Adjuvants 101: Emulsifying Agents for Pesticide Use

Whenever pesticides are used, the desire is that the pest is controlled as efficiently as possible. This means that the constituents of the pesticide spray mixture must be formulated properly and are compatible within the mixture to do the job as desired.

 

A good understanding of the chemicals in the pesticide spray mixture is the starting point. Thus:

• A pesticide is any substance or mixture of substances intended for preventing, destroying, or mitigating a pest, or used as a plant regulator, defoliant, or desiccant.
• A pesticide consists of:

• One or more active ingredients – chemicals in the pesticide that act to control the pest.
• One or more Inert ingredients – chemicals which play a key role in pesticide effectiveness and product performance.

Many pesticides are formulated using a petroleum-based solvent that does not mix well with water, which often is the carrier of the pesticide spray solution. To promote mixing of a petroleum-based pesticide formulation with water, an emulsifying agent must be one of the inert ingredients within the formulation or must be added to the mixture.

What is an emulsifying agent?

First of all, an emulsion is a mixture of two liquids where one is dispersed or suspended in the other. The liquids are categorized into two phases:

1. The continuous phase – which refers to the carrier liquid, and
2. The discontinuous phase – which refers to the liquid in suspension.

The two most common types of emulsions are: 

• Oil-in-water emulsions 

1. 1. • Water is the continuous phase and oil is the discontinuous phase.
      • The consistency of the spray mixture is usually similar to water. 
      • Milk is an example of an oil-in-water emulsion, in which the fat phase or cream forms tiny droplets within the water phase.

• Water-in-oil emulsions 

1. • Oil is the continuous phase and water is the discontinuous phase.
   • The consistency of the spray mixture is like oil, viscous, and often referred to as ‘invert’ emulsions. 
   • Margarine is an example of a water-in-oil emulsion containing droplets of water or skim milk in a blend of vegetable oils and fats.

Emulsifying agents promote the suspension of one liquid in another and help form and stabilize the emulsion.

 

Is an emulsifying agent an active or inert ingredient?

An emulsifying agent is an inert ingredient. Typically, they are odorless, nontoxic, nonirritating, chemically stable, inert, and chemically nonreactive with other chemicals in a mixture.

 

How do emulsifying agents work?

When making a simple oil and vinegar salad dressing, with enough shaking or whisking, one can make a temporary emulsion. However, in the absence of an emulsifying agent, this unstable emulsion breaks down quickly, and the oil forms a layer on top of the vinegar. Natural emulsifying agents, such as egg yolk, mustard, or honey, can prevent this separation.

Therefore, emulsifying agents work by forming physical barriers that prevent droplets from coalescing.

The molecules of an emulsifying agent have a hydrophilic (water-loving or polar) end that forms chemical bonds with water but not oil; and a hydrophobic (water-repelling or nonpolar) end that form chemical bonds with oil but not water. Thus, emulsifying agents act as the interface between two immiscible liquids, like oil and water. 

• When added to an oil-in-water emulsion, an emulsifying agent will surround the oil droplet and its nonpolar end will extend into the oil while its polar end faces the water.
• When added to a water-in-oil emulsion, the emulsifying agent’s orientation is reversed with the nonpolar end extending outward into the oil, while the polar end points into the water droplet.
• In both instances, emulsifying agents lower the interfacial tension between the water and oil phases, stabilizing the droplets and preventing them from coalescing.
  
  
  

What are the major differences among emulsifying agents? 

Emulsifying agents can be: 

• cationic – having positively charged polar head groups and used in low-to-neutral pH solutions.
• anionic – having negatively charged polar head groups and used in alkaline solutions. 
• nonionic – having uncharged polar head groups and can be used alone or in combination with charged emulsifiers to increase emulsion stability.

When charged emulsifying agents coat droplets in an oil-in-water emulsion, the positive or negative charges on the outside of the oil droplets electrostatically repel each other, helping to keep the droplets separated. 

Choosing an emulsifying agent.

Calculating the hydrophilic-lipophilic balance (HLB) of a chemical compound is an important factor when choosing an emulsifying agent to use with a pesticide formulation.

In an ideal emulsion, the emulsifying agent is equally attracted to the water phase and the oil phase. If the balance is tipped in either direction, the emulsifying agent may lose contact with the phase to which it is less attracted, causing the emulsion to break down.

Choosing the right emulsifying agent is therefore critical and calculating the HLB is a helpful step.

The hydrophilic-lipophilic balance (HLB)

HLB values vary among emulsifying agents and can be used to predict the ability to stabilize various kinds of emulsions. Generally,

• The HLB system works primarily for nonionic emulsifying agents.

• The HLB scale ranges from 0 to 20, with 10 corresponding to an emulsifying agent that is equally attracted to water and oil.
• Emulsifying agents with HLB values greater than 10 are more hydrophilic and better at stabilizing oil-in-water emulsions.
• Emulsifying agents with HLB values less than 10 are more hydrophobic and better at stabilizing water-in-oil emulsions.

Knowledge of HLB values of emulsifying agents is important in the development of emusifiable concentrates.

 

What is an emulsifiable concentrate (EC or E)?

An EC is a type of pesticide formulation that contains an active ingredient, one or more petroleum solvents, and one or more emulsifying agents, which allows the formulation to emulsify when mixed with water.

ECs are among the most versatile pesticide formulation used in a variety of industries where pests are managed.

EC product formulators choose emulsifying agents to combine with the active ingredient based upon a clear understanding of HLB.

 

 

Further Reading:

• Sun Wet Surfactant – Best Postemergence Weed Control with Herbicides 
• How Surfactants Affect Spray Droplet Size 
• Pesticides 101: Introduction to Pesticide Adjuvants and Surfactants
• Everything You Need to Know About Agricultural Wetting Agents
• I’Vod – A Superior Spray Drift Control Adjuvant 
  
  

References

• Bohmont, B.L. 2007. The Standard Pesticide User’s Guide – 7th Ed. Pearson Prentice Hall.
• United States Environmental Protection Agency (US EPA). Basic Information about Pesticide Ingredients. https://www.epa.gov/ingredients-used-pesticide-products/basic-information-about-pesticide-ingredients. Accessed, February 24, 2022.