Colloid stability

• Colloids are dispersions of small particles (1-100 nm) in another medium.
• Stability of colloidal system is due to two factors: charge present on the colloid and solvation of the colloidal particles.
• Colloidal particles form a sheath of charge on themselves, thereby preventing any attraction between opposite charges.
• An electrical double layer is formed, which prevents the coagulation of the colloid in addition to an electrolyte.
• Colloidal system is stable if the colloidal particles are present in the layers of the solvent uniformly.
• Lyophilic colloids are those colloids which are solvent loving and are quite stable as they are extensively solvated.
• A quick shaking can get the coagulated colloid back to colloidal solution.
• Lyophobic colloids are those colloids which are solvent hating and are unstable as the interactions among the colloidal particles and the solvent are very less.
• The level of screening provided by the electrolyte may permit the particles to interact, a process known as weak flocculation, but it will be relatively easy to separate them.
• This is represented as a secondary minimum and the colloidal dispersion is likely to separate over a period of time.
• The electrical potential at the shearing plane is the zeta potential.
• The region containing the double layer is sheared at some distance from the solid surface, creating a thin film associated with the solid.
• Reduction in the size of the energy barrier enables the particles to get closer together.
• Electrokinetic phenomena result from the differential movement of two phases where the interface is an electrical double layer.
• In simple terms, zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particles.
• High zeta potential indicates stability, while low zeta potential indicates flocculation.
• Lyophilic colloids are used for their protection, forming a layer around them and protecting from the attack of an electrolyte.
• Any factor which leads to the prevention of aggregation of colloids will help in stabilizing it.
• Lyophilic colloids are sometimes called protective colloids because they stabilize lyophobic colloids.
• Electric potential at the solid surface is origin of electric charge on colloidal articles.
• Frictional electrification is a method by which charge is developed on colloidal particles by mutual rubbing with molecules of dispersion medium.
• Preferential adsorption of ions from solution is a method by which anions colloid adsorbs ions common to its own lattice during preparation of colloidal solution.
• Dissociation of molecules followed by aggregation of ions is a method by which charge is developed on colloidal particles.
• In case of soap, the RCOO- groups get dissociated from Na+ ions and have tendency to aggregate into a cluster carrying –ve charge.
• When one type of the ions of the electrolyte are adsorbed on the surface of colloidal particles, it forms a 'fixed layer', attracting the opposite ions to form another layer called 'diffused layer'.
• A difference ion potential exists between the fixed layer and the diffused layer.
• Van der Waals attraction is created because of the overall interactions between the temporary dipoles of the molecules in the two interacting particles.
• The DLVO theory describes the force between charged surfaces interacting through a liquid medium.
• An important assumption in the DLVO theory is that the surface electric potential remains constant.
• The DLVO theory is valid and has been widely applied in practice, as long as the following conditions are met: dispersion is very dilute, no other force is present, and the geometry of particles is relatively simple, so that the surface properties are the same over the entire surface.
• Hamaker (1937) used Eq 2.33 to evaluate the interaction between molecules based on composition and structure of the particles.
• An electrical double layer exists around each particle.
• The DLVO theory, named after Derjaguin, Landau, Verwey and Overbeek, examines the dependence of colloid stability on the various parameters that determine the shapes and the magnitudes of interaction energies between particles.
• If the particles collide with sufficient energy to overcome that barrier, the attractive force will pull them into contact where they adhere strongly and irreversibly together.
• Steric stabilization makes the system thermodynamically stable.
• The DLVO theory combines the effects of van der Waals attraction and electrostatic repulsion due to electrical double layers.
• A solid has a point zero charge.
• The double layer is purely diffusive, so that the distributions of counter ions and charge determining ions are determined by all three forces: electrostatic force, entropic dispersion, and Brownian motion.
• The electric double layer consists of two regions: an inner region including adsorbed ions and a diffuse region in which ions are distributed according to the influence of electric forces and random thermal motion.
• Lifshiz (1956) developed a theoretical relationship for the collective interactive forces between macroscopic particles from quantum field theory that relates the interactive energy to distance of separation of particles.