Rhamnolipids Characteristics, Production, Applications, and Analysis - Biosurfactants: Research Trends and Applications - page 81

an ideal candidate for numerous applications, such as cosmetics, pharmaceutical,
petroleum, environmental, agricultural, and food industries (Bockmuehl, 2012; do
Valle Gomes and Nitschke, 2012; Lourith and Kanlayavattanakul, 2009; Mulligan
and Gibbs, 2004; Nitschke et al., 2011; Takemoto et al., 2010).
In the rhamnolipid mixture, the ratio of monorhamnolipids to dirhamnolipids
significantly affects the properties and behaviors and the properties of the rham-
nolipids. According to Cohen and Exerowa (2007), the ratio between monorham-
nolipids and dirhamnolipids in the rhamnolipid mixture affects the thickness of
the foam films and their surface electric properties, and consequently the stability
of the foams. The ratio of the monorhamnolipids and dirhamnolipids in the rham-
nolipid mixture is strain dependent; at the same time, some researchers such as
Lotfabad et al. (2010) indicate that the ratio of dirhamnolipid to monorhamnolipids
in a rhamnolipid mixture is highly substrate dependent. Therefore, by choosing an
optimal substrate and strain, rhamnolipids with the desired ratio of component, and
therefore with desirable properties, can be produced.
m iCelle C haraCteristiCs
By increasing the concentration of biosurfactants, they start forming structures such
as lamellae, vesicles, and micelles (Dahrazma et al., 2008). In water, the hydrophilic
heads create the outside layer and hydrophobic part (tail) is positioned inside; on the
other hand, in hydrophobic environment, the “inverse micelle” would be formed: the
tails would be on the surface of sphere and heads are placed inside. The shape and
the size of the micelle can be seen with transmission electron microscopy (Champion
et al., 1995). The size and the shape of micelles depend upon the size and ionic
strength of the head group and the size and shape of the hydrophobic tail. According
to Dahrazma et al. (2008), data from small-angle neutron scattering confirm that the
morphology of these aggregates is pH dependent. They concluded that as a result
of this pH dependency, the release of the metals or drugs from these pH-sensitive
vesicles could be controlled, so that they can be successfully used for drug delivery
or other applications that require a controlled release. On the other hand, pH could
be a controlling parameter on the efficacy of using biosurfactants in the process of
bioremediation. For example, Dahrazma et al. (2008) noted that an addition of 1%
NaOH increased the formation of large aggregates (>200 nm) and micelles with an
average diameter of 17 Å, but in an acidic environment (pH 5.5), addition of 1%
NaCl results in the formation of a mixture of large vesicles with an average radius of
600 Å. The reason for this occurrence could be that at higher pH, the charge densi-
ties on the carboxyl groups are increased, so the heads would be more repulsive,
increasing the heads' tendency to increase their distance from each other, therefore,
shaping a spherical structure (Ishigami and Suzuki, 1997). The other conclusion
would be that the shape and the size of these micelles are not only controlled by the
concentration of biosurfactants and the pH of the environment. It is also affected
by the presence and concentration of ions in the solution that can affect the charge
density on the carboxylic groups on the biosurfactants.
Muller (1993) studied the relation between the heat capacity of the micellization
for ionic surfactants and the temperature dependence of CMC. He found out that