Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 147

5.7.3.3 Zwitterionic surfactants

Phospholipids are perhaps the common zwitterionic surfactants used in microemulsions.
However, they are too lipophilic to be used alone and have a fairly high critical packing
parameter, making their incorporation into microemulsions difficult (Moreno et al ., 2003 ).
If used, co-surfactants are also required to increase the fluidity of the interface (Gaonkar and
Bagwe, 2002 ; Patel et al ., 2006 ).

5.7.3.4 Cationic surfactants

Presently, no cationic surfactants (e.g., cetyl trimethylammonium bromide) are used in the
manufacture of food-grade microemulsions. This class of surfactants is mostly used in
industrial chemistry applications (Joshi and Mukherjee, 2003; Zuev et al ., 2004 ).

5.7.4 Co-surfactants
A co-surfactant (e.g., propylene glycol or ethanol) can play many roles in microemulsion
formation: (a) by reducing the interfacial tension to almost zero (Attwood, 1994; Garti et al .,
2002; Yaghmur et al ., 2002 , 2004 ; de Campo et al ., 2004 a; Garti and Aserin, 2006 ); (b) by
increasing the flexibility and fluidity of the interface by positioning itself between the
surfactant tails (Dimeglio et al ., 1895; Garti et al ., 2001 ; de Campo et al ., 2004 a), which
alters the solvent properties of both the dispersed and continuous microemulsion phases;
(c) by lowering overall viscosity, which restricts formation of more rigid structures such as
gels and liquid crystals (Leung and Shah, 1987a, 1987b; Martino and Kaler, 1995; Iwanaga
et al ., 1998 ; Nagarajan and Wang, 2000 ; Garti et al ., 2001 ; Fanun, 2007 ); (d) being often
soluble in both organic and aqueous phases, co-surfactants help solubilize poorly-soluble
compounds (e.g., peptides, vitamins, etc.) (Sottmann and Strey, 1996; Krafft and Riess,
1998 ; Yaghmur et al ., 2002 ; Garti, 2003 ; de Campo et al ., 2004); (e) use of co-surfactants
can result in fully dilutable microemulsions (Martino and Kaler, 1995; Kunieda et al ., 1999 ;
Yaghmur et al ., 2002, 2005); and (f) they aid in “connecting” the w/o and o/w regions via a
bicontinuous region (Friberg et al ., 1994 ; Fanun, 2007 ), hence the transition from one
regime to another occurs without phase separation.
The inherent properties of alcohol co-surfactants will impact on usage. In particular,
alcohol chain length has a significant effect on the ionization of the surfactant and disorder
at the oil/water interface. Thus, shorter-chained alcohols will increase interfacial disorder
(flexibility) (Bansal et al ., 1979 ; Tabony et al ., 1983 ). However, toxicity concerns limit the
use of effective co-surfactants such as 1-butanol, 2-butanol and tert -butanol, which are toxic
to humans (Attwood, 1994). The most commonly used food-safe co-surfactant is ethanol,
though its use in foods limits product marketing due to regulatory requirements, which dif-
fering from country to country.

5.8 FACTORS AFFECTING PHASE BEHAVIOR

Microemulsion phase behavior and microstructure depend on two interfacial parameters,
namely the spontaneous film curvature (the optimal curvature that the surfactant film attains)
and the elasticity (bending modulus) of the surfactant film (Attwood, 1994; Sottmann and