Foam stability: The importance of film size and the micellar structuring phenomenon

The kinetic stability of SDS foam was probed using Bartsch method at concentrations above CMC (0.03 M and 0.06 M) and below CMC (0.006 M). The foam created from the higher concentration (0.06 M) of micellar solution was more kinetically stable than that produced from the 0.03 M SDS solution and showed a stepwise decrease. On the other hand, the foam produced from SDS surfactant concentration below CMC (0.006 M) did not show such a behaviour and the height remained more or less constant. The effect of the foam lamella size on the bulk foam stability was also investigated. It was found that the critical lamella area is independent of the hydrostatic pressure. And, it took longer to reach the critical lamella area in the 0.06 M SDS system than to reach the critical lamella area in the 0.03 M SDS system. We probed the foam stability by studying the stability of a single foam lamella and observed the micellar layering phenomenon. We found 5 film thickness transitions in the case of 0.06 M SDS system and 3 thickness transitions for 0.03 M SDS system. Therefore, the foam lamella formed from the 0.06 M SDS solution is kinetically more stable than that formed from the 0.03 M SDS solution because the better layering structure in the former case slows down the liquid film drainage. Therefore, this study clearly demonstrates that both the size of the foam lamella and the micellar concentration are important factors in controlling the bulk foam stability.