Macroemulsions

Macroemulsions are emulsions in which the dispersed droplets are in the micrometer size range, typically from about 0.1 to 100 micrometers. They can be oil-in-water or water-in-oil and are usually opaque due to light scattering by the droplets. Macroemulsions are commonly formed by mechanical agitation or high-shear devices and are thermodynamically unstable, tending toward phase separation over time.

The formulation of a macroemulsion includes a continuous phase (water or oil), a dispersed phase, and an emulsifying system such as surfactants, proteins, polysaccharides, or polymeric stabilizers. The stabilizer lowers interfacial tension and provides electrostatic or steric repulsion to hinder droplet coalescence. Viscosity modifiers or gelling agents may be added to reduce creaming and to tailor flow properties.

Preparation methods are broadly categorized as high-energy and low-energy. High-energy methods (such as high-pressure homogenization or rotor-stator homogenization) impart intense shear to break droplets to the desired size. Low-energy methods (such as phase inversion temperature or spontaneous emulsification) rely on formulation and thermodynamic conditions to form the emulsion. Process conditions influence droplet size, stability, and potential heating of sensitive components.

Stability in macroemulsions is primarily kinetic; droplets may coalesce, cream or sediment, and, in some cases, undergo Ostwald ripening. Stability is enhanced by selecting appropriate emulsifiers, optimization of phase volumes, and, in some systems, solid particle stabilizers (Pickering emulsions).

Applications span food (mayonnaise, lotions and sauces), cosmetics (creams, moisturizers), pharmaceuticals (topical suspensions, drug delivery emulsions), and agrochemicals. Characterization typically involves droplet-size analysis by microscopy or laser methods, rheological measurements, and accelerated stability testing.