Page 431 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 419

Figure 10.7 Different micelle structures of (A) spherical and (B) cylindrical [60].

traditional synthesized surfactant is that lower concentrations of which can provide the
same results [15,443]. In other words, introducing a relatively low concentration of
biosurfactant can induce the desired effects [31]. It can be said that biosurfactant often
shows better performance than their synthetic counterparts [444].
Aggregates of surfactant monomers known as micelles form when sufficient
amount of surfactant is added to the solution. In low concentrations, micelles have a
spherical shape containing several hundred surfactant’s monomers. The charged head
of the molecule is oriented toward the aqueous phase while the hydrophobic ends
develop a hydrophobic environment in the core of the micelle [445]. Cylindrical
micelle can be formed at higher surfactant concentrations. Fig. 10.7 shows both
spherical and cylindrical micelles.
The threshold concentration at which the micelles starts to form is referred as the
critical micelle concentration (CMC). At concentrations above CMC, addition of sur-
factant to the aqueous solution will not increase the number of monomers but results
in development of additional micelles [48].
The required surfactant concentration to form micelles is typically small and in
range of 10 2000 mg/L [445]. This depends on the parameters such as temperature,
water hardness, and also surfactant type. Moreover, as the hydrophobicity of the sur-
factant increases its CMC generally decreases [60]. Comparatively, biosurfactants exhi-
bits much lower CMC than their synthetic counterparts [51,52,446 449], which
makes it possible to use relatively lower concentrations of them compared with syn-
thetic ones [54]. This is in range of 20 2 50 mg/L [17]. As an example, 65% of the
residual kerosene from the sand-packed columns were recovered using a very low