Interaction Forces Between Aggregates

As already stated in Chap. 3, there is a significant influence of particle interaction on the macroscopic properties of colloidal suspensions (e.g. viscosity, sedimentation, light scattering, sound propagation). Moreover, the attractive part of the particle interaction ultimately causes aggregation (or coagulation) in colloidal systems. 

The kinetic of aggregation determines the morphology of the evolving aggregates and is a decisive factor for the formation and strucmre of particle networks (gelling systems). Last but not least, the aggregation kinetics is used as a measure for the stability of colloidal suspensions (cf. Sect. 5.2.4). 

---

The introduction of long-range interaction forces between colloidal particles can produce well dispersed or aggregated systems under... 

The fine particles dispersed in a liquid first collide with each other and then have a tendency to aggregate. The stability of the dispersion depends on the interaction forces between the particles during these collisions. Five such forces are ... 

By adsorption onto particles they may greatly affect colloidal interaction forces between those particles. Repulsive forces may provide long-term stability against aggregation attractive forces may allow the formation of continuous networks. 

Aggregating Particles. Repulsive colloidal interaction forces between particles hardly affect sedimentation, but the effect of attractive forces can be very strong. Aggregates naturally sediment faster than single particles. Fractal aggregates containing N particles tend to move faster than... 

Prevention of aggregation and/or coalescence of suspensions or emulsions requires fundamental understanding of the various interaction forces between the particles or droplets [3] and these will be discussed in subsequent sections. For full details one can refer to the chapters on emulsions and suspensions. 

In the above systems AAa TAS and AAy TAS and hence AG > 0. This implies thermodynamic instability and the production of suspension or emulsions by the dispersion process is nonspontaneous, i.e. energy is required to produce the smaller particles or droplets from the larger ones. In the absence of any stabilization mechanism (that will be discussed below), the smaller particles or droplets tend to aggregate and/or coalesce to reduce the total interfacial area, hence reducing the total surface energy of the system. Prevention of aggregation and/or coalescence of suspensions or emulsions requires fundamental understanding of the various interaction forces between the particles or droplets and these will be discussed in subsequent sections. 

The final part deals with applications in the food industry. Food colloids are complex multiphase systems that are mostly stabilized by naturally occurring surfactants such as lipids or proteins. Some synthetic surfactants such as sorbitan esters and their ethoxylates as well as sucrose esters are used in food emulsions. The particles or droplets in food colloids may remain as individual units suspended in the medium, but in most cases aggregation of these particles or droplets takes place forming three-dimensional structures, referred to as gels . These aggregation structures are determined by the interaction forces between the particles or droplets that are controlled by the relative magnitudes of attractive (van der Waals forces) and repulsive forces. The... 

Depending on the surface and colloid chemistry of the system, the nature and magnitude of various interaction forces between particles will change and influence colloid stabUity and the rate of aggregation. The influence of these surface forces is taken into account by the collision efficiency factor in the population balance. The collision efficiency factor for aggregates is computed as reciprocal of the modified Fuchs StabUity ratio W for two primary particles k and I [26-29] ... 

Ion-pair formation (or the formation of triplets, etc.) is a very simple kind of interaction between ions of opposite charge. As the electrolyte concentration increases and the mean distance between ions decreases, electrostatic forces are no longer the only interaction forces. Aggregates within which the ions are held together by chemical forces have certain special features (i.e., shorter interatomic distances and a higher degree of desolvation than found in ion pairs) and can form a common solvation sheath instead of the individual sheaths. These aggregates are seen distinctly in spectra, and in a number of cases their concentrations can be measured spectroscopically. 

---