Microemulsions consist of a dispersed phase of small droplets with diameters of 10 – 100 nm suspended in a continuous phase. The dispersed and continuous phases are usually an oil and water or vice-versa. It is commonly believed that microemulsions can provide superior efficacy relative to macroemulsion formulas having the same level of activities. It is believed that the small size of the emulsion droplets may allow for better transport of the pesticide active through cell membranes (plant and insect) thereby resulting in enhanced efficacy. Microemulsions are considered to be infinitely stable, thereby providing improved stability over traditional macroemulsion systems. The only real disadvantage of microemulsions is the relatively higher level of emulsifier required.
The procedure covered is a three phase process:
The solubility and emulsification properties of the pesticide are evaluated with a variety of surfactants and co-solvents. Typically a 10-20 fold excess of surfactant/co-solvent to pesticide is used for the initial study.
Solubility is evaluated by visually assessing for a clear stable solution. Emulsification is evaluated using an “invert” emulsion procedure.
- The pesticide and surfactant are mixed.
- Water is added from a burette with constant stirring (initially a water in oil emulsion is formed).
- Water addition is continued (the emulsion inverts to oil in water).
- The resultant microemulsion is studied by observing the physical appearance of the formulation as a function of time and temperature (usually upon standing at 30°C for 30 minutes).
- The most promising surfactants are those which provide thermally phase stable microemulsions.
Upon completion of phase 1, the most promising surfactants/co-solvents are blended in various ratios until an emulsifier system which provides the desired physical properties is identified. The optimized system generally consists on one main surfactant and two or more co-surfactants/co-solvents. These co-surfactants/co-solvents tend to broaden the thermal phase stability range of the microemulsion system. Ternary phase diagrams are used in this phase of development to assist in the selection of surfactant ratios.
The best formulation at this stage is often one containing one anionic surfactant and two nonionics of different types. The criteria for assessing the microemulsion properties are again physical appearance as a function of time and temperature.
Having established the best emulsifier combination, the final phase is optimizing the levels of each emulsifier component in the system. This is done by the use of a ternary phase diagram to identify the optimum emulsifier system. The Malvern Lasersizer and Turbiscan are also used to characterize these formulations.