Selecting a Dispersing Agent

The selection process suggested by Nelson (Ref 17) will soon be superseded by an ISO Standard 14887, "Dispersing Procedures for Powders in Liquids," currently in preparation with the author as convenor of the working group. A brief summary of the considerations involved in finding an effective dispersing agent is given below for the major chemical categories of particles.

Particles having a natural surface charge may be electrostatically stable to flocculation with no further treatment. If the natural charge is inadequate to prevent flocculation, these charged particles readily adsorb ionized dispersing agents that have a charge opposite that of the surface (to give a neutral particle) or multiply charged ions (which may reverse the charge on the particle).

Metal particles readily adsorb dispersing agents with highly polarizable groups and can achieve steric stabilization if coated with gelatin. Many metals react with air to form surface oxides, and many metal oxides react with water to form surface hydroxides, in which case they may be electrostatically stabilized in water if the pH is one or more units away from the isoelectric pH. Strong magnetic dipole attractions create special problems for dispersing transition metal oxide particles, which commonly have unpaired electrons that make them paramagnetic or ferromagnetic. The best approach to stabilization of magnetic particles is to provide a thick coating (for example from protein albumin) that prevents the close approach of two particles.

Highly polar materials readily adsorb dispersing agents with highly polarizable groups. A prospective dispersing agent for dispersion in aqueous systems would be sulfonated naphthalenes; for dispersion in organic systems it would be polar-substituted polyesters (acid, ester, or amine).

Nonoxide refractories consist of nonmetallic atoms held together by covalent bonds. Refractories have low values of density, polarizability, surface energy, reactivity, and adsorptivity. Because of their low polarizability, the particles are more easily dispersed in organic liquids than in water. They do not form strong flocs. A prospective dispersing agent would be a polyoxyisopropylene chain grafted to a poly-oxyethylene chain.

Polar polymers have strongly dipolar, hydrogen-bonding, or reactive groups along the chain, so their surface energies and polarizabilities are high. The particles are often incompatible with water and strongly attracted to one another, so flocculation can be a serious problem. A prospective dispersing agent would be a hydrophilic polar polymeric surfactant, which would adsorb at random points along its polymeric chain and be solvated along the rest of its chain.

Nonpolar polymers have no strongly polar groups on their repeating units. They do not require dispersing agents in nonpolar liquids. Since they have low polarizabilities, they do not form strong flocs in water, but they are not wet by water. Good dispersing agents for nonpolar polymers in water have a long nonpolar chain (that adsorbs on the polymer particles) grafted to a water-soluble chain. A prospective would be a hydrocarbon chain grafted to a polyoxyethylene chain.

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