page_title: Colloid
section_title: Stabilization (peptization)
hierarchical_section_title: Colloid / Stabilization (peptization)
caption_attribution_description: English: Diagram of steric stabilization and gel network stabilization in colloids.
context_page_description: In chemistry, a colloid is a phase separated mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid; the term colloidal suspension refers unambiguously to the overall mixture. Unlike a solution, whose solute and solvent constitute only one phase, a colloid has a dispersed phase and a continuous phase that arise by phase separation. Typically, colloids do not completely settle or take a long time to settle completely into two separated layers.
  The dispersed-phase particles have a diameter between approximately 1 and 1000 nanometers. Such particles are normally easily visible in an optical microscope, although at the smaller size range, an ultramicroscope or an electron microscope may be required. Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid.
context_section_description: The stability of a colloidal system is defined by particles remaining suspended in solution at equilibrium.
  Stability is hindered by aggregation and sedimentation phenomena, which are driven by the colloid's tendency to reduce surface energy. Reducing the interfacial tension will stabilize the colloidal system by reducing this driving force. 
  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. The electrostatic repulsion between suspended colloidal particles is most readily quantified in terms of the zeta potential, a measurable quantity describing electrical potential at the slipping plane in an electrical double layer.
  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 polymer 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.
caption: Steric and gel network stabilization.
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