Concentrated Emulsions in Agrochemicals EWs

The major advantages of EWs are their relatively low toxicity when compared with ECs, their high flash points and possibility of incorporation of adjuvants in the oil and aqueous phases. In addition, by controlling the droplet size distribution of the oil, one can enhance deposition and spreading and this may increase biological efficiency. 

The main drawback of O/W emulsions is the control of their physical stability, which needs to be controlled at various temperatures with adequate shelf life (usually a shelf life of 1-2 years is required at temperatures that can vary from —10° to 50 °C). This represents a challenge to the formulation chemist. 

Notably, however, the energy required for emulsification exceeds the thermodynamic energy AAyi2 by several orders of magnitude [39]. This is because a significant amount of energy is needed to overcome the Laplace pressure, Ap, which results from the production of a highly curved interface (small droplets), i.e. 

Five general main roles may be identified for the emulsifier. The first and most obvious is to lower y, as mentioned above. This has a direct effect on droplet size generally, the lower the interfacial tension, the smaller the droplet size. This is the case when viscous forces are predominant, whereby the droplet diameter is proportional to y. When turbulence prevails, dccy. When emulsification continues, and an equilibrium is set up between the amount adsorbed and the concentration in the continuous phase, C, the effective y depends on the surface dilational modulus, , which is given by 

The third role of the surfactant is to create interfacial tension gradients. This has been discussed before. As a result of the tangential stress dy/dz, which can build up on a pressure of the order of Pa (for y 10 mN m and droplet diameter of 1 gm), the internal circulation in the droplet is impeded or even prevented, thus facilitating droplet formation and breakup. 

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Among several applications of emulsions the most important are listed here Food emulsion, e.g. mayonnaise, salad creams, deserts, beverages, etc. Personal care and cosmetics, e.g. hand creams, lotions, hair sprays, sunscreens, etc. Agrochemicals, e.g. self-emulsifiable oils which produce emulsions on dilution with water, emulsion concentrates (EWs) and crop oil sprays. Pharmaceuticals, e.g. anaethetics of O/W emulsions, lipid emulsions, double and multiple emulsions, etc. Paints, e.g. emulsions of alkyd resins, latex emulsions, etc. Dry cleaning formulations - these may contain water droplets emulsified in the dry cleaning oil that is necessary to remove soils and clays. Bitumen emulsions - emulsions prepared stable in the containers but when applied to the road chippings they must coalesce to form a uniform film of bitumen. Emulsions in the oil industry - many crude oils contain water droplets (e.g. North Sea oil) and these must be removed by coalescence fol-... 

Steric repulsion results from the presence of adsorbed layers of surfactants and/or polymers. The use of natural and synthetic polymers (referred to as polymeric surfactants) for stabilization of suspension concentrates and emulsions (EW s) plays an important role in agrochemical formulations. Polymers are particularly important for preparation of concentrated dispersions, i.e. at high volume fraction ( j of the disperse phase,... 

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. 

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