Microemulsions in Agrochemicals

As mentioned before, for oil-insoluble agrochemicals one of the most common formulations is emulsifiable concentrates (ECs), which when added to water produce 

This section will summarise the basic principles involved in the preparation of microemulsions and the origin of their thermodynamic stability (see Chapter 10 for more details). A sub-section is devoted to emulsifier selection for both O/W and W/0 microemulsions. Physical methods that may be applied for characterization of microemulsions will be briefly described. Finally a sub-section is devoted to the possible enhancement of biological efficacy using microemulsions. The role of microemulsions in enhancing wetting, spreading and penetration will be discussed. Solubilization is also another factor that may enhance the penetration and uptake of an insoluble agrochemical. 

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Tadros, T.F. (1997) Microemulsions in agrochemicals. In C. Solans and H. Kunieda (eds), Industrial Applications of Microemulsions, Surfactant Science Series 66. Marcel Dekker, New York, pp. 199-207. 

Tadros, Th. F., Microemulsions in Agrochemicals , in Solans, C. and Kunieda, H. (Eds.), Industrial Applications of Microemulsions, Sur ctant Science Series, Vol. 66, Marcel Dekker, New Yoric, 1997. 

Surfactants find apphcation in almost all disperse systems that are utilised in areas such as paints, dyestulfs, cosmetics, pharmaceuticals, agrochemicals, fibres, and plastics. Therefore, a fundamental understanding of the physical chemistry of surface-active agents, their unusual properties, and their phase behaviour is essential for most formulation chemists. In addition, an understanding of the basic phenomena involved in the application of surfactants, such as in the preparation of emulsions and suspensions and their subsequent stabilisation, in microemulsions, in wetting, spreading and adhesion, is vitally important to arrive at the correct composition and control of the system involved [1, 2]. This is particularly the case with many formulations in the chemical industry mentioned above. 

The role of microemulsions in enhancing biological efficiency can be described in terms of the interactions at various interfaces and their effect on transfer and performance of the agrochemical. This will be described in detail below, and only a summary is given here. Application of an agrochemical as a spray involves several... 

Solubilization in micelles can provide a means of delivering otherwise insoluble drugs or improving the transport of agrochemicals across plant membranes. Some ways in which a substrate can be solubilized by a microemulsion include [236] ... 

Some agrochemicals are formulated as emulsifiable concentrates. Here active ingredients that are not very soluble in water are dissolved in a solvent that is, in turn, emulsified into the aqueous phase, either in the concentrate itself (an emulsion concentrate) or else upon dilution in the spray tank [865]. Some emulsion concentrates are designed so that when water is added to them they spontaneously emulsify to form an O/W microemulsion [225],... 

The formulations of agrochemicals cover a wide range of systems which range from simple aqueous solutions (for water-soluble actives) and self-emulsifiable oils to disperse systems of suspensions, emulsions and microemulsions. More complex formulations such as multiple emulsions and suspoemulsions (mixtures of suspensions and emulsions) are also applied in some cases. Microencapsulation of active ingredients for controlled and sustained release represents a more sophisticated approach to the formulation of agrochemicals. Solid formulations of wettable powders, grains, granules and tablets are also used in many applications. 

In particular, in pharmaceuticals, agrochemicals, foodstuffs, and in biocatalysis functional components are preferably finely dispersed in an oily or an aqueous environment. Microemulsions are eminently suitable to serve that purpose, provided that the droplets are sufficiently large to solvate the functional molecules. By way of example, the relation between the radius of a water in oil microemulsion droplet and the number of water molecules it contains is given in Table 11.3. It shows that the droplet should have a size of at least a few nanometers to be able to dissolve (hydrate) polar compounds in its interior. 

As discussed in Chapter 10, microemulsions, which may be considered as swollen micelles, are more effective in solubilization of many agrochemicals. Oil-in-water microemulsions contain a larger hydrocarbon core than surfactant micelles and hence they have a larger capacity for solubilizing lipophilic molecules such as agrochemicals. However, with polar compounds, O/W microemulsions may not be as effective as micelles of ethoxylated surfactants in solubilizing the chemical. Thus, one has to be careful in applying microemulsions without knowl-... 

Aromatic oils are quite often used in the miCToemulsion preparation. Organic liquids like benzene, toluene, hexane, cyclohexane, etc. have been used [13-17]. Such preparations are useful in chemical and agrochemical industties. In the field of pharmaceutical industries, such oils are not usable. The oils useful in pharmaceutical industries are generally of higher molecular weights and molar volumes, and they are also polar. Such properties make microemulsion formation more difficult [18]. But there have been attempts to overcome such problems [19], and a good number of reports on biocompatible microemulsions can be found in literature [20-22]. 

The important properties unique to microemulsions - thermodynamic stability, ultra-low interfacial tensions, translucence, small and tunable microstructures - make microemulsions interesting for a variety of applications. Microemulsions find application as a reaction medium for formation of polymeric and inorganic nanoparticles, for the dispersion of drugs, food stuffs, agrochemicals, and cosmetic ingredients, and in detergency, the enhancement of oil recovery from reservoirs, and the extraction of contaminated solids (17). 

It can be seen from the above short discussion that agrochemical formulations are complex multiphase systems and their preparation, stabilization and subsequent application require the application of the basic principles of colloid and interface science and this is the objective of the present part [1, 2]. It will start with a section on surfactants and the physical properties of their solutions. This is followed by a section on the interfacial aspects of agrochemical formulations including adsorption of surfactants and polymeric surfactants at the air/liquid, liquid/liquid and solid/liquid interfaces. The stabilization of dispersions, both electrostatic and steric, is discussed in the next section. The basic principles of colloid and interface science are illustrated in detail by considering emulsion concentrates (EW s) and suspension concentrates (SC s). A summary will be given on microemulsions and controlied release of agrochemical formulations. 

Several interfacial aspects must be considered when dealing with agrochemical formulations (i) Both equilibrium and dynamic aspects of adsorption of surfactants at the air/liquid interface. These aspects determine spray formation (spray droplet spectrum), impaction and adhesion of droplets on leaf surfaces as well as the various wetting and spreading phenomena, (ii) Adsorption of surfactants at the oil/water interface which determines emulsion formation and their stability. This subject is also important when dealing with microemulsions, (ill) Adsorption of surfactants and polymers at the solid/liquid interface. This is important when dealing with dispersion of agrochemical powders in liquids, preparation of suspension concentrates and their stabilization. 

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