Stability of emulsions of water in oil. I. The correlation between electrokinetic potential and stability

Experiments on W/O emulsions of moderate concentration, stabilized with oil-soluble, ionizing stabilizers, show that in these emulsions no correlation exists between stability against flocculation and electrokinetic potential. Although, according to theoretical calculations, energy barriers of over 15 kT are present, if the radius of the dispersed globules is about 1 μ and the electrokinetic potential exceeds 25 mv., they apparently do not prevent lasting contact between particles. All our emulsions flocculate rapidly, even in the presence of a surface potential considerably higher than 25 mv. A rather pronounced anticorrelation exists between the zeta potential and coalescence. It is explained as a consequence of the free mobility of the stabilizing molecules in the interface. The good stabilization against coalescence caused by some oleates of polyvalent metals is due to the formation of a thick film of partial hydrolyzates in the interface

Repeptization and the theory of electrocratic colloids

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∞ = zeδ/kT. A sol is stable, and flocs can repeptize if Z∞ > Z∞c. For Z∞ < Z∞c the traditional DLVO theory is obtained. Some examples are given to illustrate this concept. In the appendix the interaction of double layers with a constant charge is related to that at a constant potential

Stability of emulsions of water in oil III. Flocculation and redispersion of water droplets covered by amphipolar monolayers

Theoretical calculations demonstrate that emulsions of water in oil cannot be sufficiently protected against flocculation by an adsorbed layer of amphipolar molecules with an oleophilic chain of about 20 A. It is, however, expected that such flocculated emulsions can be redispersed by moderate rates of shear. This is confirmed by viscosity measurements. Non-Newtonian behavior found at low rates of shear is a consequence of flocculation. From the minimum rate of shear to reach the Newtonian region, i.e., to cause complete redispersion, the effective van der Waals' constant between the water droplets can be estimated. The rather low value of A = 0.4 × 10−14 erg is found

Repeptization and the theory of electrocratic colloids

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∞ = zeδ/kT. A sol is stable, and flocs can repeptize if Z∞ > Z∞c. For Z∞ < Z∞c the traditional DLVO theory is obtained. Some examples are given to illustrate this concept. In the appendix the interaction of double layers with a constant charge is related to that at a constant potential

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

Physical chemical studies of short-chain lecithin homologues

A simplified association model for micellisation is presented. In this model two features are incorporated; (a) There is an optimum for the change of the standard free energy per monomer upon micellisation at a certain association number. (b) At higher association numbers this free energy change becomes constant. The resulting equations for the dependence of the average micellar weight on the concentration are used to explain the experimetitally observed effects of a salting-out agent (NaCl) and of the alkyl chain length of dihexanoyl-, diheptanoyl- and dioctanoyl-lecithin

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

Stability of emulsions of water in oil. II. Charge as a factor of stabilization against flocculation

It is shown by theoretical calculations that the energy barrier between charged droplets in water-in-oil emulsions is strongly diminished when the concentration of the emulsion is not extremely low. This is a consequence of the great extension of the diffuse electrical double layer in oil. The high concentration in the sediment (or cream) therefore strongly promotes flocculation. Gravity also promotes flocculation directly in all but the most dilute W/O emulsions because the weight of the particles in higher layers transmitted by the extended double layers presses on those in the lower layers and forces them together