Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 148

Strey, 1996; Gaonkar and Bagwe, 2002). The free energy per unit area for curvature
deformation is (Strey, 1994 ; Sottmann and Strey, 1996 ; Gradzielski, 1998 ):

2

f = 2( k H − H k ) + K (5.2)

0

where H = 1/2( c 1 + c 2 )is the mean film curvature, in which c 1 and c 2 are the principal curvatures
of a regular surface at each point, K is the Gaussian curvature, H 0 is the spontaneous film
curvature, and k and k describe the elastic properties of the surfactant film (bending modulus),
which refers to the deformability of the surfactant film. For H 0 > 0, the surfactant film is convex
toward water (o/w microemulsion) whereas with H 0 values near zero, bicontinuous microemul-
sions or lamellar liquid crystalline phases are formed (Strey, 1994; Leitao et al ., 1996 ; Sottmann
and Strey, 1996). If the surfactant film is convex toward the oil phase, H 0 < 0, and a w/o
microemulsion will form. Values of H 0 may be affected by the nature of the surfactant and the
composition of the polar and apolar phases (Sottmann and Strey, 1996; Gradzielski, 1998;
Gaonkar and Bagwe, 2002). Co-surfactants are also used as “tuning parameters” to modify H 0
and to increase the flexibility of the surfactant film. These compounds may affect the
spontaneous curvature of the interface by changing the polarity of the polar and apolar phases.

5.9 PARAMETERS THAT MODIFY MICROEMULSION
STRUCTURE

In developing controlled-release matrices, the solubilization capacity of microemulsions
should be optimized, which depends on various factors, such as the critical micelle concentra-
tion, the surfactant's HLB value, the critical packing parameter and molecular compatibility
between the oil, surfactant and co-surfactant (Friberg et al ., 1994 ; Aboofazeli et al ., 1995 ; von
Corswant and Soderman, 1998 ).

5.9.1 Critical micelle concentration
Surfactants at low concentrations in solution are predominantly dispersed as monomers.
Above the critical micelle concentration (CMC), they spontaneously associate into (reverse)
micelles, either through strong interaction of their hydrophobic tails (resulting in reverse
micelles, w/o) or by hydrophilic interaction of the polar head groups (micelles, o/w). Most
micellar solutions consist of particles in the 2-5 nm range. Surfactant monomers have the
highest surface activity, yet this activity is greatly diminished once micelles are formed,
given that either the tails or heads are buried and thus unavailable to participate in surface/
interfacial tension reduction (Walz, 1998 ; Flanagan and Singh, 2006 ).

5.9.2 Critical packing parameter
The critical packing parameter (CPP) is a measure of the preferred geometry adopted by a
surfactant in solution:

pv/ la = (5.3)

o

where p is the packing parameter, v and l are the volume and length of the hydrophobic tail
and a is the optimal head group area. In solution, when the hydrophilic head group volume