In this edition of Active Insight, Steve Broadbent of Ensystex explains the science behind emulsion concentrate (EC) technology.
In this insight we will look at emulsion concentrate (EC) technology and more recent, related formulation types; emulsions-in-water (EW) and microemulsions (ME). If we look back over the last thirty years of professional pest management in Australia we will note how the industry has changed.
EC formulations in pest control
Thirty or so years ago, most of the products used by professional pest managers were emulsion concentrates. These included products for general pest control (cockroaches, ants, spiders, fleas, etc.) using actives such as propoxur, diazinon, and dichlorvos; as well as those used for termite soil treatments with the cyclodiene actives: aldrin, chlordane, heptachlor and dieldrin. Now, not only have all those actives largely disappeared from our industry, so too have emulsion concentrate formulations.
There were several reasons for the move away from emulsion concentrate formulations, all connected with the fact that emulsion concentrates are oil based products.
Negative attributes of EC formulations:
- Generally have a strong solvent odour
- Have a greater potential to affect modern synthetic surfaces and veneers
- Use solvents that tend to be flammable
- Have increased toxicity since they are more easily absorbed through the skin
- The diluted emulsions tend to soak in to porous surfaces when used as a surface spray, meaning the active ingredient is less bioavailable for the control of insect pests, meaning shorter residual performance
- The solvents can affect synthetic or rubber seals, hoses, gaskets, and pump parts.
Initially we saw emulsion concentrates replaced as surface sprays for general insect pest control with odourless and more residual formulations, such as wettable powders and more particularly suspension concentrates. As wettable powders had the drawback of leaving a visible residue, emulsion concentrates still retained their market acceptance since they did not leave visible traces. However, suspension concentrates overcame this concern and offered improved residual performance, no visible residues and odourless formulations. This resulted in the rapid demise of emulsion concentrates for indoor treatments and later for external perimeter treatments.
The cyclodiene soil treatment termiticides were replaced by chlorpyrifos and bifenthrin emulsion concentrate products. However, as suspension concentrate technologies advanced and micro-suspension concentrates were developed, the use of emulsion concentrates has largely been replaced by suspension concentrates of bifenthrin, imidacloprid and fipronil in this field of application too. Typical droplet sizes in the emulsions formed when emulsion concentrates products are diluted in water are 2 micron (modern suspension concentrates are at these levels too).
But what are emulsion concentrates?
Emulsion concentrates were initially popular as they were widely used in agriculture, and relatively easy to formulate. Most insecticide active ingredients are oil soluble, and water insoluble.
An emulsion concentrate formulation therefore usually contains a liquid active ingredient, dissolved in one or more petroleum-based solvents (which give EC formulations their strong odour). Agents known as emulsifiers or surfactants are added to allow the formulation to be mixed with water to form an emulsion in water.
The resulting emulsion therefore consists of the oil liquid dispersed (as droplets) in the water, with each liquid retaining its original identity. Emulsions in water usually have a ‘milky’ appearance. In fact, the word ‘emulsion’ is derived from the Latin verb ‘mulgeo’, meaning ‘to milk’, as milk is an emulsion of fat in water (along with other components).
The diluted emulsion is referenced as having a dispersed phase (the active in the oil-based solvent or solvents) and a continuous phase (the water). The boundary between each of the phases is called the ‘interface’. The milky appearance is created because the many phase interfaces (droplets in the water) scatter light as it passes through.
Emulsions appear white when all the light is scattered equally. If the emulsion is more dilute, higher-frequency and low-wavelength light will be scattered more, and the emulsion will appear bluer in colour, this is the Tyndall Effect. A more concentrated emulsion will distort the light toward longer wavelengths, so it appears more yellow.
The emulsifiers or surfactants used in the emulsion concentrate work by physically interacting with both the oil phase and water phase to stabilise the interface between the oil droplets (dispersed phase) and water droplets (continuous phase) and keep the oil droplets in suspension (main picture, above).
Although EC formulations have negative attributes that limit their use in pest control, emulsion concentrates are very versatile and are still widely used in agriculture.
Positive attributes of EC formulations:
- Relatively easy to handle, transport, and store
- Little agitation is required to create the emulsion
- They tend not to settle out or separate when equipment is running
- They’re not abrasive
- They do not block screens or nozzles
- There is little visible residue on treated surfaces.
A slightly more complex variation of the EC formulation is a product like Dipthor Duo, where the emulsion concentrate formulation has been designed to allow for the product to be used as both an emulsion in water, by diluting with water; or as a solution, by diluting it with oil for ULV and thermal fogging applications.
An EW (emulsion in water) concentrate is where an emulsion in water is created as a stable concentrate. The product is usually also stabilised with hydro-colloids, and protected from foaming with anti-foam agents and from freezing, due to the water component, by the addition of monopropylene glycol.
The benefit of this type of formulation is that there is now a reduced amount of the petroleum-based solvents in the product, which reduces the toxicity, flammability and the odour of the product. Once diluted with water it forms an emulsion in just the same manner as an emulsion concentrates. Emulsions in water are suitable for liquid active ingredients and also for low melting point actives which cannot be formulated in a suspension concentrate (SC) form.
A microemulsion is a clear, thermodynamically stable mixture of at least three components; a hydrophobic component (non water-soluble oil), a low melting solid (less than +50°C) or a solid dissolved in an organic solvent, a surfactant system and water. A co-surfactant (co-solvent) is often added to the mixture to increase the solubilising power of the surfactant system.
Different substances can be used as co-surfactants; mainly alcohols, amines or ether-alcohols. In contrast to an emulsion, the characteristic size of the hydrophobic or the hydrophilic droplets is typically 1 micron. In fact, microemulsions are a particular type of colloidal system. The typical dimension of the local structure explains why microemulsions are transparent.
Microemulsions are usually developed for products where the active constituents are used at low rates or when a clear solution is required when the product is applied.
Steve Broadbent, Regional Director, Ensystex