Coagulation contacts surface force

Coagulation, i.e., the process by which discrete particles come in contact with each other in the air and remain joined together by surface forces, represents another way in which aerosol diameter will increase. However, it does not alter the mass of material in the coagulated particle. 

Figure 19.10. Illustrations of the effects of hydrophobic interactions, i. e. the tendency to eliminate contacts between water and nonpolar molecules or surfaces (a) water and oil are immiscible, with a strong driving force to expel hydrocarbon molecules from water (b) self-assembly of surfactant molecules (c) other types of association of hydrocarbon chains (d) folding of proteins (e) strong adhesion between non-polar surfaces in water (f) non-wetting of water on hydrophobic surfaces (g) rapid coagulation of hydrophobic particles in water (h) attachment of hydrophobic particles to air bubbles (mechanism of froth flotation). (Redrawn from J. Israelachvili, Intermolecular and Surface Forces, with Applications to Colloidal and Biological Systems, Academic Press, London, 2nd Edn, 1991)...

The predictions of different quantitative criteria for stability-instability transitions were investigated [461], having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force was taken into account. Two different kinetic criteria were considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams were constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed [461]. 

Physical-Chemical Mechanics of Disperse Systems and Materials contains seven chapters. Section I, with four chapters, presents the basics, starting from surface forces and the contact of particles with liquids. Chapter 2 is dedicated to adsorption phenomena, accumulation of surface-active molecules at various interfaces, and the importance of surfactant s adsorption on the contact between particles. The bulk properties of particle dispersions in liquids are discussed in Chapter 3 in terms of coagulation processes and the rheological behavior. Chapter 4 describes in a comprehensive way the stability of disperse systems and emphasizes the Rehbinder effect as an important mechanism in stable colloidal systems. Section II consists of three chapters. Chapter 5 provides an introduction to the methodology of mechanical testing Chapter 6 describes in detail the structures... 

The presence of an essentially similar liquid medium and of surfactant adsorption influences the magnitude and nature of the surface forces and may result in weakened cohesion in the contacts by two to three orders of magnitude. In a lyophilized, highly concentrated system in which the particles are brought into mechanical contact, this is revealed through lower resistance to deformation, x, and results in a plasticizing of the system (see Chapters 2 and 3). When the disperse phase concentration is low, lyophilization leads to the preservation of the colloidal stability of a free-disperse system, that is, the resistance of the system to coagulation (see Chapter 4). 

DLVO theory explained major principles of coagulation of hydrosols by electrolytes and brought to common grounds all previous observations (primarily of qualitative nature) that related to individual cases and often seemed to be contradictory. In years that followed further extensions of DLVO theory that took into account the possibility of reversible particle aggregation were developed. At very small distances between particles in addition to the usual long-range interaction, molecular attraction and electrostatic repulsion, one must account for other factors that play role at a direct particle contact. The formation of peculiarly structured hydration layers in the vicinity of solid surface, the appearance of elastic forces that are responsible for the Born repulsion between surface atoms at the point of contact, the repulsion between the adsorbed surfactant molecules in contact zone between two particles, all represent the so-called non-DLVO stability factors . This means that more or less deep primary minimum remains finite. 

The basis for the introduction of the notion of contactless flotation was the analogy with the well known phenomena of colloid particle coagulation in the secondary energetic minimum. Due to the predomination of the attractive molecular forces at large distances the particles can form aggregates in which some distance between particles is preserved. Thus, there is no direct contact between particles in this type of aggregation. However the notion of "contact" is not so simple. It is sufficient to point to the fact that a water monolayer remains on the hydrophobic surface. Thus the term "contactless" is maybe not suitable. 

Steric interactions are interactions between two surfaces (particles) in the presence of adsorbed macromolecules (polymers) or other molecular or particular species. Such adsorbate layers prevent the direct contact of surfaces and, thus, restrain the attractive van-der-Waals forces and can, therefore, inhibit the coagulation of... 

---