Solubilisation and its Effect on Transport

Solubilization by micelles has been described in detail in Chapter 13, and only a summary is given here. As discussed in Chapter 13, solubilisation is the incorporation of an insoluble substance (usually referred to as the substrate) into surfactant micelles (the solubilizer). Solubilization may also be referred to as the formation of 

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- 

The presence of micelles or microemulsions will have significant effects on the biological efficacy of an insoluble pesticide. In the first instance, surfactants will affect the rate of solution of the chemical. Below the c.m.c., surfactant adsorption can aid wetting of the particles and, consequently, increases the rate of dissolution of the particles or agglomerates [8]. Above the c.m.c., the rate of dissolution is affected as a result of solubilization. According to the Noyes-Whitney relation [122], the rate of dissolution is directly related to the surface area of the particles A and the saturation solubility, Cg, i.e. 

Apart from the above effect on dissolution rate, surfactant micelles also affect the membrane permeability of the solute [8j. Solubilization can, under certain circumstances, help the transport of an insoluble chemical across a membrane. The driving force for transporting the substance through an aqueous system is always the difference in its cdiemical potential (or to a first approximation the difference in its relative saturation) between the starting point and its destination. The principal steps involved are dissolution, diffusion or convection in bulk liquid and crossing of a membrane. As mentioned above, solubilization will enhance the diffusion rate by affecting transport away from the boundary layer adjacent to the crystal [8j. However, to enhance transport the solution should remain saturated, i.e. excess solid particles must be present since an unsaturated solution has a lower activity. 

Diffusion in bulk liquid obeys Pick s first law, i.e. 

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