You know the old adage, “They go together like oil and water?” There should be a part two, “… if they had an emulsifier, they could be stable.” Oil and water might not mix alone, but with the help of an emulsifier, they can be combined into an emulsion. Emulsifiers are a substance that can be added to a solution with both oil-based and water-based liquids to allow them to mix.
In the pesticide world, surfactants are common emulsifiers that are used to stabilize tank mixes, ensuring uniform and consistent application. Emulsions break over time, but more stable emulsions remain mixed for longer periods of time allowing applicators more time to complete the job. The fascinating science behind emulsions reveals the importance of including the right emulsifier in a pesticide tank mix.
An emulsion is a mixture of polar and non-polar liquids combined through the use of an emulsifier. Polar substances have molecules with charged ends, making them attract and interact strongly with other polar molecules. Non-polar substances lack these charges and don’t easily mix with polar substances. Since oil-based substances are typically non-polar and water is polar, they do not mix without the use of an emulsifier.
Emulsions are made up of spheres of liquid contained within a layer of emulsifier such as a surfactant. Surfactants are molecules composed of both polar and non-polar components, typically a polar ‘head’ and a non-polar ‘tail’.
Together oil and water will form separate layers rather than mix. Add an emulsifier, and oil and water will mix, forming an emulsion or stable mix. Different combinations and types of oil, emulsifiers, and water can result in more or less stable emulsions.
There are two kinds of emulsions: oil-in-water (O/W) and water-in-oil (W/O). In an O/W emulsion, water is the main liquid, and oil is dispersed into tiny droplets in the water. The surfactant molecules stabilize these oil droplets by forming a layer around the oil with the polar head on the outside of the layer. This positions the surfactant molecules so the polar side touches polar water and the non-polar side touches non-polar oil.
In a W/O emulsion, oil is the main liquid, and water is dispersed into tiny droplets in the oil. The surfactant molecules stabilize these droplets by forming a layer around the water with the polar head on the inside of the layer. This positions the surfactant molecules so the polar side touches polar water and the non-polar side touches non-polar oil.
In W/O emulsions, water is dispersed as tiny drops in oil. In O/W emulsions, oil is dispersed as tiny drops in water.
Since pesticide tank mixes often contain water and oil-based products, it’s important to include the right surfactant to create a stable emulsion in the tank. The more stable the emulsion, the more time the applicator has to successfully complete the application.
When applying pesticides, spraying active ingredients uniformly across an area is essential, from both product performance and compliance standpoints. If products are not applied evenly, there will be missed spots that could result in the need for reapplication. Other areas could experience overapplication, which could have negative environmental impacts.
When an emulsion sits for too long, the liquids separate into their original layers. In pesticide tank mixes, this often means the active ingredients float to the top of the tank. While tank mixes typically don’t sit for hours or overnight, impediments like rain or equipment breakdowns can result in applications stretching over multiple days.
Agitation can help re-create an emulsion after it breaks, but agitation tools are not always accessible in the field. Products with good emulsion stability are more viable in these adverse situations. Regardless of situation, the more stable the emulsion is, the better the chances are that the tank mix will remain stable for application.
At Exacto, oil-based products such as crop oil concentrates (COCs), methylated seed oils (MSOs or MVOs), and crop oils are tested for emulsion stability. The test shows how well the surfactant can keep the oil stabilized over a 24-hour period.
1) Water is added to a graduated cylinder. Different hardness of water can be used to make the test easier or more difficult. Harder water is more challenging to create emulsions in.
2) The product to be tested is added on top.
An emulsion may form spontaneously when the product reaches the water. This ‘spontaneous emulsion’ is called a bloom. It’s important to note if a bloom occurs because it signifies an easily emulsifiable solution. Products that do not bloom will tend to settle to the bottom or top of a tank and not mix fully, which means an emulsion will be harder to form, if at all.
3) The cylinder is then inverted to mix fully.
4) A timer is started and the emulsion is observed at time zero (T0), 15 minutes, 30 minutes, 1 hour, 2 hours, and 24 hours.
5) The solution can also be reinverted after 24 hours and left to sit for 30 minutes to evaluate the ability to re-emulsify after phasing.
At each observation time, it’s important to watch for when the emulsion ‘breaks’ or separates the polar and non-polar elements into layers that can be seen. The stability of the emulsion is judged on the amount of separation visible in the solution. A solution with good stability will have very little cream and oil at the top. Solutions with larger and more visible layers is a poor emulsion. The following are the main layers observed in an emulsion test.
Emulsion: A white to off-white, uniform, liquid solution is considered an emulsion.
Cream: Cream is a viscous layer of liquid, typically at the top of the cylinder, that is often a different color than the main emulsion. Often bright white in color, it acts like cream floating to the top of milk during production.
Oil: Typically, a liquid layer of oil will form on the top of emulsions if there is not enough emulsifier (surfactant) in the product to keep the oil stabilized.
Water: Water will often appear as an emulsion, but more transparent to hazy. If the emulsion breaks completely, water will appear crystal clear, though that is rare.
At the beginning of the test (top), emulsions are formed appearing uniform white to off white. After the 24-hour period (bottom), the emulsions are evaluated for stability. The solution on the left shows extremely poor stability with a significant separation visible. The middle solution shows an acceptable emulsion with some separation on top and a reasonable emulsion beneath. The solution on the right shows a strong emulsion with very little cream and oil separated to the top.
Emulsions are characterized by the volume of the cream, oil, and other instabilities that occur over the 24-hour test. Good emulsion stability means there will be no oil and very little cream formed even after 24 hours. Bad emulsion stability means large amounts of cream will be visible after 24 hours. It can also mean un-emulsified oil is present, the emulsion below the cream is becoming thin and watery, or some other instability is present.
Emulsion stability tests can be performed on many product combinations to determine the optimum combination of oil-based adjuvant or active ingredient and surfactant. Since substances have varying levels of oiliness (polarity), certain surfactants will be better emulsifiers than others. The same reigns true for use rates; some products prefer high and others, low. The goal is all products work at the recommended use rates.
Emulsion stability is a great tool to determine surfactant product performance. Including surfactants with strong emulsion stability enhances tank mixes to ensure uniform and consistent application. Understanding the role emulsion stability plays in a tank mix can ease product decisions, mixing, and application. Products that foster good stable emulsions are key to optimizing a tank mix.
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