Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 144

energy change of mixing as a function of droplet radius and determined whether the free
energy change was positive or negative (Ruckenstein and Chi, 1975). Reiss (1975) studied
the contribution of entropy in generating thermodynamically stable microemulsions, and
distinguished between spontaneous dispersions formed due to entropy effects from micellar
solutions formed due to energetic considerations.

5.4 WHAT MAKES MICROEMULSIONS
THERMODYNAMICALLY STABLE?

Every molecular assembly theoretically organizes itself such that the Gibbs free energy
( G ) of the system is minimized (Anton, et al ., 2008). The Gibbs free energy depends on

interfacial energy (

Δ

γ Δ A ) and the change in entropy of the system upon droplet formation

( T Δ S ):

Δ=Δ−Δ GA γ T S (5.1)

where γ is the interfacial tension, Δ A is the surface area of the droplets, T is the temperature

and Δ S is the entropy of the system. In emulsions, Δ G is always positive whereas in

microemulsions G is negative or near zero, due to the interfacial tension being extremely

small given the high proportions of surfactant(s)/co-surfactant(s) (Anton et al ., 2008 ).
According to Kumar and Mittal (1999), microemulsions will form when the oil-water
interfacial tension is below 10 -2 mN/m. However, as food-grade surfactants alone cannot
normally lower

Δ

to such low levels, co-surfactants such as alcohols are often used for further
reduction. Thus, microemulsions form nearly spontaneously, in large part due to the negative

γ

Δ G resulting from the entropy ( T Δ S ) of mixing and the formation of nano-sized droplets

(Anton et al ., 2008 ).

5.5 METHODS OF MICROEMULSION FORMATION

There are numerous ways to generate microemulsions, including the low-energy and phase
inversion methods (Flanagan and Singh, 2006). With the former, microemulsions can be
formed either via: (1) dilution of an oil-surfactant mixture with water; (2) dilution of a
water-surfactant mixture with oil; or (3) mixing all components at once. In some systems,
the order of ingredient addition may determine whether a microemulsion forms or not. For
instance, Flanagan et al . (2006) established that the order of ingredient addition was crucial
in systems with soybean oil, ethoxylated mono- and di-glycerides as surfactants and a
mixture of sucrose and ethanol as the aqueous phase. Transparent microemulsions resulted
from dilution of the oil-surfactant mixtures with water along several regions in the pseudo-
ternary phase diagram. However, upon dilution of aqueous-surfactant mixtures with oil,
microemulsions only resulted when the aqueous phase:surfactant ratio was 3:2. The authors
did not explain the reason for such behavior and only mentioned the existence of a kinetic
energy barrier that inhibited microemulsion formation.
With the phase inversion method, when an aqueous solution consisting of a non-ionic
surfactant and solubilized oil is heated to, or above, the cloud point, the system separates
into a water-rich phase and a surfactant-rich phase (Saito and Shinoda, 1967, 1970; Friberg
and Venable, 1983). As the temperature is increased further, the surfactant phase coalesces