Tadros - Interfacial phenomena and colloid stability. Volume 1 : basic principles

: The Gibbs dividing line.

(1.1)

(1.2)

: Guggenheim convention of the interfacial region.

(1.3)

(1.4)

In all disperse systems such as suspensions, emulsions, foams, etc., the structure of the interfacial region determines its colloidal properties [36]. The larger the interfacial area, i.e. the larger the surface to volume ratio of the particle or droplet, the more important the role of the structure of the interfacial region. The colloid stability /instability of any disperse system is determined by the property of the interfacial region. In actual fact colloid and interface science are one individual subject. This is particularly the case with charged interfaces that form electrical double layers and those interfaces that contain adsorbed surfactants and/or polymers. With systems containing electrical double layers, repulsion between the particles or droplets takes place as a result of the overlap of double layers. This is particularly the case at low electrolyte concentrations and low valency of the indifferent electrolyte. This double layer repulsion overcomes the van der Waals attraction and at intermediate distances an energy barrier is produced that prevents approach of the particles. This barrier can reach several kT units (where k is the Boltzmann constant and T is the absolute temperature) which becomes much higher than the thermal motion ( kT) and this prevents particle aggregation (flocculation or coagulation). As the electrolyte concentration is increased, the range and magnitude of the repulsive energy is reduced and at a critical concentration (defined as the critical coagulation concentration, ccc) fast flocculation and irreversible aggregation occur. With adsorbed nonionic surfactants or polymers an adsorbed layer with thickness is produced. When the particles or droplets approach a surface-surface distance h < 2, strong repulsion occurs due to the unfavorable mixing of the adsorbed chains when these are in good solvent conditions. This repulsion is referred to as steric interaction and at distances < 2 a very sharp increase in repulsion energy occurs when h < 2. This steric repulsion overcomes the van der Waals attraction at h 2. The repulsion produced by the presence of adsorbed layers of surfactant or polymers is generally more effective than the electrostatic repulsion produced by overlap of the double layers. The stability is less sensitive to addition of moderate electrolyte concentration, provided the medium remains a good solvent for the chains.