Microemulsions in agrochemical formulations

Microemulsions are a special class of dispersions (transparent or translucent) which actually have little in common with emulsions. They are better described as swollen micelles . The term microemulsion was first introduced by Hoar and Schulman [123,124] who discovered that by titration of a milky emulsion (stabilized by soap such as potassium oleate) with a medium chain alcohol such as pentanol or hexanol, a transparent or translucent system was produced. A schematic representation of the titration method adopted by Schulman and coworkers is given below. 

Microemulsions are quite distinct from macroemulsions (EW s). With emulsions, increasing the mechanical energy and increasing surfactant concentration usually results in the formation of smaller droplets which become kinetically more stable. With microemulsions, neither mechanical energy nor increasing surfactant concentration can result in its formation. The latter is based on a specific combination of 

The PIT system provides information on the type of oil, phase volume relationships and concentration of the emulsifier. The PIT system is established on the proposition that the HLB number of a surfactant changes with temperature and that the inversion of the emulsion type occurs when the hydrophile and lipophile tendencies of the emulsifier just balance. At this temperature no emulsion is produced. From a microemulsion viewpoint the PIT has an outstanding feature since it can throw some light on the chemical type of the emulsifier needed to match a given oil. Indeed, the required HLB values for various oils estimated from the PIT system compare very favorably with those prepared using the HLB system described above. This shows a direct correlation between the HLB number and the PIT of the emulsion. 

Another important factor for control of biological efficacy is the formation of deposits after evaporation of the spray droplets, which ensure the tenacity of the particles or droplets of the agrochemical. This will prevent removal of the agrochemical from the leaf surface by falling rain. Many microemulsion systems form liquid crystalline structures after evaporation, which have high viscosity (hexagonal or lamellar liquid crystalline phases). These structures will incorporate the agrochemical particles or droplets and ensure their stickiness to the leaf surface. 

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