Influence of Surfactant Structure on the Drainage of Nonionic Surfactant Foam Films.

The stability of vertical liquid films produced from aqueous solutions of polyoxyethylene alkyl ether (CnEm) was studied by monitoring the dynamic surface tension (gammat) and aqueous core thickness of a vertical foam film (d2) measured by FT-IR. The temperature dependence of the drainage patterns of the liquid films was clarified for CnE8 (n = 10, 12, 14, and 16) solutions. It was found that the critical thickness (Drup) where the film rupture happens increased with increasing temperature in the case of C10E8 and C12E8, while Drup remarkably decreased and finally formed a black film in the case of C14E8 and C16E8 solutions within the measured temperature range. In addition, a delay in the film drainage occurred in these systems, which was caused by the stabilization of the films via the Marangoni effect proved by dynamic surface tension measurements. On the other hand, heterogeneous ethoxylated dodecyl ethers (C12Em) modified with a hydrophilic group showed a remarkable increment of Drup under dynamic conditions without the Marangoni stabilization in the foam film. Adding a hydrophobic group at the end of the polar head produces a large area per molecule and causes a large increase in the surface coverage of the air bubbles. However, a less structured surface coverage facilitated the breaking of the foam during the Ross and Miles test. These results indicate that under dynamic conditions, the Marangoni effect and the hydrophobic interaction at the surface become important for foam film rupture. Copyright 1998 Academic Press.