Stabilization (peptization)
The stability of a colloidal system is the capability of the system to remain as it is.
Stability is hindered by aggregation and by sedimentation phenomena, that determine phase separation.
Aggregation is due to the sum of the interaction forces between particles. If attractive forces (such as van der Waals forces) prevail over the repulsive ones (such as the electrostatic ones) particles aggregate in clusters.
Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.
- Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that an electrical double layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too weak to overcome the electrostatic repulsion between charged layers of the dispersing phase.
- Steric stabilization consists in covering the particles in polymers which prevents the particle to get close in the range of attractive forces.
A combination of the two mechanisms is also possible (electrosteric stabilization). All the above mentioned mechanisms for minimizing particle aggregation rely on the enhancement of the repulsive interaction forces.
Electrostatic and steric stabilization do not directly address the sedimentation/floating problem.
Particle sedimentation (and also floating, although this phenomenon is less common) arises from a difference in the density of the dispersed and of the continuous phase. The higher the difference in densities, the faster the particle settling.
- The gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation.
The method consists in adding to the colloidal suspension a green biopolymer able to form a gel network and characterized by shear thinning properties. Examples of such substances are xanthan and guar gum.
Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped. In addition, the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles.
The rheological shear thinning properties find beneficial in the preparation of the suspensions and in their use, as the reduced viscosity at high shear rates facilitates deagglomeration, mixing and in general the flow of the suspensions.
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