Colloids and Surfaces最新文献

The viscoelastic properties of concentrated polystyrene latices stabilised by ABA block copolymers of poly(ethylene oxide)-poly(propylene oxide) have been measured using oscillatory and steady-state shear measurements and osmotic pressure measurements.

For stable systems, highly concentrated polystyrene latices showed non-Newtonian flow behaviour from steady-state shear measurements. From the oscillatory shear measurements it was noted that the elastic moduli started to increase dramatically when the interparticle separation was such that the polymer layers were touching.

The osmotic pressures of these latex systems were measured and correlated to the elastic moduli measurements and to the simple scaling theory for adsorbed polymers where a qualitative agreement was found between the osmotic pressures obtained from experiment and the scaling theory.

In a complementary series of experiments, the oscillatory rheological behaviour of these systems is they were flocculated (brought about by an increase in temperature) was measured. At the onset of flocculation both the elastic and loss moduli of the dispersion increased by many orders of magnitude. It was noted that the temperature at which flocculation occurred remained constant below a volume fraction Φ of 0.5 but decreased sharply at higher volume fractions, in accord with simple statistical-mechanical theories. Moreover, the change in the elastic modulus G′ with the volume reaction after flocculation was found to be in agreement with a fractal analysis for percolating networks.

By using known thermodynamic parameters, the Briggs logarithms of the individual extraction constants for the ML⁺ complex cations (M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺; L = dibenzo-24-crown-8) in the water—nitrobenzene extraction system at 25 C have been evaluated. It has been found that these constants increase in the order Na⁺ < K⁺ < Rb⁺ < Cs⁺, i.e. in the order of increasing crystallographic radii of the alkali metal cations.

It has been shown that the electrophoretic mobility of large ion-exchanger particles (diameter, 250–1000 μm) or fibres (length, 100–1000 μm) in strong electric fields (100–1000 V cm⁻¹) exceeds the electrophoretic mobility values typical for non-conducting particles by one or two orders of magnitude. This phenomenon was called “electrophoresis of the second kind” or “superfast electrophoresis”. The mobility of such particles depends on the conductivity ratio between the particles and medium and increases linearly with the electric field gradient and the particle size. This is in contrast to classical electrophoresis which does not depend on these parameters.

The effect of electrolytes and pH on the electrophoretic mobility in strong fields is due to the change in the conductivity ratio mentioned above. Changes in the electric double layer parameters do not affect the mobility.

Electrophoresis of the second kind is due to the interaction of a strong electric field with the space charge near the surface of unipolar conducting particles. It occurs only when the conductivity of particles exceeds the conductivity of the medium. New methods to observe this phenomenon were developed.

Electrical conductivity measurements as well as adsorption investigations show that the electrical conductivity and the amount of copper ions adsorbed from aqueous solutions pass through maxima with increasing concentration of surfactant in surfactant-doped titanium gels. Electron spin resonance measurements reveal that the intensity of the signal assigned to adsorbed O²₋ depends on the surfactant content, while Fourier transform-infrared measurements provide evidence for peaks which disappear after adsorption. It is reasonable to assume that the groups responsible for these peaks represent adsorption sites.

Using the new technical possibilities of a variable speed, self-controlled-drive surface-force apparatus, the real molecular thicknesses of adsorbed protein layers between two solid mica plates have been derived.

This paper is a short review of a systematic study of the effect of oil, surfactant and cosurfactant alkyl chain length (with n, m and n_c carbon atoms in the alkyl chain, respectively) and temperature (T) on the droplet size, rate constant k_c for exchange of material between colliding droplets, and attractive interactions between droplets in water-in-oil microemulsions. In agreement will current theories on the stability of water-in-oil microemulsions, the droplet size, k_c and attractive interactions between droplets increase as n and T increase or as m and n_c decrease. Correlations have been found between droplet size, value of k_c, magnitude of interdroplet attractive interactions, appearance of electrical conductivity percolation and water solubility as one of the parameters n, m, n_c or T is changed. It has been shown, for instance, that electrical conductivity percolation appears only for systems characterized by a value of k_c larger than (1–2)·10⁹ M⁻¹ s⁻¹. It has also been shown that electrical measurements as a function of the molar concentration ratio ω (equal to [water]/[surfactant]) can give qualitative information concerning the variation of droplet size, k_c and attractive interdroplet interactions with n, m and n_c.

The variation of surface charge with pH for pure and commercially available kaolinite samples was investigated by ion adsorption and potentiometric titration. The contribution of two distinct sources of charge can be identified. The results help to explain previous work on the ion exchange capacity of kaolinite.

Surface tension-solution concentration relationships reveal that in spite of the higher efficiency of adsorption displayed by chlorcyclizine relative to ampicillin, the effectiveness of adsorption was superior for the latter. There differences in adsorption behaviour between the two drugs studied have been attributed to the differences in their chemical structure. Chlorcyclizine with its two branched hydrophobic benzyl groups exhibits a higher thermodynamic driving force for adsorption than does ampicillin with its one benzyl group.

The penetration of these drugs into phospholipid, oleic acid and their mixed monolayers has been assessed by surface pressure and surface potential measurements. Although more space is required to accommodate penetrating chlorcyclizine molecules in a spread monolayer, their penetration into an oleic acid monolayer is higher than that of ampicillin. and is most probably due to an ionic interaction between the drug cation and the oleic acid carboxylate group. The penetration into phospholipid monolayers (which are highly incompressible) and their mixed monolayers with oleic acid was superior for ampicillin at high solution concentrations. The surface potential data corroborate the surface pressure data, but indicate that the presence oleic acid molecules in the monolayers exerts an unfavourable dipolar matching on adsorbing drug molecules, the effect being superior in the case of ampicillin.

From the extraction measurements, the equilibrium distribution constant K_D(L) for the electroneutral ligand 2,2′-bipyridine between the 1,2-dichloroethane and the aqueous phases has been determined in the form log K_D(L) = 2.03 ± 0.01, where L represents the ligand. Using known thermodynamic parameters and general relations, the stability constants of the metal chelate complexes [M(bipy)₃]²⁺ (M²⁺ = Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺; bipy = 2,2′-bipyridine) in 1,2-dichloroethane saturated with water have been evaluated. It has been found that the stability constants determined in this medium are essentially higher than the corresponding stability constants of these complexes in water.

The adsorption of sodium dodecylsulfate (SDS) and ethoxylated nonylphenol (NP-12) from their mixed solutions on TiO₂ and the stability of TiO₂ dispersions in SDS—NP-12 mixed solutions have been investigated. The NP-12 alone is almost not adsorbed on the TiO₂ surface. However, the amount of NP-12 adsorbed on TiO₂ increases by more than 50 times in the presence of a certain amount of SDS in solution, and decreases again at higher concentrations of SDS. The adsorption of SDS on the surface of TiO₂ from the SDS—NP-12 mixed solutions displays similar characteristics to a lesser degree. The stability of TiO₂ dispersions in the mixed solutions of SDS and NP-12 is shown to be consistent with the adsorption results. The synergistic and antagonistic effects of SDS and NP-12 on their adsorption on TiO₂ and on the stabilization of the TiO₂ dispersions were interpreted in terms of the formation of mixed hemimicelles on the TiO₂ surface and mixed micelles in solution.