Macroemulsion
Macroemulsions are dispersed liquid-liquid, thermodynamically unstable systems with particle sizes ranging from 1 to 100 μm (orders of magnitude), which, most often, do not form spontaneously.
[2] This type of emulsion is thermodynamically unstable, but can be stabilized for a period of time with applications of kinetic energy.
[1] Surfactants (as the main emulsifiers) are used to reduce the interfacial tension between the two phases, and induce macroemulsion stability for a useful amount of time.
Emulsion in which the particles of the dispersed phase have diameters from approximately 1 to 100 μm.
Separation of the dispersed and continuous phases usually occurs within time periods from a few seconds to a few hours, depending upon the viscosity of the fluid medium and the size and density of the droplets.
Note 2: Macro-emulsions usually contain low-molecular-weight or polymeric surfactants that decrease the rates of coalescence of dispersed droplets.
Both categories will be described using a typical oil (O) and water (W) immiscible fluid pairing.
On the other hand, (W/O) involves water droplets finely dispersed in oil.
is the average radius of the newly created emulsions This equation gives the energy requirement just to separate the particles.
There are other ways to create emulsions between two liquids, such as adding one phase with droplets already being the required size.
This helps form emulsions by reducing the interfacial tension between the two phases, usually by acting as a surfactant and adsorbing to the interface.
[2] This forms a surfactant monolayer which orients itself to minimize its surface to volume ratio.
This ratio yields highly polydisperse spherical droplets in the range of 1 to 100 μm.
[2] The probability (P) of finding a certain sized droplet can be estimated for inner layer drops through the following equation:
This means that from the moment they are created, they are always reverting to their original, immiscible and separate state.
This means that, while they are continuously breaking down, it is done at such a slow pace that it is practically stable from a macroscopic perspective.
This creates a potential energy well at some distance, where the particles are in a local area of stability despite not being directly touching and therefore coalescing.
While it is energetically favorable for individual particles to coalesce due to the subsequent reduction of interfacial area, the adsorbed emulsifier prevents this.
Stability of the Macroemulsions are based on numerous environmental factors including temperature, pH, and the ionic strength of the solvent.
Flocculation occurs when the dispersed drops group together throughout the continuous phase, but don't lose their individual identities.
The driving force for flocculation is a weak van der Waals attraction between drops at large distances, which is known as the secondary energy minimum.
[2] An electrostatic repulsion between the surfaces prevents the drops from touching and merging, stabilizing the macroemulsion.
If the droplet radii are not all the same size and aggregation occurs, the flocculation rate constant is equal to
Creaming is the accumulation of drops in the dispersed phase at the top of the container.
Also, there is a greater chance of creaming at lower viscosities of the continuous phase liquid.
This process may be accelerated by adding a cosurfactant or salt or by slowly stirring the liquid solution.
They are widely utilized today in automotive, beauty, cleaning and fabric care products as well as biotechnology and manufacturing techniques.
[5] Macroemulsions are often chosen over microemulsions for automotive and industrial applications because they are less expensive, easier to dispose of, and their tendency to demulsify more quickly is often desirable for lubricants.
[6] Many skin care products, sun screens, and fabric softeners are made from silicone macroemulsions.
Different combinations of macroemulsions and surfactants are the subject of a wide range of biological research, especially in the area of cell cultures.
