Figure 2. Log-normal distribution curve
Emulsion stability
Three distinct categories of emulsion stability must be addressed:
- Will the final emulsion type remain as O/W, or will it invert to W/O as it ages?
- The most serious instability of all is a complete separation of the emulsion into a layer of pure oil sitting on top of a layer of pure water. This demulsification, or breaking of the emulsion, would, obviously, be a catastrophe.
- By far, the most commonly encountered form of instability is creaming. Often, this is mistakenly thought to involve the formation of an oil layer on top of the emulsion. However, it is the separation of the initial emulsion into two emulsions. The one at the top of the container has a much higher internal phase concentration than the original emulsion. The one at the bottom of the container has a much lower internal phase concentration than it originally had.
Stability regarding emulsion type has been partially addressed. Recall that the dispersed phase concentration plays an important role. To minimize the chances for a gradual inversion, the oil concentration should be kept as far below the level at which complete inversion occurs as practical. The specific emulsifier choice is critical. Generally, the phase in which the emulsifier has the greater solubility tends to be the external phase. If one wishes to make an O/W emulsion, the emulsifier selected should have a somewhat greater solubility in water than in oil.
These variations in emulsifier solubility are governed by the relative sizes of the hydrophilic and hydrophobic ends of the molecule. An interesting phenomenon relating to this topic can occur if the wrong formulation choices are made. A double emulsion can result. In this case, it would be a water-in-oil-in-water emulsion. Microscopic observation would show the expected oil droplets emulsified in the water, but water droplets would also be emulsified into each of those oil droplets. A situation like this is likely to result if the two phases are present in roughly equal volumes and the emulsifier has almost equal solubility in oil and water.
A complete breaking of the emulsion normally occurs in two stages. First, the dispersed phase droplets form aggregates, and the droplets become attached to each other without losing their individual identity. This process, called flocculation, destabilizes the emulsion because the large aggregates separate out more rapidly than the smaller individual droplets. However, the process is usually reversible upon application of agitation and/or the addition of a suitable surfactant. On the other hand, coalescence, the second stage of demulsification is much more serious. This is a non-reversible process in which several internal phase droplets merge together into a single large droplet.
Creaming is the most commonly encountered type of emulsion instability. It results when the inexorable work of the gravitational forces causes two liquids of different densities to stratify. This effect can be slowed by the techniques discussed in this article, but it cannot be stopped. Therefore, the real concern should be the rate of creaming. It is acknowledged that creaming will occur, but it must be slowed to a tolerable rate.
Types
Before discussing emulsion processing and evaluation, the emulsifier types must be defined. The surface-active emulsifiers, with which most processors are primarily concerned, can be subdivided into anionic, cationic, nonionic and amphoteric. These terms refer to the electrical charge that the emulsifier imparts to the dispersed-phase particles:
- Anionic = negative charge
- Cationic = positive charge
- Nonionic = electrically neutral
- Amphoteric = pH-dependent charge
The influence on emulsion stability of some important external factors depends on the electrical characteristics created by the emulsifier. For example, anionic types are effective at high pH levels, while cationic types are effective at low pH levels. The nonionic types are relatively insensitive to the emulsion’s pH.
Similarly, the stability of emulsions formed with electrically charged surfactant molecules is much more likely to be disturbed by the addition of electrolytes to the finished product. This is the reason that the addition of a trace amount of a salt often destroys some emulsions. When contact with electrolytes is expected, a nonionic emulsifier type should be used.
Processing methods
To create an emulsion, the ingredients are first combined to form a crude premix emulsion. This premix can be created in several ways:
- The emulsifier is dissolved in the continuous phase, and then the internal phase is slowly added with good agitation (most common method).
- The emulsifier can be dissolved in the internal phase before slowly adding that blend to the continuous phase under agitation.
- The emulsifier can be dissolved in the internal phase before slowly adding the continuous phase to form the premix. This means usually produces the best results, but it requires a lot of time and vigorous mixing because it involves bringing a preliminary W/O emulsion through the inversion stage to eventually form the desired O/W type.
- Another method is using a mix-order control method specifically developed by Bematek. This technique permits the injection of the product components directly into the product stream at different steps along a multistage mixing chamber.
If a mechanical shearing device such as a colloid mill or an in-line mixer is used in the finishing step, the first premix method usually produces good results.
One frequently used premix option must be mentioned separately. Many O/W emulsions are stabilized with simple soaps. In these situations, the best results are obtained by dissolving the fatty acid part of the soap in the oil phase and the alkaline part of the soap in the water phase. Then the oil is added to the water with agitation. The resulting formation of the soap directly at the interface as the two phases are combined leads to an excellent premix. Note that an in-line mixer with mix-order control ability can accomplish the simple-soap method in a continuous, single-pass system instead of using a premix tank.
Having assured a well-formulated and stable premix, the colloid mill or in-line mixer can finish the job. The zone of intense hydraulic shear forces within the colloid mill or in-line mixer head breaks the internal phase droplets apart and creates the small particle size that is generally desired. If a sufficient emulsifier is used for the enormous increase in surface area generated by this process, the final product should exhibit enhanced stability.
After preparing the emulsion, it is key to find a way to measure its quality so that some degree of consistency from one batch to another can be maintained.
Quality control
With the emulsions’ physical properties, the information to verify the results with a reliable quality control (QC) process is available. The easiest method is to put the emulsion on a shelf and observe it for creaming over time. A minimum acceptable shelf life can be a QC specification. Unfortunately, the price for this simplicity is that a poor batch might not be discovered until after the product reaches the customer. To overcome this, the creaming process can be accelerated by heating the emulsion or by centrifuging it. These results must then be related to a corresponding static creaming rate at room temperature. All these creaming-rate measurements are simple, but they are not precise.
The creaming rate of an emulsion is directly related to the size of the dispersed phase droplets. Therefore, measuring the droplet size indirectly provides the desired stability information. These methods include:
- An optical microscope for observing the maximum particle size present
- Measurements of the percentage of light transmitted (the emulsion becomes more transparent as the particle size is reduced)
- Light-scattering measurements for generating detailed particle size distribution curves
These methods are quick and accurate. After a relationship between particle size and shelf life has been established, precise predictions of the creaming rate can be made from the particle size analysis.
Another useful technique is that of viscosity measurement. Many emulsions will undergo a substantial viscosity increase as the droplet size is reduced. The amount of this increase will, therefore, be a good indicator of emulsion quality. For this reason, a viscosity measurement obtained with a device such as a Brookfield viscometer can provide an excellent QC benchmark in such cases. Many other techniques are used in specific cases. For example, cosmetic emulsions are often required to withstand several freeze/thaw cycles and to be stable at elevated temperatures before they are considered acceptable.
Conclusion
This article only covers the most basic concepts of this complex and wide-ranging topic. Many books are available that go into detail on these topics and many others.
Stephen F. Masucci is technical director for Bematek. He has been involved in the formulation, scale-up, processing and testing of emulsions and dispersions since 1973.
Chris Little is a senior manager for Bematek, and with 25 years of facility and process industry experience, he is considered an expert in process improvement and equipment reliability.
Bematek
