Emulsions in water

Definition

Emulsion in water (EW) results from the dispersion of a liquid or a low melting point active ingredient in a water continuous phase. The size of the dispersed droplets generally range from 0.5 to 2 µm.

Emulsions can be obtained by a low shear or high shear process and are usually stabilized by a thickening agent and/or hydro-colloids in order to ensure long term stability.

EW formulations, when compared to EC formulations, offer:

• Reduction of the phytotoxicity, ecotoxicity and dermal toxicity

• Higher flash point, safer in transport and storage

• Compatibility with water-based suspension concentrates

• Concentrated base in water for suspoemulsions (SE) formulations

General method of preparation

Defining suitable active ingredients

Emulsions in water are suitable for liquid active ingredients and also for low melting point actives which cannot be formulated under a suspension concentrate (SC) form. Active ingredients need to have a good chemical stability in water. Small amount of solvent can be added to obtain a solution of the active ingredient. Stabilizing agent or buffer can be added when necessary.

Manufacture description and structures of emulsions: role of emulsifiers

Different parameters are involved in the preparation of an emulsion; the energy input, the temperature, the way of introduction of different ingredients and the rheology are important to obtain a long term stable emulsion. Emulsifiers can be added to water or to actives and component mixed by a suitable stirrer (high speed mixing agitation) to achieve an average size of approx 1 µm. The use of an antifoam during the process is highly recommended to avoid the increase of viscosity due to air entrapment. The final viscosity, after degassing, can vary considerably depending on the components and the working process - it should be between 1000 and 2500 mPa.s (Brookfield 20 rpm) for long term stability. If necessary, adjust the viscosity value with a 2% water solution of RHODOPOL 23.

Effects of emulsifiers

Emulsifiers are basic components in water based concentrated emulsions. They act in the following way:

• During mixing, they allow the wetting and dispersion of the organic phase into the aqueous phase by reducing the oil/water interfacial tension.

• Provide steric and electrostatic stabilization to the micronized droplets against flocculation, aggregation or coalescence in a concentrated state, to guarantee the long term stability of emulsions in various storage conditions.

• Facilitate dispersion of the formulation during water dilution and stabilize the micronized droplets of the active ingredient in the diluted state.

• Improve compatibility with other formulations in a diluted state at the application rate.

Selection of component

Selection of emulsifiers

To be efficient, surfactants need to be strongly adsorbed or solubilised at the interface oil/water. They assure the fluidity of the emulsion by developing strong repulsive forces hindering aggregation, flocculation or coalescence. After emulsification the dispersed droplets have an inherent tendency to flocculate as the result of the attractive Van der Waals and electrostatic forces. The strong adsorption of well chosen surfactants at the interface oil/water of the droplets generates repulsive forces (steric and/or electrostatic) in order to positively balance the attractive ones. The choice of a surfactant system depends on the polarity of the oil to be emulsified, the interactions which can be created between the oil phase and the surfactant system, the nature of the continuous phase.

Determination of the HLB required

The required HLB to emulsify this active ingredient (example) is about 14 (it corresponds to the emulsion with the smallest particle size). According to the chemical nature of the active ingredient, the chemical nature of the hydrophobic part of the emulsifier has to be optimized, in order to generate the strongest affinity with the liquid to be emulsified, at around the same required HLB value.

Antifreezes

Monopropylene glycol or glycerine are the most commonly used antifreezes in the range of 50 to 80 g/l.

Thickeners

RHODOPOL, added after the mixing step, provides perfect long term storage stability in most cases.

Antifoams

The role of this additive is to avoid formation of foam during preparation, water dilution and field application. 
 

Control methods

Storage stability

The commercial formulations should be stable for at least 2 years without any significant change of viscosity and without sedimentation or phase separation. However, the presence of a supernatant water layer on the surface is acceptable, provided only slight agitation is needed for the mixture to re-disperse.

Some accelerated aging tests give a fast indication of the long term stability:

• Tropical test: 2 weeks at +54 °C (CIPAC 1 -MT 46.1.3)

• Cold stability test: 1 week at 0 °C (CIPAC 1 -MT 39)

• Stability at high temperature for two months at +45 °C

• Stability at thermal shocks: samples in sealed opaque glass bottles are submitted to temperature cycles (24 hours at -5 °C and 24 hours at +45 °C) for a one or two month period