this series. It has been found that interfacial tension goes through a minimum with pH at constant ionic strength. The ultralow value of interfacial tension is shown to result from the simultaneous adsorption of ionized and unionized acid upon the interface, and the lowering results from the formation of mixed micelles of ionized and unionized acid.

The adsorption of ABA block copolymers, where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO), on carbon black was investigated at room temperature (20 ± 2°C). Two series of block copolymers were studied: series A containing 56 PO units and 16–148 EO units per chain; series B containing 30 PO units and 30 or 77 EO units per chain. The results showed a decrease in the amount of adsorption Γ (μmol m⁻²) with increase in the ethylene oxide (EO) chain length. Adsorption increased as the number of propylene oxide (PO) units in the chain decreased. The area per molecule σ showed a linear increase with n^{$\text{1}\text{2}$}, where n is the number of EO units, indicating that adsorption is governed by the size of the PEO chain.

Concentrated dispersions of carbon black that were stabilised using the block copolymers were studied using steady state (shear stress—shear rate) and oscillatory measurements. The relative viscosity—volume fraction φ curves were compared with the theoretical curves for hard-sphere dispersions (calculated according to the Dougherty—Krieger equation) to obtain the adsorbed layer thickness as a function of φ. Storage modulus G′-φ curves showed a rapid increase above a critical volume fraction φ_cr for the carbon dispersions. With the ABA block copolymer containing the shortest EO chain (16 EO units per chain) φ_cr was significantly lower than the values obtained with the other block copolymers, indicating weak flocculation of the suspension. All other block copolymers stabilised the carbon dispersions, and φ_cr decreased with increase in the EO chain length, as expected.

The critical flocculation temperature (CFT) of carbon dispersions, stabilised using a block copolymer containing 56 PO units and 37 and 148 EO units per chain, was measured as a function of K₂SO₄ concentration using rheological measurements. The results showed that for both polymers the CFT is lower than the θ temperature of the PEO chain at the same electrolyte concentration. This indicated that the block copolymers were not as effective for stabilisation of carbon black dispersions as the previously studied ABC surfactants containing the nonylphenyl group in addition to the PPO chain, indicating that the nonylphenyl group plays a major role in anchoring the chain to the carbon surface.