Solubilisation theories

The above duplex fihn theory can explain the nature of the microemulsion. 

The surface pressures at the oil and water sides of the interface depend on the interactions of the hydrophobic and hydrophilic potions of the surfactant molecule at both sides, respectively. If the hydrophobic groups are bulky in nature relative to the hydrophihc groups, then for a flat film such hydrophobic groups tend to crowd so as to form a higher surface pressure at the oil side of the interface this results in bending and expansion at the oil side, forming a W/O microemulsion. 

An example of a surfactant with bulky hydrophobic groups is Aerosol OT (dioctyl sulphosuccinate). If the hydrophilic groups are bulky, as is the case with ethoxylated surfactants containing more than five ethylene oxide units, crowding will occur at the water side of the interface and this will produce an O/W microemulsion. 

These concepts were introduced by Shinoda and coworkers [6], who considered microemulsions to be swollen micelles that are directly related to the phase diagram of their components. 

Regions Lj and L2 are not in equilibrium, but are separated by a liquid crystalline region (a lamellar structure with equal numbers of surfactant and alcohol 

According to the duplex film theory, the interfacial tension yj is given by the following expression [5], 

Contributions to n are considered to be due to crowding of the surfactant and cosurfactant molecules and penetration of the oil phase into the hydrocarbon chains of the interface. 

According to Eq. (10.3) if i yo/yf), yj becomes negative and this leads to expansion of the interface until yj reaches a small positive value. Since (yo/w)a the order of 15-20 mN m, surface pressures of this order are required for yj to approach zero. 

At the water corner and at low alcohol concentration, normal micelles (Li) are formed since in this case there are more surfactant than alcohol molecules. At the alcohol (cosurfactant comer), inverse micelles (L2) are formed, since in this region there are more alcohol than surfactant molecules. 

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It is beyond the scope of this section to discuss the complex physico-chemical parameters of solubilisation in detail. Useful relevant works of reference are available [332-335]. It follows, however, that since solubilisation is essentially an extension of emulsification (or dispersion), the factors discussed in section 9.8.3 in regard to emulsification are also pertinent to solubilisation. Theory in this area is a useful guide but much still depends on... 

As discussed before (4) it is perhaps convenient to classify these theories into three main categories interfacial or mixed film theories, solubilisation theories and thermodynamic theories. Below a brief description of each of these classes will be given with particular emphasis on the role of surfactant nature and structure. 

The physico-chemical theory of surface activity is a vast field and no more than broad principles can be touched on here major reference sources exist for those who require more detail of the relationship between chemical structure and the various surfactant properties such as wetting, detergency and emulsification-solubilisation [32-36]. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

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