Biosurfactant-Mediated Nanoparticle Synthesis A Green and Sustainable Approach - Biosurfactants: Research Trends and Applications - page 223

cosurfactant (Eastoe et al., 2006). In general, the phase behavior of the pseudoter-
nary systems can be studied from the phase diagrams, in which the weight ratio of
two components is generally held constant (Xie et al., 2005). The formation of a
clear single phase indicates the presence of monophasic system. The various effects
such as solvent, surfactants, and cosurfactants, added electrolyte, reagent concentra-
tion, water content that influence the particle size and other effects such as nature of
templates, influence of ion/molecular adsorption that influences particle shapes were
comprehensively reviewed (Eastoe et al., 2006). The use of long-chain alcohol such
as butanol in the pseudoternary system, with rhamnolipid as biosurfactant, widened
the phase existence of the monophasic microemulsion system (Xie et al., 2006).
As surfactants are amphiphilic molecules, they are soluble both in water and
hydrocarbon. In a ternary system containing water, oil, and cosurfactant, the sur-
factant will form spherical aggregates called reverse micelles by making ion-dipole
interactions with polar cosurfactant. The polar head group orients toward the cen-
ter, while the hydrocarbon tails face outward from the core. Here the cosurfactant
acts as a spacer molecule, which minimizes the electrostatic repulsion between the
polar head groups (Cushing et al., 2004). When water is added to these suspen-
sions, they are attracted toward the center as a result of ion-dipole and dipole-dipole
interactions.
The spherical micelles are characterized by the molar water-to-surfactant ratio:

[ ]

Ω= Water
S

0

[]

The relationship between Ω 0 and micellar radius R m is

R VV

3

3

Ω

s
s

w0
s

= +

m

Σ Σ

where
V s and V w are the molar volumes of surfactant and water, respectively (Cushing
et al., 2004)
Σ s is the molar interfacial area at the surfactant-oil boundary

The size of the micelle can be varied by varying the water-to-surfactant ratio. The
higher the ratio, the larger will be the size of reverse micelles. Nevertheless, the chal-
lenge is to obtain a microemulsion system containing reverse micelles with uniform
size and shape. The frequent collisions between the small-sized reverse micelles due
to Brownian movements exclude some of the surfactant molecules into the bulk oil
phase. This results in the formation of short-lived dimers that can mutually exchange
the contents between them. The continuous collision of reverse micelles and the subse-
quent formation of short-lived dimers in a homogeneous microemulsion will facilitate
the uniform distribution of contents. The fact that contents entrapped in two reverse-
micellar microemulsion solutions can be exchanged, in turn, can necessitate a chemi-
cal reaction. So, a typical procedure for reverse-miceller-based nanoparticle synthesis