Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 143

Winsor I Winsor II Winsor III Winsor IV

Water

Oil

Figure 5.2 Mixed water-oil-surfactant structures as defined by Winsor. (Adapted from: Winsor, 1948
by permission of the Royal Society of Chemistr y (see http://dx.doi.org/10.1039/TF9484400376)

Moulik and Paul, copyright 1998 , with permission from E lsevier.)

5.3 THEORIES OF MICROEMULSION FORMATION

Three key theories to explain microemulsion formation of have been proposed: the mixed
film, solubilization and thermodynamic theories. As described, these theories are not mutually
exclusive, as elements from each can contribute to an understanding of microemulsion
formation and stability.

5.3.1 Mixed film theory
An amphiphilic film at the oil-water interface exists as a duplex film influenced by the
properties of the oil and water phases (Schulman et al ., 1959 ; Flanagan and Singh, 2006 ).
When oil molecules have the ability to associate with those within the interfacial film,
a microemulsion readily forms. For instance, a mixed system consisting of cetyl alcohol,
benzene and 2-amino-2-methyl-1-propanol (AMP) oleate does not form microemulsions
due to the lack of complementarity between these compounds. However, if benzene is
replaced by a straight-chained hydrocarbon chain (7-18 C), a microemulsion readily forms
due to the ability of the oil molecules to associate with the alcohol and AMP oleate at the
interface (Schulman et al ., 1959). It is normally assumed that microemulsion formation
requires the presence of a duplex film with differing tensions on either side of the interface,
which impart curvature and thus the possibility of forming dispersed domains or bicontinuous
structures (Prince, 1969 ).

5.3.2 Solubilization theory
This lesser-known theory states that microemulsions exist as swollen micellar systems.
Inverse (reverse) micelles can integrate water within their hydrophilic core and grow in size
beyond micellar dimensions, resulting in w/o microemulsions. In a similar manner, normal
micelles can solubilize oil and grow in size resulting in o/w microemulsions (Rance and
Friberg, 1977 ; Moulik and Paul, 1998 ; Flanagan and Singh, 2006 ).

5.3.3 Thermodynamic theory
Ruckenstein (Ruckenstein and Chi, 1975; Friberg and Venable, 1983; Flanagan and Singh,
2006) explored the stability and size of microemulsion domains based on a thermodynamic
approach where interactions (Van der Waals forces, electric double layer potential, and free
energy and entropy of formation) were considered. He studied the dependence of the free