Preventing Fioccuiation of the Dispersed Particles

Particles attract one another according to how much their interfacial energy densities differ from the intervening liquid. In an unstable dispersion, these attractive forces dominate and cause flocculation. Most size analysis techniques respond to a floc as though it were a single particle, so the particle sizes reported for a flocculated dispersion will be larger than those reported for a well-dispersed dispersion. A dispersion will be stable against flocculation if the particles repel one another or cannot approach each other closely.

Particles repel one another if they bear charges of the same sign (electrostatic stabilization) or are covered with molecules that prevent close approach (steric stabilization). The repulsion between particles may be increased by changing solution conditions or using additives called "dispersing" agents to modify particles so that they will repel one another.

Electrostatic repulsion is the simplest and generally least expensive way to stabilize a dispersion against flocculation. Surface charge can be modified by:

• pH changes that alter ionization or adsorption of surface acid and base groups

• Chemical reactions that create or deactivate ionizable surface groups

• Adsorption of ions from solution

• Ionization of an adsorbed dispersing agent

• Changing the concentration of dispersing agent to shift the adsorption equilibrium

• Using a strongly adsorbing dispersing agent to displace an unwanted adsorbate that is adsorbed less strongly

Electrostatic stabilization is most often used in aqueous systems; ionization is difficult to achieve in organic liquids.

The zeta potential characterizes the effectiveness of the charge on a particle. If the zeta potential of particles in an aqueous system (with ionic strength below 0.1 mol/L) is larger than 30 mV (positive or negative) the dispersion is generally stable against flocculation. The ionic strength is half the sum of the concentration of each ion multiplied by the square of the number of charges on that ion. It is a measure of the ability of ions in solution to form a counter-ion atmosphere about an oppositely charged particle and thus offset the effectiveness of its zeta potential.

The zeta potential for acid or base surface groups changes as a function of pH. At low pH the surface may adsorb or react with hydrogen ions to become positively charged. At high pH the surface may adsorb or react with hydroxide ions to become negatively charged. The isoelectric pH is the pH at which the zeta potential is zero. Isoelectric pH values range from 2 for silica to 6.4 for aluminum trihydroxide (gibbsite) to 9 for zinc oxide. Surface chemistry depends on the residual stresses at the surface, so the isoelectric point for a particle may depend upon its prior heat treatment, impurities, grinding, and aging in contact with chemicals. Since the zeta potential may drop by as much as 50 mV per pH unit near the isoelectric pH, it is important to keep the pH of a dispersion at least 1 pH unit away from the isoelectric pH to ensure stability against flocculation.

Since multiply charged ions are more strongly attracted to a charged surface than singly charged ions are, the effect of a millimole of MgSO4 in solution is four times that of a millimole of NaCl in diminishing the effectiveness of ionic stabilization. A change from distilled to tap water or from tap water to river water can have disastrous consequences on a process if the change introduces small quantities of multiply charged ions into a slurry that is stabilized by particle charge.

Steric repulsion arises as particles covered by a steric dispersing agent approach closely. Effective steric agents contain one or more soluble ("solvated") polymeric chain segments ("loops" or "tails") and one or more strongly adsorbed anchor ("head") groups. The typical molar mass of the soluble chain segments is 5,000 to 15,000 g/mol. When two such "hairy" particles come close together, the unfavorable energetics of trying to put both sets of solvated and moving loops and tails into the same region of solvent prevent the particles from coming close enough for the attractive force to become significant. The magnitude of steric repulsion depends on polymer-liquid interactions that influence polymer chain configurations. The hairy shell may expand or contract with temperature and liquid composition (pH, ionic strength, co-solvents). An expanded configuration generally provides the highest degree of stabilization.

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