Polyelectrolytes are charged polymers capable of stabilizing (or destabilizing) colloidal emulsions through electrostatic interactions. Their effectiveness can be dependent on molecular weight, pH, solvent polarity, ionic strength, and the hydrophilic-lipophilic balance (HLB). Stabilized emulsions are useful in many industrial processes, including deflocculation, drug delivery, petroleum waste treatment, and food technology. Types of polyelectrolytes Polyelectrolytes are made up of positively or negatively charged repeat units. The charge on a polyelectrolyte depends on the different properties of the solution, such as the degree of dissociation of the monomer units, the solvent properties, salt concentration, pH, and temperature. Polymers become charged through the dissociation of the monomer side groups. If more monomer side groups are dissociated, the polymer has a higher charge. In turn, the charge of the polymer classifies the polyelectrolyte, which can be positive (cationic) or negative (anionic). The polymer charge and ionic strength of the polyelectrolyte in question dictate how thick a polyelectrolyte layer will be. The thickness of a polyelectrolyte then affects its adsorption ability. For more information on polyelectrolyte adsorption, look here. Some examples of polyelectrolytes can be found in the table below. The properties of the polymers vary with molecular weight and degree of polymerization. Types of Emulsions The two main types of emulsions are oil-in-water (nonpolar in polar) and water-in-oil (polar in nonpolar). The difference depends upon the nature of the surfactant or polyelectrolyte in question. The hydrophilic pieces will attract the polar solvent, creating a water-in-oil emulsion and the hydrophobic pieces will attract the nonpolar solvent, creating an oil-in-water emulsion. Emulsion Stability When there is less interfacial tension between the polyelectrolyte particles and the emulsions in question, emulsions are less stable. This is because the polyelectrolyte particles penetrate the flocs in suspension less when there is less interfacial tension. Polyelectrolytes adsorb to the interface the emulsion and help stabilize it, but may or may not lower the interfacial tension. This means that the oil or water droplets will not coalesce. On their own, hydrophobic surfactants cannot stabilize an emulsion. Although they are attracted to oil, and an oil-in-water emulsion forms, the emulsion will not stay stable for long and will eventually coalesce. With the addition of a polyelectrolyte, electrostatic forces between the oil and water interface are formed and the surfactant begins to act as an “anchor” for the polyelectrolyte, stabilizing the emulsion. In addition to surfactants, nanoparticles can also help stabilize the emulsion by