Structured membrane formation via biopolymer-surfactant coacervation

Coacervation can yield membranes with different permeabilities, making it attractive for technological applications in encapsulation or separation processes. During coacervation, oppositely charged species like polymers and surfactants form complexes due to coulombic attraction. These polymer-surfactant complexes are insoluble in the aqueous phase leading to phase separation. As a result, a structured membrane can form in the contact zone between a polymer and a surfactant solution. However, the details of the structure formation processes are only poorly understood.
In this study, we use a combination of the anionic biopolymer xanthan gum and cationic CnTAB surfactants. The quasi 2D-geometry of the Hele-Shaw cell allows us to observe the membrane formation and the growth dynamics in-situ. By this, we can link the membrane properties for different surfactant types and concentrations to the underlying mass transfer and structure formation processes.
The results show that the density and permeability of the formed membrane significantly depend on surfactant chain length and concentration. In a wide range of experiments, the formation of a porous structure is observed whose characteristics depend on the process parameters. The pore formation can be explained as instability of the growing membrane surface in interaction with the supply of polymer across the depleted zone in the vicinity of the membrane front. Our insights provide the basis to control membrane thickness, density, porosity and pore structure. These properties are essential for technological applications since they determine the selectivity, permeability and mechanical stability of the membrane.