3.4. Critical micelle concentration determination
The CMC is an important characteristic of a biosurfactant and is
defined as the biosurfactant concentration requisite to form micelle.
Upon reaching the CMC, the ST remains relatively constant
due to the interface saturation with the surfactant [34]. Distilled
water was found to have the ST of 72 mN/m and the addition of
biosurfactant reduced its ST to 28 mN/m. Fig. 4 represents the
measurement of ST as a function of biosurfactant concentration.
At this point, CMC of the USTBa biosurfactant was 35 mg/L and is
in close agreement with the literature for biosurfactant from B.
subtilis [6]. Furthermore, there was no significant change in the
ST till the end of the cultivation. This observation might be due
to the attainment of CMC by the biosurfactant [34]. Generally,
the ability to reduce ST to below 35 mN/m is one of the criteria
in selecting biosurfactant-producing microorganisms and our
USTBa fulfills this requirement.