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. Macroemulsions scatter light effectively and therefore appear milky, because their droplets are greater than a wavelength of light. They are part of a larger family of emulsions along with miniemulsions (or nanoemulsions). As with all emulsions, one phase serves as the dispersing agent. It is often called the continuous or outer phase. The remaining phase(s) are disperse or inner phase(s), because the liquid droplets are finely distributed amongst the larger continuous phase droplets. This type of emulsion is thermodynamically unstable, but can be stabilized for a period of time with applications of kinetic energy. 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. Emulsions can be stabilized otherwise with polymers, solid particles (Pickering emulsions) or proteins.
Classification Macroemulsions can be divided into two main categories based on if they are a single emulsion or a double or multiple emulsion group. Both categories will be described using a typical oil (O) and water (W) immiscible fluid pairing. Single emulsions can be sub divided into two different types. For each single emulsion a single surfactant stabilizing layer exists as a buffer in between the two layers. In (O/W) oil droplets are dispersed in water. On the other hand, (W/O) involves water droplets finely dispersed in oil. Double or multiple emulsion classification is similar to single emulsion classification, except the immiscible phases are separated by at least two surfactant thin films. In a (W/O/W) combination, an immiscible oil phase exists between two separate water phases. In contrast, in an (O/W/O) combination the immiscible water phase separates two different oil phases.
Formation Macroemulsions are formed in a variety of ways. Since they are not thermodynamically stable, they do not form spontaneously and require energy input, usually in the form of stirring or shaking of some kind to mechanically mix the otherwise immiscible phases. The resulting size of the macroemulsions typically depends on how much energy was used to mix the phases, with higher-energy mixing methods resulting in smaller emulsion particles. The energy required for this can be approximated using the following equation:
Δ G e m = 3 γ V R f {\displaystyle \Delta G_{\rm {em}}=3{\gamma V \over \ R_{\rm {f}}}}
Where Δ G e m {\displaystyle \Delta G_{\rm {em}}} is the energy Input, γ {\displaystyle \gamma } is the interfacial tension between the two phases, V {\displaystyle V} is the total volume of the mixture, and R f {\displaystyle R_{\rm {f}}} is the average radius of the newly created emulsions This equation gives the energy requirement just to separate the particles. In practice the energy cost is much higher, as most of the mechanical energy is simply converted to heat rather than mixing the phases. There are other ways to create emulsions between two liquids, such as adding one phase with droplets already being the required size. An emulsifying agent of some sort is also generally required. This helps form emulsions by reducing the interfacial tension between the two phases, usually by acting as a surfactant and adsorbing to the interface. This works because most emulsifiers have a hydrophilic and hydrophobic side, which means they can bond with both the oil-like phase and the water-like phase, thus reducing the number of water-oil molecular interactions at the surface. Reducing the number of these interactions reduces the interfacial energy, thus causing the emulsions to become more stable. The concentration of surfactant needed is much higher than its critical micelle concentration (CMC). 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. The probability (P) of finding a certain sized droplet can be estimated for inner layer drops through the following equation:
P = 1 Δ R 2 π exp [ ( ln R − ln R ¯ ) 2 2 Δ R 2 ] {\displaystyle \ P={{\ 1 \over \Delta R{\sqrt {2\pi \,}}\ }\exp[{\ (\ln {R}\ -\ln {\bar {R}}\ )^{2} \over \ 2\Delta R^{2}\ \ }\ }]}
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