Forces Acting in Colloidal Systems

The particle–particle, particle–surface and surface-surface interactions are involved in daily and industrial applications. They play a crucial role in a variety of processes, such as colloidal particle aggregation, dispersion, and rheology. In colloidal systems, the behavior of disperse particles is influenced by a series of different forces acting on the whole system. These forces can act in a complex way to control the stability and the phase behavior of colloidal particles. The colloid stability is governed by the resulting energy from the balance between attractive and repulsive forces, which involves both kinetic and thermodynamic aspects. Attractive interactions come from dispersion forces, while repulsive interactions are consequences of electric double-layer forces. DLVO is the leading theory to describe the stability of colloidal systems. The interaction between the colloidal particles can also be divided into static and dynamic components, respectively related to the particle–particle static forces and the motion of particles due to the solvent movement. The macroscopic response to the attractive-repulsive balance forces is manifested in gas condensation and liquid compressibility, for instance. The instability of dispersions can lead to flocculation because of the attractive forces acting to hold the particles together. If strong enough repulsive forces act to keep the dispersed particles away from each other, the dispersion can achieve an appropriate stable state, avoiding particle flocculation. Solvation and steric interactions are usual repulsive forces that can prevent coalescence. Besides, the balance of attractive and repulsive forces in colloidal systems can be regulated by adding components that alter the particle–particle and solvent-particle interactions, such as surface agents and electrolytes, leading to the desired conditions for dispersion.