J. Th. G. Overbeek

Calculation of the electrophoretic mobility of a spherical colloid particle

A new calculation of the relation between the electrophoretic mobility and the ζ-potential of a spherical colloid particle is presented. The model consists of a rigid, electrically insulating sphere surrounded by a Gouy-Chapman double layer. The appropriate differential equations (which account for both electrophoretic retardation and relaxation effect) have been solved without approximations on an IBM 704 computer. The theoretical assumptions and the basic equations are stated. A detailed account of the solutions of the equations is published elsewhere. The present paper contains the complete results. Comparison of these results with those of Overbeek and of Booth shows that, for high ζ-potential and 0.2 < υa < 50, their approximations generally overestimate the relaxation effect. The application to practical cases (especially colloid particles in solutions of 1-1 electrolytes) is discussed extensively. Finally, the theoretical results are compared with experimental data published by others.

Recent Developments in the Understanding of Colloid Stability

Abstract The stability of suspensions and emulsions against coagulation is governed by the forces between the particles. The main forces are: van der Waals attraction, electrostatic repulsion, and the repulsion due to the interaction of adsorbed large molecules. It is stressed that coagulation and redispersion are rate phenomena. Smoluchowskis classical theory on coagulation as modified by Fuchs to include interaction forces between particles has been the basis of theories of colloid stability. Corrections to these theories are needed to account for the hampering of Brownian motion when the particles are close together. Furthermore it is pointed out that the duration of a Brownian collision is long enough to allow redistribution of the double layer and of the conformation of adsorbed macromolecules, but too short for adaptation of the surface charge or of the adsorption of macromolecules. Repeptization requires the existence of a distance of closest approach between particles which is of the order of one or two solvent molecules.

On the interpretation of electrokinetic potentials

The variation of the dielectric constant and the viscosity in the electrical double layer are considered. The Helmholtz-Smoluchowski equation for the electrophoretic mobility requires only insignificant correction for variations in the dielectric constant; the variation in the viscosity can lead to considerable corrections, which increase with surface potential and concentration. Equations are given for the relation between electrophoretic mobility and surface potential and are illustrated by two examples. The concept of the slipping plane should be replaced by a slipping layer of finite thickness. At high surface potentials, the electrophoretic mobility is independent of the potential but depends on the electrolyte concentration.

Physical chemical studies of short-chain lecithin homologues

Abstract The critical micelle concentration (CMC) of four synthetic phosphatidylcholines (containing two hexanoyl, heptanoyl, octanoyl or nonanoyl residues respectively) in aqueous solutions have been determined by surface tension measurements. The dependence of the CMC on the chain length is discussed on the basis of the mass action model for micelle formation. For the three higher homologues a contribution of 1.08 kT per CH 2 group to the standard free energy of micellisation is found. The change in this free energy in going from the dihexanoyl- to the diheptanoyllecithin is somewhat larger (1.2 kT per CH 2 group). The influence of high concentrations (several moles per liter) of simple electrolytes on the CMC is interpreted as a salting-out of nonpolar solutes in water. Contrary to expectations the effects of NaCl and Lil on the CMC of dioctanoyllecithin are not additive.

Repeptization and the theory of electrocratic colloids

Abstract The coagulation and the repeptization of electrocratic colloids can be treated in one theory provided that the appropriate boundary conditions are chosen. From this version of the DLVO theory it follows that for each sol there exists a critical value Z∞c of the double layer parameter Z∞, Z∞ = zeϕδ/kT. A sol is stable, and flocs can repeptize if Z∞ > Z∞c. For Z∞

Monodisperse colloidal systems, fascinating and useful

Abstract Monodisperse colloidal systems may form spontaneously (protein solutions, micellar solutions, micro-emulsions) or be obtained by fractionation (Perrins suspensions of gamboge) or by cleverly controlled particle growth (Au, S, latex). The controlled particle growth consists of a nucleation or seeding phase and a growth phase in which the size distribution is narrowed. The size and size distribution can be checked by a variety of techniques, such as centrifugation, light scattering, electronmicroscopy. The size distribution of micelles and micro-emulsion droplets is discussed on the basis of the thermodynamics of these systems. Monodisperse systems have many applications, most of them in checking theories of fundamental aspects of colloid science, but also several in industrial applications.

