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An emulsion is a type of coarse dispersion containing two immiscible liquid phases, where the dispersed phase is present as globules and the continuous phase is the other liquid. In cosmetic and pharmaceutical emulsions, the stability of the emulsion is the most important parameter. But there are several issues related to the instability of emulsion such as
- Flocculation and creaming
- Cohelesence and breaking
- Miscellaneous physical and chemical changes
- Phase inversion
So, in this article, we are going to see some of this instability of emulsion and ways to avoid these instabilities.
First, let’s understand why these instabilities occur. In a good emulsion, repulsive forces between dispersed particles are dominant. But when the dispersed particles get very close to each other (due to various reasons), they go into each others’ minima. And due to this, the repulsive forces become weak and attractive forces take over. This results in the instability issues mentioned earlier.
Let’s see them in detail along with ways to overcome them.
Creaming
Creaming is the accumulation of dispersed particles in dispersion on the surface of the emulsion. Although the phases separate in creaming and flocculation, these processes are reversible. So, after shaking the emulsion, the particles redisperse. This happens because the dispersed particles are still protected by the layer of the emulsifying agent. We can understand the phenomenon of creaming using Stoke’s law. According to Stoke’s equation, the velocity of creaming of individual dispersed particle (v) is:
v = d² (ρs – ρo) g / 18η0
Where,
- v is the velocity at which creaming occurs
- d is the diameter of individual dispersed particle
- ρs is the dispersed phase’s density
- ρo is the dispersion medium’s density
- η0 is the dispersion medium’s viscosity
So, our goal is to keep the value of v as little as possible. So, the creaming rate will be minimum.
Factors affecting creaming
We can conclude the following things by analyzing Stoke’s equation:
As we look at the equation, we come to know that when the density of the dispersed phase is less than the density of the dispersion medium, the value of v becomes negative. This is known as upward creaming. This generally happens in the case of o/w emulsion where oil is less dense than water.
On the contrary, when the density of the dispersed phase is greater than that of the dispersion medium, then the value of v is positive and downward creaming occurs.
The rate of creaming increases as the difference between viscosity of the dispersed phase and dispersion medium increases. Further, the lower the viscosity of a dispersion medium, the greater the creaming rate.
The diameter of dispersed particles is at the nominator. So, the greater the size of the particle, the greater will be the creaming rate. So, we need to keep the particle size as low as possible.
When we increase the force due to gravity (g) using methods like centrifugation, the rate of creaming also increases.
Theoretically, when densities of both dispersed phase and dispersion medium are the same, we can avoid creaming.
Stoke’s equation does not mention another factor affecting creaming rates and that is the homogenization of dispersed particles. When we have uneven particles, the smaller particles will get attracted to the larger ones, which causes creaming.
Solutions to overcome creaming
The following suggestions can help improve the stability of emulsion by preventing creaming, as discussed in these considerations:
Make dispersed particles homogenized: For example., in homogenized milk, it is only the homogeneity of the particles that are responsible for most of its stability.
Reduce particle size of dispersed particles: As per Stoke’s equation, the lesser the diameter lesser will be the rate of creaming. Formulators found that when the particles size was about 2 to 5 μm, the rate of creaming was even less than what was expected by Stoke’s law. This happens because, at this size range, the Brownian motion is dominant which causes repulsive forces.
Make densities of both the phases equal: As we saw earlier, if we keep both the densities equal, creaming should not happen. But in reality temperature change also changes the densities. However, formulators try to keep the difference between the densities as low as possible. Generally, the oil phase has a low density in the formulation. So, adding an oil-soluble substance increases the density of the oil phase. For pharmaceutical and food emulsions, food-grade brominated oil is used to maintain the densities.
Increase viscosity of the dispersion medium: We can increase the viscosity of the dispersion medium using viscosity improvers or thickening agents such as methylcellulose, tragacanth, sodium alginate, etc.
Coalescence and breaking
Coalescence is followed by breaking. Breaking is an irreversible process. So, we resolve it by shaking the emulsion. This happens because, unlike creaming, the protective sheath of emulsifying agent destroys and the oil tends to coalesce. Scientists have done a lot of research to study this emulsion instability.
When the dispersed particles come together and form a large globule we call this phenomenon Coalescence which further causes instability of emulsion. So, when the coalescence increases the phases separate completely, and the emulsion breaks.
Factors affecting coalescence and breaking
When the dispersed particles are non-uniformly distributed, the rate of coalescence increases. Whereas, when the particles are homogeneous and uniformly distributed, the emulsion becomes stable. Therefore uniform dispersion affects the stability of the emulsion.
Even though the viscosity has little relation with the stability of the emulsion, we consider a viscous solution to improve the stability.
The next factor is the phase-volume ratio. That means the relative volumes of oil and water phase in an emulsion. Let’s see what is the phase-volume ratio using the following diagram.
As you can see in the diagram, the pores occupy 48% of the total volume of an emulsion whereas, the globules occupy the rest 52%. Typically, this 48:52 ratio gives a stable emulsion. However, owing to the different demands from the emulsion, the volume occupied by the globules can be increased to 74%. But 74% is a critical point. That means if we exceed the globule volume beyond 74% then the globules will coalescence and the emulsion breaks.
Solutions to overcome coalescence and breaking
Determine phase-volume ratio: Many experiments have proven that when we keep phase-volume ratio 50:50 the emulsion is in the most stable form. That is why most of the pharmaceutical emulsions keep the phase-volume ratio 50:50.
Increase in zeta potential: As we know, attractive forces between the dispersed particles cause instability of emulsion. So, we can increase the zeta potential by increasing the electrostatic repulsion. For example., the addition of lecithin in perfluorocarbon emulsion. At physiological pH, lecithin has a negative charge. Lecithin adsorbs on the droplet surface giving it a negative charge and eventually increasing electrostatic repulsive forces.
Decreasing interfacial tension using a good emulsifier: In pharmaceutical or any industry, a good emulsifier is the one that is both tough and elastic and should form rapidly during emulsification.
Pharmaceutics