Journal of Colloid Science最新文献

The separation of adsorption complexes formed by the interaction of sodium dodecyl sulfate (SDS) with gelatin at pH values below the isoelectric point has been investigated. Two pH conditions were employed. In one, the pH of the reaction mixture was adjusted to 4.1 after the component solutions had been mixed. In the other, the pH of each of the component solutions was adjusted to 4.1 before the solutions were mixed and the pH of the reaction mixtures was allowed to vary with the reaction. Concentration regions of complex solubility, complex precipitation, maximum gelatin precipitation, and complex solubilization characterized these reactions. Analyses of the supernatant solutions for both gelatin and SDS indicate that the adsorption is not exclusively continuous. Under the first pH condition, the reaction mechanism appears to be all-or-none in the region of complex precipitation forming a complex in which only one-half of the cationic sites are occupied, and predominantly statistical in the region of maximum gelatin precipitation, terminating in a complex in which all the cationic sites are occupied. Solubilization of the precipitated complex is achieved by the physical adsorption of additional SDS with the hydrophilic groups oriented toward the water. Under the second pH condition, an exclusively all-or-none mechanism appears to prevail in the regions of complex precipitation and solubilizations. The reactions result in the formation of an insoluble complex in which one-half the cationic groups on the gelatin are involved and a soluble complex in which all the cationic groups are involved.

The theory of the shape of a fluid drop at a fluid-liquid interface in two-phase systems was confirmed experimentally by measuring characteristic dimensions of stabilized air bubbles at an air-water interface.

The equilibrium film at the top of the bubble may simultaneously consist of two types of film of different thickness. The condition for this situation and its effect on the permeability of the film to gases is discussed.

Deviations from the shape theory will occur when the tension of the equilibrium film is smaller than twice the normal surface tension. Calculations have been made to extend the theory to systems with such a reduced film tension.

It is demonstrated that coupling exists between the translational and rotational Brownian movements of rigid macroscopic particles possessing screwlike geometric properties. The theory is developed for the case of helicoidally isotropic particles, these being the class of bodies for which the theory adopts its most elementary form. In addition to the classical translational and rotational diffusion coefficients characterizing the intensity of the Brownian motion, it is shown that there exists yet a third independent pseudoscalar diffusivity, which quantitatively describes the degree of coupling. The cross-effect associated with this coupling coefficient manifests itself by the appearance of cross-terms in the expressions for the components of the diffusion flux vector in a six-dimensional “configuration” space when projected onto its respective three-dimensional physical and orientation subspaces.

The theory for coagulation of particles in Brownian motion is reviewed. The effects of heterogeneity in particle size, and of particle motion in a rarefied medium are examined using numerical solutions of the coagulation equations. Heterogeneity and increased values of the ratio of the mean free path of the medium to the particle radius (the Knudsen number for particles) increased the rate of coagulation. According to the results of the numerical experiments, a self-preserving function for the size distribution develops after dimensionless coagulation times of about 3. The self-preserving spectrum was found to be independent of the initial distribution after a sufficiently long time. The shape of the asymptotic distribution varied with the ratio of the mean free path of the medium to the particle radius (λ/r₁). A cumulative distribution rapidly formed which was insensitive to time, to initial conditions, and to variations in λ/r₁ up to one. The average cumulative distribution compared fairly well with an experimentally determined distribution.

It is often assumed that raising the temperature of a system increases its wettability. The contact angles of hexadecane on a fluoropolymer were measured over a 130°C. temperature range. The advancing angle was found to be independent of temperature and the receding angles increased slightly with temperature. It is postulated that large temperature effects indicate specific interactions such as adsorption or desorption.

An exact mathematical formulation of the time dependence of the surface tension γ after formation of a fresh surface with constant area has been given by Ward and Tordai (1). The rather laborious calculations are considerably simplified when dealing with detergents obeying the v. Szyskowski equation for the dependence of equilibrium surface tension on concentration. This equation holds for certain non-ionic ethylene oxide compounds. With such detergents it was found that the surface tension—time curves are well fitted by the Ward and Tordai relation. Hence one can conclude that the establishment of the adsorption equilibrium is governed by the diffusion in an undisturbed layer.

When by a rapid expansion or compression of the surface a transient increase or decrease of γ as compared to the equilibrium value γ_∞ is provoked the rate of diminishing or rising of γ during the subsequent reestablishment of the equilibrium for a given detergent solution is dependent not only on the actual deviation of γ from the equilibrium value γ_∞ but also on the preparation (prehistory). This observation can be explained by properties of the diffusion layer and complicates the treatment of surface rheology of detergent solutions.

The rheology of silica suspensions was investigated as a function of concentration, particle size, the polarity of organic solvents, and the pH and ionic strength of the aqueous phases. In addition the sedimentation volumes of quartz powder suspensions were measured in organic solvents and as a function of the pH and ionic strength of the aqueous phases. The sedimentation volumes of quartz powder suspensions were at a minimum with respect to pH at the maximum in the zeta potential-pH curve. The yield values and apparent viscosities begin to increase rapidly with pH at pH 9 in calcium hydroxide solutions and at pH 12 in sodium hydroxide solutions.

Ordinarily silica suspensions exhibit a Newtonian behavior. Einstein's equation for Newtonian systems of spherical particles in suspensions was apparently not obeyed in the case of dilute suspensions of silica sols until the deviations were accounted for by electroviscous effects and adsorbed layers of water. Non-Newtonian behaviors and large sedimentation volumes arise with nonpolar solvents and at higher pH values in aqueous media. Concentrated quartz powder suspensions exhibit a shear thickening behavior. Concentrated colloidal silica gels develop shear thinning properties.

Usually a synthetic latex contains inorganic salts from the water-soluble initiator used in polymerization and from the impurities in the surfactant used to stabilize the latex. Upon removal of this electrolyte, by dialysis, the viscosity of the latex increases and becomes markedly shear thinning, i.e., non-Newtonian. When an electrolyte such as sodium chloride is added, the viscosity decreases immediately and may even become less than the viscosity of the original latex, and the flow behavior becomes Newtonian.

Measurements show that the only significant change in the latex system is in the gegenion concentration. The particle size distribution measured on diluted latices does not change through all these viscosity changes and only a small fraction of the adsorbed surfactant is desorbed and lost upon dialysis. Upon addition of sufficient electrolyte a minimum viscosity is obtained which is that which would have been predicted by Mooney's equation.(J. Colloid Sci. 6, 162 (1951))

The use of Mooney's equation to obtain effective volume fractions from viscosities shows that the experimental data are consistent with the viewpoint that the effective volume fraction is given by the polymer volume plus the volume excluded to particles by the interaction of the charged double layers. Thus, the observed phenomenon is also consistent with a large body of colloidal theory and experiment.

The drop volume method can give precise results for the surface tension of liquids if a certain number of precautions are taken. One of the most important is the temperature control. A drop volume syringe fitted with a jacket is described; water at constant temperature circulates in the jacket. The syringe is introduced in a brass chamber and the whole system immersed in a thermostat. The closed chamber eliminates evaporation and allows the surface tension to be determined in an inert atmosphere.