Scattering of light by charged colloidal particles in salt solutions

In the interpretation of light scattering by colloidal electrolytes in salt solutions the interaction between the colloidal particles and the low molecular weight ions has to be taken into account. When fluctuation theory is applied for the derivation of a light-scattering equation, nonelectroneutral fluctuations may be neglected in most cases. The total light scattering can be split into three contributions, one due to density fluctuations, one due to concentration fluctuations in the low molecular weight components, and one due to the colloidal particles. In the last-named contribution the (usually negative) adsorption of the low molecular weight salts by the colloid is included. This can be taken into account in good approximation by using in the light-scattering equations the refractive index increment at constant chemical potential and not at constant concentrations of the other components of the system. This quantity can be measured directly in membrane equilibria or it can be calculated from concentration differences in a membrane equilibrium combined with the more usual refractive index increments at constant concentrations. The theoretical treatment is confirmed by measurements of light scattering and membrane equilibria with half-neutralized polymethacrylic acid in 0.1 M sodium halide solutions and in a few other salts. The correction on the molecular weight varies from 10% in NaF to 25% in NaI and amounts even to 45% in 0.01 M (NH4)6Mo7O24.

Electrochemistry of silver iodide the capacity of the double layer at the silver iodide-water interface

A method is described for obtaining differential double layer capacities on silver iodide. Especially the influence of the nature and concentration of indifferent electrolytes was investigated, viz., the nitrates of Li·, K·, Rb·, NH4·, H·, Tl·, Mg··, Ba··, Co··, Cd··, Pb··, La···, Th····, the fluoride, chloride, and sulfate of K·, NaClO4 in 10−3 and 10−1M solutions. Capacities showed definite analogy with those on mercury. The influence of the counter ion was in the lyotropic order. The capacity of the molecular condenser, calculated with the assumption of absence of specific adsorption, was independent of the electrolyte content.

On understanding microemulsions : II. Thermodynamics of droplet-type microemulsions

A thermodynamic theory of microemulsions containing brine, oil, an ionic surfactant, and a nonionic cosurfactant is given. Conditions for phase equilibria are derived. The treatment is limited to droplettype microemulsions with emphasis on W/O + W systems. Important features are saturation adsorption of surfactant and cosurfactant, the interfacial bending stress, the standard chemical potential of the droplets in the free energy of mixing, and a quantitative treatment of the contribution of the electric double layer to the bending stress. Numerical illustrations show good agreement with experimental data on the influence of salt and cosurfactant on droplet size and interfacial tension in W/O + W equilibria.

Physical chemical studies of short-chain lecithin homologues. II: Micellar weights of dihexanoyl- and diheptanoyllecithin

Abstract The micellar weights of dihexanoyl- and diheptanoyllecithin in aqueous solutions are calculated from light scattering and ultracentrifugation data. A monomer-micelle assocation model is used and corrections for the thermodynamic nonideality, on the basis of rigid noninteracting particles, are applied. A few experiments on the influence of high NaCI concentrations (up to 3 M) are described. Dihexanoyllecithin forms micelles with micellar weight of 15 000 to 20 000 and with rather narrow weight distributions. Diheptanoyllecithin micelles however, have broad size distributions with micellar weights of 20 000 up to about 100 000 in the concentration range studied. Micelles are assumed to be spherical or to have sphero-cylindrical shapes depending on the molecular weights. Two models are used: (1) a compact structure, where no attention is paid to the hydrocarbon-water contact (2) micelles with as little hydrocarbon-water contact as possible.