AbstractThe basic principles involved in the formulation of suspension concentrates, emulsions, suspoemulsions (mixtures of suspensions and emulsions) and microemulsions, are described. With suspension concentrates, it is essential to choose a powerful dispersing agent of the ionic or nonionic type ...
AbstractThe basic principles involved in the formulation of suspension concentrates, emulsions, suspoemulsions (mixtures of suspensions and emulsions) and microemulsions, are described. With suspension concentrates, it is essential to choose a powerful dispersing agent of the ionic or nonionic type (surfactant or polymer) which provides colloidal stability as a result of double layer, or steric repulsion or a combination of both (e.g., with polyelectrolytes). Block and graft copolymers are the best dispersants since they allow one to prepare suspensions with a high volume fraction φ. This is illustrated using Theological measurements. Once a stable suspension is prepared, it is essential to use an antisettling system to reduce separation and prevent formation of a dilatant sediment. This is achieved by addition of a high molecular weight polymer or a finely divided inert solid such as sodium montmorillonite or silica (or a combination of both). The three-dimensional gel produced in the continuous phase provides a high zero shear viscosity and an elastic modulus that are sufficient to reduce separation and settling. The physical stability of the resulting suspension concentrate can be evaluated using rheological measurements. These include steady state, transient (constant stress) and oscillatory measurements.With emulsions, it is essential to consider the process of formation of droplets from a bulk oil phase. This emulsification process is non-spontaneous and hence an energy barrier is required to prevent flocculation and coalescence. The emulsifier reduces the energy required for producing the droplets by reducing the interfacial tension and to create an interfacial tension gradient thus reducing coalescence. The factors responsible for emulsion instability are described. Several breakdown processes may be distinguished, namely, creaming (or sedimentation), flocculation, Ostwald ripening, coalescence and phase inversion. The main parameters that control these processes are described and some general principles are given to illustrate how stabilisation can be achieved.With suspoemulsions, various interactions between the particles and the droplets could be envisaged. These include homo- and hetero-flocculation, coalescence and “phase transfer”. These systems are still at an experimental stage and research is needed to understand the various breakdown processes that occur.Microemulsions offer an attractive, thermodynamically stable, system for formulation of pesticides. The mechanism of microemulsion formation and its thermodynamic stability are briefly described. This is then followed by sections describing the methods that could be applied for formulation of microemulsions and their characterisation.
AbstractThe basic principles involved in the formulation of suspension concentrates, emulsions, suspoemulsions (mixtures of suspensions and emulsions) and microemulsions, are described. With suspension concentrates, it is essential to choose a powerful dispersing agent of the ionic or nonionic type (surfactant or polymer) which provides colloidal stability as a result of double layer, or steric repulsion or a combination of both (e.g., with polyelectrolytes). Block and graft copolymers are the best dispersants since they allow one to prepare suspensions with a high volume fraction φ. This is illustrated using Theological measurements. Once a stable suspension is prepared, it is essential to use an antisettling system to reduce separation and prevent formation of a dilatant sediment. This is achieved by addition of a high molecular weight polymer or a finely divided inert solid such as sodium montmorillonite or silica (or a combination of both). The three-dimensional gel produced in the continuous phase provides a high zero shear viscosity and an elastic modulus that are sufficient to reduce separation and settling. The physical stability of the resulting suspension concentrate can be evaluated using rheological measurements. These include steady state, transient (constant stress) and oscillatory measurements.With emulsions, it is essential to consider the process of formation of droplets from a bulk oil phase. This emulsification process is non-spontaneous and hence an energy barrier is required to prevent flocculation and coalescence. The emulsifier reduces the energy required for producing the droplets by reducing the interfacial tension and to create an interfacial tension gradient thus reducing coalescence. The factors responsible for emulsion instability are described. Several breakdown processes may be distinguished, namely, creaming (or sedimentation), flocculation, Ostwald ripening, coalescence and phase inversion. The main parameters that control these processes are described and some general principles are given to illustrate how stabilisation can be achieved.With suspoemulsions, various interactions between the particles and the droplets could be envisaged. These include homo- and hetero-flocculation, coalescence and “phase transfer”. These systems are still at an experimental stage and research is needed to understand the various breakdown processes that occur.Microemulsions offer an attractive, thermodynamically stable, system for formulation of pesticides. The mechanism of microemulsion formation and its thermodynamic stability are briefly described. This is then followed by sections describing the methods that could be applied for formulation of microemulsions and their characterisation.
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