Emulsions are dispersions of droplets in an immiscible solvent that are widely used in the food, pharmaceutical and chemical industries. Analyzing the droplet size and zeta potential is a key step in optimizing emulsion stability, performance, and safety.
Introduction to emulsions
An emulsion is a mixture of two liquids that wouldn’t normally mix together. Emulsions can be “oil-in-water” emulsions that consist of oil droplets dispersed in water or some other aqueous dispersion. Mayonnaise is a common “oil-in-water” emulsion that is stabilized with lecithin obtained from egg yolk. Alternatively, there are also “water-in-oil” emulsions that are water droplets dispersed in an oil medium. Butter and margarine are common examples of water-in-oil emulsions. In addition to the examples mentioned above we also encounter other emulsions every day, including salad dressings, paints, cosmetics, creams, and lotions.
You might ask, “If oil and water don’t mix, how would an emulsion ever form?”. Similar to a vinaigrette salad dressing, emulsions can be formed, at least temporarily, by rapidly shaking or stirring an oil and water mixture together. They can also be formed by sonication, which uses ultrasonic waves to agitate dispersions and cause them to mix, as well as by homogenization, which is commonly used to process milk so that fat droplets remain dispersed and do not float to the top.
However, because there is a natural tendency for oil and water to separate, most emulsions are not stable over time and their components will separate out into two layers. That being said, stable emulsions can be generated by adding an emulsifier, which is a chemical that helps to keep the two components mixed. Emulsifiers are structurally and functionally similar to surfactants in that they have a long hydrophobic chain that extends into the oil layer and a polar head-group that interacts with the aqueous layer. Examples of common emulsifiers include mustard, egg yolk, soaps, and other surfactants.
We often think of emulsions as relatively simple, macroscale emulsions that we can see, feel, and even taste but in reality it is the nanoscale properties of the droplets dispersed in an emulsion that are the main contributors to the emulsion’s bulk properties. Specifically, the size and charge of the emulsified droplets directly affect stability, taste, safety, look and feel, and function. Thus, it is critically important to be able to accurately and quickly measure these properties of emulsions.
There are a number of scientific techniques useful for characterizing emulsions. Two of the most commonly used are dynamic light scattering (DLS), which measures the particle size and size distribution of the emulsified droplets, and electrophoretic light scattering (ELS) which measures the droplet charge/zeta potential. The droplet size of an emulsion is important to ensure the ideal look and feel of an emulsion as well as the required functionality such as the rate of drug delivery of an intravenous (IV) emulsion (Figure 1) or the dose uniformity of an inhaled drug. The zeta potential of an emulsion indicates the likelihood of the droplets to aggregate and is thus related to the overall stability of the emulsion.
Dynamic light scattering (DLS)
DLS enables the measurement of emulsion droplet size by irradiating the sample with a laser and analyzing the light that is scattered back out by the particles. This scattered light creates a speckle pattern on the detector which changes over time because the particles are moving via Brownian motion. The rate of change of the speckle pattern on the detector is related to how fast the particles are moving, which is related to the size of the particles.
The particle size within emulsions is critically important in many applications; one in particular is intravenous (IV) infusions. Drug delivery via IV is often necessary because many drugs are not stable enough to be taken orally or because they do not pass through the intestinal lining. To ensure the quality and safety of IV formulations, there are strict regulations related to the maximum particle size that can be present in IV drug formulations. Specifically, particles larger than 5 microns are considered a health risk. The DLS technique is able to measure particle sizes between 0.3 nm and 10 microns and thus is a useful technique for ensuring that IV formulations do not contain drug aggregates or contaminations from containers or storage materials. Figure 2 illustrates DLS data ensuring that there is a single population of droplet sizes and that they are less than 5 microns in diameter. Table 1 illustrates the flexibility of DLS to measure various emulsion concentrations in a highly reproducible way.
To confirm the absence of unwanted larger particles, the emulsion was analyzed further at a scattering angle of 15°. The forward scattering at 15° is the optimal angle for a sample of small particles that may also contains a few larger particles such as aggregates or contaminants. Figure 3 shows DLS data measured in forward angle, confirming that no aggregates are present.
Electrophoretic light scattering (ELS)
ELS, on the other hand, enables the measurement of an emulsion’s zeta potential or the effective charge of a droplet when dispersed. ELS functions by applying an electric field to the emulsion which causes the droplets, if charged, to migrate towards the oppositely charged electrode. The sample is irradiated with a laser and the light which is scattered by the particles is then detected. However, because the particles are moving by electrophoretic motion, the movement of the particle causes a Doppler shift (or frequency shift) in the scattered light that corresponds to the speed of the particles, known as electrophoretic mobility. The electrophoretic mobility is then correlated to the magnitude of the droplet’s charge in the emulsion.
Emulsions in the pharmaceutical Industry
The zeta potential of the droplets in an emulsion is related to the overall stability because particles that are highly charged (either positively or negatively) will repel each other in solution and thus be less likely to aggregate and eventually separate out of dispersion. On the other hand, droplets that are relatively uncharged or neutral are more likely to interact with each other rather than the solvent leading to larger particles or complete separation back into two phases. Thus, ensuring optimal droplet zeta potential is critical to ensure that the emulsion will be stable over time and that the droplet size doesn’t change or increase over the 5 micron limit.
Emulsions are fascinating and complex systems with a wide range of applications and commercial uses. Accurate characterization of emulsions, including the size and zeta potential of the dispersed droplets, is critical to ensure emulsion stability and performance. The Litesizer™ 500 instrument from Anton Paar is a useful tool for analyzing emulsions, offering a quick and accurate measure of both particle size and zeta potential.