Microemulsions packing ratio

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

The above simple theory can explain the nature of the microemulsion produced when using surfactants with different structures. For example, if the molecules have bulky hydrophobic groups such as Aerosol OT, a W/O microemulsion is produced. Conversely, if the molecule has bulky hydrophilic chains such as alcohol ethoxylates with high ethylene oxide units, an 0/W microemulsion is produced. These concepts will be rationalised using the packing ratio concept discussed in detail in Chapter 6. 

The importance of geometric packing on the nature of microemulsion has been considered in detail by Mitchell and Ninham [109]. According to these authors, the nature of the aggregate unit depends on the packing ratio, P, given by... 

Different surfactants are usually characterised by the solubility behaviour of their hydrophilic and hydrophobic molecule fraction in polar solvents, expressed by the HLB-value (hydrophilic-lipophilic-balance) of the surfactant. The HLB-value of a specific surfactant is often listed by the producer or can be easily calculated from listed increments [67]. If the water in a microemulsion contains electrolytes, the solubility of the surfactant in the water changes. It can be increased or decreased, depending on the kind of electrolyte [68,69]. The effect of electrolytes is explained by the HSAB principle (hard-soft-acid-base). For example, salts of hard acids and hard bases reduce the solubility of the surfactant in water. The solubility is increased by salts of soft acids and hard bases or by salts of hard acids and soft bases. Correspondingly, the solubility of the surfactant in water is increased by sodium alkyl sulfonates and decreased by sodium chloride or sodium sulfate. In the meantime, the physical interactions of the surfactant molecules and other components in microemulsions is well understood and the HSAB-principle was verified. The salts in water mainly influence the curvature of the surfactant film in a microemulsion. The curvature of the surfactant film can be expressed, analogous to the HLB-value, by the packing parameter Sp. The packing parameter is the ratio between the hydrophilic and lipophilic surfactant molecule part [70] ... 

The maximum of Qf is determined with respect to the two independent variables, the area per surfactant molecule of the interface, aF, and the alcohol-to-surfactant ratio in the interfacial layer, (gAs/gsih- In contrast to that in the droplet-type microemulsion, the oil-to-surfactant ratio in the interfacial layer, (goi/gsi)F, is not an independent variable but is determined by the packing constraint (eq 2.7) for flat layers. 

Compared to macro- and miniemulsions, the water uptake is lower in microemulsions. By virtue of their smaller sizes and, subsequendy, the more efficient packing of the surfactant at the interface, microemulsions are not subject to flocculation effects. In the field of microemulsions, the water uptake is defined as the molar ratio of water to surfactant, W. The corrected Wq (called taking account of the solubility of water in CO2 is defined as ... 

In essence, the packing parameter is a measure of the ratio between the effective areas occupied by the hydrophobic (5 ) and hydrophilic (a,) parts of the surfactant. This model is ideal when considering aqueous systems, but may also be applied to dispersions in oil. Depending on the surfactant structure, Pp assumes specific values and the packing constraints in the medium allow for the formation of a preferred aggregate shape configuration. This theoretical approach is also considered in microemulsion systems, as discussed below. 

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