Effect of surfactants in aerated bioreactors: Adding small amounts of detergents (surfactants) changes bubble behavior. Why can oxygen transfer rates increase in such systems?

Introduction / Context:
Oxygen transfer in gas–liquid bioreactors depends on the mass-transfer coefficient (kL a). Surfactants can alter bubble size distribution and coalescence, thereby changing interfacial area a and sometimes kL.

Given Data / Assumptions:

• Low surfactant concentrations that modify bubble coalescence behavior.
• Agitated or non-mechanical gas–liquid systems.
• All other conditions comparable.

Concept / Approach:
Surfactants lower surface tension and create interfacial films that inhibit bubble–bubble coalescence. This produces a population of smaller bubbles with larger total surface area per unit gas volume, typically increasing kL a and thus oxygen transfer rate, until excessive foaming or mass-transfer resistance at the interface offsets gains.

Step-by-Step Solution:
Note that oxygen transfer rate ∝ kL a * driving force.Surfactants discourage coalescence → more, smaller bubbles → higher a.Higher a at comparable gas flow increases kL a and oxygen transfer.Select the option that states this mechanism explicitly.

Verification / Alternative check:
Experimental kL a versus surfactant concentration curves often show an initial rise due to reduced coalescence, followed by a plateau or decline if foaming or surfactant films limit kL.

Why Other Options Are Wrong:
Enhancing coalescence or increasing surface tension would produce larger bubbles and lower a, reducing oxygen transfer.

Bubble expansion in size contradicts the commonly observed reduction in equivalent bubble diameter.

Common Pitfalls:

• Overdosing surfactant, which can cause foaming and operational issues.
• Ignoring changes to liquid rheology or antifoam effects that might counteract gains.

Final Answer:
Because surfactants discourage bubble coalescence, yielding smaller bubbles and higher interfacial area